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How is the antigen against which autoantibodies are formed, identified?

How is the antigen against which autoantibodies are formed, identified?


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Say we suspect some new autoimmune disorder in a patient, and we collect blood for serology.

How are self antibodies differentiated from normal ones in the collected sample? Once they are identified, how is the antigen towards which it is directed known?

Are there assays for this? Or is it identified by the structure of the variable region? Can a simple IHC be done on all body tissues using the purified isolated antibody to localise the protein?

I mean, what is the root for suspicion of an antigen? Like, what caused ANCAs (anti-neutrophil cytoplasmic antibodies) to be suspected in systemic lupus erythematosis (SLE)?


All of the above, basically. If it's a protein antigen, you can perform an immunoprecipitation with the auto-antibody and use any one (or more) of several methods (Western blot, various methods of sequencing, etc.) to identify the binding partner(s). DNA and RNA can be identified easily by biochemical means. You can also use ELISAs or similar methods like electrochemiluminescence which allow a greater degree of multiplexing.

Sequencing the autoantibodies' variable regions may give some information about the structure of the antigen, but not enough to actually identify the binding partner. Also, one key fact to keep in mind is that the antibody response in many autoimmune diseases is polyclonal (just as the T cell response often is), meaning there could be dozens to hundreds of different complementarity-determining regions (CDRs) to sequence, a very laborious process that may not yield much information.

Immunohistochemistry (IHC) can be useful for seeing which cell types the antibodies bind to, but it doesn't give great resolution on the subcellular level, and doesn't tell you anything about what the actual target is. Additionally, due to the amount of chemical processing a formalin-fixed paraffin-embedded (FFPE) tissue goes through, the antibody's antigen may not be exposed properly for the antibody to bind.

From what I can find, people suspected an association between ANCA and lupus due to the similarities in both diseases' clinical presentation. Here is a paper from 1996 talking about it, there are many others.


15.1: Antibodies are produced in response to antigens

  • Contributed by Clare M. O&rsquoConnor
  • Associate Professor Emeritus (Biology) at Boston College

Antibodies are proteins produced by vertebrates with adaptive immune systems capable of responding to foreign antigens. Antigens are defined as substances that stimulate the production of antibodies. Antigens are commonly able to stimulate the production of multiple kinds of antibodies, each of which recognizes a small, distinct region on the surface of the antigen known as an epitope. Antibodies are Y-shaped proteins produced by lymphocytes that bind epitopes with high affinity.

Antibodies binding to an antigen.

An antigen with three different epitopes on its surface is bound by three different antibody molecules, each of which binds a singleepitope with high affinity.

The availability of hybridoma cells that secrete large quantities of antibodies with a single specificity has greatly facilitated structural studies on antibodies. Researchers are able to harvest antibody molecules secreted by cultured hybridoma cells and to prepare crystals for X-ray diffraction. Based on a large number of crystallographic studies, we now understand the basic architecture of antibodies, more properly known as immunoglobins. The crystal structures show that immmunoglobins (Igs) are composed of three domains that are readily apparent in the crystal structure (below). The two Fab regions (antigen-binding fragments) that form the arms of the &ldquoY&rdquo are hypervariable regions involved in binding antigen. The Fc region (crystallizable fragment) that forms the base of the &ldquoY&rdquo is recognized by non-immune effector cells, such as mast cells and macrophages, which process antigen-antibody complexes. Each Ig class has a characteristic heavy chain, which gives the class its name. We are using antibodies from the IgG class of immunoglobins, which have gamma heavy chains. (IgGs are also known as gamma globulins.) IgA molecules have alpha chains, IgM molecules have mu chains, etc.

Crystal structure of an IgG antibody.

This figure is derived from Protein Data Bankentry 1IGT (Harris et al., 1997).


Introduction

Natural antibodies are formed spontaneously without specific immunisation, in germ-free conditions. They are the first line of defence of the newborn organism [1-10], though this definition does not encompass anti-Gal antibodies and/or anti-Gal natural antibodies [11]. They were discovered about half a century ago [7, 10]. As early as in 1908, a Nobel prize was awarded to Paul Ehrlich for, among other things, the hypothesis that the organism of a healthy person creates antibodies which are part of 𠇊ll antibodies” [12]. The term “natural antibodies” was first introduced by Boyden in 1963 [13]. In Poland, in the 1980s these proteins were described [10] as an important element of immunity, though it was a controversial notion, e.g., in the context of artificial antigens. Natural antibodies are the first line of defence against infections before the creation of germinal centres in which adaptive antibodies are formed [8, 12, 14]. They occur in many vertebrates, e.g. amphibians, reptiles, fish, birds, and mammals, including humans [8, 15, 16]. In humans, they consist mainly of immunoglobulin M, immunoglobulin A with isotypes IgA1 and IgA2, and IgG, mainly IgG3, but also IgG1, IgG2 and IgG4 [1, 7, 8, 15-17]. Natural antibodies are synthesised by the subpopulation of B lymphocytes, mainly B1 lymphocytes and marginal zone B cells [1, 7-9, 12, 14-21]. While describing anti-Gal antibodies and/or anti-Gal natural antibodies, some authors [22-24] point out that the antibodies are formed by about 1% of circulating B lymphocytes, and that these are IgG, IgM, IgA and IgE, which constitute about 1% of all of the immunoglobulins present in blood.

