An Essential Role for Interleukin 10 in the Function of Regulatory T Cells That Inhibit Intestinal Inflammation

A T helper cell type 1–mediated colitis develops in severe combined immunodeficient mice after transfer of CD45RB^high CD4⁺ T cells and can be prevented by cotransfer of the CD45RB^low subset. The immune-suppressive activities of the CD45RB^low T cell population can be reversed in vivo by administration of an anti-transforming growth factor β antibody. Here we show that interleukin (IL)-10 is an essential mediator of the regulatory functions of the CD45RB^low population. This population isolated from IL-10–deficient (IL-10^−/−) mice was unable to protect from colitis and when transferred alone to immune-deficient recipients induced colitis. Treatment with an anti–murine IL-10 receptor monoclonal antibody abrogated inhibition of colitis mediated by wild-type (WT) CD45RB^low CD4⁺ cells, suggesting that IL-10 was necessary for the effector function of the regulatory T cell population. Inhibition of colitis by WT regulatory T cells was not dependent on IL-10 production by progeny of the CD45RB^high CD4⁺ cells, as CD45RB^low CD4⁺ cells from WT mice were able to inhibit colitis induced by IL-10^−/− CD45RB^high CD4⁺ cells. These findings provide the first clear evidence that IL-10 plays a nonredundant role in the functioning of regulatory T cells that control inflammatory responses towards intestinal antigens.     

CD4+ T Cells Prevent Spontaneous Experimental Autoimmune Encephalomyelitis in Anti–Myelin Basic Protein T Cell Receptor Transgenic Mice

Autoimmune diseases result from a failure of tolerance. Although many self-reactive T cells are present in animals and humans, their activation appears to be prevented normally by regulatory T cells. In this study, we show that regulatory CD4⁺ T cells do protect mice against the spontaneous occurrence of experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis. Anti–myelin basic protein (MBP) TCR transgenic mice (T/R⁺) do not spontaneously develop EAE although many self-reactive T cells are present in their thymi and peripheral lymphoid organs. However, the disease develops in all crosses of T/R⁺ mice with recombination-activating gene (RAG)-1 knockout mice in which transgenic TCR-expressing cells are the only lymphocytes present (T/R⁻ mice). In this study, crosses of T/R⁺ mice with mice deficient for B cells, CD8⁺ T cells, NK1.1 CD4⁺ T (NKT) cells, γ/δ T cells, or α/β T cells indicated that α/β CD4⁺ T cells were the only cell population capable of controlling the self-reactive T cells. To confirm the protective role of CD4⁺ T cells, we performed adoptive transfer experiments. CD4⁺ T cells purified from thymi or lymph nodes of normal mice prevented the occurrence of spontaneous EAE in T/R⁻ mice. To achieve full protection, the cells had to be transferred before the recipient mice manifested any symptoms of the disease. Transfer of CD4⁺ T cells after the appearance of symptoms of EAE had no protective effect. These results indicate that at least some CD4⁺ T cells have a regulatory function that prevent the activation of self-reactive T cells.     

Positive Selection of Extrathymically Developed T Cells by Self-antigens

Most T cells develop through the thymus, where they undergo positive and negative selection. Some peripheral T cells are known to develop in the absence of thymus, but there is insufficient information about their selection. To analyze the selection of extrathymically developed T cells, we reconstituted thymectomized male or female recipient mice with bone marrow cells of mice transgenic for male H-Y antigen–specific T cell receptor (TCR). It was revealed that the T cells bearing self-antigen–specific TCR were not deleted in thymectomized male recipients. More importantly, the absence of H-Y antigen–specific T cells in thymectomized female recipients suggests positive selection of extrathymically developed T cells by the self-antigen. The extrathymically developed T cells in male mice expressed interleukin (IL)-2 receptor β chain (IL-2Rβ) and intermediate levels of CD3 (CD3^int) but were natural killer cell (NK)1.1⁻. They rapidly produced interferon γ but not IL-4 after TCR cross-linking. Furthermore, a similar pattern of cytokine production was observed in CD3^intIL-2Rβ⁺NK1.1⁻ cells in normal mice which have been shown to develop extrathymically. These results suggest that extrathymically developed CD3^intIL-2Rβ⁺NK1.1⁻ cells in normal mice are also positively selected by self-antigens.     

