Development and function of autospecific dual TCR+ T lymphocytes

Dual TCR T Cells: Identity Crisis or Multitaskers?

Dual TCR T cells are a common and natural product of TCR gene rearrangement and thymocyte development. As much as one third of the T cell population may have the capability to express two different TCR specificities on the cell surface. This discovery provoked a reconsideration of the classic model of thymic selection. Many potential roles for dual TCR T cells have since been hypothesized, including posing an autoimmune hazard, dominating alloreactive T cell responses, inducing allergy, and expanding the TCR repertoire to improve protective immunity. Yet, since the initial wave of publications following the discovery of dual TCR T cells, research in the area has slowed. In this study, we aim to provide a brief but comprehensive history of dual TCR T cell research, re-evaluate past observations in the context of current knowledge of the immune system, and identify key issues for future study.

Impact of gut microbiota on gut-distal autoimmunity: a focus on T cells

The immune system is essential for maintaining a delicate balance between eliminating pathogens and maintaining tolerance to self-tissues to avoid autoimmunity. An enormous and complex community of gut microbiota provides essential health benefits to the host, particularly by regulating immune homeostasis. Many of the metabolites derived from commensals can impact host health by directly regulating the immune system. Many autoimmune diseases arise from an imbalance between pathogenic effector T cells and regulatory T (Treg) cells. Recent interest has emerged in understanding how cross-talk between gut microbiota and the host immune system promotes autoimmune development by controlling the differentiation and plasticity of T helper and Treg cells. At the molecular level, our recent study, along with others, demonstrates that asymptomatic colonization by commensal bacteria in the gut is capable of triggering autoimmune disease by molecular mimicking self-antigen and skewing the expression of dual T-cell receptors on T cells. Dysbiosis, an imbalance of the gut microbiota, is involved in autoimmune development in both mice and humans. Although it is well known that dysbiosis can impact diseases occurring within the gut, growing literature suggests that dysbiosis also causes the development of gut-distal/non-gut autoimmunity. In this review, we discuss recent advances in understanding the potential molecular mechanisms whereby gut microbiota induces autoimmunity, and the evidence that the gut microbiota triggers gut-distal autoimmune diseases.

Multiple sclerosis: autoimmunity and viruses

PURPOSE OF REVIEW

This review will explore two new aspects of the involvement of viruses in multiple sclerosis pathogenesis. The first aspect is the complex interactions between viruses. The second aspect is the proposal of a mechanism by which autoreactive T cells are able to escape thymic selection and potentially recognize self and a pathogen.

RECENT FINDINGS

With regard to viruses, recent work has demonstrated that one virus may enhance the replication of another virus, potentially leading to an increase in inflammation and disease progression. Also, interactions between human endogenous retroviruses, which likely do not replicate, and certain herpes viruses, may also play a role in disease pathogenesis. Mechanistically, T cells expressing dual T-cell receptors would be able to recognize self and a foreign antigen specifically. Therefore, human endogenous retroviruses potentially play a role in multiple sclerosis pathogenesis, and both interactions between multiple viruses and autoreactive CD8(+) T cells with dual T-cell receptors may play a role in the pathogenesis of the disease.

SUMMARY

The complex interactions between multiple viral infections, either within the central nervous system or in the periphery, and the host immune response to viral infection may be such that a variety of viral specificities result in the activation of T cells that recognize self and induce multiple sclerosis. Therefore, it is unlikely that any one microbe will be determined to be the causative agent of multiple sclerosis as reflected by the number of potential triggering mechanisms of the disease.

Recombinase-activating gene 1-associated expression of the myelin basic protein 1-11-specific transgenic T-cell receptor in H-2b mice

We pursued a breeding strategy intended to generate disease-resistant mice with exclusive expression of the H-2(u)-restricted myelin basic protein (MBP) 1-11 peptide-specific transgenic (Tg) T-cell receptor (TCR) on the T-cell-deficient RAG1KO (H-2(b)) background. Utilizing specific screening assays for the offspring, analyses of the F1 intercross and subsequent crosses revealed that the TgTCR-associated clonotypic marker detected by the 3H12 mAb could be found only in association with the H-2(b) homozygous background in offspring possessing a functional rag1 gene. Moreover, expression of the MBP-specific TgTCR could not be found in H-2(b) homozygous offspring that were RAG1 deficient (rag1(-/-)). PCR analysis of genomic DNA from these 3H12-negative offspring verified the presence of the TCR transgenes. Thus, the presence of a functional rag1 gene was required for the expression of the MBP-specific TgTCR on the H-2(b) background. Given the role for RAG1, the results have important implications for T-cell repertoire development.

