The FAI protein of group C streptococci acts as a mucosal adjuvant by the specific targeting and activation of B cells

Adjuvants are usually required to stimulate efficient immune responses after vaccination by the mucosal route. Unfortunately, only a few molecules have been described, which exhibit this property. Thus, there is an urgent need to develop new mucosal adjuvants. Our results demonstrate that the fibrinogen-albumin-IgG receptor (FAI) from group C streptococci is a promising mucosal adjuvant. Strong antigen-specific antibody responses were stimulated at both systemic and mucosal levels when model antigens were co-administered with FAI. Immunizations performed using truncated derivatives demonstrated that the fragment encompassing the IgG- and fibrinogen-binding regions represents the minimal domain possessing adjuvant activity. FAI specifically targets B cells, thereby supporting their activation and antibody production, even in the absence of T cell help. Co-administration of antigens with FAI also resulted in elicitation of strong antibody responses in CBA/N Xid mice, which exhibit a deficiency in humoral immunity. The overcoming of their unresponsiveness by co-administration of FAI suggests that this promising adjuvant can be exploited for the establishment of vaccination strategies in patients affected by immune deficiencies of the B cell compartment.