Myeloid and lymphoid expression of C9orf72 regulates IL-17A signaling in mice.

A mutation in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Patients with ALS or FTD often develop autoimmunity and inflammation that precedes or coincides with the onset of neurological symptoms, but the underlying mechanisms are poorly understood. Here, we knocked out murine C9orf72 in seven hematopoietic progenitor compartments by conditional mutagenesis and found that myeloid lineage C9orf72 prevents splenomegaly, loss of tolerance, and premature mortality. Furthermore, we demonstrated that C9orf72 plays a role in lymphoid cells to prevent interleukin-17A (IL-17A) production and neutrophilia. Mass cytometry identified early and sustained elevation of the costimulatory molecule CD80 expressed on C9orf72-deficient mouse macrophages, monocytes, and microglia. Enrichment of CD80 was similarly observed in human spinal cord microglia from patients with C9ORF72-mediated ALS compared with non-ALS controls. Single-cell RNA sequencing of murine spinal cord, brain cortex, and spleen demonstrated coordinated induction of gene modules related to antigen processing and presentation and antiviral immunity in C9orf72-deficient endothelial cells, microglia, and macrophages. Mechanistically, C9ORF72 repressed the trafficking of CD80 to the cell surface in response to Toll-like receptor agonists, interferon-γ, and IL-17A. Deletion of Il17a in C9orf72-deficient mice prevented CD80 enrichment in the spinal cord, reduced neutrophilia, and reduced gut T helper type 17 cells. Last, systemic delivery of an IL-17A neutralizing antibody augmented motor performance and suppressed neuroinflammation in C9orf72-deficient mice. Altogether, we show that C9orf72 orchestrates myeloid costimulatory potency and provide support for IL-17A as a therapeutic target for neuroinflammation associated with ALS or FTD.

Intestinal Tr1 Cells Confer Protection against Colitis in the Absence of Foxp3+ Regulatory T Cell-Derived IL-10.

The intestinal mucosa is continually exposed to diverse microbial and dietary Ags, requiring coordinated efforts by specialized populations of regulatory T cells (Tregs) to maintain homeostasis. Suppressive mechanisms used by intestinal Tregs include the secretion of anti-inflammatory cytokines such as IL-10 and TGF-ß. Defects in IL-10 signaling are associated with severe infantile enterocolitis in humans, and mice deficient in IL-10 or its receptors develop spontaneous colitis. To determine the requirement of Foxp3+ Treg-specific IL-10 for protection against colitis, we generated Foxp3-specific IL-10 knockout (KO) mice (IL-10 conditional KO [cKO] mice). Colonic Foxp3+ Tregs isolated from IL-10cKO mice showed impaired ex vivo suppressive function, although IL-10cKO mice maintained normal body weights and developed only mild inflammation over 30 wk of age (in contrast to severe colitis in global IL-10KO mice). Protection from colitis in IL-10cKO mice was associated with an expanded population of IL-10-producing type 1 Tregs (Tr1, CD4+Foxp3-) in the colonic lamina propria that produced more IL-10 on a per-cell basis compared with wild-type intestinal Tr1 cells. Collectively, our findings reveal a role for Tr1 cells in the gut, as they expand to fill a tolerogenic niche in conditions of suboptimal Foxp3+ Treg-mediated suppression and provide functional protection against experimental colitis.

Sex-Biased T-cell Exhaustion Drives Differential Immune Responses in Glioblastoma.

Sex differences in glioblastoma (GBM) incidence and outcome are well recognized, and emerging evidence suggests that these extend to genetic/epigenetic and cellular differences, including immune responses. However, the mechanisms driving immunologic sex differences are not fully understood. Here, we demonstrate that T cells play a critical role in driving GBM sex differences. Male mice exhibited accelerated tumor growth, with decreased frequency and increased exhaustion of CD8+ T cells in the tumor. Furthermore, a higher frequency of progenitor exhausted T cells was found in males, with improved responsiveness to anti-PD-1 treatment. Moreover, increased T-cell exhaustion was observed in male GBM patients. Bone marrow chimera and adoptive transfer models indicated that T cell-mediated tumor control was predominantly regulated in a cell-intrinsic manner, partially mediated by the X chromosome inactivation escape gene Kdm6a. These findings demonstrate that sex-biased predetermined behavior of T cells is critical for inducing sex differences in GBM progression and immunotherapy response. SIGNIFICANCE: Immunotherapies in patients with GBM have been unsuccessful due to a variety of factors, including the highly immunosuppressive tumor microenvironment in GBM. This study demonstrates that sex-biased T-cell behaviors are predominantly intrinsically regulated, further suggesting sex-specific approaches can be leveraged to potentially improve the therapeutic efficacy of immunotherapy in GBM. See related commentary by Alspach, p. 1966. This article is featured in Selected Articles from This Issue, p. 1949.

