Autoreactive marginal zone B cells are spontaneously activated but lymph node B cells require T cell help

In K/BxN mice, arthritis is induced by autoantibodies against glucose-6-phosphate-isomerase (GPI). To investigate B cell tolerance to GPI in nonautoimmune mice, we increased the GPI-reactive B cell frequency using a low affinity anti-GPI H chain transgene. Surprisingly, anti-GPI B cells were not tolerant to this ubiquitously expressed and circulating autoantigen. Instead, they were found in two functionally distinct compartments: an activated population in the splenic marginal zone (MZ) and an antigenically ignorant one in the recirculating follicular/lymph node (LN) pool. This difference in activation was due to increased autoantigen availability in the MZ. Importantly, the LN anti-GPI B cells remained functionally competent and could be induced to secrete autoantibodies in response to cognate T cell help in vitro and in vivo. Therefore, our study of low affinity autoreactive B cells reveals two distinct but potentially concurrent mechanisms for their activation, of which one is T cell dependent and the other is T cell independent

Massive thymic deletion results in systemic autoimmunity through elimination of CD4+ CD25+ T regulatory cells

Incomplete deletion of KRN T cells that recognize the ubiquitously expressed self-antigen glucose-6-phosphate-isomerase (GPI) initiates an anti-GPI autoimmune cascade in K/BxN mice resulting in a humorally mediated arthritis. Transgenic (Tg) expression of a KRN T cell receptor (TCR) agonist under the major histocompatibility complex class II promoter resulted in thymic deletion with loss of anti-GPI T and B cell responses and attenuated arthritis course. However, double Tg mice succumbed to systemic autoimmunity with multiorgan inflammation and autoantibody production. Extensive thymic deletion resulted in lymphopenia and elimination of CD4(+) CD25(+) regulatory T cells (Tregs), but spared some CD4(+) T cells expressing endogenous TCR, which oligoclonally expanded in the periphery. Disease was transferred by these T cells and prevented by cotransfer of CD4(+) CD25(+) Tregs. Moreover, we extended our findings to another TCR system (anti–hen egg lysozyme [HEL] TCR/HEL mice) where similarly extensive thymic deletion also resulted in disease. Thus, our studies demonstrated that central tolerance can paradoxically result in systemic autoimmunity through differential susceptibility of Tregs and autoreactive T cells to thymic deletion. Therefore, too little or too much negative selection to a self-antigen can result in systemic autoimmunity and disease

IDO2 in Immunomodulation and Autoimmune Disease.

IDO2 is a relative of IDO1 implicated in tryptophan catabolism and immune modulation but its specific contributions to normal physiology and pathophysiology are not known. Evolutionary genetic studies suggest that IDO2 has a unique function ancestral to IDO1. In mice, IDO2 gene deletion does not appreciably affect embryonic development or hematopoiesis, but it leads to defects in allergic or autoimmune responses and in the ability of IDO1 to influence the generation of T regulatory cells. Gene expression studies indicate that IDO2 is a basally and more narrowly expressed gene than IDO1 and that IDO2 is uniquely regulated by AhR, which serves as a physiological receptor for the tryptophan catabolite kynurenine. In the established KRN transgenic mouse model of rheumatoid arthritis, where IDO1 gene deletion has no effect, IDO2 deletion selectively blunts responses to autoantigen but has no effect on responses to neoantigen challenge. In human populations, natural variations in IDO2 gene sequence that attenuate enzymatic activity have been reported to influence brain cancer control and adaptive immune responses to the IDO2 protein itself, consistent with the concept that IDO2 is involved in shaping immune tolerance in human beings. Biochemical and pharmacological studies provide further evidence of differences in IDO2 enzymology and function relative to IDO1. We suggest that IDO2 may act in a distinct manner from IDO1 as a set-point for tolerance to altered-self antigens along the self-non-self continuum where immune challenges from cancer and autoimmunity may arise

1-Methyl-tryptophan synergizes with methotrexate to alleviate arthritis in a mouse model of arthritis.

Rheumatoid arthritis (RA) is an inflammatory autoimmune disease with no known cure. Current strategies to treat RA, including methotrexate (MTX), target the later inflammatory stage of disease. Recently, we showed that inhibiting indoleamine-2,3-dioxygenase (IDO) with 1-methyl-tryptophan (1MT) targets autoantibodies and cytokines that drive the initiation of the autoimmune response. Therefore, we hypothesized that combining 1MT with MTX would target both the initiation and chronic inflammatory phases of the autoimmune response and be an effective co-therapeutic strategy for arthritis. To test this, we used K/BxN mice, a pre-clinical model of arthritis that develops joint-specific inflammation with many characteristics of human RA. Mice were treated with 1MT, MTX, alone or in combination, and followed for arthritis, autoantibodies, and inflammatory cytokines. Both 1MT and MTX were able to partially inhibit arthritis when used individually; however, combining MTX + 1MT was significantly more effective than either treatment alone at delaying the onset and alleviating the severity of joint inflammation. We went on to show that combination of MTX + 1MT did not lower inflammatory cytokine or autoantibody levels, nor could the synergistic co-therapeutic effect be reversed by the adenosine receptor antagonist theophylline or be mimicked by inhibition of polyamine synthesis. However, supplementation with folinic acid did reverse the synergistic co-therapeutic effect, demonstrating that, in the K/BxN model, MTX synergizes with 1MT by blocking folate metabolism. These data suggest that pharmacological inhibition of IDO with 1MT is a potential candidate for use in combination with MTX to increase its efficacy in the treatment of RA

