Nonmyelin-specific T cells accelerate development of central nervous system APC and increase susceptibility to experimental autoimmune encephalomyelitis

Steady-state memory-phenotype conventional CD4+ T cells exacerbate autoimmune neuroinflammation in a bystander manner via the Bhlhe40/GM-CSF axis

Memory-phenotype (MP) CD4⁺ T cells are a substantial population of conventional T cells that exist in steady-state mice, yet their immunological roles in autoimmune disease remain unclear. In this work, we unveil a unique phenotype of MP CD4⁺ T cells determined by analyzing single-cell transcriptomic data and T cell receptor (TCR) repertoires. We found that steady-state MP CD4⁺ T cells in the spleen were composed of heterogeneous effector subpopulations and existed regardless of germ and food antigen exposure. Distinct subpopulations of MP CD4⁺ T cells were specifically activated by IL-1 family cytokines and STAT activators, revealing that the cells exerted TCR-independent bystander effector functions similar to innate lymphoid cells. In particular, CCR6^high subpopulation of MP CD4⁺ T cells were major responders to IL-23 and IL-1β without MOG_35-55 antigen reactivity, which gave them pathogenic Th17 characteristics and allowed them to contribute to autoimmune encephalomyelitis. We identified that Bhlhe40 in CCR6^high MP CD4⁺ T cells as a key regulator of GM-CSF expression through IL-23 and IL-1β signaling, contributing to central nervous system (CNS) pathology in experimental autoimmune encephalomyelitis. Collectively, our findings reveal the clearly distinct effector-like heterogeneity of MP CD4⁺ T cells in the steady state and indicate that CCR6^high MP CD4⁺ T cells exacerbate autoimmune neuroinflammation via the Bhlhe40/GM-CSF axis in a bystander manner.

Emerging role of bystander T cell activation in autoimmune diseases

Autoimmune disease is known to be caused by unregulated self-antigen-specific T cells, causing tissue damage. Although antigen specificity is an important mechanism of the adaptive immune system, antigen non-related T cells have been found in the inflamed tissues in various conditions. Bystander T cell activation refers to the activation of T cells without antigen recognition. During an immune response to a pathogen, bystander activation of self-reactive T cells via inflammatory mediators such as cytokines can trigger autoimmune diseases. Other antigen-specific T cells can also be bystander-activated to induce innate immune response resulting in autoimmune disease pathogenesis along with self-antigen-specific T cells. In this review, we summarize previous studies investigating bystander activation of various T cell types (NKT, γδ T cells, MAIT cells, conventional CD4⁺, and CD8⁺ T cells) and discuss the role of innate-like T cell response in autoimmune diseases. In addition, we also review previous findings of bystander T cell function in infection and cancer. A better understanding of bystander-activated T cells versus antigen-stimulated T cells provides a novel insight to control autoimmune disease pathogenesis.

Bystander CD4+ T cells: crossroads between innate and adaptive immunity

T cells are the central mediators of both humoral and cellular adaptive immune responses. Highly specific receptor-mediated clonal selection and expansion of T cells assure antigen-specific immunity. In addition, encounters with cognate antigens generate immunological memory, the capacity for long-term, antigen-specific immunity against previously encountered pathogens. However, T-cell receptor (TCR)-independent activation, termed “bystander activation”, has also been found. Bystander-activated T cells can respond rapidly and secrete effector cytokines even in the absence of antigen stimulation. Recent studies have rehighlighted the importance of antigen-independent bystander activation of CD4⁺ T cells in infection clearance and autoimmune pathogenesis, suggesting the existence of a distinct innate-like immunological function performed by conventional T cells. In this review, we discuss the inflammatory mediators that activate bystander CD4⁺ T cells and the potential physiological roles of these cells during infection, autoimmunity, and cancer.

