The CD27/CD70 pathway negatively regulates visceral adipose tissue-resident Th2 cells and controls metabolic homeostasis.

Adipose tissue homeostasis relies on the interplay between several regulatory lineages, such as type 2 innate lymphoid cells (ILC2s), T helper 2 (Th2) cells, regulatory T cells, eosinophils, and type 2 macrophages. Among them, ILC2s are numerically the dominant source of type 2 cytokines and are considered as major regulators of adiposity. Despite the overlap in immune effector molecules and sensitivity to alarmins (thymic stromal lymphopoietin and interleukin-33) between ILC2s and resident memory Th2 lymphocytes, the role of the adaptive axis of type 2 immunity remains unclear. We show that mice deficient in CD27, a member of the tumor necrosis factor receptor superfamily, are more resistant to obesity and associated disorders. A comparative analysis of the CD4 compartment of both strains revealed higher numbers of fat-resident memory Th2 cells in the adipose tissue of CD27 knockout mice, which correlated with decreased programmed cell death protein 1-induced apoptosis. Our data point to a non-redundant role for Th2 lymphocytes in obesogenic conditions.

Chronic CD27-CD70 costimulation promotes type 1-specific polarization of effector Tregs.

Introduction: Most T lymphocytes, including regulatory T cells, express the CD27 costimulatory receptor in steady state conditions. There is evidence that CD27 engagement on conventional T lymphocytes favors the development of Th1 and cytotoxic responses in mice and humans, but the impact on the regulatory lineage is unknown. Methods: In this report, we examined the effect of constitutive CD27 engagement on both regulatory and conventional CD4+ T cells in vivo, in the absence of intentional antigenic stimulation. Results: Our data show that both T cell subsets polarize into type 1 Tconvs or Tregs, characterized by cell activation, cytokine production, response to IFN-γ and CXCR3-dependent migration to inflammatory sites. Transfer experiments suggest that CD27 engagement triggers Treg activation in a cell autonomous fashion. Conclusion: We conclude that CD27 may regulate the development of Th1 immunity in peripheral tissues as well as the subsequent switch of the effector response into long-term memory.

PHD2 Constrains Antitumor CD8+ T-cell Activity.

The prolyl hydroxylase domain/hypoxia-inducible factor (PHD/HIF) pathway has been implicated in a wide range of immune and inflammatory processes, including in the oxygen-deprived tumor microenvironment. To examine the effect of HIF stabilization in antitumor immunity, we deleted Phd2 selectively in T lymphocytes using the cre/lox system. We show that the deletion of PHD2 in lymphocytes resulted in enhanced regression of EG7-OVA tumors, in a HIF-1α-dependent manner. The enhanced control of neoplastic growth correlated with increased polyfunctionality of CD8+ tumor-infiltrating lymphocytes, as indicated by enhanced expression of IFNγ, TNFα, and granzyme B. Phenotypic and transcriptomic analyses pointed to a key role of glycolysis in sustaining CTL activity in the tumor bed and identified the PHD2/HIF-1 pathway as a potential target for cancer immunotherapy.

Agonistic anti-CD27 antibody ameliorates EAE by suppressing IL-17 production.

CD27/CD70 costimulation enhances T-cell survival, memory formation and Th1-cell differentiation and effector function. In addition to promoting Th1 responses, CD27 signaling has been shown to exert a negative regulatory role on IL-17 production, resulting in increased sensitivity of CD27 KO mice to EAE. By inducing EAE in full CD27 KO mice, and in a novel, T-cell specific CD27 KO mouse strain (CD4-Cre x CD27flox/flox ), we demonstrate herein that CD27 engagement by its natural ligand (CD70) suppresses IL-17 production in a cell autonomous fashion. We further show that CD27 engagement by an agonistic antibody given after EAE induction or at symptom onset similarly suppresses IL-17 production by activated CD4+ T cells infiltrating the inflamed CNS while IFN-γ production was unaffected, leading to an amelioration of inflammatory-related symptoms. These findings propose CD27 costimulation as a potential candidate for therapeutic manipulation to treat autoimmune and autoinflammatory diseases characterized by excessive IL-17 production.

PD-1 Is Involved in the Dysregulation of Type 2 Innate Lymphoid Cells in a Murine Model of Obesity.

