Control of tumor outgrowth by regional immunization with peptide-pulsed dendritic cells depends on epitope density

4843 Immunization route with activated, peptide-pulsed dendritic cells (DC) determines the location of antigen-specific CD8 memory populations and determines the efficacy of tumor control in different organ compartments. Thus, DC can be targeted to specific lymphoid organs to generate functionally divergent immune responses. However, no study has evaluated whether epitope density on immunizing DC differentially controls the induction of antigen-specific primary immune responses in different lymphoid compartments. Using a preclinical HLA-A2-transgenic mouse model and an endogenously-expressed antigen from tyrosinase (Tyr 369-377 ), we evaluated the immunogenicity and anti-tumor efficacy of regionally delivered DC pulsed with a range of peptide concentrations. Following intravenous injection, DC infiltrated spleen but not lymph nodes; Tyr 369-377 -specific primary and memory T cells were likewise confined to spleen. Increasing the density of Tyr 369-377 epitope on immunizing DC progressively increased the magnitude of Ag-specific CD8 + T cell response in spleen, and also enhanced the control of HLA-A2-expressing melanoma growing in the lungs. In contrast, subcutaneous delivery of peptide-pulsed DC targeted both draining peripheral lymph nodes and spleen, and antigen-specific CD8 + cells were present in both compartments. Interestingly, the optimal epitope density for the induction of primary antigen-specific CD8 + T cell responses in lymph node and control of a subcutaneously-growing HLA-A2-expressing melanoma was 3 logs lower than previously determined for spleen. Furthermore, the requirement for increased epitope density to mediate responses in spleen was independent of DC injection route. The disparity in optimal epitope density between lymphoid compartments reflects a differential kinetic of migration DC to, or retention of DC in, the lymph node and spleen. In both lymph nodes and spleen, supra-optimal epitope densities ablated Ag-specific CD8 + T cells, underscoring the need to optimize antigen density prior to clinical therapeutic application. Collectively, these data demonstrate a fundamental difference in lymph node and spleen as site for activation of anti-tumor immune responses. Supported by the American Cancer Society (DWM) and USPHS R01CA78400 (VHE).