Human CD4+ T cell recent thymic emigrants are identified by protein tyrosine kinase 7 and have reduced immune function

CD4⁺ recent thymic emigrants (RTEs) comprise a clinically and immunologically important T cell population that indicates thymic output and that is essential for maintaining a diverse αβ–T cell receptor (TCR) repertoire of the naive CD4⁺ T cell compartment. However, their frequency and function are poorly understood because no known surface markers distinguish them from older non-RTE naive CD4⁺ T cells. We demonstrate that protein tyrosine kinase 7 (PTK7) is a novel marker for human CD4⁺ RTEs. Consistent with their recent thymic origin, human PTK7⁺ RTEs contained higher levels of signal joint TCR gene excision circles and were more responsive to interleukin (IL)-7 compared with PTK7⁻ naive CD4⁺ T cells, and rapidly decreased after complete thymectomy. Importantly, CD4⁺ RTEs proliferated less and produced less IL-2 and interferon-γ than PTK7⁻ naive CD4⁺ T cells after αβ-TCR/CD3 and CD28 engagement. This immaturity in CD4⁺ RTE effector function may contribute to the reduced CD4⁺ T cell immunity observed in contexts in which CD4⁺ RTEs predominate, such as in the fetus and neonate or after immune reconstitution. The ability to identify viable CD4⁺ RTEs by PTK7 staining should be useful for monitoring thymic output in both healthy individuals and in patients with genetic or acquired CD4⁺ T cell immunodeficiencies.     

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.     

Liver Damage Preferentially Results from CD8+ T Cells Triggered by High Affinity Peptide Antigens

Little is understood of the anatomical fate of activated T lymphocytes and the consequences they have on the tissues into which they migrate. Previous work has suggested that damaged lymphocytes migrate to the liver. This study compares class I versus class II major histocompatibility complex (MHC)–restricted ovalbumin-specific T cell antigen receptor (TCR) transgenic mice to demonstrate that after in vivo activation with antigen the emergence of CD4⁻CD8⁻B220⁺ T cells occurs more frequently from a CD8⁺ precursor than from CD4⁺ T cells. Furthermore, this change in phenotype is conferred only by the high affinity native peptide antigen and not by lower affinity peptide variants. After activation of CD8⁺ cells with only the high affinity peptide, there is also a dramatically increased number of liver lymphocytes with accompanying extensive hepatocyte damage and elevation of serum aspartate transaminase. This was not observed in mice bearing a class II MHC–restricted TCR. The findings show that CD4⁻CD8⁻B220⁺ T cells preferentially derive from a CD8⁺ precursor after a high intensity TCR signal. After activation, T cells can migrate to the liver and induce hepatocyte damage, and thereby serve as a model of autoimmune hepatitis.     

Regulatory CD4+ T Cells Expressing Endogenous T Cell Receptor Chains Protect Myelin Basic Protein–specific Transgenic Mice from Spontaneous Autoimmune Encephalomyelitis

The development of T cell–mediated autoimmune diseases hinges on the balance between effector and regulatory mechanisms. Using two transgenic mouse lines expressing identical myelin basic protein (MBP)–specific T cell receptor (TCR) genes, we have previously shown that mice bearing exclusively MBP-specific T cells (designated T/R⁻) spontaneously develop experimental autoimmune encephalomyelitis (EAE), whereas mice bearing MBP-specific T cells as well as other lymphocytes (designated T/R⁺) did not. Here we demonstrate that T/R⁻ mice can be protected from EAE by the early transfer of total splenocytes or purified CD4⁺ T cells from normal donors. Moreover, whereas T/R⁺ mice crossed with B cell–deficient, γ/δ T cell–deficient, or major histocompatibility complex class I–deficient mice did not develop EAE spontaneously, T/R⁺ mice crossed with TCR-α and -β knockout mice developed EAE with the same incidence and severity as T/R⁻ mice. In addition, MBP-specific transgenic mice that lack only endogenous TCR-α chains developed EAE with high incidence but reduced severity. Surprisingly, two-thirds of MBP-specific transgenic mice lacking only endogenous TCR-β chains also developed EAE, suggesting that in T/R⁺ mice, cells with high protective activity escape TCR-β chain allelic exclusion. Our study identifies CD4⁺ T cells bearing endogenous α and β TCR chains as the lymphocytes that prevent spontaneous EAE in T/R⁺ mice.     

