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.

Cytotoxic CD8+ T cells target citrullinated antigens in rheumatoid arthritis

The immune mechanisms that mediate synovitis and joint destruction in rheumatoid arthritis (RA) remain poorly defined. Although increased levels of CD8⁺ T cells have been described in RA, their function in pathogenesis remains unclear. Here we perform single cell transcriptome and T cell receptor (TCR) sequencing of CD8⁺ T cells derived from anti-citrullinated protein antibodies (ACPA)+ RA blood. We identify GZMB⁺CD8⁺ subpopulations containing large clonal lineage expansions that express cytotoxic and tissue homing transcriptional programs, while a GZMK⁺CD8⁺ memory subpopulation comprises smaller clonal expansions that express effector T cell transcriptional programs. We demonstrate RA citrullinated autoantigens presented by MHC class I activate RA blood-derived GZMB⁺CD8⁺ T cells to expand, express cytotoxic mediators, and mediate killing of target cells. We also demonstrate that these clonally expanded GZMB⁺CD8⁺ cells are present in RA synovium. These findings suggest that cytotoxic CD8⁺ T cells targeting citrullinated antigens contribute to synovitis and joint tissue destruction in ACPA+ RA.

Randomized phase I trial of antigen-specific tolerizing immunotherapy with peptide/calcitriol liposomes in ACPA+ rheumatoid arthritis

BACKGROUNDAntigen-specific regulation of autoimmune disease is a major goal. In seropositive rheumatoid arthritis (RA), T cell help to autoreactive B cells matures the citrullinated (Cit) antigen-specific immune response, generating RA-specific V domain glycosylated anti-Cit protein antibodies (ACPA VDG) before arthritis onset. Low or escalating antigen administration under "sub-immunogenic" conditions favors tolerance. We explored safety, pharmacokinetics, and immunological and clinical effects of s.c. DEN-181, comprising liposomes encapsulating self-peptide collagen II259-273 (CII) and NF-κB inhibitor 1,25-dihydroxycholecalciferol.METHODSA double-blind, placebo-controlled, exploratory, single-ascending-dose, phase I trial assessed the impact of low, medium, and high DEN-181 doses on peripheral blood CII-specific and bystander Cit64vimentin59-71-specific (Cit-Vim-specific) autoreactive T cell responses, cytokines, and ACPA in 17 HLA-DRB1*04:01+ or *01:01+ ACPA+ RA patients on methotrexate.RESULTSDEN-181 was well tolerated. Relative to placebo and normalized to baseline values, Cit-Vim-specific T cells decreased in patients administered medium and high doses of DEN-181. Relative to placebo, percentage of CII-specific programmed cell death 1+ T cells increased within 28 days of DEN-181. Exploratory analysis in DEN-181-treated patients suggested improved RA disease activity was associated with expansion of CII-specific and Cit-Vim-specific T cells; reduction in ACPA VDG, memory B cells, and inflammatory myeloid populations; and enrichment in CCR7+ and naive T cells. Single-cell sequencing identified T cell transcripts associated with tolerogenic TCR signaling and exhaustion after low or medium doses of DEN-181.CONCLUSIONThe safety and immunomodulatory activity of low/medium DEN-181 doses provide rationale to further assess antigen-specific immunomodulatory therapy in ACPA+ RA.TRIAL REGISTRATIONAnzctr.org.au identifier ACTRN12617001482358, updated September 8, 2022.FUNDINGInnovative Medicines Initiative 2 Joint Undertaking (grant agreement 777357), supported by European Union's Horizon 2020 research and innovation programme and European Federation of Pharmaceutical Industries and Associations; Arthritis Queensland; National Health and Medical Research Council (NHMRC) Senior Research Fellowship; and NHMRC grant 2008287.

The role of posttranslational modifications in generating neo-epitopes that bind to rheumatoid arthritis-associated HLA-DR alleles and promote autoimmune T cell responses

While antibodies to citrullinated proteins have become a diagnostic hallmark in rheumatoid arthritis (RA), we still do not understand how the autoimmune T cell response is influenced by these citrullinated proteins. To investigate the role of citrullinated antigens in HLA-DR1- and DR4-restricted T cell responses, we utilized mouse models that express these MHC-II alleles to determine the relationship between citrullinated peptide affinity for these DR molecules and the ability of these peptides to induce a T cell response. Using a set of peptides from proteins thought to be targeted by the autoimmune T cell responses in RA, aggrecan, vimentin, fibrinogen, and type II collagen, we found that while citrullination can enhance the binding affinity for these DR alleles, it does not always do so, even when in the critical P4 position. Moreover, if peptide citrullination does enhance HLA-DR binding affinity, it does not necessarily predict the generation of a T cell response. Conversely, citrullinated peptides can stimulate T cells without changing the peptide binding affinity for HLA-DR1 or DR4. Furthermore, citrullination of an autoantigen, type II collagen, which enhances binding affinity to HLA-DR1 did not enhance the severity of autoimmune arthritis in HLA-DR1 transgenic mice. Additional analysis of clonal T cell populations stimulated by these peptides indicated cross recognition of citrullinated and wild type peptides can occur in some instances, while in others cases the citrullination generates a novel T cell epitope. Finally, cytokine profiles of the wild type and citrullinated peptide stimulated T cells unveiled a significant disconnect between proliferation and cytokine production. Altogether, these data demonstrate the lack of support for a simplified model with universal correlation between affinity for HLA-DR alleles, immunogenicity and arthritogenicity of citrullinated peptides. Additionally they highlight the complexity of both T cell receptor recognition of citrulline as well as its potential conformational effects on the peptide:HLA-DR complex as recognized by a self-reactive cell receptor.

