Todd Metzger

Control of central and peripheral tolerance by Aire

Summary:  The negative selection of self‐reactive thymocytes depends on the expression of tissue‐specific antigens by medullary thymic epithelial cells. The autoimmune regulator (Aire) protein plays an important role in turning on these antigens, and the absence of even one Aire‐induced tissue‐specific antigen in the thymus can lead to autoimmunity in the antigen‐expressing target organ. Recently, Aire protein has been detected in peripheral lymphoid organs, suggesting that peripheral Aire plays a complementary role here. In these peripheral sites, Aire was found to regulate the expression of a group of tissue‐specific antigens that is distinct from those expressed in the thymus. Furthermore, transgenic antigen expression in extrathymic Aire‐expressing cells (eTACs) can mediate deletional tolerance, but the immunological relevance of Aire‐dependent, endogenous tissue‐specific antigens remains to be determined.

Extrathymic Aire-Expressing Cells Are a Distinct Bone Marrow-Derived Population that Induce Functional Inactivation of CD4+ T Cells

The autoimmune regulator (Aire) is essential for prevention of autoimmunity; its role is best understood in the thymus, where it promotes self-tolerance through tissue-specific antigen (TSA) expression. Recently, extrathymic Aire-expressing cells (eTACs) have been described in murine secondary lymphoid organs, but the identity of such cells and their role in immune tolerance remains unclear. Here we have shown that eTACs are a discrete major histocompatibility complex class II (MHC II)(hi), CD80(lo), CD86(lo), epithelial cell adhesion molecule (EpCAM)(hi), CD45(lo) bone marrow-derived peripheral antigen-presenting cell (APC) population. We also have demonstrated that eTACs can functionally inactivate CD4⁺ T cells through a mechanism that does not require regulatory T cells (Treg) and is resistant to innate inflammatory stimuli. Together, these findings further define eTACs as a distinct tolerogenic cell population in secondary lymphoid organs.

Lineage tracing and cell ablation identify a post-Aire-expressing thymic epithelial cell population.

Thymic epithelial cells in the medulla (mTECs) play a critical role in enforcing central tolerance through expression and presentation of tissue-specific antigens (TSAs) and deletion of autoreactive thymocytes. TSA expression requires autoimmune regulator (Aire), a transcriptional activator present in a subset of mTECs characterized by high CD80 and major histocompatibility complex II expression and a lack of potential for differentiation or proliferation. Here, using an Aire-DTR transgenic line, we show that short-term ablation specifically targets Aire(+) mTECs, which quickly undergo RANK-dependent recovery. Repeated ablation also affects Aire(-) mTECs, and using an inducible Aire-Cre fate-mapping system, we find that this results from the loss of a subset of mTECs that showed prior expression of Aire, maintains intermediate TSA expression, and preferentially migrates toward the center of the medulla. These results clearly identify a distinct stage of mTEC development and underscore the diversity of mTECs that play a key role in maintaining tolerance.

BPIFB1 IS A LUNG-SPECIFIC AUTOANTIGEN ASSOCIATED WITH INTERSTITIAL LUNG DISEASE

Autoimmunity targeting the lung-specific antigen BPIFB1 may be important to the pathogenesis of interstitial lung disease. Seeing the Forest by Examining the Trees Sometimes looking at something too closely obscures the big picture. However, when the big picture is too big, a reductionist approach may be best. Interstitial lung disease (ILD) is a complex and heterogeneous disorder, frequently associated with autoimmune syndromes. However, due in part to this heterogeneity, it remains unclear whether autoimmunity directly contributes to ILD. Now, Shum et al. attack this question by example—connecting one form of autoimmune disease, autoimmune polyglandular syndrome type 1 (APS1), with clinical ILD. The authors screened patients with APS1 and found autoantibodies to a lung-specific protein—BPIFB1—associated with the development of ILD in APS1 patients. They then extended these findings to non-APS1–associated ILD and found that 12 to 15% of patients also had these autoantibodies. The authors then examined a potential pathogenic mechanism of these autoantibodies in a mouse model of APS1, finding that similar autoantibodies and development of ILD resulted from a defect in thymic tolerance. Indeed, autoimmune targeting of BPIFB1 could cause ILD in mice without the autoimmune defect. These results suggest not only that ILD may be an autoimmune disorder in APS1 patients but also that autoimmunity may also contribute to pathology in a broader swath of ILD patients. Interstitial lung disease (ILD) is a complex and heterogeneous disorder that is often associated with autoimmune syndromes. Despite the connection between ILD and autoimmunity, it remains unclear whether ILD can develop from an autoimmune response that specifically targets the lung parenchyma. We examined a severe form of autoimmune disease, autoimmune polyglandular syndrome type 1 (APS1), and established a strong link between an autoimmune response to the lung-specific protein BPIFB1 (bactericidal/permeability-increasing fold-containing B1) and clinical ILD. Screening of a large cohort of APS1 patients revealed autoantibodies to BPIFB1 in 9.6% of APS1 subjects overall and in 100% of APS1 subjects with ILD. Further investigation of ILD outside the APS1 disorder revealed BPIFB1 autoantibodies present in 14.6% of patients with connective tissue disease–associated ILD and in 12.0% of patients with idiopathic ILD. The animal model for APS1, Aire−/− mice, harbors autoantibodies to a similar lung antigen (BPIFB9); these autoantibodies are a marker for ILD. We found that a defect in thymic tolerance was responsible for the production of BPIFB9 autoantibodies and the development of ILD. We also found that immunoreactivity targeting BPIFB1 independent of a defect in Aire also led to ILD, consistent with our discovery of BPIFB1 autoantibodies in non-APS1 patients. Overall, our results demonstrate that autoimmunity targeting the lung-specific antigen BPIFB1 may contribute to the pathogenesis of ILD in patients with APS1 and in subsets of patients with non-APS1 ILD, demonstrating the role of lung-specific autoimmunity in the genesis of ILD.

