Tumor-induced myeloid-derived suppressor cells negatively regulate CD8⁺ T lymphocyte trafficking

The success of T cell-based immunotherapy and, unexpectedly, thermal therapy, standard chemotherapy and radiation hinges on cytotoxic T cells gaining access to tumor targets.  These observations have prompted interest in strategies to improve T cell trafficking to tumors although the mechanisms that positively or negatively regulate extravasation at tumor vascular checkpoints are poorly understood.  Here, we report that the ability of tumor vessels to respond to IL-6-dependent preconditioning regimens that boost CD8 effector T cell homing is temporally and inversely related to the expansion of myeloid-derived suppressor cells (MDSC) within the tumor microenvironment.  Using real-time intravital imaging and immunofluorescence histology, IL-6 therapies were shown to convert vessels from T cell-low to -high recruitment sites in murine tumors with minimal MDSC infiltration (i.e., CT26 colorectal, B16 melanoma, EMT6 mammary tumors). This conversion requires induction of the ICAM-1 trafficking molecule on tumor vessels. Conversely, mammary (4T1, AT-3 and PyMT-MMTV) and pancreatic (Pan02) tumors with higher MDSC burdens were refractory to IL-6 therapies, but became responsive after acute MDSC depletion.  To further investigate contributions of MDSC to poor trafficking, IL-6-responsive tumors were admixed with syngeneic CD11b⁺Gr-1⁺ MDSC isolated from spleens of tumor-bearing mice at a ratio of 2:1, thus mimicking the high MDSC burden detected in IL-6-refractive tumors.  Sustained elevation of MDSC in admixed tumors resulted in failure to support increased T cell trafficking in response to IL-6–dependent therapy. 

While suppressive effects of MDSC on effector T cell function within tumor tissues is a widely-studied topic, the impact of MDSC on naïve T cells within LN has been largely overlooked as MDSC are rare within these critical sites of immune priming.  Previous reports have shown that peripheral MDSC from tumor-bearing mice downregulate the LN homing receptor L-selectin on naive CD4⁺ and CD8⁺ T lymphocytes, but the molecular mechanisms and target cell-specificity has been unclear.  Furthermore, the biological relevance of moderate fluctuations of L-selectin is questionable as the high density of L-selectin molecules normally present on T cells could theoretically buffer against the effects of such loss during trafficking.  Using stringent murine mammary tumor models of high and moderate MDSC burden (4T1 and AT-3, respectively), we demonstrate that MDSC downregulate L-selectin on naive T and B cells post-transcriptionally via a contact-dependent mechanism.  MDSC-driven loss of L-selectin occurs within 24 hours both in vitro and in vivo, and does not appear to be species-restricted as L-selectin on human lymphocytes can also be targeted by MDSC.  By employing real-time intravital microscopy and immunofluorescence histology to visualize and assess naive CD8 T cell trafficking within vascular gateways for lymphocyte trafficking known as high endothelial venules (HEV),  we found that even moderate losses of L-selectin mediated by MDSC causes a profound reduction in the quality of lymphocyte-HEV interactions.  Ultimately, this results in significantly fewer T cells trafficking and infiltrating into the LN parenchyma.  In an in vivo vaccination model, MDSC-mediated loss of L-selectin on naive CD8 T cell and subsequent reduction in lymphocyte trafficking severely diminishes antigen-driven T cell expansion within draining LN.  These data reveal that MDSC localized outside of the LN can shape the magnitude of T cell responses within the intranodal compartment, which has unanticipated implications for systemic immunity in cancer.  Taken together, these findings identify a novel role of MDSC in subverting antitumor immunity by limiting T cell trafficking at both the LN and tumor vascular loci. 

Sharon Evans, PhD