Max Löhning

Stat6-Independent GATA-3 Autoactivation Directs IL-4-Independent Th2 Development and Commitment

The initial source of IL-4-inducing Th2 development and the mechanism of stable Th2 commitment remain obscure. We found the reduced level of IL-4 production in Stat6-deficient T cells to be significantly higher than in Th1 controls. Using a novel cell surface affinity matrix technique, we found that IL-4-secreting Stat6-deficient T cells stably expressed GATA-3 and Th2 phenotype. Introducing GATA-3 into Stat6-deficient T cells completely restored Th2 development, inducing c-Maf, Th2-specific DNase I hypersensitive sites in the IL-4 locus, and Th2 cytokine expression. The fact that GATA-3 fully reconstitutes Th2 development in Stat6-deficient T cells indicates it is a master switch in Th2 development. Finally, GATA-3 exerts Stat6-independent autoactivation, creating a feedback pathway stabilizing Th2 commitment.

The alarmin IL-33 promotes regulatory T-cell function in the intestine

FOXP3+ regulatory T cells (Treg cells) are abundant in the intestine, where they prevent dysregulated inflammatory responses to self and environmental stimuli. It is now appreciated that Treg cells acquire tissue-specific adaptations that facilitate their survival and function; however, key host factors controlling the Treg response in the intestine are poorly understood. The interleukin (IL)-1 family member IL-33 is constitutively expressed in epithelial cells at barrier sites, where it functions as an endogenous danger signal, or alarmin, in response to tissue damage. Recent studies in humans have described high levels of IL-33 in inflamed lesions of inflammatory bowel disease patients, suggesting a role for this cytokine in disease pathogenesis. In the intestine, both protective and pathological roles for IL-33 have been described in murine models of acute colitis, but its contribution to chronic inflammation remains ill defined. Here we show in mice that the IL-33 receptor ST2 is preferentially expressed on colonic Treg cells, where it promotes Treg function and adaptation to the inflammatory environment. IL-33 signalling in T cells stimulates Treg responses in several ways. First, it enhances transforming growth factor (TGF)-β1-mediated differentiation of Treg cells and, second, it provides a necessary signal for Treg-cell accumulation and maintenance in inflamed tissues. Strikingly, IL-23, a key pro-inflammatory cytokine in the pathogenesis of inflammatory bowel disease, restrained Treg responses through inhibition of IL-33 responsiveness. These results demonstrate a hitherto unrecognized link between an endogenous mediator of tissue damage and a major anti-inflammatory pathway, and suggest that the balance between IL-33 and IL-23 may be a key controller of intestinal immune responses.

Eosinophils are required for the maintenance of plasma cells in the bone marrow

Plasma cells are of crucial importance for long-term immune protection. It is thought that long-lived plasma cells survive in specialized niches in the bone marrow. Here we demonstrate that bone marrow eosinophils localized together with plasma cells and were the key providers of plasma cell survival factors. In vitro, eosinophils supported the survival of plasma cells by secreting the proliferation-inducing ligand APRIL and interleukin-6 (IL-6). In eosinophil-deficient mice, plasma cell numbers were much lower in the bone marrow both at steady state and after immunization. Reconstitution experiments showed that eosinophils were crucial for the retention of plasma cells in the bone marrow. Moreover, depletion of eosinophils induced apoptosis in long-lived bone marrow plasma cells. Our findings demonstrate that the long-term maintenance of plasma cells in the bone marrow requires eosinophils.

