T cells expressing two different T cell receptors form a heterogeneous population containing autoreactive clones

TCRα reporter mice reveal contribution of dual TCRα expression to T cell repertoire and function

It is known that a subpopulation of T cells expresses two T cell receptor (TCR) clonotypes, though the extent and functional significance of this is not established. To definitively evaluate dual TCRα cells, we generated mice with green fluorescent protein and red fluorescent protein reporters linked to TCRα, revealing that ∼16% of T cells express dual TCRs, notably higher than prior estimates. Importantly, dual TCR expression has functional consequences, as dual TCR cells predominated response to lymphocytic choriomeningitis virus infection, comprising up to 60% of virus-specific CD4⁺ and CD8⁺ T cells during acute responses. Dual receptor expression selectively influenced immune memory, as postinfection memory CD4⁺ populations contained significantly increased frequencies of dual TCR cells. These data reveal a previously unappreciated contribution of dual TCR cells to the immune repertoire and highlight their potential effects on immune responses.

Proinflammatory Dual Receptor T Cells in Chronic Graft-versus-Host Disease

Defective post-transplantation thymopoiesis is associated with chronic graft-versus-host disease (GVHD), a multiorgan pathology affecting up to 80% of patients after allogeneic hematopoietic stem cell transplantation (HSCT). Previous work demonstrated that the subset of T cells expressing 2 T cell receptors (TCRs) is predisposed to alloreactivity, driving selective and disproportionate activity in acute GVHD in both mouse models and HSCT patients. Here we investigate a potential role for this pathogenic T cell subset in chronic GVHD (cGVHD). HSCT patients with cGVHD demonstrated increased numbers of dual TCR cells in circulation. These dual receptor cells had an activated phenotype, indicating an active role in cGVHD. Notably, single-cell RNA sequencing identified the increased dual TCR cells in cGVHD as predominantly expressing Tbet, indicative of a proinflammatory phenotype. These results identify dual TCR cells as specific mediators of pathogenic inflammation underlying cGVHD and highlight Tbet-driven T cell function as a potential pathway for potential therapeutic targeting.

Lymphopenia-induced proliferation in the absence of functional Autoimmune regulator (Aire) induces colitis in mice

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is caused by mutations in Autoimmune regulator (Aire), a transcriptional regulator of negative selection in thymus. However, Aire is also expressed in periphery, but the full range of Aire's peripheral function is unknown. Here, we transferred lymphocytes from wildtype donors into lymphopenic recipients with or without functional Aire. Following cell proliferation thus took place in Aire-sufficient or deficient environment. The wildtype lymphocytes hyperproliferated and induced disease in lymphopenic Aire(-/-) but not in Aire(+/+) recipients. The disease was characterized by diarrhea, inflammation, and colitis, and in some recipients pancreatitis, gastritis, and hepatitis was also found. Our results identify Aire as an important regulator of peripheral T cell homeostasis in gastrointestinal tissues. Given a suitable trigger the absence of peripheral Aire leads to dysregulated T cell proliferation and disease.

Autoimmunity, Not a Developmental Defect, is the Cause for Subfertility of Autoimmune Regulator (Aire) Deficient Mice

Autoimmune regulator's (AIRE) best characterized role is in the generation immunological tolerance, but it is also involved in many other processes such as spermatogenesis. Loss-of-function mutations in AIRE cause a disease called autoimmune polyendocrinopathy, candidiasis and ectodermal dystrophy (APECED; also called autoimmune polyendocrinopathy syndrome type 1, APS-1) that is dominated by various autoimmune manifestations, mainly endocrinopathies. Both patients with APECED and Aire(-/-) mice suffer from varying levels of infertility, but it is not clear if it is a result of an autoimmune tissue damage or more of a developmental defect. In this study, we wanted to resolve whether or not the reduced fertility of Aire(-/-) mice is dependent on the adaptive immune system and therefore a manifestation of autoimmunity in these mice. We generated lymphopenic mice without Aire expression that were devoid of the autoimmune manifestations previously reported in immunocompetent Aire(-/-) mice. These Aire(-/-) Rag1(-/-) mice regained full fertility. This confirms that the development of infertility in Aire(-/-) mice requires a functional adaptive immune system. We also show that only the male Aire(-/-) mice are subfertile, whereas Aire(-/-) females produce litters normally. Moreover, the male subfertility can be adoptively transferred with lymphocytes from Aire(-/-) donor mice to previously fertile lymphopenic Aire(-/-) recipients. Our data show that subfertility in Aire(-/-) mice is dependent on a functional adaptive immune system thus confirming its autoimmune aetiology.