In contrast to adaptive antibodies, natural antibodies (mainly IgMs) are produced before the exposure to foreign antigens or pathogens [25]. Unlike natural antibodies, adaptive antibodies are specific to a certain antigen and are produced by B2 cells, which require the binding of the antigen to the B-cell receptor (BCR) of B2 lymphocytes and the additional “help” of T lymphocytes [25]. In mice and humans, the isotopes of natural antibodies mainly create and switch B1 lymphocytes [8]. Their numbers decrease with age, which leads to a decrease in their immunological potential [8, 14, 16, 21, 26, 27]. Regardless of these facts, it was observed that there are healthy people who do not experience changes in the concentration of natural IgMs, even when they are over 25 years old [28-30], but the numbers of natural IgGs in their bodies rise [31]. Natural antibodies also differ from adaptive antibodies in their function [8], but similarly to adaptive antibodies, they are also multi-specific (polyreactive) and they identify autoantigens and new antigenic determinants, including those formed during apoptosis or the processes of oxidation [1, 14]. The autoreactivity of natural antibodies is mainly based on their ability to bind with such particles as oxidized low-density lipoproteins (oxLDL), which occur during atherosclerosis, amyloid and tau proteins, which occur in neurodegenerative diseases, and the NGcGM3 antigens (N-glycolyl [NGc] granulocyte macrophage [GM3]), which accompany malignant neoplasms [1, 6, 8, 12, 14, 32-40]. One of the most important functions of natural antibodies, including anti-Gal antibodies and/or anti-Gal natural antibodies, is the protection against viral, bacterial, fungal and protozoan pathogens [11]. Natural antibodies also recognise phosphorylcholines – a constituent of the membranes of many cells [14], including bacteria such as Streptococcus pneumoniae. It is assumed that they ensure specific homeostasis for the organism [1, 8, 9, 11, 12, 14, 32, 37, 38, 41]. This specific state of the organism conditioned by natural antibodies is also associated with the accelerated elimination of dead and dying cells and other “remains”, including factors potentially leading to inflammation and toxic elements directly responsible for damaging cells and tissues [8, 14]. Natural antibodies are also associated with the human microbiome as there is an important relation between natural antibodies and the commensal flora of the organism, which strongly influences the variety of natural antibodies [7, 12, 16, 18, 42-46]. It has been demonstrated that already at the early stages of life a constant dynamic balance is formed between the immune system of the host and microbiome antigens, a fact which has an influence on the proper development of the immune system [12, 18, 45, 46]. This state is conditioned by the participation of autopolyreactive receptors of humoral and cell-mediated immunity of low affinity to these antibodies and commensal bacteria. This situation means that even though the antibodies are able to recognise autoantigens and commensal microorganisms, they do not destroy them [18]. It was observed that natural antibodies synthesised by B1 lymphocytes and marginal zone lymphocytes are also conditioned by the influence of T lymphocytes, including γδT lymphocytes (abundant in the digestive tract), which together with numerously produced cytokines influence the synthesis of natural antibodies [1, 14]. It was observed that in non-immunised mice γδT lymphocytes effectively influence not only the numbers of natural IgA and IgG, but also the specifics and repertoire of these proteins [1, 47]. The analysis showed that γδT lymphocytes regulate the functional activity of natural antibodies by influencing, among other things, the level of IL-4, and it was observed that IL-18 increases the production of SIgM [1, 48]. It is assumed that the level and activity of natural antibodies of humans and mice are influenced by the changes occurring in γδT lymphocytes, mainly forming due to the influence of intestinal flora [1], and the fact that they provide cytokines. They support and modulate the development of B lymphocytes and influence the formation of natural antibodies even before the immunisation of the organism [1]. It has been demonstrated that natural antibodies working in tandem with complement serve as endogenic adjuvants in the production of CD8 + lymphocytes, e.g. after vaccination against leishmaniasis [48]. Therefore, it is suggested [48] that immunological complexes created from natural antibodies can be the reason for the increase in the numbers of CD8 + lymphocytes and that they might stimulate these cells to secrete IL-4 [48].

The origin and role of natural antibodies

B1 lymphocytes, recently isolated as a new subpopulation of B lymphocytes [8, 14, 21], are generated in waves during ontogenesis, mainly in the fetal and post-fetal period. They are not created at later stages of life [14, 21, 49]. B1 lymphocytes develop outside the fetus from the yolk sac and the para-aortic splanchnopleura, in the fetal period from the bone marrow and liver, and in the first weeks after birth from the bone marrow [21, 49]. Initially, the function of B1 lymphocytes was established on the basis of the research carried out on mice, and their receptor profile (CD20 + CD27 + CD43 + CD70 − ) was determined on the basis of the analysis of umbilical cord blood and peripheral blood of adults [8, 12, 14, 50-52]. In mice, B1 lymphocytes are located mainly in the peritoneal cavity, pleural cavity, spleen, bone marrow and in small amounts in lymph nodes and blood [19, 53, 54]. These lymphocytes are different from conventional B2 lymphocytes in size. They are characterised by increased survivability ex vivo and resistance to apoptosis dependent on Fas receptors [14, 50, 51]. B1 lymphocytes are characterised by unique expression of proteins and gene transcription. They also have different signalling properties, including the reaction to phorbol ester [14]. They are characterised by higher intracellular concentration of Ca 2+ , and their development is independent of the influence of the B-cell activating factor BAFF/BLys and IL-7 [14]. However, B1 lymphocytes, similarly to B2 lymphocytes, are characterised by the basic function of B lymphocytes, i.e. the synthesis of antibodies necessary for the protection of the organism against pathogens [8, 9, 12, 14, 55]. It is assumed that the main sources of natural antibodies, both in mice and humans, are B1 lymphocytes and B lymphocytes of the marginal zone (MZ), but also other subpopulations of B lymphocytes [1, 7-9, 12, 14-21]. It is assumed that, due to their heterogeneity, B1 and MZ lymphocytes influence the occurrence of differences between the synthesised natural antibodies [14]. It is assumed that 80-90% of resting IgMs of the serum and 50% of resting serum IgAs (the major isotype of switched B-1 cell immunoglobulin) are produced by B-1 cells [12, 14, 16]. In mice, it has also been demonstrated that natural antibodies are different from adaptive antibodies in a number of ways, including their functioning [14]. Natural antibodies are polyreactive, autoreactive, and they express a relatively modest anti-microbial affinity [8, 14, 56, 57]. Polyreactivity ensures different heterology for a single antibody, though this is also conditioned by widely arranged surface antigens. As a result, their avidity increases [14]. The special manner of their conformational changes in the Fc region makes them very efficient [14]. It is assumed that the most studied natural antibodies in humans and mice are IgMs. They are germline-encoded and produced by B1a (CD5 + ) lymphocytes, mainly of the spleen, but also by B1 lymphocytes of the peritoneum and bone marrow [1, 3, 14, 21, 58, 59]. Natural antibodies are important in the prevention of illnesses, including autoimmune diseases [14], which is also observed in relation to anti-Gal antibodies and/or anti-Gal natural antibodies in Crohn’s disease and Henoch-Schönlein purpura [22]. It was also determined that natural antibodies can be used in the treatment of older people [8, 14, 50]. In other examples, they are used in the case of degenerative diseases associated with the accumulation of toxic particles and in the treatment of bacterial infections [8, 14, 60]. Born et al. and Ugorski suggest that the existence of natural antibodies has been programmed in ontogenesis in such a way as to enable mammals to develop normally by ensuring all the necessary functions and protection against common pathogens at the time when there are no adaptive antibodies [1, 10]. Such a state influences tissue homeostasis and immunological balance important in the case of infectious diseases and anticancer protection [1, 7, 8, 12, 14, 32, 37-39]. Formed during an infection, the state is influenced by the fact that, importantly, the main component of natural antibodies recognises phosphorylcholines (PC), which are a key constituent of cell membranes of Gram-positive and Gram-negative bacteria, as well as protozoa and fungi [7, 14, 48]. It is currently suggested that natural antibodies also recognise the PC of membranes of apoptotic cells and oxidised lipids [5, 7, 14, 61-65]. Such recognition of antigens by natural antibodies, mainly apoptotic cells, is conditioned by their other element – phosphatidylcholines (PtC) – which are a key constituent of aging (apoptotic) erythrocyte membranes present in elderly people [14, 66]. This specific recognition of apoptotic cells by natural antibodies may also lead to the excessive activation of the immune system and chronic inflammation [7]. On the other hand, participation of natural antibodies in the removal of apoptotic cells leads to a decrease of the inflammation [7, 67]. Natural antibodies also recognise low-density lipoproteins, protecting the organism against atherosclerosis because oxLDL lead to the formation of atherosclerotic plaque, inflammation and in consequence to cardiovascular diseases. Furthermore, natural antibodies bind with the oxidised main form of lipoproteins – apolipoprotein B100 [8, 14, 38]. These antibodies also participate in the prevention of tumours [8, 68]. By binding with the tumour antigen NGcGM3 – which is present, e.g. during lung cancer – they may lead to the elimination of cancer cells. Binding with a tumour antigen proceeds via a mechanism dependent on the complement and/or an oncosis-like mechanism dependent on the complement [8, 33, 48, 68, 69]. The participation of natural antibodies was also observed in reactions with the Thomsen-Friedenreich tumour antigen [1, 32, 70], the ganglioside of neurons in the Guillain-Barré syndrome [1, 71] and the amyloid present in the Alzheimer disease [1, 8, 26, 39, 72]. It is suggested that, due to this array of functions, natural antibodies might also serve as biomarkers in the clinical studies of these states [12, 14, 63]. It is accepted that the increased frequency of many diseases, including the illnesses common in older people, is associated with a decrease in the number of antibodies [8, 14, 26, 48]. Therefore, more and more often, in order to treat these states, it is recommended to use intravenous immunoglobulins (IVIG) which are built mainly from IgGs, including natural IgGs, and trace amounts of polyclonal natural antibodies – IgMs and IgAs [1, 7, 12, 35, 39, 41, 73-78]. It has been demonstrated that a therapy using IVIG is very effective in patients with autoimmune diseases and the systemic inflammatory response syndrome (SIRS) [79]. Currently, due to the features of natural antibodies, mainly IgMs, there are attempts to use these antibodies along with IgAs in IVIG, which should increase the anti-inflammatory capabilities of the preparation [79].

