Tumour cells evade immune surveillance by upregulating the surface expression of programmed death-ligand 1 (PD-L1), which interacts with programmed death-1 (PD-1) receptor on T cells to elicit the immune checkpoint response. Anti-PD-1 antibodies have shown remarkable promise in treating tumours, including metastatic melanoma. However, the patient response rate is low. A better understanding of PD-L1-mediated immune evasion is needed to predict patient response and improve treatment efficacy. Here we report that metastatic melanomas release extracellular vesicles, mostly in the form of exosomes, that carry PD-L1 on their surface. Stimulation with interferon-γ (IFN-γ) increases the amount of PD-L1 on these vesicles, which suppresses the function of CD8 T cells and facilitates tumour growth. In patients with metastatic melanoma, the level of circulating exosomal PD-L1 positively correlates with that of IFN-γ, and varies during the course of anti-PD-1 therapy. The magnitudes of the increase in circulating exosomal PD-L1 during early stages of treatment, as an indicator of the adaptive response of the tumour cells to T cell reinvigoration, stratifies clinical responders from non-responders. Our study unveils a mechanism by which tumour cells systemically suppress the immune system, and provides a rationale for the application of exosomal PD-L1 as a predictor for anti-PD-1 therapy.

Immune homeostasis is essential for the normal functioning of the immune system, and its breakdown leads to fatal inflammatory diseases. We report here the identification of a member of the tumor necrosis factor-alpha-induced protein-8 (TNFAIP8) family, designated TIPE2, that is required for maintaining immune homeostasis. TIPE2 is preferentially expressed in lymphoid tissues, and its deletion in mice leads to multiorgan inflammation, splenomegaly, and premature death. TIPE2-deficient animals are hypersensitive to septic shock, and TIPE2-deficient cells are hyper-responsive to Toll-like receptor (TLR) and T cell receptor (TCR) activation. Importantly, TIPE2 binds to caspase-8 and inhibits activating protein-1 and nuclear factor-kappaB activation while promoting Fas-induced apoptosis. Inhibiting caspase-8 significantly blocks the hyper-responsiveness of TIPE2-deficient cells. These results establish that TIPE2 is an essential negative regulator of TLR and TCR function, and its selective expression in the immune system prevents hyperresponsiveness and maintains immune homeostasis.

Summary Regulatory T (Treg) cells play essential roles in maintaining immune homeostasis. While Foxp3 expression marks the commitment of progenitors to Treg lineage, how Treg cells are generated during lymphocyte development remains enigmatic. Both NFAT and Smad have been implicated in Foxp3 gene activation, but mice deficient in them are reported to have normal Treg cell numbers. We report here that c-Rel controls the development of Treg cells by promoting the formation of a Foxp3-specific enhanceosome that contains c-Rel, p65, NFAT, Smad, and CREB. Although Smad and CREB first bind to Foxp3 enhancers, they later move to the promoter to form the c-Rel enhanceosome. Consequently, c-Rel-deficient mice have up to ten-fold reductions in Treg cells, and c-Rel-deficient T cells are significantly compromised in Treg differentiation. Thus, Treg development is controlled by a c-Rel enhanceosome, and strategies targeting Rel/NF-κB can be effective for manipulating Treg function.

TRAIL, the tumor necrosis factor-related apoptosis-inducing ligand, selectively induces apoptosis of tumor cells, but not most normal cells. Its role in normal, nontransformed tissues is not clear. We report here that mice deficient in TRAIL have a severe defect in thymocyte apoptosis-thus, thymic deletion induced by T cell receptor ligation is severely impaired. TRAIL-deficient mice are also hypersensitive to collagen-induced arthritis and streptozotocin-induced diabetes and develop heightened autoimmune responses. Thus, TRAIL mediates thymocyte apoptosis and is important in the induction of autoimmune diseases.

Toll-like receptors (TLRs) trigger the production of inflammatory cytokines and shape adaptive and innate immunity to pathogens. We report the identification of B cell leukemia (Bcl)-3 as an essential negative regulator of TLR signaling. By blocking ubiquitination of p50, a member of the nuclear factor (NF)-kB family, Bcl-3 stabilizes a p50 complex that inhibits gene transcription. As a consequence, Bcl-3-deficient mice and cells were found to be hypersensitive to TLR activation and unable to control responses to lipopolysaccharides. Thus, p50 ubiquitination blockade by Bcl-3 limits the strength of TLR responses and maintains innate immune homeostasis. These findings indicate that the p50 ubiquitination pathway can be selectively targeted to control deleterious inflammatory diseases.