Citrullination only infrequently impacts peptide binding to HLA class II MHC

It has been hypothesized that HLA class II alleles associated with rheumatoid arthritis (RA) preferentially present self-antigens altered by post-translational modification, such as citrullination. To understand the role of citrullination we tested four RA-associated citrullinated epitopes and their corresponding wild-type version for binding to 28 common HLA class II. Binding patterns were variable, and no consistent impact of citrullination was identified. Indeed, in one case citrullination significantly increased binding compared to the WT peptide, in another citrullination was associated with a reduction in promiscuity by 40%. For a more comprehensive analysis, we tested over 200 citrullinated peptides derived from vimentin and collagen II for their capacity to bind the RA-associated shared epitope alleles DRB1*01:01 and DRB1*04:01. The overall effect of citrullination on binding was found to be relatively minor, and only rarely associated with 3-fold increases or decreases in affinity. Previous studies have suggested that citrullination of MHC anchor residues, in particular P4, is associated with generation of novel RA-associated epitopes. However, analysis of the predicted MHC-binding cores of all peptides tested found that in modified peptides with increased binding affinity the citrullinated residue was predicted to occupy an anchor position in only a minority of cases. Finally, we also show that identification of citrullinated peptide binders could be facilitated by using the NetMHCIIpan 3.1 algorithm, representing citrullination as a wildcard. Our studies identify a total of 117 citrullinated peptides that bound RA-associated alleles with an affinity of 1000 nM or better.

SLAT promotes TCR-mediated, Rap1-dependent LFA-1 activation and adhesion through interaction of its PH domain with Rap1

SLAT (also known as DEF6) promotes T cell activation and differentiation by regulating NFAT-Ca2+ signaling. However, its role in TCR-mediated inside-out signaling, which induces integrin activation and T cell adhesion, a central process in T cell immunity and inflammation, has not been explored. Here, we show that SLAT is crucial for TCR-induced adhesion to ICAM-1 and LFA-1 affinity maturation in CD4+ T cells. Mechanistic studies revealed that SLAT interacts, through its PH domain, with a key component of inside-out signaling, namely the active form of the small GTPase Rap1. This interaction has been further shown to facilitate the interdependent recruitment of Rap1 and SLAT to the T cell immunological synapse upon TCR engagement. Furthermore, a SLAT mutant lacking its PH domain drastically inhibited LFA-1 activation and CD4+ T cell adhesion. Finally, we established that a constitutively active form of Rap1, which is present at the plasma membrane, rescues the defective LFA-1 activation and ICAM-1 adhesion in SLAT-deficient (Def6−/−) T cells. These findings ascribe a new function to SLAT, and identify Rap1 as a target of SLAT function in TCR-mediated inside-out signaling.

Association of the EF-hand and PH domains of the guanine nucleotide exchange factor SLAT with IP₃ receptor 1 promotes Ca²⁺ signaling in T cells

The guanine nucleotide exchange factor SLAT (SWAP-70-like adaptor of T cells) regulates T cell activation and differentiation by enabling Ca(2+) release from intracellular stores in response to stimulation of the T cell receptor (TCR). We found a TCR-induced association between SLAT and inositol 1,4,5-trisphosphate (IP3) receptor type 1 (IP3R1). The N-terminal region of SLAT, which contains two EF-hand motifs that we determined bound Ca(2+), and the SLAT pleckstrin homology (PH) domain independently bound to IP3R1 by associating with a conserved motif within the IP3R1 ligand-binding domain. Disruption of the SLAT-IP3R1 interaction with cell-permeable, IP3R1-based fusion peptides inhibited TCR-stimulated Ca(2+) signaling, activation of the transcription factor NFAT (nuclear factor of activated T cells), and production of cytokines, suggesting that this interaction is required for optimal T cell activation. The finding that SLAT is an IP3R1-interacting protein required for T cell activation suggests that this interaction could be a potential target for a selective immunosuppressive drug.

