Overcoming Obstacles to Naïve T Lymphocyte Trafficking in Anti-Tumor Immunity
Muhitch, Jason B.
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Tumor-draining lymph nodes (TdLN) are the primary sites of generation of anti-tumor immunity, and thus, have emerged as key contributors to the efficacy of cancer treatment during chemotherapy, radiation, surgery, and immunotherapy. A largely overlooked locus for initiating anti-tumor immunity are the vascular gateways termed high endothelial venules (HEV). High rates of trafficking in HEV ensure that rare naïve T cells specific for a given antigen undergo immune activation within LN. Here, we investigated whether lymphocyte entry across HEV was impaired in TdLN HEV and if impediments to T cell trafficking could be overcome therapeutically with preconditioning regimens. Trafficking of naïve and central memory CD8+ T cells was found to be diminished by ∼50% in TdLN in preclinical murine models of melanoma, colon, and breast cancer. Intravital microscopy further pinpointed a deficiency in the multistep adhesion cascade that governs naive T cell entry across HEV. Specifically, TdLN HEV were shown to exhibit an intrinsic defect in the ability to support the transition from slow rolling interactions to the firm arrest step required for T cell extravasation. The molecular defect in TdLN HEV mapped to suboptimal expression of the homeostatic chemokine CCL21, which is vital for triggering ICAM-1/2-mediated firm arrest in HEV. Tumors exerted localized field effects by reducing CCL21 expression exclusively in LN HEV in the lymphatic drainage basin immediately proximal to the tumor. In mice with large tumor burdens, naïve T cells were also found to be defective in L-selectin expression which reduced the quality of rolling interactions necessary for efficient T cell trafficking in HEV. We further examined if TdLN HEV could be targeted for improved trafficking using therapeutic preconditioning regimens. Administration of systemic thermal therapy (STT; core temperature elevated to 39.5 ± 0.5°C for 6 h) as well as chemotherapeutic immunodepletion protocols (cyclophosphamide) overcame intrinsic defects in the firm arrest step in HEV, thus restoring CD8+ T cell trafficking in TdLN. Investigation of the mechanism of action for STT revealed that improved trafficking was mediated by increases in the intravascular density of CCL21 and ICAM-1 on HEV. Moreover, the proadhesive activities of STT in HEV were shown to be orchestrated by two independent cytokine signaling pathways, i.e., lymphotoxin-dependent induction of CCL21 and IL-6-dependent upregulation of ICAM-1. Collectively, these findings suggest that therapeutic targeting of HEV gateways may be a novel strategy to overcome immune evasion in cancer immunity and immunotherapy.