Interleukin 2–mediated Uncoupling of T Cell Receptor α/β from CD3 Signaling

T cell activation and clonal expansion is the result of the coordinated functions of the receptors for antigen and interleukin (IL)-2. The protein tyrosine kinase p56^lck is critical for the generation of signals emanating from the T cell antigen receptor (TCR) and has also been demonstrated to play a role in IL-2 receptor signaling. We demonstrate that an IL-2–dependent, antigen-specific CD4⁺ T cell clone is not responsive to anti-TCR induced growth when propagated in IL-2, but remains responsive to both antigen and CD3ε-specific monoclonal antibody. Survival of this IL-2–dependent clone in the absence of IL-2 was supported by overexpression of exogenous Bcl-xL. Culture of this clonal variant in the absence of IL-2 rendered it susceptible to anti-TCR–induced signaling, and correlated with the presence of kinase-active Lck associated with the plasma membrane. The same phenotype is observed in primary, resting CD4⁺ T cells. Furthermore, the presence of kinase active Lck associated with the plasma membrane correlates with the presence of ZAP 70–pp21ζ complexes in both primary T cells and T cell clones in circumstances of responsive anti-TCR signaling. The results presented demonstrate that IL-2 signal transduction results in the functional uncoupling of the TCR complex through altering the subcellular distribution of kinase-active Lck.     

Essential and Partially Overlapping Role of CD3γ and CD3δ for Development of αβ and γδ T Lymphocytes

CD3γ and CD3δ are two highly related components of the T cell receptor (TCR)–CD3 complex which is essential for the assembly and signal transduction of the T cell receptor on mature T cells. In gene knockout mice deficient in either CD3δ or CD3γ, early thymic development mediated by pre-TCR was either undisturbed or severely blocked, respectively, and small numbers of TCR-αβ⁺ T cells were detected in the periphery of both mice. γδ T cell development was either normal in CD3δ^−/− mice or partially blocked in CD3γ^−/− mice. To examine the collective role of CD3γ and CD3δ in the assembly and function of pre-TCR and in the development of γδ T cells, we generated a mouse strain with a disruption in both CD3γ and CD3δ genes (CD3γδ^−/−). In contrast to mice deficient in either CD3γ or CD3δ chains, early thymic development mediated by pre-TCR is completely blocked, and TCR-αβ⁺ or TCR-γδ⁺ T cells were absent in the CD3γδ^−/− mice. Taken together, these studies demonstrated that CD3γ and CD3δ play an essential, yet partially overlapping, role in the development of both αβ and γδ T cell lineages.     

CD4+ regulatory T cells require CTLA-4 for the maintenance of systemic tolerance

Cytotoxic T lymphocyte antigen-4 (CTLA-4) plays a critical role in negatively regulating T cell responses and has also been implicated in the development and function of natural FOXP3⁺ regulatory T cells. CTLA-4–deficient mice develop fatal, early onset lymphoproliferative disease. However, chimeric mice containing both CTLA-4–deficient and –sufficient bone marrow (BM)–derived cells do not develop disease, indicating that CTLA-4 can act in trans to maintain T cell self-tolerance. Using genetically mixed blastocyst and BM chimaeras as well as in vivo T cell transfer systems, we demonstrate that in vivo regulation of Ctla4^−/− T cells in trans by CTLA-4–sufficient T cells is a reversible process that requires the persistent presence of FOXP3⁺ regulatory T cells with a diverse TCR repertoire. Based on gene expression studies, the regulatory T cells do not appear to act directly on T cells, suggesting they may instead modulate the stimulatory activities of antigen-presenting cells. These results demonstrate that CTLA-4 is absolutely required for FOXP3⁺ regulatory T cell function in vivo.     

Uncovering a novel role of PLCβ4 in selectively mediating TCR signaling in CD8+ but not CD4+ T cells

Because of their common signaling molecules, the main T cell receptor (TCR) signaling cascades in CD4⁺ and CD8⁺ T cells are considered qualitatively identical. Herein, we show that TCR signaling in CD8⁺ T cells is qualitatively different from that in CD4⁺ T cells, since CD8α ignites another cardinal signaling cascade involving phospholipase C β4 (PLCβ4). TCR-mediated responses were severely impaired in PLCβ4-deficient CD8⁺ T cells, whereas those in CD4⁺ T cells were intact. PLCβ4-deficient CD8⁺ T cells showed perturbed activation of peripheral TCR signaling pathways downstream of IP₃ generation. Binding of PLCβ4 to the cytoplasmic tail of CD8α was important for CD8⁺ T cell activation. Furthermore, GNAQ interacted with PLCβ4, mediated double phosphorylation on threonine 886 and serine 890 positions of PLCβ4, and activated CD8⁺ T cells in a PLCβ4-dependent fashion. PLCβ4-deficient mice exhibited defective antiparasitic host defense and antitumor immune responses. Altogether, PLCβ4 differentiates TCR signaling in CD4⁺ and CD8⁺ T cells and selectively promotes CD8⁺ T cell–dependent adaptive immunity.     

