Anagen hair bulb immune privilege (IP) collapse is a key underlying event leading to inflammatory hair loss in alopecia areata (AA). Although the hair follicle (HF) organ culture assay provides an excellent platform for the identification of IP protective/restorative drugs for AA management, it is inadequate for high-throughput screening purposes. Thus, we aimed at developing in vitro models suitable for the latter goal. Interferon-γ (IFNγ), key cytokine driving AA, was used to mimic IP collapse-like responses, i.e. upregulation of MHC class Ia and II (MHCI/II) expression in human outer root sheath (ORSK) keratinocytes and epidermal keratinocytes (NHEK), although the latter do not enjoy IP in vivo. Model validation was performed employing the IP guardian, α-melanocyte stimulating hormone (αMSH), and the immunosuppressive drug, Tacrolimus, which is known to protect and restore HF IP ex vivo, as well as the known AA-therapeutic, JAK inhibitor Tofacitinib. Secretion of the IFNγ-inducible chemokine CXCL10 into the medium was evaluated by ELISA along with the expression of classical IP markers by quantitative immunofluorescence and qRT-PCR. As expected, IFNγ significantly increased mRNA and protein expression of MHCI and MHCII, and increased CXCL10 release into the medium in both NHEK and ORSK. Tofacitinib significantly prevented the up-regulation of IP-collapse markers and CXCL10 secretion in both cell types whilst Tacrolimus and αMSH successfully prevented MHCI/II increase exclusively in ORSK. In a therapeutic setting, Tofacitinib and Tacrolimus also rescued the IFNγ-mediated increase in MHC I and II protein expression. Our data reveal that both NHEK and ORSK can be used to screen drugs interfering with IFNg signaling, however only ORSK are suitable model for the testing of IP protective/restorative candidate drugs targeting other pathways.

The transcriptional events that control T cell tolerance are still poorly understood. To investigate why tolerant T cells fail to produce interleukin (IL)-2, we analyzed the regulation of NFkappaB-mediated transcription in CD4(+) T cells after tolerance induction in vivo. We demonstrate that a predominance of p50-p50 homodimers binding to the IL-2 promoter kappaB site in tolerant T cells correlated with repression of NFkappaB-driven transcription. Impaired translocation of the p65 subunit in tolerant T cells was a result from reduced activation of IkappaB kinase and poor phosphorylation and degradation of cytosolic IkappaBs. Moreover, tolerant T cells expressed high amounts of the p50 protein. However, the increased expression of p50 could not be explained by activation-induced de novo synthesis of the precursor p105, which was constitutively expressed in tolerant T cells. We also demonstrate the exclusive induction of the IkappaB protein B cell lymphoma 3 (Bcl-3) in tolerant T cells as well as its specific binding to the NFkappaB site. These results suggest that the cellular ratio of NFkappaB dimers, and thus the repression of NFkappaB activity and IL-2 production, are regulated at several levels in tolerant CD4(+) T cells in vivo.

BACKGROUND:Histone deacetylase inhibitors (HDACIs) induce hyperacetylation of core histones modulating chromatin structure and affecting gene expression. These compounds are also able to induce growth arrest, cell differentiation, and apoptotic cell death of tumor cells in vitro as well as in vivo. Even though several genes modulated by HDAC inhibition have been identified, those genes clearly responsible for the biological effects of these drugs have remained elusive. We investigated the pharmacological effect of the HDACI and potential anti-cancer agent Trichostatin A (TSA) on primary T cells.METHODS:To ascertain the effect of TSA on resting and activated T cells we used a model system where an enriched cell population consisting of primary T-cells was stimulated in vitro with immobilized anti-CD3/anti-CD28 antibodies whilst exposed to pharmacological concentrations of Trichostatin A.RESULTS:We found that this drug causes a rapid decline in cytokine expression, accumulation of cells in the G1 phase of the cell cycle, and induces apoptotic cell death. The mitochondrial respiratory chain (MRC) plays a critical role in the apoptotic response to TSA, as dissipation of mitochondrial membrane potential and reactive oxygen species (ROS) scavengers block TSA-induced T-cell death. Treatment of T cells with TSA results in the altered expression of a subset of genes involved in T cell responses, as assessed by microarray gene expression profiling. We also observed up- as well as down-regulation of various costimulatory/adhesion molecules, such as CD28 and CD154, important for T-cell function.CONCLUSIONS:Taken together, our findings indicate that HDAC inhibitors have an immunomodulatory potential that may contribute to the potency and specificity of these antineoplastic compounds and might be useful in the treatment of autoimmune disorders.

