HIV-1/SIV Humoral Responses in External Secretions

The protective function of mucosal HIV-1- or SIV-specific antibodies against viral infection has stimulated extensive studies of their Ig isotype association with differences in specificity and in effector functions. In contrast to many mucosally acquired microbial infections in which the humoral responses are dominated by induction of secretory IgA (S-IgA), HIV-1/SIV infections stimulate vigorous IgG responses in sera as well as in external secretions but low IgA virus-specific antibodies although the total levels of IgA in these fluids remain unaltered. The diminished or even absent IgA responses to HIV-1/SIV and to other mucosal antigens in external secretions and their replacement with IgG is likely to influence the functionality of mucosal barriers and eliminate antiinflammatory effector functions of IgA antibodies. Furthermore, the polymeric character of S-IgA with 4-8 antigen-binding sites, exquisite resistance to proteolysis and anti-inflammatory potential are of great advantage in mucosal protection. The markedly different effector functions of mucosal antibodies of IgG and IgA isotypes must be considered in the design of HIV-1 vaccines to stimulate S-IgA responses at sites of virus entry and IgG responses in the systemic compartment.