Locally renewing resident synovial macrophages provide a protective barrier for the joint.

Macrophages are considered to contribute to chronic inflammatory diseases such as rheumatoid arthritis1. However, both the exact origin and the role of macrophages in inflammatory joint disease remain unclear. Here we use fate-mapping approaches in conjunction with three-dimensional light-sheet fluorescence microscopy and single-cell RNA sequencing to perform a comprehensive spatiotemporal analysis of the composition, origin and differentiation of subsets of macrophages within healthy and inflamed joints, and study the roles of these macrophages during arthritis. We find that dynamic membrane-like structures, consisting of a distinct population of CX3CR1+ tissue-resident macrophages, form an internal immunological barrier at the synovial lining and physically seclude the joint. These barrier-forming macrophages display features that are otherwise typical of epithelial cells, and maintain their numbers through a pool of locally proliferating CX3CR1- mononuclear cells that are embedded into the synovial tissue. Unlike recruited monocyte-derived macrophages, which actively contribute to joint inflammation, these epithelial-like CX3CR1+ lining macrophages restrict the inflammatory reaction by providing a tight-junction-mediated shield for intra-articular structures. Our data reveal an unexpected functional diversification among synovial macrophages and have important implications for the general role of macrophages in health and disease.

RELMα-expressing macrophages protect against fatal lung damage and reduce parasite burden during helminth infection.

Alternatively activated macrophages (AAMs) can contribute to wound healing, regulation of glucose and fat metabolism, resolution of inflammation, and protective immunity against helminths. Their differentiation, tissue distribution, and effector functions are incompletely understood. Murine AAMs express high levels of resistin-like molecule (RELM) α, an effector protein with potent immunomodulatory functions. To visualize RELMα+ macrophages (MΦs) in vivo and evaluate their role in defense against helminths, we generated RELMα reporter/deleter mice. Infection with the helminth Nippostrongylus brasiliensis induced expansion of RELMα+ lung interstitial but not alveolar MΦs in a STAT6-dependent manner. RELMα+ MΦs were required for prevention of fatal lung damage during primary infection. Furthermore, protective immunity was lost upon specific deletion of RELMα+ MΦs during secondary infection. Thus, RELMα reporter/deleter mice reveal compartmentalization of AAMs in different tissues and demonstrate their critical role in resolution of severe lung inflammation and protection against migrating helminths.

Selective expression of constitutively activated STAT6 in intestinal epithelial cells promotes differentiation of secretory cells and protection against helminths.

Intestinal epithelial cells (IECs) constitute an important barrier between host and pathogen. Immune mechanisms that provide protection against gastrointestinal helminths often require IL-4Rα-induced activation of STAT6-regulated genes in IECs. However, it is not known whether STAT6 activation in IECs enhances protective immunity against helminths. Furthermore, the regulation of proliferation and differentiation processes of the intestinal epithelium by IEC-intrinsic STAT6 signaling remains unclear. To address these questions, we generated mice with specific expression of a constitutively active version of STAT6 in IECs. These VillinCre_STAT6vt mice show accumulation of secretory IECs, increased proliferation of IECs and lengthening of the small intestine. They rapidly expelled Nippostrongylus brasiliensis worms even in the absence of T cells. Furthermore, primary infection with Heligmosomoides polygyrus resulted in larval trapping in the submucosa and the fecundity of adult worms was severely impaired. Our results reveal an important IEC-intrinsic role of STAT6-regulated genes for intestinal homeostasis and protective immunity against helminths.

RelB regulates Th17 differentiation in a cell-intrinsic manner.

The role of the alternative NF-κB pathway is mainly attributed to the lymphoid organ formation and blood cancer. However, its involvement in lymphocyte differentiation is not clearly defined. Recently, we have shown that uncontrolled activation of alternative NF-κB in mice lacking the NF-κB inhibitory protein p100 (p100-/- mice) hinders plasmablast proliferation and diminishes T cell independent responses. Here we show that hyperactivation of this pathway leads to a cell-intrinsic T cell defects. p100-deficient T helper cells displayed both an activation and a proliferation defect in vitro. In addition, memory T cell formation was impaired in vivo. Moreover, p100-/- T cells failed to polarize into T helper 17 cells. This phenotype was dependent on increased RelB activation and suboptimal RORγt expression. Thus, our results demonstrate that RelB acts as a negative regulator of T cell activation and Th17 development. Targeting this pathway therefore could be beneficial in Th17-mediated pathologies.

Th2 and eosinophil responses suppress inflammatory arthritis.

Th2-eosinophil immune responses are well known for mediating host defence against helminths. Herein we describe a function of Th2-eosinophil responses in counteracting the development of arthritis. In two independent models of arthritis, Nippostrongylus brasiliensis infection leads to Th2 and eosinophil accumulation in the joints associated with robust inhibition of arthritis and protection from bone loss. Mechanistically, this protective effect is dependent on IL-4/IL-13-induced STAT6 pathway. Furthermore, we show that eosinophils play a central role in the modulation of arthritis probably through the increase of anti-inflammatory macrophages into arthritic joints. The presence of these pathways in human disease is confirmed by detection of GATA3-positive cells and eosinophils in the joints of rheumatoid arthritis patients. Taken together, these results demonstrate that eosinophils and helminth-induced activation of the Th2 pathway axis effectively mitigate the course of inflammatory arthritis.

NF-κB2/p100 deficiency impairs immune responses to T-cell-independent type 2 antigens.

Formation of the splenic marginal zone (MZ) depends on the alternative NF-κB signaling pathway. Recently, we reported that unrestricted activation of this pathway in NF-κB2/p100-deficient (p100(-/-) ) knock-in mice alters the phenotype of MZ stroma and B cells. Here, we show that lack of the p100 inhibitor resulted in an expansion of both MZ B and peritoneal B-1 cells. However, these cells failed to generate proliferating blasts in response to T-cell-independent type 2 (TI-2) Ags, correlating with dampened IgM and absent IgG3 responses. This phenotype was in part due to increased activity of the NF-κB subunit RelB. Moreover, p100(-/-) →B6 BM chimeras were more susceptible to infection by encapsulated Streptococcus pneumoniae bacteria, pathogens that induce TI-2 responses. In contrast to the TI-2 defect, p100 deficiency did not impair immune responses to the TI-1 Ag LPS and p100(-/-) MZ B cells showed normal Ag transportation into B-cell follicles. Furthermore, p100(-/-) MZ B and B-1 cells failed to respond to TI-2 Ags in the presence of WT accessory cells. Thus, NF-κB2/p100 deficiency caused a predominant B-cell-intrinsic TI-2 defect that could largely be attributed to impaired proliferation of plasmablasts. Importantly, p100 was also necessary for efficient defense against clinically relevant TI-2 pathogens.