Contributions of conventional and heavy-chain IgG to immunity in fetal, neonatal, and adult alpacas.

In addition to conventional immunoglobulins, camelids produce antibodies that do not incorporate light chains into their structures. These so-called heavy-chain (HC) antibodies have incited great interest in the biomedical community, as they have considerable potential for biotechnological and therapeutic application. Recently, we have begun to elucidate the immunological functions of HC antibodies, yet little is known about their significance in maternal immunity or about the B lymphocytes that produce them. This study describes the application of isotype-specific reagents toward physiological assessments of camelid IgGs and the B cells that produce them. We document the specificities of monoclonal antibodies that distinguish two conventional IgG1 isotypes and two HC IgG3 variants produced by alpacas. Next, we report that the relative concentrations of five isotypes are similar in serum, milk, and colostrum; however, following passive transfer, the concentrations of HC IgG2 and IgG3 declined more rapidly than the concentration of conventional IgG1 in the sera of neonates. Finally, we assessed the distribution of B cells of distinct isotypes within lymphoid tissues during fetal and adult life. We detected IgG1, IgG2, and IgG3 in lymphocytes located in lymph node follicles, suggesting that HC B cells affinity mature and/or class switch. One IgG3 isotype was present in B cells located in ileal Peyer's patches, and one conventional IgG1 isotype was detected in splenic marginal zone B cells. Our findings contribute to the growing body of knowledge pertaining to HC antibodies and are compatible with functional specialization among conventional and HC IgGs in the alpaca.