Herpes simplex virus 2 (HSV-2) and, to a lesser extent, HSV-1 cause the majority of sexually transmitted genital ulcerative disease. No effective prophylactic vaccine is currently available. Replication-defective HSV stimulates immune responses in animals but produces no progeny virus, making it potentially useful as a safe form of live vaccine against HSV. Because it does not replicate and spread in the host, however, replicationdefective virus may have relatively limited capacity to solicit professional antigen presentation. We previously demonstrated that in mice devoid of B7-1 and B7-2 costimulation molecules, replication-defective HSV-2 encoding B7-1 or B7-2 induces stronger immune responses and protection against HSV-2 challenge than immunization with replication-defective virus alone. Here, we vaccinated wild-type mice fully competent to express endogenous B7 costimulation molecules with replication-defective HSV-2 or replication-defective virus encoding B7-2 and compared their capacities to protect against vaginal HSV-2 infection and disease. Replication-defective virus encoding B7-2 induced more IFN-␥-producing CD4 T cells than did replication-defective virus alone. Immunization with B7-2-expressing virus decreased challenge virus replication in the vaginal mucosa, genital and neurological disease, and mortality more effectively than did immunization with the parental replication-defective virus. Prior immunization with B7-expressing, replication-defective virus also effectively suppressed infection of the nervous system compared to immunization with the parental virus. Thus, B7 costimulation molecules expressed at the site of HSV infection can enhance vaccine efficacy even in a fully immunocompetent host.

The interaction between B7 costimulation molecules on antigen-presenting cells and CD28 on antigenresponsive T cells is essential for T-cell activation and maturation of immune responses to herpes simplex virus (HSV) infection. Vaccine-induced immune responses also depend upon adequate upregulation of B7 costimulation molecules, but this signal may be limiting for replication-defective virus vaccines. We investigated whether expression of B7 costimulation molecules by a prototypical replication-defective antiviral vaccine could enhance immune responses to the vaccine and whether B7-1 and B7-2 would be similarly effective. We altered an ICP8؊ replication-defective strain of HSV type 2 (HSV-2), 5BlacZ, to encode either murine B7-1 or B7-2. B7 molecule expression was detected on the surface of cells infected in vitro and at the RNA level in tissue of immunized mice. Immunization of B7-1/B7-2 knockout mice with B7-encoding virus modestly expanded the number of gamma interferon-producing T cells and significantly augmented class-switched HSV-specific antibody responses compared with the parental virus. Mice immunized with either B7-expressing virus showed less replication of challenge virus in the genital mucosa than mice immunized with 5BlacZ, markedly fewer signs of genital and neurological disease, and little weight loss. Virtually all mice immunized with B7-encoding virus survived challenge with a large dose of HSV-2, whereas most 5BlacZ-immunized mice succumbed to infection. These results indicate that protective immune responses can be enhanced by the inclusion of host B7 costimulation molecules in a prototypical replication-defective HSV vaccine against HSV-2 genital infection and that B7-1 and B7-2 induce immune responses with similar capacities to fight HSV-2 infection.

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