Professional Memory CD4+ T Lymphocytes Preferentially Reside and Rest in the Bone Marrow

CD4(+) T lymphocytes are key to immunological memory. Here we show that in the memory phase of specific immune responses, most of the memory CD4(+) T lymphocytes had relocated into the bone marrow (BM) within 3-8 weeks after their generation-a process involving integrin alpha2. Antigen-specific memory CD4(+) T lymphocytes highly expressed Ly-6C, unlike most splenic CD44(hi)CD62L(-) CD4(+) T lymphocytes. In adult mice, more than 80% of Ly-6C(hi)CD44(hi)CD62L(-) memory CD4(+) T lymphocytes were in the BM. In the BM, they associated to IL-7-expressing VCAM-1(+) stroma cells. Gene expression and proliferation were downregulated, indicating a resting state. Upon challenge with antigen, they rapidly expressed cytokines and CD154 and efficiently induced the production of high-affinity antibodies by B lymphocytes. Thus, in the memory phase of immunity, memory helper T cells are maintained in BM as resting but highly reactive cells in survival niches defined by IL-7-expressing stroma cells.

Expression of ICOS In Vivo Defines CD4+ Effector T Cells with High Inflammatory Potential and a Strong Bias for Secretion of Interleukin 10

The studies performed to date analyzed the overall participation of the inducible costimulator (ICOS) in model diseases, but did not yield information on the nature and function of ICOS-expressing T cells in vivo. We examined ICOS+ T cells in the secondary lymphoid organs of nonmanipulated mice, in the context of an “unbiased” immune system shaped by environmental antigens. Using single cell analysis, ICOSlow cells were found to be loosely associated with the early cytokines interleukin (IL)-2, IL-3, IL-6, and interferon (IFN)-γ. ICOSmedium cells, the large majority of ICOS+ T cells in vivo, were very tightly associated with the synthesis of the T helper type 2 (Th2) cytokines IL-4, IL-5, and IL-13, and these cells exhibited potent inflammatory effects in vivo. In contrast, ICOShigh T cells were highly and selectively linked to the anti-inflammatory cytokine IL-10. Overall, these data seem to indicate that ICOS cell surface density serves as a regulatory mechanism for the release of cytokines with different immunological properties. Further in vivo functional experiments with in vitro–activated T cells strongly suggested that the ICOS+ population, although representing in vivo only around 10% of T cells bearing early or late activation markers, nevertheless encompasses virtually all effector T cells, a finding with major diagnostic and therapeutic implications.

The microRNA miR-182 is induced by IL-2 and promotes clonal expansion of activated helper T lymphocytes

After being activated by antigen, helper T lymphocytes switch from a resting state to clonal expansion. This switch requires inactivation of the transcription factor Foxo1, a suppressor of proliferation expressed in resting helper T lymphocytes. In the early antigen-dependent phase of expansion, Foxo1 is inactivated by antigen receptor–mediated post-translational modifications. Here we show that in the late phase of expansion, Foxo1 was no longer post-translationally regulated but was inhibited post-transcriptionally by the interleukin 2 (IL-2)-induced microRNA miR-182. Specific inhibition of miR-182 in helper T lymphocytes limited their population expansion in vitro and in vivo. Our results demonstrate a central role for miR-182 in the physiological regulation of IL-2-driven helper T cell–mediated immune responses and open new therapeutic possibilities.

