Intranasal immunization with SAG1 and nontoxic mutant heat-labile enterotoxins protects mice against Toxoplasma gondii

Effective protection against intestinal pathogens requires both mucosal and systemic immune responses. Intranasal administration of antigens induces t hese responses but generally fails to trigger a strong protective immunity. Mucosal adjuvants can significantly enhance the immunogenicities of intran asally administered antigens. Cholera toxin (CT) and heat-labile enterotoxi n (LT) are strong mucosal adjuvants with a variety of antigens. Moreover, t he toxicities of CT and LT do not permit their use in humans. Two nontoxic mutant LTs, LTR72 and LTK63, were tested with Toxoplasma gondii SAG1 protei n in intranasal vaccination of CBA/J mice. Vaccination with SAG1 plus LTR72 or LTK63 induced strong systemic (immunoglobulin G [IgG]) and mucosal (IgA ) humoral responses. Splenocytes and mesenteric lymph node cells from mice immunized with LTR72 plus SAG1, but not those from mice immunized with LTK6 3 plus SAG1, responded to restimulation with a T. gondii lysate antigen in vitro. Gamma interferon and interleukin 2 (IL-2) production by splenocytes and IL-2 production by mesenteric lymph node cells were observed in vitro a fter antigen restimulation, underlying a Th1-like response. High-level prot ection as assessed by the decreased load of cerebral cysts after a challeng e with the 76K strain of T.gondii was obtained in the group immunized with LTR72 plus SAG1 and LTK63 plus SAG1, They were as well protected as the mic e immunized with the antigen plus native toxins. This is the first report s howing protection against a parasite by using combinations of nontoxic muta nt LTs and SAG1 antigen. These nontoxic mutant LTs are now attractive candi dates for the development of mucosally delivered vaccines.