IL-1β–driven osteoclastogenic Tregs accelerate bone erosion in arthritis

IL-1β is a proinflammatory mediator with roles in innate and adaptive immunity. Here we show that IL-1β contributes to autoimmune arthritis by inducing osteoclastogenic capacity in Tregs. Using mice with joint inflammation arising through deficiency of the IL-1 receptor antagonist (Il1rn^–/–), we observed that IL-1β blockade attenuated disease more effectively in early arthritis than in established arthritis, especially with respect to bone erosion. Protection was accompanied by a reduction in synovial CD4⁺Foxp3⁺ Tregs that displayed preserved suppressive capacity and aerobic metabolism but aberrant expression of RANKL and a striking capacity to drive RANKL-dependent osteoclast differentiation. Both Il1rn^–/– Tregs and wild-type Tregs differentiated with IL-1β accelerated bone erosion upon adoptive transfer. Human Tregs exhibited analogous differentiation, and corresponding RANKL^hiFoxp3⁺ T cells could be identified in rheumatoid arthritis synovial tissue. Together, these findings identify IL-1β–induced osteoclastogenic Tregs as a contributor to bone erosion in arthritis.     

TLR4 signaling in effector CD4+ T cells regulates TCR activation and experimental colitis in mice

TLRs sense various microbial products. Their function has been best characterized in DCs and macrophages, where they act as important mediators of innate immunity. TLR4 is also expressed on CD4⁺ T cells, but its physiological function on these cells remains unknown. Here, we have shown that TLR4 triggering on CD4⁺ T cells affects their phenotype and their ability to provoke intestinal inflammation. In a model of spontaneous colitis, Il10^–/–Tlr4^–/– mice displayed accelerated development of disease, with signs of overt colitis as early as 8 weeks of age, when compared with Il10^–/– and Il10^–/–Tlr9^–/– mice, which did not develop colitis by 8 months. Similar results were obtained in a second model of colitis in which transfer of naive Il10^–/–Tlr4^–/– CD4⁺ T cells into Rag1^–/– recipients sufficient for both IL-10 and TLR4 induced more aggressive colitis than the transfer of naive Il10^–/– CD4⁺ T cells. Mechanistically, LPS stimulation of TLR4-bearing CD4⁺ T cells inhibited ERK1/2 activation upon subsequent TCR stimulation via the induction of MAPK phosphatase 3 (MKP-3). Our data therefore reveal a tonic inhibitory role for TLR4 signaling on subsequent TCR-dependent CD4⁺ T cell responses.     

Contrasting roles for all-trans retinoic acid in TGF-β–mediated induction of Foxp3 and Il10 genes in developing regulatory T cells

Extrathymic induction of regulatory T (T reg) cells is essential to the regulation of effector T cell responses in the periphery. In addition to Foxp3, T reg cell expression of suppressive cytokines, such as IL-10, is essential for peripheral tolerance, particularly in the intestines. TGF-β has been shown to induce expression of Foxp3 as well as IL10 and the vitamin A metabolite; all-trans retinoic acid (RA [at-RA]) has been found to enhance the former. We report that in contrast to its enhancement of TGF-β–mediated Foxp3 induction, at-RA potently inhibits the TGF-β–mediated induction of Il10 in naive CD4 T cells. Thus, mucosal DC subsets that are active producers of at-RA inhibit induction of Il10 in naive CD4 T cells while promoting induction of Foxp3. Accordingly, mice with vitamin A deficiency have increased numbers of IL-10–competent T reg cells. Activation of DCs by certain Toll-like receptors (TLRs), particularly TLR9, suppresses T cell induction of Foxp3 and enables induction of Il10. Collectively, our data indicate that at-RA has reciprocal effects on the induction of Foxp3 and Il10 in developing CD4⁺ T reg cells and suggest that TLR9-dependent inhibition of at-RA production by antigen-presenting cells might represent one mechanism to promote the development of IL-10–expressing T cells.     

