Graham Le Gros

Cutaneous immunosurveillance and regulation of inflammation by group 2 innate lymphoid cells

Type 2 immunity is critical for defense against cutaneous infections but also underlies the development of allergic skin diseases. We report the identification in normal mouse dermis of an abundant, phenotypically unique group 2 innate lymphoid cell (ILC2) subset that depended on interleukin 7 (IL-7) and constitutively produced IL-13. Intravital multiphoton microscopy showed that dermal ILC2 cells specifically interacted with mast cells, whose function was suppressed by IL-13. Treatment of mice deficient in recombination-activating gene 1 (Rag1−/−) with IL-2 resulted in the population expansion of activated, IL-5-producing dermal ILC2 cells, which led to spontaneous dermatitis characterized by eosinophil infiltrates and activated mast cells. Our data show that ILC2 cells have both pro- and anti-inflammatory properties and identify a previously unknown interactive pathway between two innate populations of cells of the immune system linked to type 2 immunity and allergic diseases.

Differential T Cell Function and Fate in Lymph Node and Nonlymphoid Tissues

The functions and fate of antigen-experienced T cells isolated from lymph node or nonlymphoid tissues were analyzed in a system involving adoptive transfer of in vitro–activated T cells into mice. Activated T cells present in the lymph nodes could be stimulated by antigen to divide, produce effector cytokines, and migrate to peripheral tissues. By contrast, activated T cells that had migrated into nonlymphoid tissues (lung and airway) produced substantial effector cytokines upon antigen challenge, but were completely unable to divide or migrate back to the lymph nodes. Therefore, activated T cells can undergo clonal expansion in the lymph node, but are recruited and retained as nondividing cells in nonlymphoid tissues. These distinct regulatory events in lymph node and nonlymphoid tissues reveal simple key mechanisms for both inducing and limiting T cell immunity.

The role of CTLA-4 in the regulation of T cell immune responses

Over the past few years a great deal of research has examined how T cell‐dependent immune responses are initiated and subsequently regulated. Ligation of the TCR with an antigenic peptide bound to an MHC protein on a professional APC provides the crucial antigen‐specific stimulus required for T cell activation. Interaction of CD28 with CD80 or CD86 molecules on APC initiates a costimulatory or second signal within the T cell which augments and sustains T cell activation initiated through the TCR. However, recently it has become clear that T cell immune responses are a result of a balance between stimulatory and inhibitory signals. Cytotoxic T lymphocyte‐associated molecule‐4 (CTLA‐4) is a cell surface molecule that is expressed nearly exclusively on CD4+ and CD8+ T cells. Investigation into the role of CTLA‐4 in the regulation of T cell immune responses has revealed that CTLA‐4 is a very important molecule involved in the maintenance of T cell homeostasis. In the present review, evidence for the proposed inhibitory role of CTLA‐4 is examined and a model suggesting a role for CTLA‐4 in both early and late stages of T cell activation is presented.

Immune Responses of IL-4, IL-5, IL-6 Deficient Mice

Cytokines are the principal soluble regulators of the immune system and the hematopoietic system (Paul & Seder 1994). In vivo, cytokines contribute to a complex network of pleiotropic and redundantly acting molecules, which makes them difficult to study. Within the past 4 years, gene targeting has been used to poke holes into this network. The list of mice rendered deficient for cytokines and their receptor is growing (Table I). In contrast to previous ideas, from these efforts we have learned that cytokines do not play unique roles in the development of the immune system and steady state hematopoiesis. Exceptions involve IL-7/IL-7R and TNF-p (LT-a), which dramatically affect early T and B cell precursors and the formation of lymph nodes, respectively (De-Togni et al. 1994, Peschon et al. 1994). However, impairment, or minimally a modulation, of immune and/or infiammatory responses has been shown to occur in each of these cytokine/receptor deficient mice, which clearly demonstrates that no cytokine is dispensable or completely redundant in vivo. This review summarises the effects of either an IL-4, IL-5 or IL-6 gene deletion on the immune response of mice to several distinct kinds of immune stimuli

c-Jun NH2-Terminal Kinase (JNK)1 and JNK2 Have Distinct Roles in CD8+ T Cell Activation

The c-Jun NH2-terminal kinase (JNK) signaling pathway is induced by cytokines and stress stimuli and is implicated in cell death and differentiation, but the specific function of this pathway depends on the cell type. Here we examined the role of JNK1 and JNK2 in CD8+ T cells. Unlike CD4+ T cells, the absence of JNK2 causes increased interleukin (IL)-2 production and proliferation of CD8+ T cells. In contrast, JNK1-deficient CD8+ T cells are unable to undergo antigen-stimulated expansion in vitro, even in the presence of exogenous IL-2. The hypoproliferation of these cells is associated with impaired IL-2 receptor α chain (CD25) gene and cell surface expression. The reduced level of nuclear activating protein 1 (AP-1) complexes in activated JNK1-deficient CD8+ T cells can account for the impaired IL-2 receptor α chain gene expression. Thus, JNK1 and JNK2 play different roles during CD8+ T cell activation and these roles differ from those in CD4+ T cells.

