The significance of heavy-chain antibodies to camelid immunity
Camelids produce IgG isotypes that do not conform to the rules governing conventional antibody structures. Typically, immunoglobulins combine homodimeric heavy and light chains to produce characteristic tetrameric structures. Unlike IgG1, the camelid IgG2 and IgG3 isotypes do not incorporate light chains into their structures. These unusual antibodies, generally referred to as heavy-chain antibodies (HCAbs), comprise approximately 50% of serum IgGs, compatible with a significant role in camelid immunity. Hitherto, the effector functions of camelid HCAbs remain largely undefined primarily due to the dearth of available isotype-specific reagents. As it was our objective to investigate camelid HCAbs, we produced and characterized monoclonal antibodies that discern among the IgG isotypes produced by both llamas and alpacas. These reagents were employed in affinity chromatography, serologic analyses, virus neutralization assays, flow cytometry, and immunohistochemistry to differentiate between the B cell sub-populations that synthesize conventional and heavy-chain IgGs, determine whether expression of the different IgG isotypes was regulated differently, and to investigate the clinical and physiological relevance of HCAbs. Our investigations led to the discovery of a novel IgG3 protein, and the production of immunological reagents that discern among two conventional IgG1, IgG2 and the two IgG3 HCAbs. We have documented that the B cell sub-populations developed within similar lymphoid compartments during gestation. At birth, HCAbs obtained through colostrum contributed to the passive transfer of immunity that is critical in protecting the newborn until maturity of its immune system. Throughout life, the B cell sub-populations continued to occupy similar compartments, although, B cells that locate within splenic marginal zones expressed one sub-isotype of IgG1 exclusively. Also, follicular B cells within adult, ileal Peyer's patches expressed only the novel IgG3. In response to pathogen infections, camelid B cells elicited IgG2 to helminths and IgG3 to viral infections: IgG1 expression was ubiquitous. Anti-viral IgG2 were induced only by hyperimmunization. While IgG3 proved to be as potent as IgG1 in neutralizing activities, anti-viral IgG2 was ineffective. The data presented here document for the first time a dichotomy in the effector functions of camelid IgG2 and IgG3 HCAbs.
Heavy-chain antibodies; camelid; anti-nematode IgGs; anti-viral IgGs; B cell development; B cell distribution; colostrum; IgG-specific monoclonal antibodies
dissertation or thesis