Ectopic expression of neural autoantigen in mouse liver suppresses experimental autoimmune neuroinflammation by inducing antigen-specific Tregs

Tregs are important mediators of immune tolerance to self antigens, and it has been suggested that Treg inactivation may cause autoimmune disease. Therefore, immunotherapy approaches that aim to restore or expand autoantigen-specific Treg activity might be beneficial for the treatment of autoimmune disease. Here we report that Treg-mediated suppression of autoimmune disease can be achieved in vivo by taking advantage of the ability of the liver to promote immune tolerance. Expression of the neural autoantigen myelin basic protein (MBP) in the liver was accomplished stably in liver-specific MBP transgenic mice and transiently using gene transfer to liver cells in vivo. Such ectopic MBP expression induced protection from autoimmune neuroinflammation in a mouse model of multiple sclerosis. Protection from autoimmunity was mediated by MBP-specific CD4⁺CD25⁺Foxp3⁺ Tregs, as demonstrated by the ability of these cells to prevent disease when adoptively transferred into nontransgenic mice and to suppress conventional CD4⁺CD25^– T cell proliferation after antigen-specific stimulation with MBP in vitro. The generation of MBP-specific CD4⁺CD25⁺Foxp3⁺ Tregs in vivo depended on expression of MBP in the liver, but not in skin, and occurred by TGF-β–dependent peripheral conversion from conventional non-Tregs. Our findings indicate that autoantigen expression in the liver may generate autoantigen-specific Tregs. Thus, targeting of autoantigens to hepatocytes may be a novel approach to prevention or treatment of autoimmune diseases.     

Identification of heme oxygenase-1–specific regulatory CD8+ T cells in cancer patients

Treg deficiencies are associated with autoimmunity. Conversely, CD4⁺ and CD8⁺ Tregs accumulate in the tumor microenvironment and are associated with prevention of antitumor immunity and anticancer immunotherapy. Recently, CD4⁺ Tregs have been much studied, but little is known about CD8⁺ Tregs and the antigens they recognize. Here, we describe what we believe to be the first natural target for CD8⁺ Tregs. Naturally occurring HLA-A2–restricted CD8⁺ T cells specific for the antiinflammatory molecule heme oxygenase-1 (HO-1) were able to suppress cellular immune responses with outstanding efficacy. HO-1–specific CD8⁺ T cells were detected ex vivo and in situ among T cells from cancer patients. HO-1–specific T cells isolated from the peripheral blood of cancer patients inhibited cytokine release, proliferation, and cytotoxicity of other immune cells. Notably, the inhibitory effect of HO-1–specific T cells was far more pronounced than that of conventional CD4⁺CD25⁺CD127^– Tregs. The inhibitory activity of HO-1–specific T cells seemed at least partly to be mediated by soluble factors. Our data link the cellular stress response to the regulation of adaptive immunity, expand the role of HO-1 in T cell–mediated immunoregulation, and establish a role for peptide-specific CD8⁺ T cells in regulating cellular immune responses. Identification of potent antigen-specific CD8⁺ Tregs may open new avenues for therapeutic interventions in both autoimmune diseases and cancer.     

Engagement of Cytotoxic T Lymphocyte–associated Antigen 4 (CTLA-4) Induces Transforming Growth Factor β (TGF-β) Production by Murine CD4+ T Cells

Evidence indicates that cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) may negatively regulate T cell activation, but the basis for the inhibitory effect remains unknown. We report here that cross-linking of CTLA-4 induces transforming growth factor β (TGF-β) production by murine CD4⁺ T cells. CD4⁺ T helper type 1 (Th1), Th2, and Th0 clones all secrete TGF-β after antibody cross-linking of CTLA-4, indicating that induction of TGF-β by CTLA-4 signaling represents a ubiquitous feature of murine CD4⁺ T cells. Stimulation of the CD3–T cell antigen receptor complex does not independently induce TGF-β, but is required for optimal CTLA-4–mediated TGF-β production. The consequences of cross-linking of CTLA-4, together with CD3 and CD28, include inhibition of T cell proliferation and interleukin (IL)-2 secretion, as well as suppression of both interferon γ (Th1) and IL-4 (Th2). Moreover, addition of anti–TGF-β partially reverses this T cell suppression. When CTLA-4 was cross-linked in T cell populations from TGF-β1 gene–deleted (TGF-β1^−/−) mice, the T cell responses were only suppressed 38% compared with 95% in wild-type mice. Our data demonstrate that engagement of CTLA-4 leads to CD4⁺ T cell production of TGF-β, which, in part, contributes to the downregulation of T cell activation. CTLA-4, through TGF-β, may serve as a counterbalance for CD28 costimulation of IL-2 and CD4⁺ T cell activation.     