T-cell avidity and tuning: the flexible connection between tolerance and autoimmunity

Thymic T-cell selection mechanisms generate a cross-reactive, self-MHC restricted peripheral T-cell pool. Affinity and avidity are of profound influence on this selection and the generation of immunity. Autoreactive T cells can escape thymic deletion by lowering their avidity and retain this "tuned" state in the periphery. Upon activation, tuned T cells can cause autoimmunity, while immunotherapeutic strategies may be hampered by existing T-cell tolerance. The regulation of T-cell avidity and tuning therefore determines the balance between tolerance and autoimmunity and should be taken into account in the design of therapeutic strategies aimed at T-cell reactivity.

Fas-dependent elimination of nonselected CD8 cells and lpr disease

MHC/self peptide interactions with cognate coreceptor/TCR complexes are central to homeostasis of the T cell repertoire. Recent reports have also underlined the critical role of IL-15/IL-2 cytokines in regulating this homeostatic process. In this study, we investigate mechanisms that regulate potentially autoreactive CD8 cells that have escaped intrathymic selection. These cells, upon exit from the thymus, express high levels of CD44, B220, and the IL-15R/IL-2R, and undergo fas-dependent apoptosis. Defects in fas signaling allow increased IL-15/IL-2-dependent survival of these CD44/B220(+) CD8(+) as well as the double-negative T cells characteristic of lpr disease.

Autoreactive T cells revealed in the normal repertoire: escape from negative selection and peripheral tolerance

Self-reactive T cells are known to be eliminated by negative selection in the thymus or by the induction of tolerance in the periphery. However, developmental pathways that allow self-reactive T cells to inhabit the normal repertoire are not well-characterized. In this investigation, we made use of anti-small nuclear ribonucleoprotein particle (snRNP) Ig transgenic (Tg) mice (2-12 Tg) to demonstrate that autoreactive T cells can be detected and activated in both normal naive mice and autoimmune-prone MRL lpr/lpr mice. In contrast, autoreactive T cells of nonautoimmune Tg mice are tolerized by Tg B cells in the periphery. In adoptive transfer studies, autoreactive T cells from MRL lpr/lpr mice can stimulate autoantibody synthesis in nonautoimmune anti-snRNP Tg mice. Transferred CD4 T cells migrate to regions of the spleen proximal to the B cell follicles, suggesting that cognate B cell-T cell interactions are critical to the autoimmune response. Taken together, our studies suggest that anti-snRNP B cells are important APCs for T cell activation in autoimmune-prone mice. Additionally, we have demonstrated that anti-snRNP B cell anergy in nonautoimmune mice may be reversed by appropriate T cell help.

CD8+ T lymphocytes in double alpha beta TCR transgenic mice. I. TCR expression and thymus selection in the absence or in the presence of self-antigen

We derived Rag2-deficient mice bearing two rearranged alphabeta TCR transgenes, one specific for the HY male Ag and the second specific for the gp33-41 peptide of lymphocytic choriomeningitis virus, both restricted to the MHC H-2D(b) class I molecule. We found that, in female double transgenic (DTg) mice, most CD8 T cells express only the TCRbeta chain from the aHY transgene. By comparing the mRNA species for both beta-chains, we observed that in T cells from DTg mice the aHY TCRbeta chain transcripts are abundant, whereas the anti-lymphocytic choriomeningitis virus TCRbeta chain transcripts are rare. In contrast to TCRbeta chain expression, most of the T cells from DTg mice express two TCRalpha chains. We examined the thymus selection of the dual-receptor CD8 T cells in the presence of self-Ag. We found that the presence of a second TCRalpha chain allows a significant number of CD8 T cells expressing a self-reactive receptor to escape central deletion and migrate to the peripheral pools of male mice. Differences in TCR and coreceptor expression between female and male MoaHY and DTg mice suggest that peripheral T cell survival requires an optimal level of signaling, which implies a process of "adaptation" of lymphocyte populations to the host environment.