During the development of immune responses to pathogens, self-antigens, or environmental allergens, naive CD4(+) T cells differentiate into subsets of effector cells including Th1, Th2, and Th17 cells. The differentiation into these subsets is controlled by specific transcription factors. The activity of these effector cells is limited by nTregs and iTregs, whose differentiation and maintenance are dependent on the transcription factor Foxp3. The regulation of autoimmune diseases mediated by Th1 and Th17 cells by Tregs has been studied and reviewed extensively. However, much less has been presented about the interplay between Tregs and Th2 cells and their contribution to allergic disease. In this perspective, we discuss the regulation of Th2 cells by Tregs and vice versa, focusing on the interplay between the IL-4-activated STAT6/GATA3 pathway and Foxp3.

Th2 cells induce asthma through the secretion of cytokines. Two such cytokines, IL-4 and IL-13, are critical mediators of many features of this disease. They both share a common receptor subunit, IL-4Rα, and signal through the STAT6 pathway. STAT6−/− mice have impaired Th2 differentiation and reduced airway response to allergen. Transferred Th2 cells were not able to elicit eosinophilia in response to OVA in STAT6−/− mice. To clarify the role of STAT6 in allergic airway inflammation, we generated mouse bone marrow (BM) chimeras. We observed little to no eosinophilia in OVA-treated STAT6−/− mice even when STAT6+/+ BM or Th2 cells were provided. However, when Th2 cells were transferred to STAT6×Rag2−/− mice, we observed an eosinophilic response to OVA. Nevertheless, the expression of STAT6 on either BM-derived cells or lung resident cells enhanced the severity of OVA-induced eosinophilia. Moreover, when both the BM donor and recipient lacked lymphocytes, transferred Th2 cells were sufficient to induce the level of eosinophilia comparable with that of wild-type (WT) mice. The expression of STAT6 in BM-derived cells was more critical for the enhanced eosinophilic response. Furthermore, we found a significantly higher number of CD4+CD25+ Foxp3+ T cells (regulatory T cells [Tregs]) in PBS- and OVA-treated STAT6−/− mouse lungs compared with that in WT animals suggesting that STAT6 limits both naturally occurring and Ag-induced Tregs. Tregs obtained from either WT or STAT6−/− mice were equally efficient in suppressing CD4+ T cell proliferation in vitro. Taken together, our studies demonstrate multiple STAT6-dependent and -independent features of allergic inflammation, which may impact treatments targeting STAT6.

BackgroundThe IL-4 receptor α (IL-4Rα) chain has a broad expression pattern and participates in IL-4 and IL-13 signaling, allowing it to influence several pathological components of allergic lung inflammation. We previously reported that IL-4Rα expression on both bone marrow-derived and non-bone marrow-derived cells contributed to the severity of allergic lung inflammation. There was a correlation between the number of macrophages expressing the IL-4Rα, CD11b, and IAd, and the degree of eosinophilia in ovalbumin challenged mice. The engagement of the IL-4Rα by IL-4 or IL-13 is able to stimulate the alternative activation of macrophages (AAM). The presence of AAM has been correlated with inflammatory responses to parasites and allergens. Therefore, we hypothesized that IL-4Rα+ AAM play an active role in allergic lung inflammation. To directly determine the role of AAM in allergic lung inflammation, M-CSF-dependent macrophages (BMM) were prepared from the bone-marrow of IL-4Rα positive and negative mice and transferred to IL-4RαxRAG2-/- mice. Wild type TH2 cells were provided exogenously.ResultsMice receiving IL-4Rα+/+ BMM showed a marked increase in the recruitment of eosinophils to the lung after challenge with ovalbumin as compared to mice receiving IL-4Rα-/- BMM. As expected, the eosinophilic inflammation was dependent on the presence of TH2 cells. Furthermore, we observed an increase in cells expressing F4/80 and Mac3, and the AAM marker YM1/2 in the lungs of mice receiving IL-4Rα+/+ BMM. The BAL fluid from these mice contained elevated levels of eotaxin-1, RANTES, and CCL2.ConclusionsThese results demonstrate that transfer of IL-4Rα + macrophages is sufficient to enhance TH2-driven, allergic inflammation. They further show that stimulation of macrophages through IL-4Rα leads to their alternative activation and positive contribution to the TH2-driven allergic inflammatory response in the lung. Since an increase in AAM and their products has been observed in patients with asthma exacerbations, these results suggest that AAM may be targeted to alleviate exacerbations.