Utilizing primary HLA-G+ extravillous trophoblasts and HLA-G+ EVT-like cell lines to study maternal-fetal interactions.

Fetal extravillous trophoblasts (EVTs) are the most invasive cells of the placenta and play a key role in modulating maternal immune responses. Here, we present a protocol to purify and culture human leukocyte antigen-G (HLA-G)+ EVTs. We describe steps for tissue dissection, tissue digestion, density gradient centrifugation, and cell sorting, and we provide detailed methods to determine EVT function. HLA-G+ EVTs are isolated from two maternal-fetal interfaces: the chorionic membrane and the basalis/villous tissue. This protocol allows in-depth functional investigation of maternal immune interactions with HLA-G+ EVTs. For complete details on the use and execution of this protocol, please refer to Papuchova et al. (2020),1 Salvany-Celades et al. (2019),2 Tilburgs et al. (2015),3 Tilburgs et al. (2015),4 van der Zwan et al. (2018).5.

Impact of COVID-19 on Patients with Inflammatory Bowel Disease.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first identified in Wuhan, China, in late 2019. Responsible for the ongoing coronavirus disease 2019 (COVID-19) pandemic, SARS-CoV-2 is one of three structurally similar beta-coronaviruses that can cause a strong upregulation of cytokines referred to as cytokine release syndrome (CRS). Unresolved CRS leads to respiratory symptoms, including pneumonia, and in more severe cases, acute respiratory distress syndrome (ARDS). Although COVID-19 is widely known for these hallmark respiratory symptoms, it also impacts the gut, causing gastrointestinal (GI) tract inflammation and diarrhea. COVID-19's GI symptoms may be due to the high intestinal expression of angiotensin converting enzyme-2 receptors, which are for the binding of SARS-CoV-2 viral particles. Reports have shown that SARS-CoV-2 can be passed through fecal matter, with one study finding that 48.1% of COVID-19 patients expressed viral SARS-CoV-2 mRNA in their stool. Given that the GI tract is a target tissue affected by COVID-19, this causes concern for those with underlying GI pathologies, such as inflammatory bowel disease (IBD). Regrettably, there have been only limited studies on the impact of COVID-19 on gut health, and the impact of COVID-19 on intestinal inflammation among IBD patients remains unclear. In particular, questions regarding susceptibility to SARS-CoV-2 infection, clinical impact of COVID-19 on IBD, and the potential influence of age, sex, and immunosuppressant medications are still poorly understood. An improved understanding of these issues is needed to address the unique risks of COVID-19 among IBD patients, as well as the potential impact of SARS-CoV-2 on the host intestinal microbiota.

Maternal-fetal conflict averted by progesterone- induced FOXP3+ regulatory T cells.

Pregnancy stimulates an intricately coordinated assortment of physiological changes to accommodate growth of the developing fetus, while simultaneously averting rejection of genetically foreign fetal cells and tissues. Despite increasing evidence that expansion of immune-suppressive maternal regulatory T cells enforces fetal tolerance and protects against pregnancy complications, the pregnancy-associated signals driving this essential adaptation remain poorly understood. Here we show that the female reproductive hormone, progesterone, coordinates immune tolerance by stimulating expansion of FOXP3+ regulatory T cells. Conditional loss of the canonical nuclear progesterone receptor in maternal FOXP3+ regulatory T cells blunts their proliferation and accumulation, which is associated with fetal wastage and decidual infiltration of activated CD8+ T cells. Reciprocally, the synthetic progestin 17α-hydroxyprogesterone caproate (17-OHPC) administered to pregnant mice reinforces fetal tolerance and protects against fetal wastage. These immune modulatory effects of progesterone that promote fetal tolerance establish a molecular link between immunological and other physiological adaptions during pregnancy.

Sam68 contributes to intestinal inflammation in experimental and human colitis.