In vivo studies fail to reveal a role for IL-4 or STAT6 signaling in Th2 lymphocyte differentiation

The expression of interleukin-4 (IL-4) is viewed as the hallmark of a Th2 lymphocyte, whereas the subsequent action of IL-4 and IL-13, mediated through the STAT6 signaling pathway, is seen as a prerequisite for the full development of Th2 immune responses to parasites and allergens. G4 mice, whose IL-4 gene locus contains the fluorescent reporter eGFP, were used to quantify the number of Th2 cells that develop during Nippostrongylus brasiliensis- or allergen-induced immune responses under conditions where IL-4 or STAT6 was absent. Here, we show that deletion of IL-4 or STAT6 had little impact on the number or timing of appearance of IL-4-producing Th2 cells. These data indicate that in vivo differentiation of naïve CD4 T cells to Th2 status often occurs independently of IL-4 and STAT6 and that recently described pathways of Th2 cell differentiation may explain how allergens and parasites selectively induce Th2-mediated immunity.

Animal Model of Nippostrongylus brasiliensis and Heligmosomoides polygyrus

Animal models of Nippostrongylus brasiliensis and Heligmosomoides polygyrus infection are powerful tools for the investigation of the basic biology of immune responses and protective immunity. In particular, they model the induction and maintenance of Th2 type immune responses and exhibit all the requisite hallmarks of CD4 T cell-dependent IgE production, eosinophilia, mastocytosis, and mucus production. This chapter describes simple, cost-effective techniques for using and maintaining these easy-to-work-with parasites in the context of a modern laboratory.

T cell-derived IL-3 plays key role in parasite infection-induced basophil production but is dispensable for in vivo basophil survival

Enhanced basophil production is often associated with T(h)2-related conditions such as parasite infections or allergic inflammations. Our previous study demonstrated that T cell activation is necessary to promote basophil production in Nippostrongylus brasiliensis (Nb)-infected mice. Yet, mechanisms underlying how T cells aid infection-induced basophil production are not clear. In this report, we show that IL-3 produced by T cells activated by the infection enhances basophil production in Nb-infected mice. IL-3-deficient mice or Rag2-/- recipients of IL-3-deficient T cells but not of wild-type T cells failed to support basophil production following the Nb infection. Interestingly, although IL-3 was critical for preventing basophil apoptosis in vitro, IL-3 had little contribution to basophil survival and proliferation in vivo. Collectively, these results highlight a novel mechanism by which activation of adaptive immune components induces basophil production but not basophil survival via IL-3 production.

Basophils Are the Major Producers of IL-4 during Primary Helminth Infection

IL-4 production by leukocytes is a key regulatory event that occurs early in the type 2 immune response, which induces allergic reactions and mediates expulsion of parasites. CD4+ T cells and basophils are thought to be the key cell types that produce IL-4 during a type 2 response. In this study, we assessed the relative contribution of both CD4+ T cell- and basophil–IL-4 production during primary and secondary responses to Nippostrongylus brasiliensis using a murine IL-4–enhanced GFP reporter system. During infection, IL-4–producing basophils were detected systemically, and tissue recruitment occurred independent of IL-4/STAT6 signaling. We observed that basophil recruitment to a tissue environment was required for their full activation. Basophil induction in response to secondary infection exhibited accelerated kinetics in comparison with primary infection. However, total basophil numbers were not enhanced, as predicted by previous models of protective immunity. Overall, the induction and migration of IL-4–producing basophils into peripheral tissues was found to be a prominent characteristic of the primary but not memory responses to N. brasiliensis infection, in which CD4+ T cells were identified as the major source of IL-4. Whereas basophils were the major initial producers of IL-4, we determined that normal Th2 differentiation occurs independently of basophils, and depletion of basophils led to an enhancement of inflammatory cell recruitment to the site of infection.

CTLA4 ligation attenuates AP-1, NFAT and NF-κB activity in activated T cells

CTLA4 is currently viewed as a late‐appearing T cell surface receptor which is able to inhibit the proliferation of activated T cells. We sought to identify how CTLA4 ligation exerts these anti‐proliferative effects by studying its influence on the activities of the relevant nuclear transcription factors AP‐1, NFAT and NF‐κB. We found that cross‐linking CTLA4 on activated T cells completely blocks AP‐1 and NFAT transcription factor activity before any effects on T cell proliferation can be observed, with NF‐κB activity affected to a lesser degree. The suppression of AP‐1 and NFAT transcriptional activity correlates with reduced levels of AP‐1 and NFAT DNA binding as early as 10 h after T cell activation, prior to detectable up‐regulation of CTLA4 on the T cell surface. Additionally, inhibitory effects on T cell proliferation only occurred when CTLA4 molecules were ligated in proximity to signaling TCR complexes, and inhibition of transcription factor DNA binding and activity was observed in the absence of CD28 stimulation. CTLA4 can thus act early during T cell activation to reduce the activity of several key nuclear transcription factors important for continued T cell proliferation and differentiation.