The characteristics and role of IgM natural antibodies

The M class immunoglobulins are the most studied natural antibodies. They commonly occur in healthy people and are well represented in the circulation after birth [12, 48, 80]. The antibodies of this class are one of the main immunoglobulins in the organism, and they are the earliest to form out of all the antibodies during ontogenesis [55]. Both in humans and animals, they manifest large amounts of SIgM (300-800 μg/ml for mice and 400-2300 μg/ml for humans) [81]. Most natural IgMs are formed by B1 lymphocytes, but also by B lymphocytes of the marginal zone of the spleen [3, 7, 12, 14, 16, 21, 58, 59, 80, 82, 83]. These antibodies probably account for about 80% of all IgMs circulating in the organism [12, 16, 21] and regardless of their immunological function [14, 48, 55] they also have an influence on the development of B lymphocytes [1]. In mice and humans, IgMs are mainly encoded in the germline, and they are formed by B1a (CD5 + ) lymphocytes [1, 21]. However, there are a lot of data suggesting that, apart from B1a (CD5 + ) lymphocytes, B1b (CD5 - ) lymphocytes are also responsible for the synthesis of IgMs [21]. The production of natural IgMs may also be increased through the activation of LPS/TLR or by activation of the B-cell receptors by pathogens [16, 48]. It was also observed [55] that natural IgMs undergo expression in epithelial cells stimulated by a TLR9 agonist, and due to this fact it is assumed that these cells are also an important source of natural IgMs. Similarly to adaptive IgMs, natural antibodies of the M class are pentameric molecules. However, in patients suffering from autoimmune diseases [84] and chronic liver diseases, natural IgMs may occur mainly in the monomeric form [85]. Natural IgMs have an Fc receptor specific for IgMs (FCMR), which is a transmembrane protein with the particle mass of about 41 kDa. However, after the process of O-glycosylation in the extracellular domain, the protein mass might increase to 60 kDa [81]. The receptor also undergoes expression in many immune system cells, including B CD19 + lymphocytes, T CD4 + /CD8 + lymphocytes and NK CD56 + /CD3 - cells [86]. More prominently, it undergoes overexpression in the case of chronic lymphocytic leukaemia and because of that it may serve as a specific marker for this illness [86]. Polyvalent natural IgMs with 10 antigen binding sites recognise many structures, including proteins, carbohydrates, phospholipids and nucleic acids. They participate in the identification of apoptotic cells thanks to the changes to which the membrane of the cells is subjected. One of those changes is oxidation, which may enable the recognition of the main lipid group of phosphorylcholine [5, 12, 48, 64, 82, 87]. The role which natural IgMs play in the process of phagocytosis of apoptotic cells leads to the confirmation of their anti-inflammatory effect. This feature has been confirmed in mice with a deficit of natural M-class antibodies. In this case, the potential anti-inflammatory properties of IgMs in relation to the membranes of apoptotic cells has been demonstrated [2, 12, 87]. It has to be added that the clearance of apoptotic cells is associated not only with the removal of cellular bodies, but also with the protection against potentially harmful factors, namely the high-mobility group box protein 1 (HMGB – 1) and heat shock proteins (HSP) [12] – the elements that form hazard factors – DAMP (damage-associated molecular patterns) [88]. In vitro research showed that the anti-apoptotic properties of natural IgMs in relation to the PC determinant inhibit the expression of cytokines by means of toll-like receptors (TLR) and the activation of mitogen-activated protein kinases, which are elements of the innate cell immunity [2, 12]. Natural IgMs may also induce specific anti-inflammatory signal pathways, which are dependent on the induction of the immunosuppressive phosphatase mitogen activated protein kinase phosphatase 1 (also known as MKP-1 or DUSP-1) in dendritic cells derived from the bone marrow [5, 12, 64]. Currently, it is assumed that the removal of apoptotic bodies from the organism is associated with the functioning of SIgM, the functioning of which is dependent on numerous elements of the immune system, including the IgM Fcα/μR receptor and the complement component C1q [7, 58, 80, 89]. Moreover, this process is also associated with natural IgMs, because these antibodies combine with the complement component C1q, leading to a situation in which apoptotic bodies are directed towards macrophages [7, 58, 80, 89], which destroy them in the process of efferocytosis [90-92]. Therefore, thanks to such polyreactivity, they strengthen the effectiveness against infection by using phagocytosis and other processes [3, 5, 64, 80, 93]. Activation of the complement cascade, by lectin which binds mannose or by a constituent of the C1q complement, may also take place when the targets of natural IgMs are the PC epitopes of apoptotic cells [2, 3, 5, 9, 12, 48, 64]. It has been demonstrated that the serum of newborns is characterised by high reactivity of natural IgMs towards many self-antigens, viral ssDNA and LDL particles [6, 12, 35, 36, 94], common bacterial antigens, phospholipids and some cell membrane proteins [1, 9, 21, 94]. Numerous studies show that autoimmune diseases can be blocked by natural autoantibodies of the IgM isotype that mask the antigens originating from pathogenic autoimmune T lymphocytes [6]. In mice which were deprived of the ability to create SIgM, it was observed that the animals were predisposed to develop “pathogenic” IgG autoantibodies and lupus-like autoimmunity [12, 95, 96]. Therefore, it is concluded that natural IgMs protect the organism against autoimmune diseases [12], and this protective characteristic of IgM is also true for cardiovascular system illnesses. [12, 15, 21]. Natural IgMs may participate in the feedback inhibition of B1 cell differentiation [97]. This function of IgM was observed in mice with a deficiency of secreted IgM and those with insufficient FcμR [97-99]. It was observed that the number of B1 lymphocytes in the MZ of the spleen significantly increased in mice with insufficient IgM, whereas the concentration of IgM increased in the animals with a deficiency of FcμR. This phenomenon may indicate that the level of natural IgMs maintains the homeostasis of B cells through binding FcμR [97-99]. The participation of IgM in the protection against atherosclerosis was confirmed