Sam68 is an RNA-binding protein with an adaptor role in signal transduction. Our previous work identified critical proinflammatory and apoptotic functions for Sam68, downstream of the TNF/TNFR1 and TLR2/3/4 pathways. Recent studies have shown elevated Sam68 in inflamed tissues from rheumatoid arthritis and ulcerative colitis (UC) patients, suggesting that Sam68 contributes to chronic inflammatory diseases. Here, we hypothesized that deletion of Sam68 is protective against experimental colitis in vivo, via reductions in TNF-associated inflammatory signaling. We used Sam68 knockout (KO) mice to study the role of Sam68 in experimental colitis, including its contributions to TNF-induced inflammatory gene expression in three-dimensional intestinal organoid cultures. We also studied the expression of Sam68 and inflammatory genes in colon tissues of UC patients. Sam68 KO mice treated with an acute course of DSS exhibited significantly less weight loss and histopathological inflammation compared to wild-type controls, suggesting that Sam68 contributes to experimental colitis. Bone marrow transplants showed no pathologic role for hematopoietic cell-specific Sam68, suggesting that non-hematopoietic Sam68 drives intestinal inflammation. Gene expression analyses showed that Sam68 deficiency reduced the expression of proinflammatory genes in colon tissues from DSS-treated mice, as well as TNF-treated three-dimensional colonic organoids. We also found that inflammatory genes, such as TNF, CCR2, CSF2, IL33 and CXCL10, as well as Sam68 protein, were upregulated in inflamed colon tissues of UC patients. This report identifies Sam68 as an important inflammatory driver in response to intestinal epithelial damage, suggesting that targeting Sam68 may hold promise to treat UC patients.

Sex matters: impact on pathogenesis, presentation and treatment of inflammatory bowel disease.

Inflammatory bowel disease (IBD), as do most chronic inflammatory disorders, displays unique features and confers different risk factors in male and female patients. Importantly, sex-based differences in IBD exist for epidemiological incidence and prevalence among different age groups, with men and women developing distinct clinical symptoms and disparity in severity of disease. In addition, the presentation of comorbidities in IBD displays strong sex differences. Notably, particular issues exclusive to women's health, including pregnancy and childbirth, require specific considerations in female patients with IBD of childbearing age that can have a substantial influence on clinical outcomes. This Review summarizes the latest findings regarding sex-based differences in the epidemiology, clinical course, comorbidities and response to current therapies in patients with IBD. Importantly, the latest basic science discoveries in this area of investigation are evaluated to provide insight into potential mechanisms underlying the influence of sex on disease pathogenesis, as well as to design more personalized and efficacious care, in patients with IBD.

Impaired estrogen signaling underlies regulatory T cell loss-of-function in the chronically inflamed intestine.

Signaling of 17ß-estradiol (estrogen) through its two nuclear receptors, α and ß (ERα, ERß), is an important mechanism of transcriptional regulation. Although ERs are broadly expressed by cells of the immune system, the mechanisms by which they modulate immune responses remain poorly understood. ERß-specific signaling is reduced in patients with chronic inflammatory diseases, including systemic lupus erythematosus and inflammatory bowel disease, and our previous work suggests that dysregulation of ERß-specific signaling contributes to enhanced intestinal inflammation in female SAMP/YitFC mice, a spontaneous model of Crohn's disease-like ileitis. The present study builds on these prior observations to identify a nonredundant, immunoprotective role for ERß-specific signaling in TGF-ß-dependent regulatory T cell (Treg) differentiation. Using a strain of congenic SAMP mice engineered to lack global expression of ERß, we observed dramatic, female-specific exacerbation of intestinal inflammation accompanied by significant reductions in intestinal Treg frequency and function. Impaired Treg suppression in the absence of ERß was associated with aberrant overexpression of Tsc22d3 (GILZ), a glucocorticoid-responsive transcription factor not normally expressed in mature Tregs, and ex vivo data reveal that forced overexpression of GILZ in mature Tregs inhibits their suppressive function. Collectively, our findings identify a pathway of estrogen-mediated immune regulation in the intestine, whereby homeostatic expression of ERß normally functions to limit Treg-specific expression of GILZ, thereby maintaining effective immune suppression. Our data suggest that transcriptional cross-talk between glucocorticoid and steroid sex hormone signaling represents an important and understudied regulatory node in chronic inflammatory disease.

IL-33 promotes recovery from acute colitis by inducing miR-320 to stimulate epithelial restitution and repair.