Tissue-resident-like CD4+ T cells secreting IL-17 control Mycobacterium tuberculosis in the human lung

T cell immunity is essential for the control of tuberculosis (TB), an important disease of the lung, and is generally studied in humans using peripheral blood cells. Mounting evidence, however, indicates that tissue-resident memory T cells (Trms) are superior at controlling many pathogens, including Mycobacterium tuberculosis (M. tuberculosis), and can be quite different from those in circulation. Using freshly resected lung tissue, from individuals with active or previous TB, we identified distinct CD4⁺ and CD8⁺ Trm-like clusters within TB-diseased lung tissue that were functional and enriched for IL-17–producing cells. M. tuberculosis–specific CD4⁺ T cells producing TNF-α, IL-2, and IL-17 were highly expanded in the lung compared with matched blood samples, in which IL-17⁺ cells were largely absent. Strikingly, the frequency of M. tuberculosis–specific lung T cells making IL-17, but not other cytokines, inversely correlated with the plasma IL-1β levels, suggesting a potential link with disease severity. Using a human granuloma model, we showed the addition of either exogenous IL-17 or IL-2 enhanced immune control of M. tuberculosis and was associated with increased NO production. Taken together, these data support an important role for M. tuberculosis–specific Trm-like, IL-17–producing cells in the immune control of M. tuberculosis in the human lung.     

Vitamin D3 attenuates Th2 responses to Aspergillus fumigatus mounted by CD4+ T cells from cystic fibrosis patients with allergic bronchopulmonary aspergillosis

Allergic bronchopulmonary aspergillosis (ABPA) is caused by a dominant Th2 immune response to antigens derived from the opportunistic mold Aspergillus, most commonly Aspergillus fumigatus. It occurs in 4%–15% of patients with cystic fibrosis (CF); however, not all patients with CF infected with A. fumigatus develop ABPA. Therefore, we compared cohorts of A. fumigatus–colonized CF patients with and without ABPA to identify factors mediating tolerance versus sensitization. We found that the costimulatory molecule OX40 ligand (OX40L) was critical in driving Th2 responses to A. fumigatus in peripheral CD4⁺ T cells isolated from patients with ABPA. In contrast, CD4⁺ T cells from the non-ABPA cohort did not mount enhanced Th2 responses in vitro and contained a higher frequency of TGF-β–expressing regulatory T cells. Heightened Th2 reactivity in the ABPA cohort correlated with lower mean serum vitamin D levels. Further, in vitro addition of 1,25 OH-vitamin D₃ substantially reduced DC expression of OX40L and increased DC expression of TGF-β. This in vitro treatment also resulted in increased Treg TGF-β expression and reduced Th2 responses by CD4⁺ T cells from patients with ABPA. These data provide rationale for a therapeutic trial of vitamin D to prevent or treat ABPA in patients with CF.     

Depletion of FOXP3+ regulatory T cells promotes hypercholesterolemia and atherosclerosis

Atherosclerosis is a chronic inflammatory disease promoted by hyperlipidemia. Several studies support FOXP3-positive regulatory T cells (Tregs) as inhibitors of atherosclerosis; however, the mechanism underlying this protection remains elusive. To define the role of FOXP3-expressing Tregs in atherosclerosis, we used the DEREG mouse, which expresses the diphtheria toxin (DT) receptor under control of the Treg-specific Foxp3 promoter, allowing for specific ablation of FOXP3⁺ Tregs. Lethally irradiated, atherosclerosis-prone, low-density lipoprotein receptor–deficient (Ldlr^–/–) mice received DEREG bone marrow and were injected with DT to eliminate FOXP3⁺ Tregs. Depletion of Tregs caused a 2.1-fold increase in atherosclerosis without a concomitant increase in vascular inflammation. These mice also exhibited a 1.7-fold increase in plasma cholesterol and an atherogenic lipoprotein profile with increased levels of VLDL. Clearance of VLDL and chylomicron remnants was hampered, leading to accumulation of cholesterol-rich particles in the circulation. Functional and protein analyses complemented by gene expression array identified reduced protein expression of sortilin-1 in liver and increased plasma enzyme activity of lipoprotein lipase, hepatic lipase, and phospholipid transfer protein as mediators of the altered lipid phenotype. These results demonstrate that FOXP3⁺ Tregs inhibit atherosclerosis by modulating lipoprotein metabolism.     

