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In addition to antigen-specific signals mediated through the T-cell receptor, T cells also require antigen nonspecific costimulation for activation. The B7 family of molecules on antigen-presenting cells, which include B7-1 (CD80) and B7-2 (CD86), play important roles in providing costimulatory signals required for development of antigen-specific immune responses. Hemagglutinin B (HagB) is a nonfimbrial adhesin of the periodontopathic microorganism Porphyromonas gingivalis and is thought to be involved in the attachment of the bacterium to host tissues. However, the immune mechanisms involved in responses to HagB and their roles in pathogenesis have yet to be elucidated. Therefore, the purpose of this study was to determine the role of B7 costimulatory molecules on T-helper-cell differentiation for the induction of immune responses to HagB. Mice deficient in either or both of the costimulatory molecules B7-1 and B7-2 were used to explore their role in immune responses to HagB after subcutaneous immunization. B7-1(-/-) mice had levels of immunoglobulin G (IgG) anti-HagB antibody activity in serum similar to those of wild-type mice, whereas lower serum IgG anti-HagB antibody responses were seen in B7-2(-/-) mice. Moreover, significantly lower numbers of IgG antibody-secreting cells and lower levels of CD4(+)-T-cell proliferation were observed in B7-2(-/-) mice compared to wild-type mice. No serum IgG response to HagB was detected in B7-1/B7-2(-/-) mice. Analysis of the subclass of the serum IgG responses and the cytokines induced in response to HagB revealed that B7-2(-/-) mice had significantly lower IgG1 and higher IgG2a anti-HagB antibody responses compared to wild-type mice. The B7-2(-/-) mice also had significantly reduced levels of interleukin-4 (IL-4) and IL-5 and enhanced level of gamma interferon. Furthermore, assessment of B7-1 and B7-2 expression on B cells and macrophages derived from wild-type BALB/c mice after in vitro stimulation with HagB revealed a predominant upregulation in the expression of the B7-2 costimulatory molecule on B cells and macrophages. Essentially no change was seen in the expression of B7-1. Taken together, these results suggest a critical role for B7, especially B7-2, for the preferential induction of a Th2-like response to HagB.

Porphyromonas gingivalis, a gram-negative, black-pigmented anaerobe, is among the microorganisms implicated in the etiology of adult periodontal disease. This bacterium possesses a number of factors, including hemagglutinins, of potential importance in virulence. Our laboratory has shown the induction of protection to P. gingivalis infection after subcutaneous immunization with recombinant hemagglutinin B (rHagB). The purpose of this study was to determine if humoral antibody responses are induced after intranasal (i.n.) immunization of rHagB and if monophosphoryl lipid A (MPL), a nontoxic derivative of the lipid A region of lipopolysaccharide, acts as a mucosal adjuvant and potentiates responses to rHagB. Further, the effects of MPL on the nature of the response to HagB and on the costimulatory molecules B7-1 and B7-2 on different antigen-presenting cells (APC) were evaluated. Groups of BALB/c mice were immunized three times (2-week intervals) by the i.n. route with HagB (20 microg) alone or with MPL (25 microg). A group of nonimmunized mice served as control. Serum and saliva samples were collected prior to immunization and at approximately 2-week intervals and evaluated for serum immunoglobulin G (IgG) and IgG subclass and for salivary IgA antibody activity by enzyme-linked immunosorbent assay. Mice immunized with rHagB plus MPL had significantly higher salivary IgA (P < 0.05) and serum IgG (P < 0.05) anti-HagB responses than mice immunized with rHagB alone. The IgG1 and IgG2a subclass responses seen in mice immunized with rHagB plus MPL were significantly higher (P < 0.05) than those seen in mice immunized with rHagB only. Further, the IgG2a/IgG1 ratio in the latter group was approximately 1, whereas in mice immunized with rHagB plus MPL the ratio was <1. These results provide evidence for the participation of T helper (Th) 1 and Th2 cells in responses to rHagB and that MPL potentiates a type 2 response to HagB. MPL was also shown to preferentially up-regulate B7-2 expression on B cells, whereas a preferential increase in B7-1 costimulatory molecule was seen on macrophages and dendritic cells. These results provide evidence that MPL exerts a differential regulation in the expression of costimulatory molecules on APC.