Unresolved issues in theories of autoimmune disease using myocarditis as a framework

Many theories of autoimmune disease have been proposed since the discovery that the immune system can attack the body. These theories include the hidden or cryptic antigen theory, modified antigen theory, T cell bypass, T cell-B cell mismatch, epitope spread or drift, the bystander effect, molecular mimicry, anti-idiotype theory, antigenic complementarity, and dual-affinity T cell receptors. We critically review these theories and relevant mathematical models as they apply to autoimmune myocarditis. All theories share the common assumption that autoimmune diseases are triggered by environmental factors such as infections or chemical exposure. Most, but not all, theories and mathematical models are unifactorial assuming single-agent causation of disease. Experimental and clinical evidence and mathematical models exist to support some aspects of most theories, but evidence/models that support one theory almost invariably supports other theories as well. More importantly, every theory (and every model) lacks the ability to account for some key autoimmune disease phenomena such as the fundamental roles of innate immunity, sex differences in disease susceptibility, the necessity for adjuvants in experimental animal models, and the often paradoxical effect of exposure timing and dose on disease induction. We argue that a more comprehensive and integrated theory of autoimmunity associated with new mathematical models is needed and suggest specific experimental and clinical tests for each major theory that might help to clarify how they relate to clinical disease and reveal how theories are related.

MHC-mismatched mixed chimerism mediates thymic deletion of cross-reactive autoreactive T cells and prevents insulitis in nonobese diabetic mice

Type 1 diabetic NOD mice have defects in both thymic negative selection and peripheral regulation of autoreactive T cells, and induction of mixed chimerism can effectively reverse these defects. Our recent studies suggest that MHC-mismatched mixed chimerism mediates negative selection of autoreactive thymocytes in wild-type NOD and TCR-transgenic NOD.Rag1(+/+).BDC2.5 mice. However, it remains unknown how mismatched I-A(b) MHC class II can mediate deletion of autoreactive T cells positively selected by I-A(g7). In the present study, we directly tested the hypothesis that mismatched MHC class II in mixed chimeras mediates deletion of cross-reactive autoreactive thymocytes. We first identify that transgenic BDC2.5 T cells from NOD.Rag1(+/+).BDC2.5 but not NOD.Rag1(-/-).BDC2.5 mice possess cross-reactive TCRs with endogenous TCRα-chains; MHC-mismatched H-2(b) but not matched H-2(g7) mixed chimerism mediates thymic deletion of the cross-reactive transgenic T cells in NOD.Rag1(+/+).BDC2.5 mice. Second, by transplanting T cell-depleted (TCD) bone marrow (BM) cells from NOD.Rag1(+/+).BDC2.5 or NOD.Rag1(-/-).BDC2.5 mice into lethally irradiated MHC-mismatched H-2(b) C57BL/6 or MHC-matched congenic B6.H-2(g7) recipients, we demonstrate that NOD.Rag1(+/+).BDC2.5 BM-derived cross-reactive transgenic T cells, but not NOD.Rag1(-/-).BDC2.5 BM-derived non-cross-reactive transgenic T cells, can be positively selected in MHC-mismatched H-2(b) thymus. Third, by cotransplanting NOD.Rag1(+/+).BDC2.5 TCD BM cells with BM cells from MHC-mismatched T cell-deficient C57BL/6 mice into lethally irradiated MHC-matched B6.H-2(g7) recipients, we demonstrate that thymic deletion of the cross-reactive transgenic T cells is dependent on MHC-mismatched donor BM-derived APCs but not on donor BM-derived T cells. Taken together, our studies indicate that MHC-mismatched mixed chimerism can mediate thymic deletion of cross-reactive autoreactive T cells that express more than one TCR.