by a study on mice. In this case, natural IgMs probably removed oxidised low-density lipoproteins and “pathogenic” lipids [12, 34, 38, 40]. Furthermore, natural IgMs inhibit the progression of atherosclerotic changes in cases of hypercholesterolaemia in mice suffering from the deficiency of apolipoprotein E (ApoE) [12, 100]. Their protective role was also registered in patients subjected to haemolysis and those with acute coronary syndromes. It was observed that when the level of natural IgMs in these patients was low, the mortality rate was increased [12, 63, 101]. Due to this fact, it is assumed that a high concentration of these antibodies decreases the risk of cardiovascular diseases, such as myocardial infarction (heart attack) and stroke [5, 12, 37, 64]. It is assumed that natural IgMs can serve as markers in studies that allow one to determine the occurrence of these illnesses, mainly disorders associated with atherosclerosis [12]. In mice, it was determined that natural IgMs are also a basic element which forms the protective barrier against microorganisms because a reduction of IgMs leads to infection in these animals [7, 12]. It has been demonstrated that the IgM class antibody can also cross-react with epitopes associated with the pathogenic bacteria Porphyromonas gingivalis, which are responsible for inflammation in the oral cavity [12, 102]. This antibody also protects the human organism against infection with Pseudomonas aeruginosa, Streptococcus pneumoniae and influenza virus [1]. An increase in their levels is also observed during infections caused by Mycoplasma pneumonia and Epstein-Barr virus [103-105]. These natural antibodies also support host immunity to infections with the Pneumocystis fungus [106]. It was observed that they also have an influence on the recognition of the antigens of this type of fungi via dendritic cells, increasing their migration to lymphatic nodes and causing intense differentiation of Th2 and Th17 cells during infection with this pathogen [106]. Thus, it can be assumed that natural IgMs are responsible for many physiological processes, including the homeostasis of tissues, the modulation of the immunological response and the apoptotic clearance of cells [12, 14, 21, 48]. It is probable that these antibodies are an important factor which makes it possible to survive the conditions of chronic inflammation [12], but, at the same time, they strengthen the inflammatory state by increasing the secretion of, e.g. crystals of uric acid, which amplify the inflammation by recruiting neutrophils [101]. As mentioned above, natural IgMs protect humans against autoimmune diseases [12], probably including systemic lupus erythematosus. They are also responsible for the prevention of cardiovascular diseases, including atherosclerosis [5, 7, 12, 21, 37], and old-age illnesses [14].

The characteristics and role of natural antibodies of the IgG and IgA class

In terms of natural antibodies, apart from the widely known IgM, there are also natural IgAs and IgGs [8, 12, 17, 45, 46]. Natural IgGs are further divided into subclasses IgG1, IgG2, IgG4 and the highest level of IgG3 [1, 7, 8, 16, 17]. IgG antibodies are the only isotypes that can cross the barrier of the placenta in order to ensure fetal immunity [7, 9, 19]. IgGs are created by various subpopulations of B lymphocytes, and it is supposed [1, 7-9, 12, 14-21] that B1 lymphocytes and B lymphocytes of spleen MZ are mainly responsible for the expression of IgG2 [7, 8, 12, 16]. Both natural IgGs and IgMs bind with the same phylogenetically conserved antigens and in mice about 15-20% of IgGs were found to be polyreactive, similarly to IgAs [16]. In contrast to IgMs, natural IgGs are inactive after birth. In mice, it has been demonstrated that B lymphocytes begin the production of IgGs after exposure to intestinal bacteria or foreign antigens [7, 8, 12, 16, 42, 73]. It was reported that in humans it can take as long as two years before the concentration of natural IgGs in the serum becomes high enough to be noticeable [107]. It is also suggested that autoreactive T lymphocytes may activate B lymphocytes to produce natural IgGs in adults [16]. It was observed that the level of polyreactive anti-dsDNA IgGs increases after various infections and that the same anti-dsDNA IgGs cross-react with the antigens of microorganisms, including bacteria [16, 43, 108]. The potential role of natural IgGs in controlling inflammation has been demonstrated, based on a complex that formed in haemolytic conditions, between IgG antibodies and haemoglobin [7]. It has been shown in vitro [7] that purified natural IgGs derived from the serum of healthy people recognise Gram-positive and Gram-negative bacteria by means of pattern recognition receptors (PRRs), such as ficolin or mannose-binding lectin (MBL). It is known that, by binding with polysaccharide residues on microorganisms, C-type lectin causes the formation of a complex which includes natural IgGs and lectin, strengthening the process of bacterial phagocytosis via FcγR1 of, e.g. monocytes [17]. These PRR:PRR interaction-mediated mechanisms induce even stronger immunological responses [7]. In this case, an interaction between IgG and ficolin occurs, amplifying the aforementioned phagocytosis [7, 17, 109]. Interestingly, the characteristic complex of IgG-ficolin occurs in both humans and mice, which proves the fundamental importance of IgGs in the immunological protection of the organism [7, 76]. Both natural IgGs and adaptive IgGs react with PRRs via the C region of the H chain, which indicates that Fc is important in the transmission of host defence information, regardless of the origin and specificity of IgGs [7]. Natural IgGs also specifically cooperate with lectin in the struggle against infections caused by Pseudomonas aeruginosa and Staphylococcus aureus [7, 17]. The human anti-PC IgGs mainly represent the IgG2 subclass, whereas anti-MDA (malondialdehyde) represent IgG1 or IgG3 subclasses, the latter having a higher potential to include the complement cascade and to engage in the activation of the Fc immunoglobulin receptor [5, 7, 12, 64]. It was observed that anti-MDA IgGs do not show high expression apart from in patients with inflammatory diseases [5, 12, 64], and that anti-PC IgGs are present even in healthy people [12]. Therefore, it is assumed that these natural IgGs participate in both the regulation of inflammatory states and in the protection of the organism against pathogens, fulfilling the role of innate immunity [7].

