<b><i>Introduction:</i></b> As has been shown previously, various protein-modifying agents can change the antigen-binding properties of immunoglobulins. However, induced polyspecificity of human secretory immunoglobulin A (sIgA) has not been previously characterized in detail. <b><i>Methods:</i></b> In the present study, human secretory immunoglobulin A (IgA) was exposed to buffers with acidic pH, to free heme, or to pro-oxidative ferrous ions, and the antigen-binding behavior of the native and modified IgA to viral and bacterial antigens was compared using Western blotting and enzyme-linked immunosorbent assay. The ability of these agents to modulate the antigen-binding properties of human sIgA toward a wide range of pathogen peptides was investigated using an epitope microarray. <b><i>Results:</i></b> We have shown that acidic pH, heme, and pro-oxidative ferrous ions influenced the binding of secretory IgA in opposite directions (either increasing or decreasing); however, the strongest effect was observed when using buffers with low pH. This fraction had the highest number of affected reactivities; most of them were increased and most of the new ones were toward common pathogens. <b><i>Conclusions:</i></b> Thus, it was shown that all investigated treatments can alter to some degree the antigen-binding of secretory IgA, but acidic pH has the most potentially beneficial effect by increasing binding to a largest number of common pathogens’ antigens.

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Recent studies have shown that polyspecific antibodies play an important role in the frontline defense against the dissemination of pathogens in the pre-immune host. Interestingly, antigen-binding polyspecificity can not only be inherent, but also acquired post-translationally. The ability of individual monoclonal IgE and IgG antibodies to acquire polyspecificity following contact with protein-modifying agents has been studied in detail. However, to the best of our knowledge this property of human IgA has previously been described only cursorily. In the present study pooled human serum IgA and two monoclonal IgA antibodies were exposed to buffers with acidic pH, to free heme or to pro-oxidative ferrous ions, and the antigen-binding behavior of the native and modified IgA to viral and bacterial antigens were compared using Western blotting and ELISA. We observed a dose-dependent increase in reactivity to several bacterial extracts and to pure viral antigens. This newly described property of IgA may have therapeutic potential as has already been shown for pooled IgG with induced polyspecificity.

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It is time to celebrate the 125th anniversary of the first successful attempt to develop and use a specific high-titer antitoxic serum for treating diphtheria, a deadly infectious disease. This was followed by major advances in passive immunotherapy 75 years ago (production of pooled human IgG for subcutaneous injection) and 50 years ago (widespread technology for producing immunoglobulin preparations for intravenous administration). More than 200 tons of pooled human IgG are produced per year worldwide. The preparation is used primarily for IgG substitution in patients with primary and secondary immunodeficiencies, as well as for an immunomodulating treatment of a growing number of autoimmune and inflammatory diseases. These preparations contain the pooled IgG antibody repertoire of a large population of healthy plasma donors. This repertoire includes antibodies that neutralize pathogens and their factors of virulence, anti-idiotypic antibodies, and antibodies to other foreign and own proteins, as well as to carbohydrate antigens. Naturally polyspecific antibodies that are present in all healthy individuals play an important role as a first-line defense against bacteria and viruses. After exposure to protein-modifying agents, some IgG molecules can acquire the ability to bind novel structurally unrelated antigens. This phenomenon is referred to as induced polyspecificity. The list of these protein-modifying molecules was shown to include low-pH buffers, free heme, pro-oxidative ferrous ions, reactive oxygen species, etc. Such modified antibody preparations may have a therapeutic potential, since their administration to animals with experimental sepsis or aseptic systemic response syndromes significantly improved survival rates, while the same dose of the native preparation had no effect. We also hypothesize that the aggressive protein-modifying molecules released in sites of inflammation and tissue damage could also modify the antigen-binding behavior of surface immunoglobulin B cell receptors and the structurally related T cell receptors. This "specificity editing" of both types of receptors may play a major role in the body's defense mechanisms.

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