A regulatory circuit controlled by extranuclear and nuclear retinoic acid receptor α determines T cell activation and function

Ligation of retinoic acid receptor alpha (RARα) by RA promotes varied transcriptional programs associated with immune activation and tolerance, but genetic deletion approaches suggest the impact of RARα on TCR signaling. Here, we examined whether RARα would exert roles beyond transcriptional regulation. Specific deletion of the nuclear isoform of RARα revealed an RARα isoform in the cytoplasm of T cells. Extranuclear RARα was rapidly phosphorylated upon TCR stimulation and recruited to the TCR signalosome. RA interfered with extranuclear RARα signaling, causing suboptimal TCR activation while enhancing FOXP3+ regulatory T cell conversion. TCR activation induced the expression of CRABP2, which translocates RA to the nucleus. Deletion of Crabp2 led to increased RA in the cytoplasm and interfered with signalosome-RARα, resulting in impaired anti-pathogen immunity and suppressed autoimmune disease. Our findings underscore the significance of subcellular RA/RARα signaling in T cells and identify extranuclear RARα as a component of the TCR signalosome and a determinant of immune responses.

Ablation of BATF Alleviates Transplant Rejection via Abrogating the Effector Differentiation and Memory Responses of CD8+ T Cells

Allogeneic CD8⁺ T cells are prominently involved in allograft rejection, but how their effector differentiation and function are regulated at a transcriptional level is not fully understood. Herein, we identified the basic leucine zipper ATF-like transcription factor (BATF) as a key transcription factor that drives the effector program of allogeneic CD8⁺ T cells. We found that BATF is highly expressed in graft-infiltrating CD8⁺ T cells, and its ablation in CD8⁺ T cells significantly prolonged skin allograft survival in a fully MHC-mismatched transplantation model. To investigate how BATF dictates allogeneic CD8⁺ T cell response, BATF^-/- and wild-type (WT) CD8⁺ T cells were mixed in a 1:1 ratio and adoptively transferred into B6.Rag1^-/- mice 1 day prior to skin transplantation. Compared with WT CD8⁺ T cells at the peak of rejection response, BATF^-/- CD8⁺ T cells displayed a dysfunctional phenotype, evident by their failure to differentiate into CD127^-KLRG1⁺ terminal effectors, impaired proliferative capacity and production of pro-inflammatory cytokines/cytotoxic molecules, and diminished capacity to infiltrate allografts. In association with the failure of effector differentiation, BATF^-/- CD8⁺ T cells largely retained TCF1 expression and expressed significantly low levels of T-bet, TOX, and Ki67. At the memory phase, BATF-deficient CD8⁺ T cells displayed impaired effector differentiation upon allogeneic antigen re-stimulation. Therefore, BATF is a critical transcriptional determinant that governs the terminal differentiation and memory responses of allogeneic CD8⁺ T cells in the transplantation setting. Targeting BATF in CD8⁺ T cells may be an attractive therapeutic approach to promote transplant acceptance.

Infant T cells are developmentally adapted for robust lung immune responses through enhanced T cell receptor signaling

Infants require coordinated immune responses to prevent succumbing to multiple infectious challenges during early life, particularly in the respiratory tract. The mechanisms by which infant T cells are functionally adapted for these responses are not well understood. Here, we demonstrated using an in vivo mouse cotransfer model that infant T cells generated greater numbers of lung-homing effector cells in response to influenza infection compared with adult T cells in the same host, due to augmented T cell receptor (TCR)–mediated signaling. Mouse infant T cells showed increased sensitivity to low antigen doses, originating at the interface between T cells and antigen-bearing accessory cells—through actin-mediated mobilization of signaling molecules to the immune synapse. This enhanced signaling was also observed in human infant versus adult T cells. Our findings provide a mechanism for how infants control pathogen load and dissemination, which is important for designing developmentally targeted strategies for promoting immune responses at this vulnerable life stage.