Repeated exposures to both microbial and innocuous Ags in vivo have been reported to both eliminate and tolerize T cells after their initial activation and expansion. The remaining tolerant T cells have been shown to suppress the response of naive T cells in vitro. This feature is reminiscent of natural CD4(+)CD25(-) regulatory T cells. However, it is not known whether the regulatory function of in vivo-tolerized T cells is similar to the function of natural CD4(+)CD25(-) regulatory T cells. In this study, we demonstrate that CD4(+)CD25(-) as well as CD4(+)CD25(-) T cells isolated from mice treated with superantigen three consecutive times to induce tolerance were functionally comparable to natural CD4(+)CD25(-) regulatory T cells, albeit more potent. The different subpopulations of in vivo-tolerized CD4(+) T cells efficiently down-modulated costimulatory molecules on dendritic cells, and their suppressive functions were strictly cell contact dependent. Importantly, we demonstrate that conventional CD4(+)CD25(-) T cells could also be induced to acquire regulatory functions by the same regimen in the absence of natural regulatory T cells in vivo, but that such regulatory cells were functionally different.

Repeated exposures to both microbial and innocuous Ags in vivo have been reported to both eliminate and tolerize T cells after their initial activation and expansion. The remaining tolerant T cells have been shown to suppress the response of naive T cells in vitro. This feature is reminiscent of natural CD4+CD25+ regulatory T cells. However, it is not known whether the regulatory function of in vivo-tolerized T cells is similar to the function of natural CD4+CD25+ regulatory T cells. In this study, we demonstrate that CD4+CD25+ as well as CD4+CD25− T cells isolated from mice treated with superantigen three consecutive times to induce tolerance were functionally comparable to natural CD4+CD25+ regulatory T cells, albeit more potent. The different subpopulations of in vivo-tolerized CD4+ T cells efficiently down-modulated costimulatory molecules on dendritic cells, and their suppressive functions were strictly cell contact dependent. Importantly, we demonstrate that conventional CD4+CD25− T cells could also be induced to acquire regulatory functions by the same regimen in the absence of natural regulatory T cells in vivo, but that such regulatory cells were functionally different.

Costimulatory molecules of the B7 family regulate the activation of T lymphocytes. T cell activation is promoted by binding of B7 molecules to CD28 and inhibited by binding to CTLA-4 (CD152). The balance between positive signals through CD28 and negative signals through CTLA-4 is critical for the fate of the T cell and is subject to tight regulation. Recent in vitro and in vivo studies have significantly advanced our understanding of the function of the CTLA-4 receptor. The results of these experiments suggest that CTLA-4 is critical for the induction of self-tolerance, and that it may have distinct signaling functions in resting and activated T cells. In resting T cells, CTLA-4 crosslinking leads to cell-cycle arrest, whereas in activated T cells, CTLA-4 crosslinking induces apoptosis. In this article, we will review the physiologic functions of the CTLA-4 receptor.

The CTLA-4 receptor is a critical inhibitory regulator of T cell proliferation and effector function. However, the mechanisms through which CTLA-4 modulates the activation of T cells remain uncertain. Initial studies, using activated human T cells, have suggested that CTLA-4 crosslinking may induce apoptosis. However, more recent experiments have demonstrated that crosslinking of the CTLA-4 receptor on the surface of resting murine T cells blocks cell cycle progression without inducing apoptosis. Here we provide evidence that CTLA-4 crosslinking on the surface of activated murine CD4(+) T lymphocytes leads to death of a substantial fraction of the cells whereas in resting CD4(+) T cells the same stimulation conditions induce cell cycle arrest without apoptosis. Cell death induced by CTLA-4 stimulation occurs independently of Fas and therefore may involve a novel pathway. CTLA-4-mediated apoptosis may be a means of terminating the function of previously stimulated T cells. Exploitation of this mechanism also may provide a therapeutic strategy to eliminate alloreactive or autoreactive T cells.