TGF-β prevents T follicular helper cell accumulation and B cell autoreactivity

T follicular helper (Tfh) cells contribute to the establishment of humoral immunity by controlling the delivery of helper signals to activated B cells; however, Tfh development must be restrained, as aberrant accumulation of these cells is associated with positive selection of self-reactive germinal center B cells and autoimmunity in both humans and mice. Here, we show that TGF-β signaling in T cells prevented Tfh cell accumulation, self-reactive B cell activation, and autoantibody production. Using mice with either T cell–specific loss or constitutive activation of TGF-β signaling, we demonstrated that TGF-β signaling is required for the thymic maturation of CD44⁺CD122⁺Ly49⁺CD8⁺ regulatory T cells (Tregs), which induce Tfh apoptosis and thus regulate this cell population. Moreover, peripheral Tfh cells escaping TGF-β control were resistant to apoptosis, exhibited high levels of the antiapoptotic protein BCL2, and remained refractory to regulation by CD8⁺ Tregs. The unrestrained accumulation of Tfh cells in the absence of TGF-β was dependent on T cell receptor engagement and required B cells. Together, these data indicate that TGF-β signaling restrains Tfh cell accumulation and B cell–associated autoimmunity and thereby controls self-tolerance.     

Definition of target antigens for naturally occurring CD4(+) CD25(+) regulatory T cells

The antigenic targets recognized by naturally occurring CD4⁺ CD25⁺ regulatory T cells (T reg cells) have been elusive. We have serologically defined a series of broadly expressed self-antigens derived from chemically induced mouse sarcomas by serological identification of antigens by recombinant expression cloning (SEREX). CD4⁺ CD25⁺ T cells from mice immunized with SEREX-defined self-antigens had strong suppressive activity on peptide-specific proliferation of CD4⁺ CD25⁻ T cells and CD8⁺ T cells. The suppressive effect was observed without in vitro T cell stimulation. Foxp3 expression in these CD4⁺ CD25⁺ T cells from immunized mice was 5–10 times greater than CD4⁺ CD25⁺ T cells derived from naive mice. The suppressive effect required cellular contact and was blocked by anti-glucocorticoid–induced tumor necrosis factor receptor family–related gene antibody. In vitro suppressive activity essentially disappeared 8 wk after the last immunization. However, it was regained by in vitro restimulation with cognate self-antigen protein but not with control protein. We propose that SEREX-defined self-antigens such as those used in this study represent self-antigens that elicit naturally occurring CD4⁺ CD25⁺ T reg cells.     

Central memory CD8+ T lymphocytes mediate lung allograft acceptance

Memory T lymphocytes are commonly viewed as a major barrier for long-term survival of organ allografts and are thought to accelerate rejection responses due to their rapid infiltration into allografts, low threshold for activation, and ability to produce inflammatory mediators. Because memory T cells are usually associated with rejection, preclinical protocols have been developed to target this population in transplant recipients. Here, using a murine model, we found that costimulatory blockade–mediated lung allograft acceptance depended on the rapid infiltration of the graft by central memory CD8⁺ T cells (CD44^hiCD62L^hiCCR7⁺). Chemokine receptor signaling and alloantigen recognition were required for trafficking of these memory T cells to lung allografts. Intravital 2-photon imaging revealed that CCR7 expression on CD8⁺ T cells was critical for formation of stable synapses with antigen-presenting cells, resulting in IFN-γ production, which induced NO and downregulated alloimmune responses. Thus, we describe a critical role for CD8⁺ central memory T cells in lung allograft acceptance and highlight the need for tailored approaches for tolerance induction in the lung.     

Amelioration of arthritis through mobilization of peptide-specific CD8+ regulatory T cells

Current therapies to treat autoimmune disease focus mainly on downstream targets of autoimmune responses, including effector cells and cytokines. A potentially more effective approach would entail targeting autoreactive T cells that initiate the disease cascade and break self tolerance. The murine MHC class Ib molecule Qa-1b (HLA-E in humans) exhibits limited polymorphisms and binds to 2 dominant self peptides: Hsp60_p216 and Qdm. We found that peptide-induced expansion of tetramer-binding CD8⁺ Tregs that recognize Qa-1–Hsp60_p216 but not Qa-1–Qdm strongly inhibited collagen-induced arthritis, an animal model of human rheumatoid arthritis. Perforin-dependent elimination of autoreactive follicular Th (T_FH) and Th17 cells by CD8⁺ Tregs inhibited disease development. Infusion of in vitro–expanded CD8⁺ Tregs increased the efficacy of methotrexate treatment and halted disease progression after clinical onset, suggesting an alternative approach to this first-line treatment. Moreover, infusion of small numbers of Qa-1–Hsp60_p216–specific CD8⁺ Tregs resulted in robust inhibition of autoimmune arthritis, confirming the inhibitory effects of Hsp60_p216 peptide immunization. These results suggest that strategies designed to expand Qa-1–restricted (HLA-E–restricted), peptide-specific CD8⁺ Tregs represent a promising therapeutic approach to autoimmune disorders.     