The transcriptional regulator Aire coopts the repressive ATF7ip-MBD1 complex for the induction of immunotolerance

The maintenance of immunological tolerance requires the deletion of self-reactive T cells in the thymus. The expression of genes encoding tissue-specific antigens (TSAs) by thymic epithelial cells is critical for this process and depends on activity of the transcriptional regulator Aire; however, the molecular mechanisms Aire uses to target loci encoding TSAs are unknown. Here we identified two Aire-interacting proteins known to be involved in gene repression, ATF7ip and MBD1, that were required for Aires targeting of loci encoding TSAs. Moreover, Mbd1−/− mice developed pathological autoimmunity and had a defect in Aire-dependent thymic expression of genes encoding TSAs, which underscores the importance of Aires interaction with the ATF7ip-MBD1 protein complex in maintaining central tolerance.

Spontaneous Development of Autoimmune Uveitis Is CCR2 Dependent

Development of novel strategies to treat noninfectious posterior uveitis is an ongoing challenge, in part because of limited availability of animal models that mimic the naturally occurring disease in humans. Mice deficient in the autoimmune regulatory gene Aire develop a spontaneous T-cell and macrophage-mediated autoimmune uveitis that closely recapitulates human endogenous uveitis and thus provide a useful model for mechanistic and therapeutic investigations. Lymphocytic and mononuclear infiltration of the retina in Aire knockout (KO) mice triggers the onset of uveitis from initial retinal inflammation to eventual destruction of the neuroretina with loss of photoreceptors. The C-C chemokine receptor type 2 protein (CCR2) functions in directing monocyte and macrophage migration to inflamed tissues via interaction with monocyte chemotactic proteins. Using the Aire KO mouse model, we demonstrated an essential role for CCR2 in the pathogenesis of autoimmune-mediated uveitis. Loss of functional CCR2 effectively reduced immune cell infiltration and rescued the retina from destruction. CCR2-dependent migration of bone marrow-derived cells provided the driving force for retinal inflammation, with CCR2-expressing mononuclear cells contributing to retinal damage via recruitment of CD4(+) T cells. These studies identify the CCR2 pathway as a promising therapeutic target that may prove an effective approach to treat uveitis associated with autoimmunity.

Myocarditis: a defect in central immune tolerance?

Myocarditis, or inflammation of the heart, is a potentially devastating disease that can result from both viral infection and autoimmune attack of self antigens in the heart. In the current issue of the JCI, Lv and colleagues use a genetically susceptible mouse model to show that myocarditis is a T cell-mediated autoimmune disease that occurs due to insufficient thymic negative selection of α-myosin-reactive T cells.

Thymic tuft cells promote an IL-4-enriched medulla and shape thymocyte development

The thymus is responsible for generating a diverse yet self-tolerant pool of T cells1. Although the thymic medulla consists mostly of developing and mature AIRE+ epithelial cells, recent evidence has suggested that there is far greater heterogeneity among medullary thymic epithelial cells than was previously thought2. Here we describe in detail an epithelial subset that is remarkably similar to peripheral tuft cells that are found at mucosal barriers3. Similar to the periphery, thymic tuft cells express the canonical taste transduction pathway and IL-25. However, they are unique in their spatial association with cornified aggregates, ability to present antigens and expression of a broad diversity of taste receptors. Some thymic tuft cells pass through an Aire-expressing stage and depend on a known AIRE-binding partner, HIPK2, for their development. Notably, the taste chemosensory protein TRPM5 is required for their thymic function through which they support the development and polarization of thymic invariant natural killer T cells and act to establish a medullary microenvironment that is enriched in the type 2 cytokine, IL-4. These findings indicate that there is a compartmentalized medullary environment in which differentiation of a minor and highly specialized epithelial subset has a non-redundant role in shaping thymic function.A comprehensive analysis of the thymic medulla identifies a tuft-cell-like thymic epithelial cell population that is necessary for shaping thymic function.