Regulatory T (Treg) cells act as terminators of T cell immuniy during acute phase of viral infection; however, their role and suppressive mechanism in chronic viral infection are not completely understood. In this study, we compared the phenotype and function of Treg cells during acute or chronic infection with lymphocytic choriomeningitis virus. Chronic infection, unlike acute infection, led to a large expansion of Treg cells and their upregulation of programmed death-1 (PD-1). Treg cells from chronically infected mice (chronic Treg cells) displayed greater suppressive capacity for inhibiting both CD8+ and CD4+ T cell proliferation and subsequent cytokine production than those from naive or acutely infected mice. A contact between Treg and CD8+ T cells was necessary for the potent suppression of CD8+ T cell immune response. More importantly, the suppression required cell-specific expression and interaction of PD-1 on chronic Treg cells and PD-1 ligand on CD8+ T cells. Our study defines PD-1 upregulated on Treg cells and its interaction with PD-1 ligand on effector T cells as one cause for the potent T cell suppression and proposes the role of PD-1 on Treg cells, in addition to that on exhausted T cells, during chronic viral infection.

BackgroundRegulatory T (Treg) cells have an immunosuppressive function in cancer, but the underlying mechanism of immunosuppression in the tumor microenvironment (TME) is unclear.MethodsWe compared the phenotypes of T cell subsets, including Treg cells, obtained from peripheral blood, malignant effusion, and tumors of 103 cancer patients. Our primary focus was on the expression of immune checkpoint (IC)-molecules, such as programmed death (PD)-1, T-cell immunoglobulin and mucin-domain containing (TIM)-3, T cell Ig and ITIM domain (TIGIT), and cytotoxic T lymphocyte antigen (CTLA)-4, on Treg cells in paired lymphocytes from blood, peritumoral tissue, and tumors of 12 patients with lung cancer. To identify the immunosuppressive mechanisms acting on tumor-infiltrating Treg cells, we conducted immunosuppressive functional assays in a mouse model.ResultsCD8+, CD4+ T cells, and Treg cells exhibited a gradual upregulation of IC-molecules the closer they were to the tumor. Interestingly, PD-1 expression was more prominent in Treg cells than in conventional T (Tconv) cells. In lung cancer patients, higher levels of IC-molecules were expressed on Treg cells than on Tconv cells, and Treg cells were also more enriched in the tumor than in the peri-tumor and blood. In a mouse lung cancer model, IC-molecules were also preferentially upregulated on Treg cells, compared to Tconv cells. PD-1 showed the greatest increase on most cell types, especially Treg cells, and this increase occurred gradually over time after the cells entered the TME. PD-1 high-expressing tumor-infiltrating Treg cells displayed potent suppressive activity, which could be partially inhibited with a blocking anti-PD-1 antibody.ConclusionsWe demonstrate that the TME confers a suppressive function on Treg cells by upregulating IC-molecule expression. Targeting IC-molecules, including PD-1, on Treg cells may be effective for cancer treatment.

Inhibition of immune checkpoint proteins like programmed death 1 (PD-1) is a promising therapeutic approach for several cancers, including non-small cell lung cancer (NSCLC). Although PD-1 ligand (PD-L1) expression is used to predict anti-PD-1 therapy responses in NSCLC, its accuracy is relatively less. Therefore, we sought to identify a more accurate predictive blood biomarker for evaluating anti-PD-1 response. We evaluated the frequencies of T cells, B cells, natural killer (NK) cells, polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs), mononuclear myeloid-derived suppressor cells (M-MDSCs), and Lox-1+ PMN-MDSCs in peripheral blood samples of 62 NSCLC patients before and after nivolumab treatment. Correlation of immune-cell population frequencies with treatment response, progression-free survival, and overall survival was also determined. After the first treatment, the median NK cell percentage was significantly higher in responders than in non-responders, while the median Lox-1+ PMN-MDSC percentage showed the opposite trend. NK cell frequencies significantly increased in responders but not in non-responders. NK cell frequency inversely correlated with that of Lox-1+ PMN-MDSCs after the first treatment cycle. The NK cell-to-Lox-1+ PMN-MDSC ratio (NMR) was significantly higher in responders than in non-responders. Patients with NMRs ≥ 5.75 after the first cycle had significantly higher objective response rates and longer progression-free and overall survival than those with NMRs <5.75. NMR shows promise as an early predictor of response to further anti-PD-1 therapy.

Tumor blood vessels are leaky and immature, which causes inadequate blood supply to tumor tissues resulting in hypoxic microenvironment and promotes metastasis. Here we have explored tumor vessel modulating activity of Sac-1004, a recently developed molecule in our lab, which directly potentiates VE-cadherin-mediated endothelial cell junction. Sac-1004 could enhance vascular junction integrity in tumor vessels and thereby inhibit vascular leakage and enhance vascular perfusion. Improved perfusion enabled Sac-1004 to have synergistic anti-tumor effect on cisplatin-mediated apoptosis of tumor cells. Interestingly, characteristics of normalized blood vessels namely reduced hypoxia, improved pericyte coverage and decreased basement membrane thickness were readily observed in tumors treated with Sac-1004. Remarkably, Sac-1004 was also able to inhibit lung and lymph node metastasis in MMTV and B16BL6 tumor models. This was in correlation with a reduction in epithelial-to-mesenchymal transition of tumor cells with considerable diminution in expression of related transcription factors. Moreover, cancer stem cell population dropped substantially in Sac-1004 treated tumor tissues. Taken together, our results showed that direct restoration of vascular junction could be a significant strategy to induce normalization of tumor blood vessels and reduce metastasis.

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