Despite significant advances in the treatment of localized disease, survival for patients with sarcoma relapse and/or metastasis remains poor. Oncolytic herpes simplex virus (oHSV) therapy is a promising platform currently in clinical trials, and our lab has previously shown that human sarcoma cell lines and xenografts are sensitive to oHSV therapy. In addition to direct oncolysis, virotherapy promotes an antitumor cytotoxic T cell response that is important for oHSV efficacy, however the role of innate immunity in both limiting viral oncolysis and establishing the T-cell dependent vaccine-like effects of oHSV virotherapy remains undefined. Such immunological studies have been hampered by the low permissivity of mouse tumors for HSV infection. To facilitate such studies, we have characterized three implantable mouse rhabdomyosarcoma (RMS) tumor cell lines for their permissiveness to oHSV oncolysis and their suitability as immunocompetent models for the study of the innate and adaptive immune responses to oHSV therapy. Similar to human RMS, murine RMS tumor cell lines are susceptible to HSV entry. Murine RMS models displayed a range of permissiveness to oHSV replication and tumor cell death in vitro, with the most permissive murine tumor being comparable to the least permissive human RMS lines. The in vivo antitumor effect also ranged in these models from no or modest response to complete tumor regression and protection from tumor re-challenge, which was partially dependent on T-cells. Interestingly, the in vitro permissiveness to oHSV oncolysis was not predictive of the in vivo anti-tumor effect. The differential in vivo response to oHSV in these models was not explained by differences in interferon responsiveness, which was intact in all models, or in vivo virus replication, which was minimal in all models. Therefore, we further interrogated the innate and adaptive immune microenvironment and found some striking differences in the cytokine environment and composition of the tumor-associated myeloid cell component. The most resistant model displayed a high baseline level of immunosuppressive cytokines IL10 and TGF-β and a cellular infiltrate dominated by CD4+ T cells and tumor-associated macrophages. In contrast, sensitive models displayed higher baseline levels of TNF-α, IFN-γ, and CD8+ T cells and a strong neutrophilic influx, which correlated with higher expression of the major neutrophil-recruiting chemokine CXCL2. Further studies in a sarcoma xenograft model indicate that neutrophils play an important role in tumor shrinkage during oHSV therapy as the anti-tumor effect of oHSV was prevented by neutrophil depletion. This is in stark contrast to our previously reported work in this model where depletion of all CD11b+ cells (including both TAM and neutrophils) enhanced the therapeutic effect of oHSV. Together these data suggest that individual myeloid cell subsets exert opposing influences on the anti-tumor response to oHSV therapy and raise the possibility that the therapeutic effect of oHSV is determined not by susceptibility or permissiveness to oHSV oncolysis but rather by the influence of oHSV on tumor cell-microenvironment interactions.

Citation Format: Jennifer L. Leddon, Chun-Yu Chen, Mark Currier, Pin-Yi Wang, Francesca Jung, Kevin Cripe, Kellie Haworth, Timothy Eubank, David Hildeman, Timothy Cripe. The tumor immunologic microenvironment influences antitumor efficacy of oncolytic herpes simplex virotherapy. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr A74.