On the other hand, natural IgAs are a group of immunoglobulins consisting of two subclasses, IgA1 and IgA2, which are present in mucosal surfaces [8, 12, 14]. In their case, it has been demonstrated [7] that a percentage of natural IgAs are formed by B1 lymphocytes at the newborn stage [7, 12, 44-46] and that, together with natural IgMs, they recognise autoantigens and bind with homologous molecules produced by different microbes [12]. It was observed that natural IgAs along with natural IgMs are important factors responsible for a variety of microbes which settle in the human intestine – the microbiome [12, 45, 46]. It has been shown that the isotypes of IgA inhibit inflammation by participating in it via a reaction with the Fc type 1 receptor (FcαR1/CD89), but the manner in which they take part in this process is a matter of debate [7]. The regulating role of natural IgAs has also been demonstrated in patients who suffered from a selective deficiency of IgAs, when the concentration of both IgA1 and IgA2 was significantly decreased or totally absent but, at the same time, the concentration of IgMs and IgGs remained normal [78]. In such a case, apart from a higher frequency of occurrence of infections in the respiratory tract and digestive tract, the patient was more susceptible to autoimmune disorders, allergy-related illnesses, haematological diseases, arthritis, chronic liver inflammation, ulcerative colitis and Crohn’s disease.


Advantages and Disadvantages of Using Polyclonal Antibodies

  • Production is quicker
  • It is inexpensive
  • Tolerant of minor changes of antigen. Polyclonal antibodies are less sensitive to antigen changes than monoclonal antibodies.
  • Have a choice of producing antibodies in different animals.
  • Chances of getting a better response to the antigen are increased, and can be tried with various animal sources as the secondary antibody produced recognizes different epitopes on the same antigen.
  • Moderately easy to purify while using the high-affinity chromatography methods.

Disadvantages

  • An amplified chance for cross-reaction and false positives.
  • Non-specific interaction with the considerable antigen heterogeneity within the antibody pool.
  • The life span of the host animal is limited.
  • Multiple epitopes make it essential to check the immunogen sequence for any cross-reactivity.
  • Multiple animals have to be immunized against the same antigen.
  • Antibody response depends on the host animal.
  • Sometimes requires multiple control samples to arrive at meaningful conclusions.

Applications of Polyclonal Antibodies

Polyclonal antibodies are a mixture of heterogeneous products produced by different B cell clones. They can recognize and bind to many different epitopes of a single antigen. Polyclonal antibodies are produced by injecting an immunogen into an animal.

After being injected with a specific antigen to elicit a primary immune response, the animal is given a secondary even tertiary immunization to produce higher titers of antibodies against the particular antigen. After immunization, polyclonal antibodies can be obtained straight from the serum or purified to get a free solution from other serum proteins.

The ability of antibodies to selectively bind a specific epitope present on a chemical, carbohydrate, protein or nucleic acid has been thoroughly exploited through the years, as evidenced by the broad spectrum of research and clinical applications in which they are utilized. Applications include simple qualitative and quantitative analyses to ascertain the following:

  • Whether an epitope is present within a solution, cell, tissue, or organism, and if so, where
  • Methods to facilitate purification of an antigen, antigen-associated molecules, or cells expressing an antigen and
  • Techniques that use antibodies bind to mediate and modulate physiological effects for research, diagnostic, or therapeutic purposes.

The applications listed in Table 1 are by no means exhaustive. Still, they illustrate that the versatility of an antibody-x2y is frequently limited only by the user’s imagination and determination.

PurposeApplications relative to antigen
SolubilizedIntact tissues/cellsorganism
Analysis (qualitative or quantitative)Immunoblot (Western blot)FACS b analysisImmunoimaging (SPECT b and PET b)
Immunoprecipitation
Proteomics/antibody microarraySandwich ELISAImmunofluorescence
Proteomics/antibody microarrayImmunohistochemistry
X-ray crystallography
Purification and/or enrichmentImmunoaffinity purificationFACS and MACS
Mediation and/or modulationCatalysis-abzymesNeutralize activityNeutralize activity
Activate signalingDeplete cell types to alter phenotype
Proteomics/intrabodiesImmunotherapy

A particular rundown of applications in which polyclonal antibodies(PAbs), monoclonal antibodies (MAbs), their pieces and forms, either play a fundamental part or have had a massive effect in the essential exploration process. With the exception of imaging, immunotherapy, immunohistochemistry, and x-ray crystallography, regardless of whether to utilize PAb or MAb, relies upon the setting wherein the application is being utilized and the staff’s specialized capacities using them.

ELISA, protein connected immunosorbent examine ELISPOT, compound connected immunospot test FACS, fluorescence-enacted cell checking MACS, attractive actuated cell arranging PET, positron discharge tomography SPECT, single-photon outflow automated tomography.