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.     

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.     

Subsets of Transgenic T Cells That Recognize CD1 Induce or Prevent Murine Lupus: Role of Cytokines

T cells with T cell receptor (TCR) transgenes that recognized CD1 on syngeneic B cells stimulated B cells to secrete immunoglobulins in vitro. The CD4⁺, CD8⁺, or CD4⁻CD8⁻ T cells from the spleen of the TCR transgenic BALB/c donors induced lupus with anti–double stranded DNA antibodies, proteinuria, and immune complex glomerulonephritis in irradiated BALB/c nude mice reconstituted with nude bone marrow. Injection of purified CD4⁻CD8⁻ T cells from the marrow of transgenic donors prevented the induction of lupus by the transgenic T cells. Transgenic T cells that induced lupus secreted large amounts of interferon (IFN)-γ and little interleukin (IL)-4, and those that prevented lupus secreted large amounts of IL-4 and little IFN-γ or IL-10.     

MHCII-independent CD4+ T cells protect injured CNS neurons via IL-4

A body of experimental evidence suggests that T cells mediate neuroprotection following CNS injury; however, the antigen specificity of these T cells and how they mediate neuroprotection are unknown. Here, we have provided evidence that T cell–mediated neuroprotection after CNS injury can occur independently of major histocompatibility class II (MHCII) signaling to T cell receptors (TCRs). Using two murine models of CNS injury, we determined that damage-associated molecular mediators that originate from injured CNS tissue induce a population of neuroprotective, IL-4–producing T cells in an antigen-independent fashion. Compared with wild-type mice, IL-4–deficient animals had decreased functional recovery following CNS injury; however, transfer of CD4⁺ T cells from wild-type mice, but not from IL-4–deficient mice, enhanced neuronal survival. Using a culture-based system, we determined that T cell–derived IL-4 protects and induces recovery of injured neurons by activation of neuronal IL-4 receptors, which potentiated neurotrophin signaling via the AKT and MAPK pathways. Together, these findings demonstrate that damage-associated molecules from the injured CNS induce a neuroprotective T cell response that is independent of MHCII/TCR interactions and is MyD88 dependent. Moreover, our results indicate that IL-4 mediates neuroprotection and recovery of the injured CNS and suggest that strategies to enhance IL-4–producing CD4⁺ T cells have potential to attenuate axonal damage in the course of CNS injury in trauma, inflammation, or neurodegeneration.     

Genetic Regulation of Commitment to Interleukin 4 Production by a CD4+ T Cell–intrinsic Mechanism

The dysregulated expression of interleukin 4 (IL-4) can have deleterious effects on the outcome of infectious and allergic diseases. Despite this, the mechanisms by which naive T cells commit to IL-4 expression during differentiation into mature effector cells remain incompletely defined. As compared to cells from most strains of mice, activated CD4⁺ T cells from BALB mice show a bias towards IL-4 production and T helper 2 commitment in vitro and in vivo. Here, we show that this bias arises not from an increase in the amount of IL-4 produced per cell, but rather from an increase in the proportion of CD4⁺ T cells that commit to IL-4 expression. This strain-specific difference in commitment was independent of signals mediated via the IL-4 receptor and hence occurred upstream of potential autoregulatory effects of IL-4. Segregation analysis of the phenotype in an experimental backcross cohort implicated a polymorphic locus on chromosome 16. Consistent with a role in differentiation, expression of the phenotype was CD4⁺ T cell intrinsic and was evident as early as 16 h after the activation of naive T cells. Probabilistic gene activation is proposed as a T cell–intrinsic mechanism capable of modulating the proportion of naive T cells that commit to IL-4 production.     

CD5 Expression Is Developmentally Regulated By T Cell Receptor (TCR) Signals and TCR Avidity

Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4⁻CD8⁻ (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56^lck. At the CD4⁺CD8⁺ (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR–major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4⁺ or CD8⁺ (single positive, SP) thymocytes is induced by engagement of the α/β-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR–MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.