Defective and/or delayed wound healing has been implicated in the pathogenesis of several chronic inflammatory disorders, including inflammatory bowel disease (IBD). The resolution of inflammation is particularly important in mucosal organs, such as the gut, where restoration of epithelial barrier function is critical to reestablish homeostasis with the interfacing microenvironment. Although IL-33 and its receptor ST2/ILRL1 are known to be increased and associated with IBD, studies using animal models of colitis to address the mechanism have yielded ambiguous results, suggesting both pathogenic and protective functions. Unlike those previously published studies, we focused on the functional role of IL-33/ST2 during an extended (2-wk) recovery period after initial challenge in dextran sodium sulfate (DSS)-induced colitic mice. Our results show that during acute, resolving colitis the normal function of endogenous IL-33 is protection, and the lack of either IL-33 or ST2 impedes the overall recovery process, while exogenous IL-33 administration during recovery dramatically accelerates epithelial restitution and repair, with concomitant improvement of colonic inflammation. Mechanistically, we show that IL-33 stimulates the expression of a network of microRNAs (miRs) in the Caco2 colonic intestinal epithelial cell (IEC) line, especially miR-320, which is increased by >16-fold in IECs isolated from IL-33-treated vs. vehicle-treated DSS colitic mice. Finally, IL-33-dependent in vitro proliferation and wound closure of Caco-2 IECs is significantly abrogated after specific inhibition of miR-320A. Together, our data indicate that during acute, resolving colitis, IL-33/ST2 plays a crucial role in gut mucosal healing by inducing epithelial-derived miR-320 that promotes epithelial repair/restitution and the resolution of inflammation.

Epithelial-specific Toll-like Receptor (TLR)5 Activation Mediates Barrier Dysfunction in Experimental Ileitis.

BACKGROUND: A large body of evidence supports a central role of TLR5 and its natural ligand, flagellin, in Crohn's disease (CD), with the precise mechanism(s) still unresolved. METHODS: We investigated the role of flagellin/TLR5 in SAMP1/YitFc (SAMP) mice, a spontaneous model of Crohn's disease-like ileitis. RESULTS: Ileal Tlr5 and serum antiflagellin IgG antibodies were increased in SAMP before the onset of inflammation and during established disease; these trends were abrogated in the absence of colonizing commensal bacteria. Irradiated SAMP receiving either wild-type (AKR) or SAMP bone marrow (BM) developed severe ileitis and displayed increased ileal Tlr5 compared with AKR recipients of either SAMP or AKR bone marrow, neither of which conferred ileitis, suggesting that elevated TLR5 in native SAMP is derived primarily from a nonhematopoietic (e.g., epithelial) source. Indeed, ileal epithelial TLR5 in preinflamed SAMP was increased compared with age-matched AKR and germ-free SAMP. TLR5-specific ex vivo activation of SAMP ileal tissues decreased epithelial barrier resistance, indicative of increased permeability, and was accompanied by altered expression of the tight junction proteins, claudin-3, occludin, and zonula occludens-1. CONCLUSIONS: Our results provide evidence that aberrant, elevated TLR5 expression is present in the ileal epithelium of SAMP mice, is augmented in the presence of the gut microbiome, and that TLR5 activation in response to bacterial flagellin results in a deficiency to maintain appropriate epithelial barrier integrity. Together, these findings represent a potential mechanistic pathway leading to the exacerbation and perpetuation of chronic gut inflammation in experimental ileitis and possibly, in patients with Crohn's disease.

IL-33 Drives Eosinophil Infiltration and Pathogenic Type 2 Helper T-Cell Immune Responses Leading to Chronic Experimental Ileitis.

Although a clear association has been established between IL-33 and inflammatory bowel disease, mechanistic studies to date, primarily using acute murine models of colitis, have yielded contradicting results, demonstrating both pathogenic and protective roles. We used a well-characterized, spontaneous model of inflammatory bowel disease [ie, SAMP1/YitFc (SAMP) mice] to investigate the role of IL-33 during chronic intestinal inflammation. Our results showed marked eosinophil infiltration into the gut mucosa with increased levels of eotaxins and type 2 helper T-cell (Th2) cytokines as disease progressed and became more severe, which could be reversed upon either eosinophil depletion or blockade of IL-33 signaling. Exogenous IL-33 administration recapitulated these effects in ilea of uninflamed (parental) control AKR/J mice. Human data supported these findings, showing colocalization and up-regulation of IL-33 and eosinophils in the colonic mucosa of inflammatory bowel disease patients versus noninflamed controls. Finally, colonization of commensal flora by fecal material transplantation into germ-free SAMP and the presence of the gut microbiome induced IL-33, subsequent eosinophil infiltration, and mounting of Th2 immune responses, leading to exacerbation of chronic intestinal inflammation characteristic of SAMP mice. These data demonstrate a pathogenic role for IL-33-mediated eosinophilia and activation of Th2 immunity in chronic intestinal inflammation that is dependent on the gut microbiome. Targeting IL-33 may represent a novel therapeutic approach to treat patients with inflammatory bowel disease.