The Alarmin Interleukin-33 Drives Protective Antiviral CD8+ T Cell Responses

Sound the Alarm When small protein fragments or nucleic acids derived from an invading pathogen are detected by pattern recognition receptors on immune cells, the innate immune response is triggered. This event activates cells of the adaptive immune system, and together, both responses clear the infection. Infections also induce the release of “danger-associated molecular patterns,” or alarmins, from the host as a result of tissue damage. Whether these are also important for the ensuing immune response is less clear. Bonilla et al. (p. 984, published online 2 February) report that the alarmin, interleukin-33, is required for optimal cytotoxic CD8+ T cells responses and antiviral immunity in mice. In virus-infected mice deficient in IL-33 or its receptor, IL-33 is essential for signaling CD8+ T cells to expand, produce multiple cytokines and acquire cytotoxic capabilities. These results showed that endogenous material, independently of pathogen-derived molecules, are also required for antiviral immunity. A danger signal released from dying cells is required for antiviral immunity in mice. Pathogen-associated molecular patterns decisively influence antiviral immune responses, whereas the contribution of endogenous signals of tissue damage, also known as damage-associated molecular patterns or alarmins, remains ill defined. We show that interleukin-33 (IL-33), an alarmin released from necrotic cells, is necessary for potent CD8+ T cell (CTL) responses to replicating, prototypic RNA and DNA viruses in mice. IL-33 signaled through its receptor on activated CTLs, enhanced clonal expansion in a CTL-intrinsic fashion, determined plurifunctional effector cell differentiation, and was necessary for virus control. Moreover, recombinant IL-33 augmented vaccine-induced CTL responses. Radio-resistant cells of the splenic T cell zone produced IL-33, and efficient CTL responses required IL-33 from radio-resistant cells but not from hematopoietic cells. Thus, alarmin release by radio-resistant cells orchestrates protective antiviral CTL responses.

Aggravation of viral hepatitis by platelet-derived serotonin.

More than 500 million people worldwide are persistently infected with hepatitis B virus or hepatitis C virus. Although both viruses are poorly cytopathic, persistence of either virus carries a risk of chronic liver inflammation, potentially resulting in liver steatosis, liver cirrhosis, end-stage liver failure or hepatocellular carcinoma. Virus-specific T cells are a major determinant of the outcome of hepatitis, as they contribute to the early control of chronic hepatitis viruses, but they also mediate immunopathology during persistent virus infection. We have analyzed the role of platelet-derived vasoactive serotonin during virus-induced CD8+ T cell–dependent immunopathological hepatitis in mice infected with the noncytopathic lymphocytic choriomeningitis virus. After virus infection, platelets were recruited to the liver, and their activation correlated with severely reduced sinusoidal microcirculation, delayed virus elimination and increased immunopathological liver cell damage. Lack of platelet-derived serotonin in serotonin-deficient mice normalized hepatic microcirculatory dysfunction, accelerated virus clearance in the liver and reduced CD8+ T cell–dependent liver cell damage. In keeping with these observations, serotonin treatment of infected mice delayed entry of activated CD8+ T cells into the liver, delayed virus control and aggravated immunopathological hepatitis. Thus, vasoactive serotonin supports virus persistence in the liver and aggravates virus-induced immunopathology.

Inverse correlation between IL-7 receptor expression and CD8 T cell exhaustion during persistent antigen stimulation.

Persistence is a hallmark of infection by viruses such as HIV, hepatitis B virus, hepatitis C virus and LCMV. In the case of LCMV, persistence may often be associated with exhaustion of CD8+ T cells. We demonstrate here that persistent antigen suppressed IL‐7Rα expression and this correlated with T cell exhaustion and reduced expression of the anti‐apoptotic molecule B cell leukemia/lymphoma 2 (Bcl‐2). In contrast, exposure to short‐lived antigen only temporarily suppressed IL‐7Rα expression, failed to induce T cell exhaustion, and primed T cells. Persistent antigen also suppressed IL‐7Rα expression on primed T cells and this correlated with exhaustion of a previously stable primed T cell population. These findings suggest that antigen longevity regulates T cell fate.

Regulation of Expression of IL-4 Alleles: Analysis Using a Chimeric GFP/IL-4 Gene

CD4 cells from mice heterozygous for an IL-4 and a GFP/IL-4 gene frequently express a single allele. Analysis of IL-4 or GFP production by cells from recently primed Th2 cells indicates that essentially all are competent to transcribe either allele but have a low probability of doing so. By contrast, long-term Th2 clones show distinct and heritable ratios in the proportion of cells that express IL-4 or GFP. We conclude that in the course of Th2 priming an early efficient event renders both alleles capable of being inefficiently transcribed; a second, less frequent event occurs that renders one allele more competent, accounting for the differential expression of IL-4 and GFP in different clones.