Evaluation of intra- and extra-epithelial secretory IgA in chlamydial infections

Immunoglobulin A is an important mucosal antibody that can neutralize mucosal pathogens by either preventing attachment to epithelia (immune exclusion) or alternatively inhibit intra-epithelial replication following transcytosis by the polymeric immunoglobulin receptor (pIgR). Chlamydia trachomatis is a major human pathogen that initially targets the endocervical or urethral epithelium in women and men, respectively. As both tissues contain abundant secretory IgA (SIgA) we assessed the protection afforded by IgA targeting different chlamydial antigens expressed during the extra- and intra-epithelial stages of infection. We developed an in vitro model using polarizing cells expressing the murine pIgR together with antigen-specific mouse IgA, and an in vivo model using pIgR(-/-) mice. Secretory IgA targeting the extra-epithelial chlamydial antigen, the major outer membrane protein, significantly reduced infection in vitro by 24% and in vivo by 44%. Conversely, pIgR-mediated delivery of IgA targeting the intra-epithelial inclusion membrane protein A bound to the inclusion but did not reduce infection in vitro or in vivo. Similarly, intra-epithelial IgA targeting the secreted protease Chlamydia protease-like activity factor also failed to reduce infection. Together, these data suggest the importance of pIgR-mediated delivery of IgA targeting extra-epithelial, but not intra-epithelial, chlamydial antigens for protection against a genital tract infection.