Dysregulated intrahepatic CD4+ T-cell activation drives liver inflammation in ileitis-prone SAMP1/YitFc mice.

BACKGROUND AND AIMS: Liver inflammation is a common extraintestinal manifestation of inflammatory bowel disease (IBD); however, whether liver involvement is a consequence of a primary intestinal defect or results from alternative pathogenic processes remains unclear. Therefore, we sought to determine the potential pathogenic mechanism(s) of concomitant liver inflammation in an established murine model of IBD. METHODS: Liver inflammation and immune cell subsets were characterized in ileitis-prone SAMP1/YitFc (SAMP) and AKR/J (AKR) control mice, lymphocyte-depleted SAMP (SAMPxRag-1-/-), and immunodeficient SCID recipient mice receiving SAMP or AKR donor CD4+ T-cells. Proliferation and suppressive capacity of CD4+ T-effector (Teff) and T-regulatory (Treg) cells from gut-associated lymphoid tissue (GALT) and livers of SAMP and AKR mice were measured. RESULTS: Surprisingly, prominent inflammation was detected in 4-wk-old SAMP livers, prior to histologic evidence of ileitis, while both disease phenotypes were absent in age-matched AKRs. SAMP liver disease was characterized by abundant infiltration of lymphocytes, required for hepatic inflammation to occur, a Th1-skewed environment, and phenotypically-activated CD4+ T-cells. SAMP intrahepatic CD4+ T-cells also had the ability to induce liver and ileal inflammation when adoptively transferred into SCID recipients, whereas GALT-derived CD4+ T-cells produced milder ileitis, but not liver inflammation. Interestingly, SAMP intrahepatic CD4+ Teff cells showed increased proliferation compared to both SAMP GALT- and AKR liver-derived CD4+ Teff cells, while SAMP intrahepatic Tregs were decreased among CD4+ T-cells and impaired in in vitro suppressive function compared to AKR. CONCLUSIONS: Activated intrahepatic CD4+ T-cells induce liver inflammation and contribute to experimental ileitis via locally-impaired hepatic immunosuppressive function.

Regulatory cell populations in the intestinal mucosa.

PURPOSE OF REVIEW: The purpose of this review is to summarize recent advances in the fields of intestinal T-regulatory cell (Treg) and tolerogenic dendritic cell subsets. Under homeostatic conditions, CD4(+) Tregs and tolerogenic dendritic cells function to maintain mucosal tolerance. Loss of immune homeostasis is the primary cause of intestinal abnormalities, including inflammatory bowel disease. Thus, an improved understanding of cellular mechanisms promoting tolerance will be critical for the development of more efficacious therapies to treat chronic intestinal inflammation. RECENT FINDINGS: Significant progress has been made in the past year in the study of mucosal Treg and dendritic cell populations. In particular, efforts have focused on the migration and differentiation of these cells in the intestinal mucosa, the functional consequences of cross-talk with the intestinal microbiome, mechanisms by which tolerogenic dendritic cells take up antigen, and regulation of retinoic acid synthesis. SUMMARY: Recent studies examining tolerogenic cell populations of the intestinal mucosa highlight the progress in understanding the function, regulation, and cross-talk of Treg and dendritic cell populations, and their interactions with the gut microbiota. Scientific advances in these areas will undoubtedly lead to the development of more effective therapeutic strategies for intestinal abnormalities such as inflammatory bowel disease.

Regulation generation: the suppressive functions of human regulatory T cells.