Beryllium-specific CD4+ T cells induced by chemokine neoantigens perpetuate inflammation

Discovering dominant epitopes for T cells, particularly CD4⁺ T cells, in human immune-mediated diseases remains a significant challenge. Here, we used bronchoalveolar lavage (BAL) cells from HLA-DP2–expressing patients with chronic beryllium disease (CBD), a debilitating granulomatous lung disorder characterized by accumulations of beryllium-specific (Be-specific) CD4⁺ T cells in the lung. We discovered lung-resident CD4⁺ T cells that expressed a disease-specific public CDR3β T cell receptor motif and were specific to Be-modified self-peptides derived from C-C motif ligand 4 (CCL4) and CCL3. HLA-DP2–CCL/Be tetramer staining confirmed that these chemokine-derived peptides represented major antigenic targets in CBD. Furthermore, Be induced CCL3 and CCL4 secretion in the lungs of mice and humans. In a murine model of CBD, the addition of LPS to Be oxide exposure enhanced CCL4 and CCL3 secretion in the lung and significantly increased the number and percentage of CD4⁺ T cells specific for the HLA-DP2–CCL/Be epitope. Thus, we demonstrate a direct link between Be-induced innate production of chemokines and the development of a robust adaptive immune response to those same chemokines presented as Be-modified self-peptides, creating a cycle of innate and adaptive immune activation.     

Toll-like receptor 4–mediated lymphocyte influx induces neonatal necrotizing enterocolitis

The nature and role of the intestinal leukocytes in necrotizing enterocolitis (NEC), a severe disease affecting premature infants, remain unknown. We now show that the intestine in mouse and human NEC is rich in lymphocytes that are required for NEC development, as recombination activating gene 1–deficient (Rag1^–/–) mice were protected from NEC and transfer of intestinal lymphocytes from NEC mice into naive mice induced intestinal inflammation. The intestinal expression of the lipopolysaccharide receptor TLR4, which is higher in the premature compared with full-term human and mouse intestine, is required for lymphocyte influx through TLR4-mediated upregulation of CCR9/CCL25 signaling. TLR4 also mediates a STAT3-dependent polarization toward increased proinflammatory CD3⁺CD4⁺IL-17⁺ and reduced tolerogenic Foxp3⁺ Treg lymphocytes (Tregs). Th17 lymphocytes were required for NEC development, as inhibition of STAT3 or IL-17 receptor signaling attenuated NEC in mice, while IL-17 release impaired enterocyte tight junctions, increased enterocyte apoptosis, and reduced enterocyte proliferation, leading to NEC. Importantly, TLR4-dependent Th17 polarization could be reversed by the enteral administration of retinoic acid, which induced Tregs and decreased NEC severity. These findings identify an important role for proinflammatory lymphocytes in NEC development via intestinal epithelial TLR4 that could be reversed through dietary modification.     

CRTAM controls residency of gut CD4+CD8+ T cells in the steady state and maintenance of gut CD4+ Th17 during parasitic infection

