Myeloid-Derived Suppressor Cells and Reduces the Immunosuppressive Activity Oligonucleotides Induces the Differentiation Intratumoral Injection of CpG

Immunostimulatory CpG oligonucleotides (ODN) activate cells that express TLR9 and have been shown to improve the host's response to tumor Ags. Unfortunately, the immunosuppressive microenvironment that surrounds many cancers inhibits Ag-specific cellular responses and thus interferes with CpG-mediated immunotherapy. Myeloid-derived suppressor cells (MDSC) represent an important constituent of this immunosuppressive milieu. Large numbers of MDSC are present in and near tumor sites where they inhibit the activity of Ag-specific T and NK cells. Current studies indicate that the delivery of CpG ODN directly into the tumor bed reduces the immunosuppressive activity of monocytic (CD11b + , Ly6G – , Ly6C high) MDSC. Monocytic MDSC express TLR9 and respond to CpG stimulation by 1) losing their ability to suppress T cell function, 2) producing Th1 cytokines, and 3) differentiating into macrophages with tumoricidal capability. These findings provide insight into a novel mechanism by which CpG ODN contribute to tumor regression, and they support intratumoral injection as the optimal route for their delivery. S ynthetic oligodeoxynucleotides (ODN) containing unme-thylated CpG motifs mimic the ability of bacterial DNA to stimulate the innate immune system. CpG ODN trigger cells that express TLR9, thereby promoting the maturation and improving the function of professional APCs while supporting the generation of Ag-specific B cells and CTL (1–3). Preclinical and clinical trials indicate that CpG ODN have potent immunosti-mulatory effects that enhance the host's response to cancer (4, 5). Kawarada et al. (6) and Heckelsmiller et al. (7) showed that CpG ODN facilitated the induction of tumor-specific immunity and memory (6, 7). This involved both improved plasmacytoid den-dritic cell (DC) entry into the tumor site and the activation of tumor-specific CD8 + CTL and NK cells. This activity was optimized by direct injection of CpG ODN into the tumor, as CpG DNA was far less effective when delivered systemically (6, 7). Virtually all studies to date have examined the effect of CpG ODN on nascent tumor foci and tumors ,5 mm in diameter. This work extends those studies to better understand the effect of CpG ODN on tumors of clinically relevant size (.1 cm diameter). Despite evidence that tumor-specific CTL are expanded in the periphery, immune-mediated tumor destruction is difficult to achieve by any form of immunotherapy. For example, CpG ODN administered alone or in combination with vaccines promote the induction of tumor-specific cellular and humoral immune responses yet rarely lead to prolonged tumor regression (4, 5, 8). …