Abstract 3537: Manipulation of local and systemic immune suppression by GPI-anchored immune stimulatory proteins

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Each day in the United States alone, approximately 3400 people are diagnosed while another 1500 people die from cancer. Breast cancer is the most common form of cancer among females and accounts for the second leading cause of death among female cancer patients. In order to be effective in a therapeutic setting, current immunotherapies must be able to overcome the multiple layers of tumor-induced immune suppression such as modulation of MHC expression and costimulatory molecules (CD80/CD86), expression of inhibitory molecules PDL-1 and GITRL and the secretion of immunosuppressive factors such as VEGF and TGF-β into the tumor microenvironment. These immunosuppressive factors promote the recruitment, differentiation and expansion of regulatory immune cells such as myeloid derived suppressor cells (MDSCs) and regulatory T cells (Tregs) into the tumor microenvironment. Using the 4T07 murine breast cancer model we investigated the effects of expressing GPI-anchored immune stimulatory molecules (GPI-ISMs), namely cytokines (IL-2, IL-12) and the costimulatory protein B7-1, on the surface of the tumor cells. BALB/c mice were challenged subcutaneously (s.c.) with either wild-type 4T07 cells (4T07-WT) or 4T07 cells expressing GPI-ISMs. We observed significant splenomegaly in the mice challenged with 4T07-WT cells relative to the mice challenged with 4T07 cells expressing GPI-ISMs. This observed splenomegaly correlated with tumor size and a 4-5 fold increase in the percentage of splenic CD11b+Gr1+ MDSCs indicating the role of active immune suppression in the tumorigenicity of 4T07 breast cancer cells. We then conducted studies to analyze the effect of GPI-ISMs on infiltrating cells into the tumor microenvironment as well as in the spleen and draining lymph nodes (dLNs). Three groups of mice were challenged (s.c.) with the following cells mixed in a 1:1 ratio with BD Matrigel™ (a solubilized basement membrane preparation derived from a mouse sarcoma): 4T07-WT, 4T07-B7/IL-12 or PBS (control). Seven days post challenge, the Matrigel/tumor, spleen and dLNs were harvested from the mice, digested and analyzed for cellular infiltrates by flow cytometry. We observed that the expression of GPI-ISMs on the surface of tumor cells led to reduced angiogenesis as evidenced by a reduced level of blood vessels and decreased presence of CD4+CD25+FOXP3+ regulatory T cells and CD11b+Gr1+ MDSCs locally at the tumor site and dLNs as well as systemically in the spleen. Additionally, there was a decrease in CD8+PD1+ exhausted T cells at the tumor site. Along with the inhibition of immune suppressive cell populations, the GPI-ISMs increased the presence of CD4+ and CD8+ T cells as well as dendritic cells and B cells. These observations suggest that components of the active immune suppression evident in this model can be inhibited by expressing GPI-ISMs on the surface of the 4T07 tumor cells and could be effective in a therapeutic setting. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3537. doi:1538-7445.AM2012-3537