Immune cells that become activated in the absence of antigen stimulation could be harnessed in the fight against infection, autoimmunity, and cancer. Je-Min Choi and colleagues from Hanyang University in Seoul, South Korea, review how the immune system can deploy helper T cells through an unusual process called bystander activation. Most T cells become activated only after receptors on their surface bind to specific cognate antigen. In contrast, bystander T cells are activated non-specifically in response to cytokines and other pro-inflammatory mediators. Studies have shown that this cell population has a variety of protective and pathogenic functions, for example, guarding against multiple sclerosis, aggravating the symptoms of parasitic infections and promoting antitumor immunity. A better understanding of these immune cells could lead to new therapeutic options for these diseases.

Pathogenic function of bystander-activated memory-like CD4+ T cells in autoimmune encephalomyelitis

T cells generate antigen-specific immune responses to their cognate antigen as a hallmark of adaptive immunity. Despite the importance of antigen-specific T cells, here we show that antigen non-related, bystander memory-like CD4⁺ T cells also significantly contribute to autoimmune pathogenesis. Transcriptome analysis demonstrates that interleukin (IL)-1β- and IL-23-prime T cells that express pathogenic T_Η17 signature genes such as RORγt, CCR6, and granulocyte macrophage colony-stimulating factor (GM-CSF). Importantly, when co-transferred with myelin-specific 2D2 TCR-transgenic naive T cells, unrelated OT-II TCR-transgenic memory-like T_H17 cells infiltrate the spinal cord and produce IL-17A, interferon (IFN)-γ, and GM-CSF, increasing the susceptibility of the recipients to experimental autoimmune encephalomyelitis in an IL-1 receptor-dependent manner. In humans, IL-1R1^high memory CD4⁺ T cells are major producers of IL-17A and IFN-γ in response to IL-1β and IL-23. Collectively, our findings reveal the innate-like pathogenic function of antigen non-related memory CD4⁺ T cells, which contributes to the development of autoimmune diseases.

T cells express specific T cell receptors (TCR) to recognise antigens and initiate adaptive immune responses. Here the authors show, in a mouse model of autoimmune encephalomyelitis, that memory-like CD4 T cells expressing unrelated TCR can also infiltrate the spinal cord and contribute to autoimmunity.

TGF-β signaling via Smad4 drives IL-10 production in effector Th1 cells and reduces T-cell trafficking in EAE

Effector Th1 cells perpetuate inflammatory damage in a number of autoimmune diseases, including MS and its animal model EAE. Recently, a self-regulatory mechanism was described in which effector Th1 cells produce the immunomodulatory cytokine IL-10 to dampen the inflammatory response in both normal and autoimmune inflammation. While the presence of TGF-β has been suggested to enhance and stabilize an IFN-γ(+) IL-10(+) phenotype, the molecular mechanism is poorly understood. Additionally, in the context of adoptive transfer EAE, it is unclear whether IL-10 acts on the transferred Th1 cells or on endogenous host cells. In the present study, using myelin-specific TCR-Tg mice, we show that repetitive Ag stimulation of effector Th1 cells in the presence of TGF-β increases the population of IFN-γ(+) IL-10(+) cells, which correlates with a decrease in EAE severity. Additionally, TGF-β signaling causes binding of Smad4 to the IL-10 promoter, providing molecular evidence for TGF-β-mediated IL-10 production from Th1 effector cells. Finally, this study demonstrates that IL-10 not only reduces encephalitogenic markers such as IFN-γ and T-bet on Th1 effector cells expressing the IL-10R but also prevents recruitment of both transferred and host-derived inflammatory T cells. These data establish a regulatory mechanism by which highly activated Th1 effector cells modulate their pathogenicity through the induction of IL-10.

