Blockade of surface-bound TGF-β on regulatory T cells abrogates suppression of effector T cell function in the tumor microenvironment

Abstract

Regulatory T cells (Tregs) suppress antitumor immunity by inhibiting the killing of tumor cells by antigen-specific CD8+ T cells. To better understand the mechanisms involved, we used ex vivo three-dimensional (3D) collagen-fibrin gel cultures of dissociated B16 melanoma tumors. This system recapitulated the in vivo suppression of antimelanoma immunity, rendering the dissociated tumor cells resistant to killing by cocultured activated, antigen-specific T cells. Immunosuppression was not observed when tumors excised from Treg-depleted mice were cultured in this system. Experiments with neutralizing antibodies showed that blocking transforming growth factor–β (TGF-β) also prevented immunosuppression. Immunosuppression depended on cell-cell contact or cellular proximity because soluble factors from the collagen-fibrin gel cultures did not inhibit tumor cell killing by T cells. Moreover, intravital, two-photon microscopy showed that tumor-specific Pmel-1 effector T cells physically interacted with tumor-resident Tregs in mice. Tregs isolated from B16 tumors alone were sufficient to suppress CD8+ T cell–mediated killing, which depended on surface-bound TGF-β on the Tregs. Immunosuppression of CD8+ T cells correlated with a decrease in the abundance of the cytolytic protein granzyme B and an increase in the cell surface amount of the immune checkpoint receptor PD-1. These findings suggest that contact between Tregs and antitumor T cells in the tumor microenvironment inhibits antimelanoma immunity in a TGF-β–dependent manner and highlight potential ways to inhibit intratumoral Tregs therapeutically.

Role of the immunosuppressive microenvironment in immunotherapy

“…Tregs are commonly found in solid tumors and promote immunosuppression by several mechanisms including secretion of immunosuppressive cytokines, competing for activating cyokines with effector cells, and after direct cellular contact with infiltrating effector cells 40, 41. However, elevated levels of Treg infiltrates in tumors can accompany elevated levels of effector T cell infiltrates and thus are not absolute indicators of an overall immunosuppressive TME 42 .…”

Targeting cytokines secreted by CD4+CD25highCD127low regulatory T cells inhibits ovarian cancer progression

“…21 Despite the evidence that Tregs inhibit tumour immunity, the question remains as to where and through what mechanism do Tregs inhibit the anti-tumour immune response. 8 Notably, we identified that the percentage of CD4 + CD25 + CD127 low Tregs was significantly increased and was related to lymphatic invasion in EOC. The percentage of CD4 + CD25 high CD127 low Tregs was significantly increased in normal CD4 + T cells after coculturing with OVCAR3.…”

“…The efficacy of many immunotherapeutic approaches that target T cell co‐inhibitory and co‐stimulatory receptors correlate with an altered balance in the ratio of effector T cells to Tregs in favour of the effector cells . Despite the evidence that Tregs inhibit tumour immunity, the question remains as to where and through what mechanism do Tregs inhibit the anti‐tumour immune response . Notably, we identified that the percentage of CD4 + CD25 + CD127 low Tregs was significantly increased and was related to lymphatic invasion in EOC.…”

“…In humans, Tregs represent cells that maintain homeostasis and are defined as a subset of CD4 + T cells that express high levels of CD25 (IL‐2 receptor α‐chain), forkhead box P3 (FoxP3), and a low level of CD127 (alpha chain of the IL‐7 receptor) . Tregs can inhibit tumour antigen‐specific T cell responses through several mechanisms, including the release of suppressive cytokines [such as transforming growth factor–β (TGF‐β), interleukin‐10 (IL‐10) and IL‐35] . Other possible mechanisms of Treg‐mediated T cell suppression include IL‐2 deprivation via high surface CD25 expression and release of soluble CD25 …”

“…7 Tregs can inhibit tumour antigen-specific T cell responses through several mechanisms, including the release of suppressive cytokines [such as transforming growth factor-β (TGF-β), interleukin-10 (IL-10) and . 8 Other possible mechanisms of Treg-mediated T cell suppression include IL-2 deprivation via high surface CD25 expression and release of soluble CD25. 9 Studies have reported that matrix metalloproteinases (MMPs) regulated the occurrence and development of malignant tumours by participating in cellular processes of proliferation, apoptosis and angiogenesis.…”