Retention of lymphocytes in the intestinal mucosa requires specialized chemokine receptors and adhesion molecules. We find that both CD4⁺CD8⁺ and CD4⁺ T cells in the intestinal epithelium, as well as CD8⁺ T cells in the intestinal mucosa and mesenteric lymph nodes, express the cell adhesion molecule class I–restricted T cell–associated molecule (Crtam) upon activation, whereas the ligand of Crtam, cell adhesion molecule 1 (Cadm1), is expressed on gut CD103⁺DCs. Lack of Crtam–Cadm1 interactions in Crtam^−/− and Cadm1^−/− mice results in loss of CD4⁺CD8⁺ T cells, which arise from mucosal CD4⁺ T cells that acquire a CD8 lineage expression profile. After acute oral infection with Toxoplasma gondii, both WT and Crtam^−/− mice mounted a robust TH1 response, but markedly fewer TH17 cells were present in the intestinal mucosa of Crtam^−/− mice. The almost exclusive TH1 response in Crtam^−/− mice resulted in more efficient control of intestinal T. gondii infection. Thus, Crtam–Cadm1 interactions have a major impact on the residency and maintenance of CD4⁺CD8⁺ T cells in the gut mucosa in the steady state. During pathogenic infection, Crtam–Cadm1 interactions regulate the dynamic equilibrium between newly formed CD4⁺ T cells and their retention in the gut, thereby shaping representation of disparate CD4⁺ T cell subsets and the overall quality of the CD4⁺ T cell response.Class I–restricted T cell–associated molecule (Crtam) is an Ig-like cell surface protein that was originally found on activated NKT cells (Kennedy et al., 2000), NK cells, and CD8⁺ T cells (Arase et al., 2005; Boles et al., 2005; Galibert et al., 2005) and shown to bind the cell adhesion molecule 1 (Cadm1, also known as Nectin like [Necl] 2; Arase et al., 2005; Boles et al., 2005; Galibert et al., 2005). Cadm1 is a cell surface molecule of the nectin and Necl families that is expressed on CD8α DCs (Galibert et al., 2005; Poulin et al., 2010), epithelial cells, neurons, and tumor cells (Sakisaka and Takai, 2004; Mizutani et al., 2011). Crtam–Cadm1 interactions strengthen NK cell and CD8⁺ T cell effector functions (Arase et al., 2005; Boles et al., 2005; Galibert et al., 2005; Murakami, 2005) and promote the retention of virus-specific CD8⁺ T cells within LNs (Takeuchi et al., 2009). One report proposed that Crtam is essential for the establishment of CD4⁺ T cell polarization after TCR engagement, a process which blocks CD4⁺ T cell division and induces the capacity to secrete IFN-γ, IL-17, and IL-22 (Yeh et al., 2008).The immune system associated with the gastrointestinal mucosa comprises large numbers of dispersed lymphoid cells that reside in the epithelium and the underlying lamina propria. Intraepithelial lymphocytes (IELs) and lamina propria lymphocytes (LPLs) include antigen-experienced CD8⁺ and CD4⁺ T cells, γδ T cells, various subsets of innate lymphoid cells (ILCs), and IgA-secreting plasma cells (Jabri and Ebert, 2007; Cerutti, 2008; Cheroutre et al., 2011; Sheridan and Lefrançois, 2011; Spits et al., 2013). Homing and residency of IELs and LPLs in the mucosa requires specialized chemokine receptors, such as CCR9, CCR6, and CXCR6, which detect chemokines released by gut epithelial cells (CCL25, CCL20, and CXCL16, respectively; Johansson-Lindbom and Agace, 2007). Integrins, like CD103 (α_E) and α₄β₇, also play an essential role in promoting homing and retention of IELs and LPLs in the mucosa by binding E-cadherin and MAdCAM-1 on epithelial cells and vascular endothelial cells, respectively (Johansson-Lindbom and Agace, 2007).T cell acquisition of homing and adhesion molecules is induced by T cell interaction with DCs (Mora et al., 2008; Villablanca et al., 2011). Among the disparate subsets of DC in the intestinal lamina propria and mesenteric LNs (mLN), the CD103⁺ DC subset produces retinoic acid (RA), which induces the gut homing receptors CCR9 and α₄β₇ on lymphocytes (Coombes et al., 2007; Mora et al., 2008; Villablanca et al., 2011). Gut-associated CD103⁺ DCs also produce TGF-β, which induces the expression of CD103 on T cells (Coombes et al., 2007; Mora et al., 2008; Villablanca et al., 2011). In addition to imprinting gut-homing capacity on T cells, gut CD103⁺ DCs control the differentiation of CD4⁺ T cells by priming regulatory CD4⁺ T cells during the steady state (Mucida et al., 2007) and TH1 and TH17 cells during inflammation (DePaolo et al., 2011; Hall et al., 2011).Here, we investigated the impact of Crtam–Cadm1 interaction in the intestinal immune system. We find that Crtam is expressed upon activation on all CD8⁺ T cells of the intestinal mucosa and mLN, intraepithelial CD4⁺ T cells, and intraepithelial CD4⁺CD8⁺ T cells, whereas Cadm1 is expressed on gut CD103⁺ DCs. Crtam–Cadm1 interactions have a major impact on the maintenance of intraepithelial CD4⁺CD8⁺ T cells and a limited influence on the presence of mucosal CD4⁺ and CD8⁺ T cells. Crtam^−/− and Cadm1^−/− mice almost completely lacked CD4⁺CD8⁺ T cells in the intestinal epithelium under steady-state conditions and had fewer CD4⁺ and CD8⁺ T cells in the intestinal mucosa than WT mice. CD4⁺CD8⁺ T cells arise from CD4⁺ T cells that acquire a CD8 T cell lineage gene expression profile upon reaching the intestinal mucosa (Mucida et al., 2013; Reis et al., 2013). Therefore, we further investigated the role of Crtam–Cadm1 interactions in governing CD4⁺ T cell homing and maintenance in the intestinal mucosa, and found that Crtam^−/− CD4⁺ T cells reconstituted the intestinal CD4⁺ T cell and CD4⁺CD8⁺ T cell subsets less effectively than did WT CD4⁺ T cells after transfer into Rag1^−/− mice. Moreover, fewer intestinal CD4⁺ T cells in Crtam^−/− mice expressed gut homing, adhesion, and retention molecules typical of their counterparts in WT mice (including CCR9, CD103, and CD69), and hence adhered less firmly to the intestinal mucosa.Crtam deficiency did not affect the intrinsic capacity of naive CD4⁺ T cells to differentiate into IFN-γ– or IL-17–producing CD4⁺ T cells in vitro. After acute oral infection with Toxoplasma gondii, both WT and Crtam^−/− mice mounted a robust TH1 response and cleared the intestinal infection. However, a preferential reduction of TH17 cells was evident within the intestinal mucosa CD4⁺ T cells in the Crtam^−/− mice. The almost exclusive TH1 response in Crtam^−/− mice resulted in more efficient clearance of intestinal infection. Antibody blockade of IL-17 in WT mice orally infected with T. gondii recapitulated the enhanced host response of Crtam^−/− mice. Thus, the defects in T cell gut homing and maintenance in Crtam^−/− mice preferentially affected TH17, whereas TH1 were relatively unaffected. Because the differentiation of CD4⁺CD8⁺ T cells from CD4⁺ T cells skews CD4⁺ T cell cytokine production toward IFN-γ at the expenses of IL-17 production (Mucida et al., 2013; Reis et al., 2013), the need for continuous replacement of lost CD4⁺CD8⁺ T cells in Crtam^−/− mice may result in a relative depletion of TH17 cells. Together, these results demonstrate that Crtam–Cadm1 interactions have a major impact on the residency and maintenance of CD4⁺CD8⁺ T cells in the gut mucosa in the steady-state. During mucosal responses against pathogenic infection, Crtam–Cadm1 interactions may be required to retain a balanced representation of disparate CD4⁺ T cell subsets, thereby influencing the overall quality of the CD4⁺ T cell response.     