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Autoantibodies

Circulating antibodies elicited by the patient’s own immune system after exposure to cancer proteins are emerging as promising biomarkers for the early detection of cancer. An advantage of autoantibodies as biomarkers is their production in large quantities despite the presence of a relatively small amount of corresponding antigen. Autoantibodies are also expected to have persistent concentrations and long half-lives due to limited proteolysis and clearance. The immune system constantly monitors the body for the invasion of microorganisms and foreign molecules. A tightly regulated network of antibodies, T-lymphocytes, antigen-presenting cells, cytokines, and microenvironment signals secures the development of an appropriately targeted immune response to combat infections. Foreign extracellular and surface antigens are recognized by B-lymphocytes, which respond by secreting antibodies. To mount a sustained antibody response, B cells require an additional signal from T helper cells, which present the relevant antigen as peptide fragments 15–25 amino acids in length that are in complex with major histocompatibility complex (MHC) class II. Antigens can also stimulate CD8+ T lymphocytes. These cells are activated by intracellular and membrane proteins that are processed by the endogenous processing pathway and presented as peptides 8–10 amino acids in complex with MHC class I. The 2 systems are highly coordinated, and in most cases, high affinity immunoglobulin G (IgG) antibody responses require recognition of the antigen by both B and T lymphocytes. During the initial development of the immune system, more than half of the newly generated B cell receptors are estimated to be capable of binding autoantigens. Most autoreactive B cells are eliminated during B cell maturation, however, preventing mature B cells from reacting with self-molecules. This selection provides the basis for the development of self-tolerance, the ability of the immune system to recognize and ignore the body’s own cells and tissues. Sometimes this mechanism fails and the immune system reacts with one’s own antigens as a consequence of over-expression, mutations, changes in protein half-lives, misfolding, aberrant degradation of self-proteins, or altered post-translational modifications (eg, glycosylation and phosphorylation) of the protein. Autoantibodies have long been recognized in autoimmune diseases, including systemic lupus erythematosus, myasthenia gravis, and rheumatoid arthritis. In some of the diseases, autoantibodies play a central role in its pathogenesis (eg, myasthenia gravis), whereas their role in others is less clear. Nevertheless, the existence and detection of autoantibodies is an important element in establishing an accurate diagnosis. In rheumatoid arthritis, for example, the test for an anti-IgG antibody, also known as the rheumatoid factor, is useful and has a sensitivity of approximately 80%.


Immunoglobulins-Antigen-Antibody reactions and Selected Tests

The combining site of an antibody is located in the Fab portion of the molecule and is constructed from the hypervariable regions of the heavy and light chains. X-Ray crystallography studies of antigen-antibody interactions show that the antigenic determinant nestles in a cleft formed by the combining site of the antibody as illustrated in Figure 1. Thus, our concept of antigen-antibody reactions is one of a key (i.e. the antigen) which fits into a lock (i.e. the antibody).

Non-covalent Bonds

The bonds that hold the antigen to the antibody combining site are all non-covalent in nature. These include hydrogen bonds, electrostatic bonds, Van der Waals forces and hydrophobic bonds. Multiple bonding between the antigen and the antibody ensures that the antigen will be bound tightly to the antibody.

Since antigen-antibody reactions occur via non-covalent bonds, they are by their nature reversible.

Antibody affinity is the strength of the reaction between a single antigenic determinant and a single combining site on the antibody. It is the sum of the attractive and repulsive forces operating between the antigenic determinant and the combining site of the antibody as illustrated in Figure 2.

Affinity is the equilibrium constant that describes the antigen-antibody reaction as illustrated in Figure 3.

Avidity is a measure of the overall strength of binding of an antigen with many antigenic determinants and multivalent antibodies. Avidity is influenced by both the valence of the antibody and the valence of the antigen. Avidity is more than the sum of the individual affinities. This is illustrated in Figure 4.

To repeat, affinity refers to the strength of binding between a single antigenic determinant and an individual antibody combining site whereas avidity refers to the overall strength of binding between multivalent antigens and antibodies.

SPECIFICITY AND CROSS REACTIVITY

Specificity refers to the ability of an individual antibody combining site to react with only one antigenic determinant or the ability of a population of antibody molecules to react with only one antigen. In general, there is a high degree of specificity in antigen-antibody reactions. Antibodies can distinguish differences in:

The primary structure of an antigen

Isomeric forms of an antigen

Secondary and tertiary structure of an antigen

Cross reactivity refers to the ability of an individual antibody combining site to react with more than one antigenic determinant or the ability of a population of antibody molecules to react with more than one antigen.

Factors affecting measurement of antigen-antibody reactions

The only way that one knows that an antigen-antibody reaction has occurred is to have some means of directly or indirectly detecting the complexes formed between the antigen and antibody. The ease with which one can detect antigen-antibody reactions will depend on a number of factors.

Affinity
The higher the affinity of the antibody for the antigen, the more stable will be the interaction. Thus, the ease with which one can detect the interaction is enhanced.

Avidity
Reactions between multivalent antigens and multivalent antibodies are more stable and thus easier to detect.

Physical form of the antigen
The physical form of the antigen influences how one detects its reaction with an antibody. If the antigen is a particulate, one generally looks for agglutination of the antigen by the antibody. If the antigen is soluble one generally looks for the precipitation of the antigen after the production of large insoluble antigen-antibody complexes.


Agglutination Tests

Agglutination/Hemagglutination
When the antigen is particulate, the reaction of an antibody with the antigen can be detected by agglutination (clumping) of the antigen. The general term agglutinin is used to describe antibodies that agglutinate particulate antigens. When the antigen is an erythrocyte the term hemagglutination is used. All antibodies can theoretically agglutinate particulate antigens but IgM, due to its high valence, is particularly good agglutinin and one sometimes infers that an antibody may be of the IgM class if it is a good agglutinating antibody.

Qualitative agglutination test
Agglutination tests can be used in a qualitative manner to assay for the presence of an antigen or an antibody. The antibody is mixed with the particulate antigen and a positive test is indicated by the agglutination of the particulate antigen. (Figure 7).

For example, a patient’s red blood cells can be mixed with antibody to a blood group antigen to determine a person’s blood type. In a second example, a patient’s serum is mixed with red blood cells of a known blood type to assay for the presence of antibodies to that blood type in the patient’s serum.
Quantitative agglutination test
Agglutination tests can also be used to measure the level of antibodies to particulate antigens. In this test, serial dilutions are made of a sample to be tested for antibody and then a fixed number of red blood cells or bacteria or other such particulate antigen is added. Then the maximum dilution that gives agglutination is determined. The maximum dilution that gives visible agglutination is called the titer. The results are reported as the reciprocal of the maximal dilution that gives visible agglutination. Figure 8 illustrates a quantitative hemagglutination test.

Prozone effect – Occasionally, it is observed that when the concentration of antibody is high (i.e. lower dilutions), there is no agglutination and then, as the sample is diluted, agglutination occurs (See Patient 6 in Figure 8). The lack of agglutination at high concentrations of antibodies is called the prozone effect. Lack of agglutination in the prozone is due to antibody excess resulting in very small complexes that do not clump to form visible agglutination.

Applications of agglutination tests

i. Determination of blood types or antibodies to blood group antigens.

ii. To assess bacterial infections

e.g. A rise in titer of an antibody to a particular bacterium indicates an infection with that bacterial type. N.B. a fourfold rise in titer is generally taken as a significant rise in antibody titer.

