Therapeutic vaccines for the treatment of cancer are an attractive alternative to some of the conventional therapies that are currently used. More importantly, vaccines could be very useful to prevent recurrences when applied after primary therapy. Unfortunately, most therapeutic vaccines for cancer have performed poorly due to the low level of immune responses that they induce. Previous work done in our laboratory in cancer mouse models demonstrated that vaccines consisting of synthetic peptides representing minimal CD8 T cell epitopes administered i.v. mixed with poly-IC and anti-CD40 antibodies (TriVax) were capable of inducing massive T cell responses similar to those found during acute infections. We now report that some peptides are capable of inducing similarly large T cell responses after vaccination with poly-IC alone (BiVax). The results show that amphiphilic peptides are more likely to function as strong immunogens in BiVax and that systemic immunizations (i.v. or i.m.) were more effective than local (s.c.) vaccine administration. The immune responses induced by BiVax were found to be effective against established tumors in two mouse cancer models. The roles of various immune related pathways such as type-I IFN, CD40 costimulation, CD4 T cells, TLRs and the MDA5 RNA helicase were examined. The present findings could facilitate the development of simple and effective subunit vaccines for diseases where CD8 T cells provide a therapeutic benefit.

IntroductionBecause cytotoxic T lymphocytes (CTLs) have the ability to recognize and kill tumor cells, considerable efforts are being devoted to the development of T-cell immunotherapies for cancer. [1][2][3] CTLs express the CD8 coreceptor and recognize antigen on tumor cells as peptide/major histocompatibility class I (MHC-I) complexes. As a consequence of antigen recognition, CD8 CTLs exert antitumor function via the perforin-granzyme cytolytic pathway or through cytokines such as interferon gamma (IFN␥) and tumor necrosis factor alpha (TNF␣), which exhibit cytostatic activity. The MHC-I-binding peptides recognized by tumorreactive CD8 T lymphocytes are usually derived from genes preferentially expressed by transformed cells or from tissuedifferentiation antigens. The identification of MHC-I-binding peptides that serve as tumor-rejection CD8 T-cell epitopes has opened the door to developing synthetic peptide cancer vaccines. 4 The discovery of melanoma T-cell epitopes for humans and mice has led to studies assessing the utility of peptide vaccines for the treatment of established disease states. In many of these studies, promising, but ultimately not outstanding, therapeutic effects were attained, indicating that the use of synthetic peptides alone, with commonly used adjuvants such as incomplete Freund adjuvant, or in combination with cytokines constitute relatively weak and ineffective vaccines. [5][6][7][8] Thus, several groups, including ours, have focused on optimizing peptide vaccines with the use of Toll-like receptor agonists and costimulatory antibodies as immunologic adjuvants. [9][10][11][12][13][14] Our goal was to design a peptide immunization strategy that generates T-cell responses similar to those observed during an acute viral infection, in which 10% to 50% of all CD8 T cells are specific for the pathogen. We recently described a vaccine that we call TriVax (named for its 3 components: synthetic peptide, polyriboinosinic-polyribocytidylic acid [poly-IC], and anti-CD40 antibody), which achieved our stated goal. 15,16 In addition to generating large CD8 T-cell responses to a melanosomal epitope (Trp2 180 ), significant therapeutic effects (60% long-term survival) were observed against 3-day established B16 melanomas. The therapeutic effect of TriVax disappeared when CD8 T cells were depleted with antibodies or in perforin-deficient mice. Conversely, the elimination of CD4 T lymphocytes and natural killer cells had no significant effect. 16 These results indicated that the major effector mechanism of TriVax is mediated by classic CD8 CTLs through perforin-mediated lysis of tumor cells. Nevertheless, the therapeutic effect of TriVax decreased if vaccination was administered in more advanced disease states, even though large numbers of functional CD8 T cells were detected in the tumor-bearing mice, suggesting that immune-suppressive activity at the tumor site was responsible for the tumor's evasion from the T cells. During these studies, we observed that the therapeutic effectiveness of TriVax was...

Purpose Epitope-based cancer vaccines capable of inducing CD8 T cell responses to tumor-associated antigens (TAAs) expressed by tumor cells have been considered as attractive alternatives for the treatment of some types of cancer. However, reliable TAAs have not been identified for most malignant diseases, limiting the development of epitope-based vaccines. Herein, we report that the combinatorial therapy of polyinosinic-polycytidylic acid (poly-IC) and anti-programmed death-ligand 1 (PD-L1) monoclonal antibody (mAb) can be implemented with good results for tumors where no known TAAs have been identified. Experimental Design Three cancer mouse models (melanoma, lung, and colon) were used to evaluate therapeutic efficacy and examine the immunological mechanisms of the poly-IC/anti-PD-L1 mAb therapy. Results The combined administration of poly-IC and anti-PD-L1 mAb into tumor-bearing mice generated potent immune responses resulting in the complete eradication or remarkable reduction of tumor growth. In some instances, the poly-IC/anti-PD-L1 mAb therapy induced long-lasting protection against tumor rechallenges. The results indicate that CD8 T cells but not CD4 T cells or NK cells mediated the therapeutic efficacy of this combinatorial therapy. Experiments using genetically-deficient mice indicate that the therapeutic efficacy of this combinatorial therapy depended in part by the participation of type-I interferon, whereas interferon-γ did not appear to play a major role. Conclusions The overall results suggest that immunotherapy consisting of the combination of poly-IC/anti-PD-L1 mAb could be a promising new approach for treating cancer patients, especially those instances where no reliable TAAs are available as a therapeutic vaccine.