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BackgroundOne of the significant tumor immune escape mechanisms and substantial barrier for successful immunotherapy is tumor-mediated inhibition of immune response through cell-to-cell or receptor/ligand interactions. Programmed death receptor-1 (PD-1) interaction with its ligands, PD-L1 and PD-L2, is one of the important strategies that many tumors employ to escape immune surveillance. Upon PD-Ls binding to PD-1, T cell receptor (TCR) signaling is dampened, causing inhibition of proliferation, decreased cytokine production, anergy and/or apoptosis. Thus PD-Ls expression by tumor cells serves as a protective mechanism, leading to suppression of tumor-infiltrating lymphocytes in the tumor microenvironment. Lm-LLO immunotherapies have been shown to be therapeutically effective due to their ability to induce potent antigen-specific immune responses. However, it has been demonstrated that infection with Lm leads to up-regulation of PD-L1 on mouse immune cells that can inhibit effector T cells through PD-1/PD-L1 pathway.MethodsTherapeutic and immune efficacy of Listeria-based vaccine (Lm-LLO-E7) in combination with anti-PD-1 antibody was tested in E7 antigen expressing TC-1 mouse tumor model. Tumor growth, survival, as well as peripheral and tumor-infiltrating immune cell profiles after immunotherapy were assessed.ResultsHere we demonstrate that the combination of an Lm-LLO immunotherapy with anti-PD-1 antibody that blocks PD-1/PD-L1 interaction, significantly improves immune and therapeutic efficacy of treatment in TC-1 mouse tumor model. Importantly, we show that in addition to significant reduction of regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSC) in both spleen and tumor microenvironment that are mediated solely by the Lm-LLO immunotherapy, the addition of anti-PD-1 antibody to the treatment results in significant increase of antigen-specific immune responses in periphery and CD8 T cell infiltration into the tumor. As a result, this combinational treatment leads to significant inhibition of tumor growth and prolonged survival/complete regression of tumors in treated animals.We also demonstrate that in vitro infection with Lm results in significant upregulation of surface PD-L1 expression on human monocyte-derived dendritic cells suggesting the translational capacity of this finding.ConclusionsOur findings demonstrate that combination of Lm-LLO-based vaccine with blocking of PD-1/PD-L1 interaction is a feasible approach with clinical translation potential that can lead to overall enhancement of the efficacy of anti-tumor immunotherapy.

In a randomized prospective study we tested the toxicity and efficacy of recombinant alpha-2 interferon in the treatment of Kaposi's sarcoma associated with the acquired immunodeficiency syndrome. High doses (50 X 10(6) U/m2 body surface area, intravenously) or low doses (1 X 10(6) U/m2, subcutaneously) of recombinant alpha-2 interferon were administered to 20 patients for 5 days/wk, every other week, for four treatment cycles. Therapy was well tolerated subjectively and caused only mild hematologic and hepatic toxicity at both dose levels. No consistent or sustained changes were seen in immunologic variables during or after treatment. Six patients with Kaposi's sarcoma, four at high dose and two at low dose, had objective responses (complete or partial) to treatment. However, therapy did not appear to eradicate cytomegalovirus carriage or prevent opportunistic infections related to cytomegalovirus.

HPV infection is a direct cause of neoplasia and malignancy. Cellular immunologic activity against cells expressing HPV E6 and E7 is sufficient to eliminate the presence of dysplastic or neoplastic tissue driven by HPV infection. Live attenuatedListeria monocytogenes-(Lm-) based immunotherapy (ADXS11-001) has been developed for the treatment of HPV-associated diseases. ADXS11-001 secretes an antigen-adjuvant fusion (Lm-LLO) protein consisting of a truncated fragment of theLmprotein listeriolysin O (LLO) fused to HPV-16 E7. In preclinical models, this construct has been found to stimulate immune responses and affect therapeutic outcome. ADXS11-001 is currently being evaluated in Phase 2 clinical trials for cervical intraepithelial neoplasia, cervical cancer, and HPV-positive head and neck cancer. The use of a live attenuated bacterium is a more complex and complete method of cancer immunotherapy, as over millenniaLmhas evolved to infect humans and humans have evolved to prevent and reject this infection over millennia. This evolution has resulted in profound pathogen-associated immune mechanisms which are genetically conserved, highly efficacious, resistant to tolerance, and can be uniquely invoked using this novel platform technology.

For decades Listeria monocytogenes has been used as a model of host-disease immunology, and a considerable body of knowledge has been amassed regarding the complex immune response to L. monocytogenes. Attenuated strains of L. monocytogenes are currently being assessed as therapeutic bacterial vectors to present tumor-associated antigens to the immune system for the clinical treatment of cancer. L. monocytogenes immunotherapy utilizes many synchronous and disparate action mechanisms that stimulate innate and cell-mediated adaptive immunity while reducing immunosuppressive influences in the tumor microenvironment. Other effects not typically associated with immunotherapy include the stimulation of myeloid hematopoiesis and vascular changes that enable chemotaxis. Preliminary clinical results using L. monocytogenes bearing the HPV oncogene E7 indicate good tolerability and a strong efficacy signal, warranting further development. This article reviews the current status of L. monocytogenes as a cancer immunotherapeutic and the complex immune responses that underlie L. monocytogenes immunotherapy.