Proper regulation of immune homeostasis is necessary to limit inflammation and prevent autoimmune and chronic inflammatory diseases. Many autoimmune diseases, such as psoriasis, are driven by vicious cycles of activated T cells that are unable to be suppressed by regulatory T cells. Effective suppression of auto-reactive T cells by regulatory T cells (Treg) is critical for the prevention of spontaneous autoimmune disease. Psoriatic Treg cells have been observed to a defect in their capacity to regulate, which clearly contributes to psoriasis pathogenesis. A challenge for translational research is the development of novel therapeutic interventions for autoimmune diseases that will result in durable remissions. Understanding the mechanism(s) of dysregulated T cell responses in autoimmune disease will allow for the development of future therapeutic strategies that may be employed to specifically target pathogenic, proinflammatory cells. Several reports have demonstrated a pathogenic role for Thl and Thl7 cells in psoriasis as well as other autoimmune diseases. Similarly, several laboratories have independently demonstrated functional defects in regulatory T cells isolated from patients with numerous divergent autoimmune diseases. One primary challenge of research in autoimmune diseases is therefore to restore the balance between chronic T cell activation and impairment of Treg suppressor mechanisms. To this end, it is critical to develop an understanding of the many suppressive mechanisms employed by Treg cells in hopes of developing more targeted therapeutic strategies for Treg-mediated autoimmune diseases.

Stat3 phosphorylation mediates resistance of primary human T cells to regulatory T cell suppression.

Human autoimmune diseases are characterized by systemic T cell dysfunction, resulting in chronically activated Th1 and Th17 cells that are inadequately suppressed by regulatory T cells (Tregs). IL-6, which is overexpressed in tissue and serum of patients with autoimmune diseases, inhibits human Treg function. We sought to determine the mechanism for the antitolerogenic properties of IL-6 by examining the signaling pathways downstream of IL-6R in primary human T cells. Inhibition of Stat3 signaling in MLCs containing IL-6 restores Treg-mediated suppression, demonstrating that IL-6-mediated loss of Treg suppression requires phosphorylation of Stat3. Cultures in which either effector T cells (Teffs) or Tregs were pretreated with Stat3 inhibitors indicate that phosphorylated (p)Stat3 is required in both T cell populations for IL-6-mediated reversal of Treg function. IL-21, which signals preferentially through pStat3, also reverses Treg suppression, in contrast to IL-27 and IFN-γ, which signal preferentially through Stat1 and do not inhibit Treg function. Interestingly, both Teffs and Tregs respond to IL-6 stimulation through strong Stat3 phosphorylation with minimal MAPK/Erk activation and moderate Stat1 phosphorylation. Finally, Teffs stimulated strongly through the TCR are also resistant to suppression by Tregs and show concurrent Stat3 phosphorylation. In these cultures, inhibition of pStat3 restores functional suppression by Tregs. Taken together, our findings suggest that an early dominance of Stat3 signaling, prior to subsequent T cell activation, is required for the loss of functional Treg suppression and that kinase-specific inhibitors may hold therapeutic promise in the treatment of autoimmune and chronic inflammatory diseases.

IL-6 signaling in psoriasis prevents immune suppression by regulatory T cells.

T memory/effector cells (Tmem/eff) isolated from psoriatic patients are chronically activated and poorly suppressed by regulatory T cells (Treg). The proinflammatory cytokine IL-6, which signals through Stat3, allows escape of Tmem/eff cells from Treg-mediated suppression in a murine system. We show here that IL-6 protein is markedly elevated and most highly expressed by CD31(+) endothelial cells and CD11c(+) dermal dendritic cells (DCs) in lesional psoriatic skin. We hypothesized that exposure to high IL-6 in lesional tissue may lead to the dampened Treg function observed in psoriasis patients. Indeed, we found that IL-6, but not other Stat3-activating cytokines, was necessary and sufficient to reverse human T cell suppression by Treg in an in vitro model using activated DCs as a source of IL-6. IL-6Ralpha and gp130 expression was significantly elevated in psoriatic effector T cells compared with normal controls. Overall, IL-6Ralpha expression on Treg exceeded that of effector T cells, and both populations phosphorylated Stat3 in response to IL-6. Phosphorylation of Stat3 in T cells contributes to Th17 differentiation and we identify cells within lesional tissue that coexpress CD3, IL-17, and IL-6, indicating that Th17 cells are present in vivo within the psoriatic Tmem/eff population and contribute to IL-6-mediated resistance to Treg suppression. Taken together, T lymphocytes trafficking into lesional psoriatic skin encounter high IL-6 from endothelial cells, DCs, and Th17 cells, enabling cutaneous T cell escape from Treg suppression and Th17 participation in inflammation. Targeting IL-6 signaling pathways in psoriasis may rebalance Treg/T effector activity and ameliorate disease.