Practical considerations
Although the test is easy to perform, it is only semi-quantitative.

Direct Coomb’s Test
When antibodies bind to erythrocytes, they do not always result in agglutination. This can result from the antigen/antibody ratio being in antigen excess or antibody excess or in some cases electrical charges on the red blood cells preventing the effective cross linking of the cells. These antibodies that bind to but do not cause agglutination of red blood cells are sometimes referred to as incomplete antibodies. In no way is this meant to indicate that the antibodies are different in their structure, although this was once thought to be the case. Rather, it is a functional definition only. In order to detect the presence of non-agglutinating antibodies on red blood cells, one simply adds a second antibody directed against the immunoglobulin (antibody) coating the red cells. This anti-immunoglobulin can now cross link the red blood cells and result in agglutination. This test is illustrated in Figure 10 and is known as the Direct Coomb’s test.

Applications
These include detection of anti-rhesus factor (Rh) antibodies. Antibodies to the Rh factor generally do not agglutinate red blood cells. Thus, red cells from Rh+ children born to Rh- mothers, who have anti-Rh antibodies, may be coated with these antibodies. To check for this, a direct Coombs test is performed. To see if the mother has anti-Rh antibodies in her serum an Indirect Coombs test is performed.
Hemagglutination Inhibition
The agglutination test can be modified to be used for the measurement of soluble antigens. This test is called hemagglutination inhibition. It is called hemagglutination inhibition because one measures the ability of soluble antigen to inhibit the agglutination of antigen-coated red blood cells by antibodies. In this test, a fixed amount of antibodies to the antigen in question is mixed with a fixed amount of red blood cells coated with the antigen (see passive hemagglutination above). Also included in the mixture are different amounts of the sample to be analyzed for the presence of the antigen. If the sample contains the antigen, the soluble antigen will compete with the antigen coated on the red blood cells for binding to the antibodies, thereby inhibiting the agglutination of the red blood cells. as illustrated in Figure 12.

By serially diluting the sample, you can quantitate the amount of antigen in your unknown sample by its titer. This test is generally used to quantitate soluble antigens and is subject to the same practical considerations as the agglutination test.


Precipitation tests

Radial Immunodiffusion (Mancini)
In radial immunodiffusion antibody is incorporated into the agar gel as it is poured and different dilutions of the antigen are placed in holes punched into the agar. As the antigen diffuses into the gel, it reacts with the antibody and when the equivalence point is reached a ring of precipitation is formed as illustrated in Figure 13.


The diameter of the ring is proportional to the log of the concentration of antigen since the amount of antibody is constant. Thus, by running different concentrations of a standard antigen one can generate a standard cure from which one can quantitate the amount of an antigen in an unknown sample. Thus, this is a quantitative test. If more than one ring appears in the test, more than one antigen/antibody reaction has occurred. This could be due to a mixture of antigens or antibodies. This test is commonly used in the clinical laboratory for the determination of immunoglobulin levels in patient samples.
Immunoelectrophoresis
In immunoelectrophoresis, a complex mixture of antigens is placed in a well punched out of an agar gel and the antigens are electrophoresed so that the antigen are separated according to their charge. After electrophoresis, a trough is cut in the gel and antibodies are added. As the antibodies diffuse into the agar, precipitin lines are produced in the equivalence zone when an antigen/antibody reaction occurs as illustrated in Figure 14.


This tests is used for the qualitative analysis of complex mixtures of antigens, although a crude measure of quantity (thickness of the line) can be obtained. This test is commonly used for the analysis of components in a patient’ serum. Serum is placed in the well and antibody to whole serum in the trough. By comparisons to normal serum, one can determine whether there are deficiencies on one or more serum components or whether there is an overabundance of some serum component (thickness of the line). This test can also be used to evaluate purity of isolated serum proteins.
Countercurrent electrophoresis
In this test the antigen and antibody are placed in wells punched out of an agar gel and the antigen and antibody are electrophoresed into each other where they form a precipitation line as illustrated in Figure 15. This test only works if conditions can be found where the antigen and antibody have opposite charges. This test is primarily qualitative, although from the thickness of the band you can get some measure of quantity. Its major advantage is its speed.


Radioimmunoassay (RIA)/Enzyme Linked Immunosorbent Assay (ELISA)

Radioimmunoassays (RIA) are assays that are based on the measurement of radioactivity associated with immune complexes. In any particular test, the label may be on either the antigen or the antibody. Enzyme Linked Immunosorbent Assays (ELISA) are those that are based on the measurement of an enzymatic reaction associated with immune complexes. In any particular assay, the enzyme may be linked to either the antigen or the antibody.

Competitive RIA/ELISA for Ag Detection
The method and principle of RIA and ELISA for the measurement of antigen is shown in Figure 16. By using known amounts of a standard unlabeled antigen, one can generate a standard curve relating radioactivity (cpm) (Enzyme) bound versus amount of antigen. From this standard curve, one can determine the amount of an antigen in an unknown sample.

The key to the assay is the separation of the immune complexes from the remainder of the components. This has been accomplished in many different ways and serves as the basis for the names given to the assay:

Precipitation with ammonium sulphate
Ammonium sulphate (33 – 50% final concentration) will precipitate immunoglobulins but not many antigens. Thus, this can be used to separate the immune complexes from free antigen. This has been called the Farr Technique

Anti-immunoglobulin antibody
The addition of a second antibody directed against the first antibody can result in the precipitation of the immune complexes and thus the separation of the complexes from free antigen.

Immobilization of the Antibody
The antibody can be immobilized onto the surface of a plastic bead or coated onto the surface of a plastic plate and thus the immune complexes can easily be separated from the other components by simply washing the beads or plate (Figure 17). This is the most common method used today and is referred to as Solid phase RIA or ELISA. In the clinical laboratory, competitive RIA and ELISA are commonly used to quantitate serum proteins, hormones, drugs metabolites.

Non-competitive RIA/ELISA for Ag or Ab
Non-competitive RIA and ELISAs are also used for the measurement of antigens and antibodies. In Figure 18, the bead is coated with the antigen and is used for the detection of antibody in the unknown sample. The amount of labeled second antibody bound is related to the amount of antibody in the unknown sample. This assay is commonly employed for the measurement of antibodies of the IgE class directed against particular allergens by using a known allergen as antigen and anti-IgE antibodies as the labeled reagent. It is called the RAST test (radioallergosorbent test). In Figure 19, the bead is coated with antibody and is used to measure an unknown antigen. The amount of labeled second antibody that binds is proportional to the amount of antigen that bound to the first antibody.

