Chemically Synthesized Lipid A as an Adjuvant to Augment Immune Responses to Type B Conjugate Vaccine.

We previously identified spp. as a commensal bacterium that resides in lymphoid tissues, including Peyer's patches. We found that -derived lipopolysaccharide acted as a weak agonist of Toll-like receptor four due to the unique structure of lipid A, which lies in the core of lipopolysaccharide. This feature allowed the use of chemically synthesized lipid A as a safe synthetic vaccine adjuvant that induces Th17 polarization to enhance systemic IgG and respiratory IgA responses to T-cell-dependent antigens (e.g., ovalbumin and pneumococcal surface protein A) without excessive inflammation. Here, we examined the adjuvant activity of lipid A on a influenzae B conjugate vaccine that contains capsular polysaccharide polyribosyl ribitol phosphate (PRP), a T-cell-independent antigen, conjugated with the T-cell-dependent tetanus toxoid (TT) antigen (i.e., PRP-TT). When mice were subcutaneously immunized with PRP alone or mixed with TT, lipid A did not affect PRP-specific IgG production. In contrast, PRP-specific serum IgG responses were enhanced when mice were immunized with PRP-TT, but these responses were impaired in similarly immunized T-cell-deficient nude mice. Furthermore, TT-specific-but not PRP-specific-T-cell activation occurred in mice immunized with PRP-TT together with lipid A. In addition, coculture with lipid A promoted significant proliferation of and enhanced antibody production by B cells. Together, these findings suggest that lipid A exerts an adjuvant activity on thymus-independent Hib polysaccharide antigen in the presence of a T-cell-dependent conjugate carrier antigen.