Recent progress in mucosal immunology and vaccine development

Mucosal surfaces are covered with specialized epithelial cells that serve as physical barriers to the environment and prevent exogenous challenges by pathogens and soluble antigens. 1 Functionally independent of the systemic immune compartments, the mucosal immune system has developed its own highly organized lymphoid tissues. Mucosal tissues maintain homeostasis by mounting specialized anti-inflammatory immune defenses, such as the production of secretory IgA (SIgA) antibodies and the induction of tolerance against innocuous soluble substances and commensal bacteria. When antigens are administered with appropriate adjuvants or attenuated live vaccines via mucosal routes (oral, nasal, sublingual, ocular, genital or rectal), the mucosal immune system can trigger both humoral and cell-mediated immune protection not only in mucosal sites but also systemically. 2 Furthermore, owing to mucosal homing properties, local mucosal immunization leads to antigen-specific T- and B-cell responses at both local and distal mucosal sites. Nevertheless, most pathogens can still effectively invade by crossing the host’s mucosal membranes. Hence, effective vaccines that exert protective effects at mucosal surfaces are much needed.