Blockade of B7H1 by Monoclonal Antibodies Enhances Therapeutic Immunity of DKK1 Vaccine in Multiple Myeloma

Abstract 2941

Current approaches for vaccination and immunotherapy are often capable of eliciting strong T-cell responses against tumor antigens. However, negative regulatory mechanisms within the tumor microenvironment inhibit antitumor T-cell function, leading to evasion from immune attack. One inhibitory mechanism is upregulation of programmed death-ligand 1 (PD-L1/B7H1) expressed on tumor or stromal cells, which binds to programmed death-1 (PD-1) on activated T cells. PD-1 and B7H1 engagement results in diminished antitumor T-cell responses and correlates with poor outcome in murine and human cancers.

To cure mice with large tumor burdens by administering tumor vaccines, we hypothesized that it is necessary not only to enhance the immunogenicity of the vaccines, but also to target the suppressive tumor microenvironment and break immune suppression on effector cells. Therefore, we explored combinational treatments in a myeloma setting by using vaccination and anti-B7H1 blocking mAbs to enhance clinical efficacy of cancer vaccines. To establish mouse tumor models for evaluation of tumor-associated B7H1 in the inhibition of T-cell immunity, first, we examined the expression of B7H1 in four murine myeloma cell lines that originated from Balb/c mice. Results showed that all the myeloma cells expressed a negative T-cell costimulatory molecule B7H1, and blocking surface B7H1 by using a specific mAb (M5H1) enabled tumor-specific CTLs from vaccinated mice to be more efficient at lysing the tumor cells. Thus these results suggest that B7H1 on tumor cells and PD-1 on T cells may form a B7H1/PD-1 molecular shield to prevent lysis by T cells and actively inhibit the cytolytic function of tumor-specific CTLs by expressing B7H1. Next, we evaluated the therapeutic efficacy of DKK1-DNA vaccine in mice with established myeloma. In these experiments, DKK1-DNA vaccine plus CpG was used as a standard vaccine. Anti-B7H1 (M5H1) mAb was used to block negative T-cell signaling in mice receiving DKK1-DNA vaccination. Balb/c mice were first inoculated subcutaneously with the myeloma cells (1 million cells per mouse). Ten days after tumor inoculation, mice were immunized with three injections of the vaccines on days 10, 14, and 17 after tumor inoculation. Mice were also injected intraperitoneally with B7H1 (200 μg per mouse per treatment) on days 1, 4, 7, and 10 following the first vaccination. Mice receiving injections of PBS, isotype IgG (200 μg per mouse per treatment), or CpG- (50 μg per mouse per treatment) alone were used as controls. Results show that mice receiving DKK1-DNA vaccine (plus CpG) together with B7H1 induced more robust tumor growth inhibition (P < 0.01, compared with PBS control mice). The survival of DKK1-CpG vaccine- and vaccine plus B7H1 antibody-treated mice were 60% and 80%, respectively. Thus, our data showed that B7H1 blocking mAbs can further enhance the therapeutic efficacy of the vaccine.

Finally, we investigated the mechanism of therapeutic efficacy of vaccine enhanced by anti-B7H1 mAbs. Our results showed that the percentages of IL-10-secreting or Foxp3⁺ Tregs in the spleens and tumors are significantly decreased in mice vaccinated with DKK1 DNA plus CpG in combination with anti-B7H1 antibodies (P < 0.01, compared with mice receiving DKK1 DNA alone or vaccine plus CpG). These results indicate that myeloma cells are able to induce or recruit regulatory T cells, and the beneficial effects of anti-B7H1 mAbs were derived from their ability to reduce the numbers of Tregs in tumor-bearing mice. Taken together, these results support the concept that B7H1/PD-1 forms a molecular shield to prevent destruction by CTLs and implicate new approaches for immunotherapy of human cancers by B7H1 blocking mAbs.