The ability to rearrange dual TCRs enhances positive selection, leading to increased Allo- and Autoreactive T cell repertoires

Thymic selection is designed to ensure TCR reactivity to foreign Ags presented by self-MHC while minimizing reactivity to self-Ags. We hypothesized that the repertoire of T cells with unwanted specificities such as alloreactivity or autoreactivity are a consequence of simultaneous rearrangement of both TCRα loci. We hypothesized that this process helps maximize production of thymocytes capable of successfully completing thymic selection, but results in secondary TCRs that escape stringent selection. In T cells expressing two TCRs, one TCR can mediate positive selection and mask secondary TCR from negative selection. Examination of mice heterozygous for TRAC (TCRα(+/-)), capable of only one functional TCRα rearrangement, demonstrated a defect in generating mature T cells attributable to decreased positive selection. Elimination of secondary TCRs did not broadly alter the peripheral T cell compartment, though deep sequencing of TCRα repertoires of dual TCR T cells and TCRα(+/-) T cells demonstrated unique TCRs in the presence of secondary rearrangements. The functional impact of secondary TCRs on the naive peripheral repertoire was evidenced by reduced frequencies of T cells responding to autoantigen and alloantigen peptide-MHC tetramers in TCRα(+/-) mice. T cell populations with secondary TCRs had significantly increased ability to respond to altered peptide ligands related to their allogeneic ligand as compared with TCRα(+/-) cells, suggesting increased breadth in peptide recognition may be a mechanism for their reactivity. Our results imply that the role of secondary TCRs in forming the T cell repertoire is perhaps more significant than what has been assumed.

Incomplete TCR-β allelic exclusion accelerates spontaneous autoimmune arthritis in K/BxN TCR transgenic mice

Allelic exclusion of antigen receptor loci is a fundamental mechanism of immunological self-tolerance. Incomplete allelic exclusion leads to dual T-cell receptor (TCR) expression and can allow developing autoreactive αβ T lymphocytes to escape clonal deletion. Because allelic exclusion at the TCR-β locus is more stringent than at the TCR-α locus, dual TCR-β expression has not been considered a likely contributor to autoimmunity. We show here that incomplete TCR-β allelic exclusion permits developing thymocytes bearing the autoreactive, transgene-encoded KRN TCR to be positively selected more efficiently, thereby accelerating the onset of spontaneous autoimmune arthritis. Our findings highlight dual TCR-β expression as a mechanism that can enhance the maturation of autoreactive pathogenic T cells and lead to more rapid development of autoimmune disease.

IL-7 dysregulation and loss of CD8+ T cell homeostasis in the monogenic human disease autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a monogenic autoimmune disease that is caused by mutations in the AIRE gene. Murine studies have linked AIRE to thymocyte selection and peripheral deletional tolerance, but the pathogenesis of the human disease remains unclear. In this study, we show that APECED patients have elevated IL-7 levels and a drastically decreased expression of IL-7R on CD8(+) T cells. This is associated with increased proliferation and a decreased expression of the negative TCR regulator CD5 in the CD45RO(-) subset. The CD45RO(-) cells also display oligoclonal expansions, decreased expression of the lymph node homing factors CCR7 and CD62L, and increased expression of perforin, consistent with the accumulation of highly differentiated effector cells. The CD45RO(-)CCR7(+)CD8(+) population of cells with markers characteristic of naive phenotype is also skewed, as shown by decreased expression of CD5 and increased expression of perforin. The putative CD31(+) recent thymic emigrant population is likewise affected. These data are consistent with IL-7 dysregulation inducing a decreased threshold of TCR signaling and self-antigen-driven proliferation, probably in synergy with the failed thymic selection. The resultant loss of CD8(+) T cell homeostasis is likely to play a significant role in the pathogenesis of APECED. Our findings may also hold lessons for other diseases in which the IL-7-IL-7R pathway has emerged as a risk factor.