Brief homogeneous TCR signals instruct common iNKT progenitors whose effector diversification is characterized by subsequent cytokine signaling

Innate-like T cell populations expressing conserved TCRs play critical roles in immunity through diverse developmentally acquired effector functions. Focusing on the prototypical lineage of invariant natural killer T (iNKT) cells, we sought to dissect the mechanisms and timing of fate decisions and functional effector differentiation. Utilizing induced expression of the semi-invariant NKT cell TCR on double positive thymocytes, an initially highly synchronous wave of iNKT cell development was triggered by brief homogeneous TCR signaling. After reaching a uniform progenitor state characterized by IL-4 production potential and proliferation, effector subsets emerged simultaneously, but then diverged toward different fates. While NKT17 specification was quickly completed, NKT1 cells slowly differentiated and expanded. NKT2 cells resembled maturing progenitors, which gradually diminished in numbers. Thus, iNKT subset diversification occurs in dividing progenitor cells without acute TCR input but utilizes multiple active cytokine signaling pathways. These data imply a two-step model of iNKT effector differentiation.

MST1/2 Balance Immune Activation and Tolerance by Orchestrating Adhesion, Transcription, and Organelle Dynamics in Lymphocytes

The STE20-like serine/threonine kinases MST1 and MST2 (MST1/2) are mammalian homologs of Hippo in flies. MST1/2 regulate organ size by suppressing the transcription factor YAP, which promotes proliferation. MST1 is predominantly expressed in immune cells, where it plays distinct roles. Here, we review the functions of MST1/2 in immune cells, uncovered by a series of recent studies, and discuss the connection between MST1/2 function and immune responses. MST1/2 regulate lymphocyte development, trafficking, survival, and antigen recognition by naive T cells. MST1/2 also regulate the function of regulatory T cells and effector T cell differentiation, thus acting to balance immune activation and tolerance. Interestingly, MST1/2 elicit these functions not by the “canonical” Hippo pathway, but by the non-canonical Hippo pathway or alternative pathways. In these pathways, MST1/2 regulates cellular processes relating to immune response, such as chemotaxis, cell adhesion, immunological synapse, gene transcriptions. Recent advances in our understanding of the molecular mechanisms of these processes have revealed important roles of MST1/2 in regulating cytoskeleton remodeling, integrin activation, and vesicular transport in lymphocytes. We discuss the significance of the MST1/2 signaling in lymphocytes in the regulation of organelle dynamics.

Role of PD-1 during effector CD8 T cell differentiation

PD-1 (programmed cell death-1) is the central inhibitory receptor regulating CD8 T cell exhaustion during chronic viral infection and cancer. Interestingly, PD-1 is also expressed transiently by activated CD8 T cells during acute viral infection, but the role of PD-1 in modulating T cell effector differentiation and function is not well defined. To address this question, we examined the expression kinetics and role of PD-1 during acute lymphocytic choriomeningitis virus (LCMV) infection of mice. PD-1 was rapidly up-regulated in vivo upon activation of naive virus-specific CD8 T cells within 24 h after LCMV infection and in less than 4 h after peptide injection, well before any cell division had occurred. This rapid PD-1 expression by CD8 T cells was driven predominantly by antigen receptor signaling since infection with a LCMV strain with a mutation in the CD8 T cell epitope did not result in the increase of PD-1 on antigen-specific CD8 T cells. Blockade of the PD-1 pathway using anti-PD-L1 or anti-PD-1 antibodies during the early phase of acute LCMV infection increased mTOR signaling and granzyme B expression in virus-specific CD8 T cells and resulted in faster clearance of the infection. These results show that PD-1 plays an inhibitory role during the naive-to-effector CD8 T cell transition and that the PD-1 pathway can also be modulated at this stage of T cell differentiation. These findings have implications for developing therapeutic vaccination strategies in combination with PD-1 blockade.

Uncoupling T-cell expansion from effector differentiation in cell-based immunotherapy

Adoptive cellular immunotherapy (ACT) is a potentially curative therapy for patients with advanced cancer. Eradication of tumor in mouse models and humans correlates with both a high dose of adoptively transferred cells and cells with a minimally differentiated phenotype that maintain replicative capacity and multipotency. We speculate that response to ACT not only requires transfer of cells with immediate cytolytic effector function to kill the bulk of fast-growing tumor but also transfer of tumor-specific cells that maintain an ability for self-renewal and the capacity to produce a continual supply of cytolytic effector progeny until all malignant cells are eliminated. Current in vitro methods to expand cells to sufficient numbers and still maintain a minimally differentiated phenotype are hindered by the biological coupling of clonal expansion and effector differentiation. Therefore, a better understanding of the physiologic mechanism that couples cell expansion and differentiation in CD8(+) T cells may improve the efficacy of ACT.