Protein kinase C-η controls CTLA-4-mediated regulatory T cell function

Regulatory T cells (T_reg cells), which maintain immune homeostasis and self-tolerance, form an immunological synapse (IS) with antigen-presenting cells (APCs). However, signaling events at the T_reg IS remain unknown. Here we show that protein kinase C-η (PKC-η) associated with CTLA-4 and was recruited to the T_reg IS. PKC-η-deficient T_reg cells displayed defective suppressive activity, including suppression of tumor immunity but not autoimmune colitis. Phosphoproteomic analysis revealed an association between CTLA-4-PKC-η and the GIT-PIX-PAK complex, an IS-localized focal adhesion complex. Defective activation of this complex in PKC-η-deficient T_reg cells was associated with reduced CD86 depletion from APCs by T_reg cells. These results reveal a novel CTLA-4-PKC-η signaling axis required for contact-dependent suppression, implicating this pathway as a potential cancer immunotherapy target.

SLAT-IP3R interaction: implications and functional relevance in T cell activation and calcium signaling (P1172)

SWAP-70-like adaptor of T cells (SLAT) is a guanine nucleotide exchange factor for Rho GTPases that regulates the activation and differentiation of CD4+ T cells by controlling the Ca2+-NFAT signaling pathway. In response to T cell receptor (TCR)/CD28 costimulation, SLAT promotes calcium signaling by regulating calcium release from endoplasmic reticulum (ER) stores. Hence, SLAT-deficient T cells display a severe defect in Ca2+ release from the ER, despite normal activation of PLCg1 and intact production of inositol-1,4,5-trisphosphate (IP3). Here we report a direct and TCR-induced association between SLAT and the (IP3) receptor type 1 (IP3R1), an ER-resident calcium channel, which mediates calcium release from intracellular stores. Using biochemical approaches, we found that both a N-terminal domain, containing an EF-hand motif, and the pleckstrin homology (PH) domain of SLAT bind directly a defined ~20 amino acid region in the regulatory domain of IP3R1. Subcellular fractionation analysis showed that SLAT is present in the ER compartment. Altogether, these data prompt us to hypothesize that SLAT is required for proper IP3R function in T cells, reflecting its direct interaction with the IP3R. Current studies, including strategies aimed at blocking and disrupting the SLAT-IP3R1 interaction, are aimed at determining the biological significance of this novel interaction and its importance for the overall TCR/CD28-induced Ca2+ signaling pathway in T lymphocytes.

The role of pTα/pre-TCR in susceptibility to autoimmunity (P1464)

Autoimmune diseases result from a breakdown in self-tolerance, with a major contribution of pathogenic self-reactive T cells. Individuals with a particular autoimmune disease are often at risk of developing additional autoimmunity directed towards other target tissues. These observations suggest that there might be a mechanism leading to a general defect in self-tolerance, resulting in increased susceptibility to autoimmune disease. Tolerance towards self is initially established in the thymus during the process of thymic selection. Developing thymocytes are first immature CD4 and CD8 double negative thymocytes. Cells with a productive TCRβ gene rearrangement will express a pre-TCR/CD3 complex on the cell surface by combining a TCRβ chain with the pTα chain. Signaling through the pre-TCR, β-selection, plays a crucial role in the generation αβT cells, by enabling further maturation to the CD4 and CD8 double positive stage. The crucial role of pTα at β-selection together with the genetic localization of the ptcra gene and the fact that variations in pTα will affect all developing T cells, suggests that pTα could be a common factor leading to a general defect in self-tolerance. We have discovered differences in the isoform expression pattern and amino acid sequence of pTa between NOD and B6 mice. These findings are consistent with the hypothesis that alterations in the pre-TCR might affect selection, giving a modified TCR repertoire and increased susceptibility to autoimmunity.

Signalling via MHC class II molecules modifies the composition of GEMs in APC

Major histocompatibility complex (MHC) class II molecules are responsible for peptide presentation to helper T lymphocytes and as such play an essential role in the immune response. These molecules transmit intracellular signals leading to diverse consequences in B lymphocytes including proliferation and apoptosis. Recent studies have revealed that glycolipid enriched membrane microdomains (GEMs) behave as signalling platforms for a variety of lymphocyte receptors. We have quantified human leucocyte antigen (HLA)-DR molecules localized in GEMs in human B lymphocytes. Use of a model imitating the interaction of HLA-DR with a T-cell receptor (TCR) modified the constituents of the HLA-DR-enriched GEMs. Confocal microscopy demonstrated a recruitment of HLA-DR and the ganglioside GM1 at the site of HLA-DR interaction with the stimulating ligand. Moreover, cholesterol depletion efficiently impaired this recruitment. Co-localizing proteins detected in HLA-DR-enriched GEMs include protein kinase C (PKC)-delta and actin. These data reveal that MHC class II antigens are localized in GEMs in mature human B lymphocytes and indicates that the formation of the immunological synapse regulates the composition of HLA-DR enriched GEMs in the antigen presenting cell (APC).