Tests for Cell Associated Antigens

Immunofluorescence
Immunofluorescence is a technique whereby an antibody labeled with a fluorescent molecule (fluorescein or rhodamine or one of many other fluorescent dyes) is used to detect the presence of an antigen in or on a cell or tissue by the fluorescence emitted by the bound antibody.

Direct Immunofluorescence
In direct immunofluorescence, the antibody specific to the antigen is directly tagged with the fluorochrome (Figure 20).

Indirect Immunofluorescence
In indirect immunofluorescence, the antibody specific for the antigen is unlabeled and a second anti-immunoglobulin antibody directed toward the first antibody is tagged with the fluorochrome (Figure 21). Indirect fluorescence is more sensitive than direct immunofluorescence since there is amplification of the signal.

Flow Cytometry
Flow cytometry is commonly used in the clinical laboratory to identify and enumerate cells bearing a particular antigen. Cells in suspension are labeled with a fluorescent tag by either direct or indirect immunofluorescence. The cells are then analyzed on the flow cytometer.


Figure 22 illustrates the principle of flow cytometry. In a flow cytometer, the cells exit a flow cell and are illuminated with a laser beam. The amount of laser light that is scattered off the cells as they passes through the laser can be measured, which gives information concerning the size of the cells. In addition, the laser can excite the fluorochrome on the cells and the fluorescent light emitted by the cells can be measured by one or more detectors.

The type of data that is obtained from the flow cytometer is shown in Figure 23. In a one parameter histogram, increasing amount of fluorescence (e.g. green fluorescence) is plotted on the x axis and the number of cells exhibiting that amount of fluorescence is plotted on the y axis. The fraction of cells that are fluorescent can be determined by integrating the area under the curve. In a two parameter histogram, the x axis is one parameter (e.g. red fluorescence) and the y axis is the second parameter (e.g. green fluorescence). The number of cells is indicated by the contour and the intensity of the color.

rx-24.jpg (17140 bytes) Figure 24
PowerPoint animation of figure 24 of this figure

Antigen/antibody complexes can also be measured by their ability to fix complement because an antigen/antibody complex will “consume” complement if it is present, whereas free antigens or antibodies do not. Tests for antigen/antibody complexes that rely on the consumption of complement are termed complement fixation tests and are used to quantitate antigen/antibody reactions. This test will only work with complement fixing antibodies (IgG and IgM are best).
The principle of the complement fixation test is illustrated in Figure 24. Antigen is mixed with the test serum to be assayed for antibody and antigen/antibody complexes are allowed to form. A control tube in which no antigen is added is also prepared. If no antigen/antibody complexes are present in the tube, none of the complement will be fixed. However, if antigen/antibody complexes are present, they will fix complement and thereby reduce the amount of complement in the tube. After allowing complement fixation by any antigen/antibody complexes, a standard amount of red blood cells, which have been pre-coated with anti-erythrocyte antibodies is added. The amount of antibody-coated red blood cells is predetermined to be just enough to completely use up all the complement initially added, if it were still there. If all the complement was still present (i.e. no antigen/antibody complexes formed between the antigen and antibody in question), all the red cells will be lysed. If antigen/antibody complexes are formed between the antigen and antibody in question, some of the complement will be consumed and, thus, when the antibody-coated red cells are added not all of them will lyse. By simply measuring the amount of red cell lysis by measuring the release of hemoglobin into the medium, one can indirectly quantitate antigen/antibody complexes in the tube. Complement fixation tests are most commonly used to assay for antibody in a test sample but they can be modified to measure antigen.


Dendritic Cells

Dendritic cells reside in the skin, lymph nodes, and tissues throughout the body. Most dendritic cells are antigen-presenting cells. That is, they ingest, process, and present antigens, enabling helper T cells to recognize the antigen. Dendritic cells present antigen fragments to T cells in the lymph nodes.

Another type of dendritic cell, the follicular dendritic cell, is present in lymph nodes and presents unprocessed (intact) antigen that has been linked with antibody (antibody-antigen complex) to B cells. Follicular dendritic cells help B cells respond to an antigen.

After T and B cells are presented with the antigen, they become activated.


REAP: A platform to identify autoantibodies that target the human exoproteome

Autoantibodies that recognize extracellular proteins (the “exoproteome”) exert potent biological effects but have proven challenging to detect with existing screening technologies. Here, we developed Rapid Extracellular Antigen Profiling (REAP) as a technique for comprehensive, high-throughput discovery of exoproteome-targeting autoantibodies. With REAP, patient samples are applied to a genetically-barcoded library containing 2,688 human extracellular proteins displayed on the surface of yeast. Antibody-coated cells are isolated by magnetic selection and deep sequencing of their barcodes is used to identify the displayed antigens. To benchmark the performance of REAP, we screened 77 patients with autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED). REAP sensitively and specifically detected known autoantibody reactivities in APECED in addition to numerous previously unidentified reactivities. We further screened 106 patients with systemic lupus erythematosus (SLE) and identified novel autoantibody reactivities against a diverse set of antigens including growth factors, extracellular matrix components, cytokines, and immunomodulatory proteins. Several of these responses were associated with disease severity and specific clinical manifestations of SLE and exerted potent functional effects on cell signaling ex vivo. These findings demonstrate the utility of REAP to atlas the expansive landscape of exoproteome-targeting autoantibodies and their impacts on patient health outcomes.


Key Points

Natural antibodies or autoantibodies, particularly IgM, that react with self-molecules occur in normal individuals and display a moderate affinity but high avidity for self-antigens

High-affinity, somatically mutated, class-switched IgG autoantibodies reflect a pathologic process in which homeostatic pathways related to cell clearance, antigen-receptor signaling or cell effector functions are disturbed

The mechanisms involved in immune-complex-mediated tissue injury include engagement of FcγRs and activation of complement, as well as internalization and activation of Toll-like receptors

Autoantibodies might be detectable long before disease onset and serve as biomarkers enabling diagnosis and targeting of therapeutic intervention

In organ-specific autoimmune diseases, autoantibodies directly injure target organs in systemic autoimmune diseases, they can also bind to different self-molecules and cause disease through the formation of immune complexes

Research is needed to clarify why certain antigens are targeted in different autoimmune diseases and how some antibodies activate, whereas others inhibit, immune responses


Regulatory Cells

The adaptive immune system is carefully regulated by several different cell populations. Helper T cells that promote immune responses are described earlier. Other T cells are called regulatory T cells (Treg cells). These secrete a mixture of cytokines that inhibit conventional immune responses. They serve to turn off an immune response once it has completed its task and the invading microorganism is eliminated. Treg cells also play a central role in preventing the development of autoimmunity.

For More Information

Also see pet health content regarding adaptive immunity in dogs, cats, and horses.