Tumor-associated Tregs obstruct antitumor immunity by promoting T cell dysfunction and restricting clonal diversity in tumor-infiltrating CD8+ T cells

BACKGROUND

Accumulation of regulatory T cells (Treg) has been described to often correlate with poor prognosis in many solid tumors. How Treg presence impinges on limited functionality and clonal composition of tumor-associated CD8 +T cells has important implications for their therapeutic targeting in the tumor microenvironment. In the present study, we investigated how accumulation of Tregs contributes to T cell dysfunction and clonal constriction of tumor-infiltrating CD8 +T cells.

METHODS

Resected melanoma and lung adenocarcinoma tissues from tumor-bearing mice or patients were analyzed. The proportions and phenotype as well as clonal diversity of tumor-associated CD8 +T cells were evaluated by flow cytometry and single-cell T-cell receptor (TCR) sequencing, respectively, at early or advanced tumor stages or under Treg depletion conditions. Furthermore, antigen-specific T cells were evaluated on adoptive transfer into tumor-bearing mice in the presence or absence of anti-CTLA-4 antibody or CTLA-4 Ig. Lastly, tumor-bearing mice were treated with anti-KLRG1 antibody and/or bromodomain inhibitor JQ1 with interleukin (IL)-2 immune complexes to determine therapeutic efficacy.

RESULTS

We demonstrate that the emergence of exhaustion-like phenotype and impaired effector functionality in tumor-associated CD8 +T cells is positively correlated with Treg accumulation in the tumor bed and this dysfunctional phenotype becomes reversed on Treg reduction in murine melanoma and lung cancer models. Heightened tumor-associated Treg-expressed CTLA-4 is key to emergence and sustenance of this phenotype. Furthermore, TCR sequencing revealed a clonal shrinkage of tumor-infiltrating CD8 +T cells as tumor progressed, which was associated with reduced survival profile concomitant to increasing Treg proportions. Limited IL-2 availability was a key mechanism contributing to this peripheral repertoire reshaping as Treg depletion improved IL-2 levels, rescued CD8 +T cell viability, and improved their clonal diversity. Finally, targeted reduction of tumor but not peripheral Tregs through JQ1 and/or anti-KLRG1 antibody significantly improved antitumor response in melanoma-bearing mice when supplemented with IL-2 immune complexes.

CONCLUSION

Collectively, our study reveals a bimodal program enacted by Tregs to support T cell dysfunction in the tumor bed and highlights a promising therapeutic regimen for localized reprogramming of the tumor microenvironment to curb Treg impairment of antitumor CD8 +T cell response in favor of improved antitumor immunity.

Identification and characterization of a unique KLRG1-expressing subset of CD4+FOXP3+ Tregs in Non-small cell lung cancer

It is well established that CD4+FOXP3+ T regulatory cells (Tregs) contribute to dampening anti-tumor responses. Despite our improved understanding of their role in cancer, there remains a knowledge gap with respect to the clonal composition and heterogeneity of tumor-infiltrating Tregs in solid cancers. We addressed this outstanding issue by conducting comprehensive phenotypic profiling of Tregs present in lung adenocarcinomas of a genetically engineered mouse model of non-small cell lung cancer (NSCLC) as well as those in resected tumors of NSCLC patients. Multi-parameter flow cytometric analysis revealed that unlike the peripheral tissues, the tumor harbors a distinct sub-population of Tregs that express the co-inhibitory receptor, KLRG1. Compared to their negative counterparts, the KLRG1+ Treg subset exhibited heightened expression of a number of Treg signature proteins as well as higher levels of activation and memory molecular markers suggesting that they are a highly activated and differentiated Treg pool that is recruited to, or induced in the tumor microenvironment. Consistent with this phenotype, these KLRG1+ Tregs were superior in their capacity to suppress T cell proliferation relative to the KLRG1-cells. Collectively, these findings demonstrate that the tumor microenvironment in non-small cell lung cancer harbors a unique Treg sub-population that is characterized by dominant expression of KLRG1, and which represents a clonal pool with the most potent inhibition of T cell responses. These studies highlight the installment of distinct Treg subsets in NSCLC that have implications for regulation of anti-tumor responses.