Abstract 273: TLR8 agonist VTX-2337 (motolimod) decreases monocytic myeloid-derived suppressor cells by inducing differentiation to mature antigen-presenting cells (2015)

Abstract There is increasing evidence that myeloid-derived suppressor cells (MDSCs) contribute to the progression of cancer by inhibiting tumor-directed immune responses and producing mediators that promote the growth and survival of tumor cells. While present at low numbers in the peripheral blood of healthy individuals, levels of circulating MDSC increase in cancer patients as tumors produce factors that drive both the expansion and recruitment of these immune cells. MDSCs are a heterogeneous population and include both granulocytic MDSC (G-MDSC) and monocytic MDSC (M-MDSC) sub-populations, each of which can inhibit T cell function through arginase activity and the production of nitric oxide. Novel approaches to decrease MDSC expansion, or inhibit their immunosuppressive functions, hold great promise for augmenting anti-tumor immunity. In this study, we examined the effect of VTX-2337, a selective TLR8 agonist, on MDSC function. We initially measured the expression of TLR8 in RNA extracted from FACS-sorted MDSC isolated from peripheral blood mononuclear cells (PBMCs) of three healthy donors. TLR8 levels found in the M-MDSC subpopulation were comparable to those found in monocytes and myeloid dendritic cells (mDC), and are known to express high levels of TLR8 protein. To assess the M-MDSC response to TLR8 activation, PBMCs from healthy donors, were treated overnight in medium alone, low dose (167 nM) or high dose (500 nM) VTX-2337. Treatment with VTX-2337 resulted in a significant loss of the M-MDSC (HLA-DR-CD14+) population. Interestingly, we also observed that older (&amp;gt;45 years) donors demonstrated a greater loss of the M-MDSC sub-population than younger (&amp;lt;45 years) donors. In contrast, there was no significant difference in the recovery of the G-MDSC subtype (Lin-HLA-DR-CD33+) following VTX-2337 treatment. Additionally, treatment with either the TLR7 agonist imiquimod, or TLR9 agonist CpG ODN2006, did not result in a loss of the M-MDSC population. We hypothesized that the selective decrease in the M-MDSC subpopulation following TLR8 activation may be attributed to their differentiation into mature antigen-presenting cells. To test this hypothesis, we cultured CFSE-labelled MACS-separated HLA-DR-CD14+ M-MDSC and autologous PBMC with VTX-2337 or medium only (control). TLR8 activation of labelled M-MDSC cells resulted in HLA-DR expression and promoted their differentiation into HLA-DR+CD14+ monocytes. Whether VTX-2337 activation can also decrease the immunosuppressive and/or tumor-promoting activities of MDSC is currently under investigation. Our finding, where TLR8-activated M-MDSC can differentiate into mature antigen-presenting cells is highly novel, and suggests the potential for using VTX-2337 to modulate MDSCs in cancer patients and enable a more effective, immune response to tumors. Citation Format: Zina J. Rutnam, Yushe Dang, Gregory Diestch, Hailing Lu, Yi Yang, Robert Hershberg, Mary L. Disis. TLR8 agonist VTX-2337 (motolimod) decreases monocytic myeloid-derived suppressor cells by inducing differentiation to mature antigen-presenting cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 273. doi:10.1158/1538-7445.AM2015-273