Recent observations clearly highlight the critical role of type 2 innate lymphoid cells in maintaining the homeostasis of adipose tissues in humans and mice. This cell population promotes beiging and limits adiposity directly and indirectly by sustaining a Th2-prone environment enriched in eosinophils and alternatively activated macrophages. Accordingly, the number and function of type 2 innate lymphoid cells (ILC2s) are strongly impaired in obese individuals. In this work, we identify the PD-1-PD-L1 pathway as a factor leading to ILC2 destabilization upon high-fat feeding resulting in impaired tissue metabolism. Tumor necrosis factor (TNF) appears to play a central role, triggering interleukin-33 (IL-33)-dependent PD-1 expression on ILC2s and recruiting and activating PD-L1hi M1 macrophages. PD-1 blockade partially restores the type 2 innate axis, raising the possibility of restoring tissue homeostasis.

Apolipoproteins L control cell death triggered by TLR3/TRIF signaling in dendritic cells.

Apolipoproteins L (ApoLs) are Bcl-2-like proteins expressed under inflammatory conditions in myeloid and endothelial cells. We found that Toll-like receptor (TLR) stimuli, particularly the viral mimetic polyinosinic:polycytidylic acid (poly(I:C)), specifically induce ApoLs7/11 subfamilies in murine CD8α(+)  dendritic cells (DCs). This induction requires the TLR3/TRIF (where TRIF is TIR domain containing adapter-inducing interferon ß) signaling pathway and is dependent on IFN-ß in all ApoLs subfamilies except for ApoL7c. Poly(I:C) treatment of DCs is also associated with induction of both cell death and autophagy. ApoLs expression is related to promotion of DC death by poly(I:C), as ApoLs7/11 knockdown increases DC survival and ApoLs7 are associated with the anti-apoptotic protein Bcl-xL (where Bcl-xL is B-cell lymphoma extra large). Similarly, in human monocyte-derived DCs poly(I:C) induces both cell death and the expression of ApoLs, principally ApoL3. Finally, the BH3-like peptide of ApoLs appears to be involved in the DC death-promoting activity. We would like to propose that ApoLs are involved in cell death linked to activation of DCs by viral stimuli.

Cutting Edge: Hypoxia-Inducible Factor 1 Negatively Regulates Th1 Function.

Tissue hypoxia can occur in physiological and pathological conditions. When O2 availability decreases, the transcription factor hypoxia-inducible factor (HIF)-1α is stabilized and regulates cellular adaptation to hypoxia. The objective of this study was to test whether HIF-1α regulates T cell fate and to define the molecular mechanisms of this control. Our data demonstrate that Th1 cells lose their capacity to produce IFN-γ when cultured under hypoxia. HIF-1α(-/-) Th1 cells were insensitive to hypoxia, underlining a critical role for HIF-1α. Our results point to a role for IL-10, as suggested by the increased IL-10 expression at low O2 levels and the unchanged IFN-γ production by IL-10-deficient Th1 cells stimulated in hypoxic conditions. Accordingly, STAT3 phosphorylation is increased in Th1 cells under hypoxia, leading to enhanced HIF-1α transcription, which, in turn, may inhibit suppressor of cytokine signaling 3 transcription. This positive-feedback loop reinforces STAT3 activation and downregulates Th1 responses that may cause collateral damage to the host.

Thymus-derived regulatory T cells restrain pro-inflammatory Th1 responses by downregulating CD70 on dendritic cells.

The severity and intensity of autoimmune disease in immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) patients and in scurfy mice emphasize the critical role played by thymus-derived regulatory T cells (tTregs) in maintaining peripheral immune tolerance. However, although tTregs are critical to prevent lethal autoimmunity and excessive inflammatory responses, their suppressive mechanism remains elusive. Here, we demonstrate that tTregs selectively inhibit CD27/CD70-dependent Th1 priming, while leaving the IL-12-dependent pathway unaffected. Immunized mice depleted of tTregs showed an increased response of IFN-γ-secreting CD4(+) T cells that was strictly reliant on a functional CD27/CD70 pathway. In vitro studies revealed that tTregs downregulate CD70 from the plasma membrane of dendritic cells (DCs) in a CD27-dependent manner. CD70 downregulation required contact between Tregs and DCs and resulted in endocytosis of CD27 and CD70 into the DC. These findings reveal a novel mechanism by which tTregs can maintain tolerance or prevent excessive, proinflammatory Th1 responses.