Infiltration of CD4+ lymphocytes into the brain contributes to neurodegeneration in a mouse model of Parkinson disease

Parkinson disease (PD) is a neurodegenerative disorder characterized by a loss of dopamine-containing neurons. Mounting evidence suggests that dopaminergic cell death is influenced by the innate immune system. However, the pathogenic role of the adaptive immune system in PD remains enigmatic. Here we showed that CD8⁺ and CD4⁺ T cells but not B cells had invaded the brain in both postmortem human PD specimens and in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD during the course of neuronal degeneration. We further demonstrated that MPTP-induced dopaminergic cell death was markedly attenuated in the absence of mature T lymphocytes in 2 different immunodeficient mouse strains (Rag1^–/– and Tcrb^–/– mice). Importantly, similar attenuation of MPTP-induced dopaminergic cell death was seen in mice lacking CD4 as well as in Rag1^–/– mice reconstituted with FasL-deficient splenocytes. However, mice lacking CD8 and Rag1^–/– mice reconstituted with IFN-γ–deficient splenocytes were not protected. These data indicate that T cell–mediated dopaminergic toxicity is almost exclusively arbitrated by CD4⁺ T cells and requires the expression of FasL but not IFNγ. Further, our data may provide a rationale for targeting the adaptive arm of the immune system as a therapeutic strategy in PD.