The regulation of anti -double -stranded DNA B cells in healthy and autoimmune mice

A hallmark of systemic lupus erythematosus is the presence of anti-double-stranded (ds) DNA Abs that are absent in healthy individuals. To identify the mechanisms involved in the regulation of anti-dsDNA B cells in non-autoimmune mice and the steps leading to the production of these Abs in autoimmune mice, we have compared the phenotype and localization of anti-dsDNA B cells in autoimmune-prone (MRL+/+, MRL-lpr/lpr , and bcl-2 Tg) mice with that in non-autoimmune-prone (BALB/c) mice. To increase the frequency of anti-DNA B cells so that they can be identified and tracked, we have utilized a H-chain Tg that can pair with endogenous L-chains to generate anti-single-stranded (ss) DNA, anti-dsDNA, and non-DNA B cells, allowing us to study the regulation of anti-dsDNA B cells in the context of a diverse B cell repertoire. We have found that anti-dsDNA B cells are actively regulated in BALB/c mice as indicated by the lack of their Ig in the serum, their developmental arrest, increased turnover rate, and accumulation at the T/B interface of the splenic follicle. In the MRL genetic background, anti-dsDNA B cells are no longer developmentally arrested, suggesting an intrinsic B cell defect conferred by MRL background genes. With intact Fas, they continue to exhibit follicular exclusion; however, in the presence of the lpr/lpr mutation, anti-dsDNA B cells are now present in the follicle and their Ig becomes present in the serum. These data suggest that MRL mice are defective in maintaining the developmental arrest of autoreactive B cells and indicate a role for Fas in restricting entry into the follicle. In contrast, the presence of a bcl-2 Tg increased the lifespan of anti-dsDNA B cells, but did not alter the other features of tolerance. This suggests that the serum anti-dsDNA Abs present in bcl-2 Tg mice were not due to a breakdown in central tolerance, as was found for MRL-lpr/lpr mice. Instead, we provide evidence that these Abs originate from B cells that have transited a GC, in that they are somatically mutated and clonally expanded. Together these data directly show that a breakdown in the regulation of anti-dsDNA B cells can occur at two levels: in the generation of the primary repertoire in the BM or in the formation of the modified repertoire during GC maturation

The regulation of anti -double -stranded DNA B cells in healthy and autoimmune mice

A hallmark of systemic lupus erythematosus is the presence of anti-double-stranded (ds) DNA Abs that are absent in healthy individuals. To identify the mechanisms involved in the regulation of anti-dsDNA B cells in non-autoimmune mice and the steps leading to the production of these Abs in autoimmune mice, we have compared the phenotype and localization of anti-dsDNA B cells in autoimmune-prone (MRL+/+, MRL-lpr/lpr , and bcl-2 Tg) mice with that in non-autoimmune-prone (BALB/c) mice. To increase the frequency of anti-DNA B cells so that they can be identified and tracked, we have utilized a H-chain Tg that can pair with endogenous L-chains to generate anti-single-stranded (ss) DNA, anti-dsDNA, and non-DNA B cells, allowing us to study the regulation of anti-dsDNA B cells in the context of a diverse B cell repertoire. We have found that anti-dsDNA B cells are actively regulated in BALB/c mice as indicated by the lack of their Ig in the serum, their developmental arrest, increased turnover rate, and accumulation at the T/B interface of the splenic follicle. In the MRL genetic background, anti-dsDNA B cells are no longer developmentally arrested, suggesting an intrinsic B cell defect conferred by MRL background genes. With intact Fas, they continue to exhibit follicular exclusion; however, in the presence of the lpr/lpr mutation, anti-dsDNA B cells are now present in the follicle and their Ig becomes present in the serum. These data suggest that MRL mice are defective in maintaining the developmental arrest of autoreactive B cells and indicate a role for Fas in restricting entry into the follicle. In contrast, the presence of a bcl-2 Tg increased the lifespan of anti-dsDNA B cells, but did not alter the other features of tolerance. This suggests that the serum anti-dsDNA Abs present in bcl-2 Tg mice were not due to a breakdown in central tolerance, as was found for MRL-lpr/lpr mice. Instead, we provide evidence that these Abs originate from B cells that have transited a GC, in that they are somatically mutated and clonally expanded. Together these data directly show that a breakdown in the regulation of anti-dsDNA B cells can occur at two levels: in the generation of the primary repertoire in the BM or in the formation of the modified repertoire during GC maturation

IDO2: A Pathogenic Mediator of Inflammatory Autoimmunity

Indoleamine 2,3-dioxygenase 2 (IDO2), a homolog of the better-studied tryptophan-catabolizing enzyme IDO1, is an immunomodulatory molecule with potential effects on various diseases including cancer and autoimmunity. Here, we review what is known about the direct connections between IDO2 and immune function, particularly in relationship to autoimmune inflammatory disorders such as rheumatoid arthritis and lupus. Accumulating evidence indicates that IDO2 acts as a pro-inflammatory mediator of autoimmunity, with a functional phenotype distinct from IDO1. IDO2 is expressed in antigen-presenting cells, including B cells and dendritic cells, but affects inflammatory responses in the autoimmune context specifically by acting in B cells to modulate T cell help in multiple model systems. Given that expression of IDO2 can lead to exacerbation of inflammatory responses, IDO2 should be considered a potential therapeutic target for autoimmune disorders