Early influx of macrophages determines susceptibility to experimental allergic encephalomyelitis in Dark Agouti (DA) rats

Experimental allergic encephalomyelitis (EAE) is characterized by inflammatory infiltrates of myelin antigen(s) specific T cells and consecutive demyelination. Injection of encephalitogen into the footpads induces disease in genetically susceptible Dark Agouti rats (DA) but not in Albino Oxford (AO) rats although mild inflammatory infiltrates are observed in both strains early after disease induction. In addition, only DA rats develop disease when cells from (AO×DA) F(1) hybrids are passively transferred into sub-lethally radiated AO and DA parent hosts. The aim of the study was therefore to examine the participation of accessory cells, macrophages, dendritic cells and microglia in EAE development at the level of the target tissue in these two strains using specific membrane markers. We demonstrate here that in the induction phase of EAE in DA rats, macrophages (CD68(+); CD45(hi)CD11b(+)) are the first detectable infiltrating cells in the subpial regions of the spinal cord but were not found in AO rats. During the same period, resident microglial cells which are of the ramified variety are observed in both DA and AO rats. In DA rats at the peak of disease, when profuse influx of T cells is seen, macrophages and dendritic cells appear in the parenchyma of the CNS. In addition, at that time, microglial cells are activated. FACS analyses also reveal a significant increase in CD45(hi)CD11c(+) dendritic cells and CD45(hi)D11b(+) macrophages compared with levels in naïve and immunized AO rats. During resolution of disease in DA rats, the expression of microglia and macrophage markers is comparable with those in naïve non-immunized DA and immunized AO rats. We conclude that an initial influx of macrophages is indispensible for the development of EAE in DA rats. The presence of dendritic cells and myeloid dendritic cells at the peak of disease supports the role of these cells in EAE especially in relapses and chronicity. The activation pattern of microglia in DA rats does not indicate their role as antigen presenting cells in disease induction since they are ramified at the induction phase and only become activated after the overwhelming influx of T cells.

Encephalitogenic T-cells increase numbers of CNS T-cells regardless of antigen specificity by both increasing T-cell entry and preventing egress

This study utilized an adoptive transfer model of experimental autoimmune encephalomyelitis (EAE) induction in mice to characterize the mechanisms involved in CNS accumulation of transferred and host T-cells. Using a flow cytometric technique, we examined phenotypic characteristics of CNS T-cells following disease initiation and the role of T-cell activation in CNS invasion and retention. Host T-cell activation increased cell recruitment and EAE severity. CNS antigen specific T-cells were required to induce T-cell retention within the CNS. Once retention was initiated, CNS T-cells were retained regardless of specificity. This study characterizes mechanisms involved in CNS accumulation of T-cells during EAE pathogenesis.

Host T cells are the main producers of IL-17 within the central nervous system during initiation of experimental autoimmune encephalomyelitis induced by adoptive transfer of Th1 cell lines

Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, has long been thought to be mediated by Th1 CD4(+) T cells. Using adoptive transfer techniques, transfer of CNS specific Th1 T cells was sufficient to induce EAE in naive mice. However, recent studies found a vital role for IL-17 in induction of EAE. These studies suggested that a fraction of IL-17-producing T cells that contaminate Th1 polarized cell lines are largely responsible for initiation of EAE. In this study, we tracked the appearance and cytokine production capacity of adoptively transferred cells within the CNS of mice throughout EAE disease. IL-17-producing, adoptively transferred cells were not enriched over the low percentages present in vitro. Thus, there was no selective recruitment and/or preferential proliferation of adoptively transferred IL-17-producing cells during the induction of EAE. Instead a large number of CNS infiltrating host T cells in mice with EAE were capable of producing IL-17 following ex vivo stimulation. The IL-17-producing T cells contained both alphabeta and gammadelta TCR(+) T cells with a CD4(+)CD8(-) or CD4(-)CD8(-) phenotype. These cells concentrated within the CNS within 3 days of adoptive transfer, and appeared to play a role in EAE induction as adoptive transfer of Th1 lines derived from wild-type mice into IL-17-deficient mice induced reduced EAE clinical outcomes. This study demonstrates that an encephalitogenic Th1 cell line induces recruitment of host IL-17-producing T cells to the CNS during the initiation of EAE and that these cells contribute to the incidence and severity of disease.