BackgroundCD4+ T helper type 2 (TH2) cells, their cytokines IL-4, IL-5 and IL-13 and the transcription factor STAT6 are known to regulate various features of asthma including lung inflammation, mucus production and airway hyperreactivity and also drive alternative activation of macrophages (AAM). However, the precise roles played by the IL-4/IL-13 receptors and STAT6 in inducing AAM protein expression and modulating specific features of airway inflammation are still unclear. Since TH2 differentiation and activation plays a pivotal role in this disease, we explored the possibility of developing an asthma model in mice using T cells that were differentiated in vivo.ResultsIn this study, we monitored the activation and proliferation status of adoptively transferred allergen-specific naïve or in vivo primed CD4+ T cells. We found that both the naïve and in vivo primed T cells expressed similar levels of CD44 and IL-4. However, in vivo primed T cells underwent reduced proliferation in a lymphopenic environment when compared to naïve T cells. We then used these in vivo generated effector T cells in an asthma model. Although there was reduced inflammation in mice lacking IL-4Rα or STAT6, significant amounts of eosinophils were still present in the BAL and lung tissue. Moreover, specific AAM proteins YM1 and FIZZ1 were expressed by epithelial cells, while macrophages expressed only YM1 in RAG2-/- mice. We further show that FIZZ1 and YM1 protein expression in the lung was completely dependent on signaling through the IL-4Rα and STAT6. Consistent with the enhanced inflammation and AAM protein expression, there was a significant increase in collagen deposition and smooth muscle thickening in RAG2-/- mice compared to mice deficient in IL-4Rα or STAT6.ConclusionsThese results establish that transfer of in vivo primed CD4+ T cells can induce allergic lung inflammation. Furthermore, while IL-4/IL-13 signaling through IL-4Rα and STAT6 is essential for AAM protein expression, lung inflammation and eosinophilia are only partially dependent on this pathway. Further studies are required to identify other proteins and signaling pathways involved in airway inflammation.

The concept that macrophages play an active role in inflammatory responses began its development in the late 1800s with the now iconic studies by Elie Metchnikoff using starfish larvae and Daphnia [reviewed in Kaufmann SHE: Nat Immunol 2008;9:705–712 and Cavaillon JM: J Leukoc Biol 2011;90:413–424]. Based on his observation of the phagocyte response to a foreign body (rose thorn) and yeast, he proposed that phagocytes acted in host defense and were active participants in the inflammatory process. Flash forward more than 100 years and we find that these basic tenets hold true. However, it is now appreciated that macrophages come in many different flavors and can adopt a variety of nuanced phenotypes depending on the tissue environment in which the macrophage is found. In this brief review, we discuss the role of one type of macrophage termed the alternatively activated macrophage (AAM), also known as the M2 type of macrophage, in regulating allergic lung inflammation and asthma. Recent studies using mouse models of allergic lung inflammation and samples from human asthma patients contribute to the emerging concept that AAMs are not just bystanders of the interleukin (IL)-4- and IL-13-rich environment found in allergic asthma but are also active players in orchestrating allergic lung disease.

Insulin Receptor Substrate (IRS)-2 is an adaptor protein that becomes tyrosine phosphorylated in response to IL-4 and IL-13 resulting in activation of the PI-3′ kinase/Akt pathway. While the contribution of IL-4 and IL-13 to allergic lung inflammation has been studied extensively, the functional significance of the IRS2 pathway is unclear. To examine the role of IRS2 in allergic disease, we evaluated responses in IRS2-deficient mice. Deficiency of IRS2 resulted in a substantial increase in expression of a subset of genes associated with alternatively activated macrophages (AAM) in response to IL-4 or IL-13 in vitro. Moreover, IRS2+/− and IRS2−/− mice developed enhanced pulmonary inflammation, accumulation of eosinophils and AAM, and airway and vascular remodeling upon allergen stimulation in comparison to IRS2+/+ mice; this enhanced response was in part macrophage intrinsic. Loss of IRS2 led to greater phosphorylation of Akt and ribosomal S6 protein in the basal state and upon IL-4 stimulation. Thus, we identify a critical negative regulatory loop downstream of IRS2, demonstrating a previously unrecognized role for IRS2 in suppressing allergic lung inflammation and remodeling.

The TH2 cytokines, IL-4 and IL-13, play critical roles in inducing allergic lung inflammation and drive the alternative activation of macrophages (AAM). Although both cytokines share receptor subunits, IL-4 and IL-13 have differential roles in asthma pathogenesis: IL-4 regulates TH2 cell differentiation, while IL-13 regulates airway hyperreactivity and mucus production. Aside from controlling TH2 differentiation, the unique contribution of IL-4 signaling via the Type I receptor in airway inflammation remains unclear. Therefore, we analyzed responses in mice deficient in gamma c (γc) to elucidate the role of the Type I IL-4 receptor. OVA primed CD4+ OT-II T cells were adoptively transferred into RAG2−/− and γc −/− mice and allergic lung disease was induced. Both γc −/− and γcxRAG2−/− mice developed increased pulmonary inflammation and eosinophilia upon OVA challenge, compared to RAG2−/− mice. Characteristic AAM proteins FIZZ1 and YM1 were expressed in lung epithelial cells in both mouse strains, but greater numbers of FIZZ1+ or YM1+ airways were present in γc −/− mice. Absence of γc in macrophages, however, resulted in reduced YM1 expression. We observed higher TH2 cytokine levels in the BAL and an altered DC phenotype in the γc −/− recipient mice suggesting the potential for dysregulated T cell and dendritic cell (DC) activation in the γc-deficient environment. These results demonstrate that in absence of the Type I IL-4R, the Type II R can mediate allergic responses in the presence of TH2 effectors. However, the Type I R regulates AAM protein expression in macrophages.

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