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SummaryInterleukin-33 (IL-33) is an immunoregulatory cytokine that moderately suppresses experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis (MS). However, poor pharmacokinetics and toxicity hinder its clinical translation. To address these limitations, we develop an activity-attenuated IL-33 by recombinant fusion to serum albumin (SA). SA-IL-33 exhibits reduced toxicity and prolonged residence in the secondary lymphoid organs (SLOs), sites of T cell priming in autoimmunity, compared to wild-type (WT) IL-33. Prophylactic SA-IL-33 administration prevents EAE with superior efficacy to WT IL-33 and comparable efficacy to fingolimod (FTY720), a Food and Drug Administration (FDA)-approved MS drug. Therapeutic SA-IL-33 treatment also reduces disease severity in both chronic and relapsing-remitting EAE. SA-IL-33 modulates immunity in EAE by suppressing CD45+ cell infiltration (including myelin-reactive T helper 17 [TH17] cells) in the spinal cord, while expanding type 2 immune cells (including type 2 innate lymphoid cells [ILC2s], ST2+ regulatory T cells [Tregs], T helper 2 [TH2] cells, and M2-polarized macrophages) in the SLOs. These findings suggest that SA-IL-33 is a promising therapeutic for neuroinflammatory diseases. Graphical abstract Highlights•Fusion of serum albumin (SA) to interleukin-33 (IL-33) attenuates its activity and toxicity•Engineered SA-IL-33 exhibits prolonged residence in the secondary lymphoid organs (SLOs)•SA-IL-33 treatment both prevents the onset of and reduces established neuroinflammation in mice•Cytokine therapy suppresses TH17 cells in the CNS and promotes immunoregulation in the SLOs The clinical utility of interleukin-33 is hindered by poor pharmacokinetics and toxicity. Budina et al. develop a fusion of serum albumin and interleukin-33 (SA-IL-33) with reduced toxicity and prolonged lymph node residence. SA-IL-33 prevents the onset of and suppresses established inflammation-mediated paralysis in mice, demonstrating promise as a therapeutic for neuroinflammatory diseases.

Francisella tularensis (Ft), a Gram-negative bacterium that causes tularemia, possesses a non-inflammatory atypical LPS (LPSFt) that is highly immunogenic through unknown mechanism. We previously showed that immunization with LPSFt, a type 2 T-independent (TI) antigen, elicits protective LPSFt-specific IgM (IgMFt) and IgG3Ft by B1 cells in a mechanism dependent on the IL-5 produced by innate lymphoid cells type 2 (ILC2). Here, we examined the role of complement in the B1 cells’ response against LPSFt. C3-/-, C1q-/- and C4-/- mice immunized with LPSFt failed to produce IgMFt and IgG3Ft. In contrast, the response of Cfb-/- and Mbl1/Mbl2-/- mice was comparable to that of WT mice. Thus, activation of the classical complement cascade, but not the alternative or the Mannose Binding Lectin pathway, is required for activation of B1 cells and production of LPSFt-specific antibodies. Complement activation generates the C3d fragment, which opsonizes antigens for recognition by complement receptor-2 (CR2), and the C3a and C5a anaphylatoxins. Our results show that C3d opsonized LPSFt and that the response to immunization was dependent on CR2 expression by B1 cells. Importantly, the response to LPSFt immunization was also drastically decreased in C3ar1-/-, but not in C5ar1-/- mice. C3a induced IL-5 in ILC2, which supported B1 cells activation. Decreased antibody production in C3ar1-/- and Cr2-/- mice correlated with increased susceptibility to tularemia. Together, these results demonstrate that the high immunogenicity of LPSFt depends on two effector mechanisms triggered by activation of the classical complement pathway: 1) tagging of LPSFt with C3d fragment, leading to its interaction with CR2 expressed by B1 cells; 2) production of the anaphylatoxin C3a that stimulated IL-5 secretion by ILC2. Our study increases our understanding of the B1 cells’ response to TI-2 antigens and identifies two complement effector mechanisms that can be harnessed for therapeutic interventions. Author summaryThe lipopolysaccharide (LPS) of the bacterium Francisella tularensis strongly stimulates B cells for antibody production independently of T cell help through unknown mechanism. In the present study we examined the role of the complement cascade in this process. We found that production of antibodies against this LPS depends on activation of the classical complement pathway but not the MBL-dependent lectin or the alternative pathways. Following complement activation, LPS became tagged with the C3d complement fragment leading to its interaction with the complement receptor CR2 expressed by B cells. Complement activation also resulted in production of the anaphylatoxin C3a that was required for B cells activation, possibly through induction of IL-5 by innate lymphoid cells 2. Our study increases our understanding of the B cells’ response to T-independent antigens and identifies two complement effector mechanisms that can be harnessed for therapeutic interventions.

Crystalline silica (CS) particle exposure leads to silicosis which is characterized as progressive fibrosis. Fibroblasts are vital effector cells in fibrogenesis. Emerging studies have identified immune sentinel roles for fibroblasts in chronic disease, while their immune-modulatory roles in silicosis remain unclear. Herein, we show that group 2 innate lymphoid cell (ILC2) conversion to ILC1s is closely involved in silicosis progression, which is mediated by activated fibroblasts via interleukin (IL)−18. Mechanistically, Notch3 signaling in mechanics-activated fibroblasts modulates IL-18 production via caspase 1 activity. The mouse-specific Notch3 knockout in fibroblasts retards pulmonary fibrosis progression that is linked to attenuated ILC conversion. Our results indicate that activated fibroblasts in silicotic lungs are regulators of ILC2–ILC1 conversion, associated with silicosis progression via the Notch3–IL-18 signaling axis. This finding broadens our understanding of immune-modulatory mechanisms in silicosis, and indicates potential therapeutic targets for lung fibrotic diseases. Crystalline silica particle exposure in the airways can lead to lung silicosis and progressive fibrosis. Here the authors use mouse silicosis models to show mechanics activated fibroblasts promote conversion of ILC2 to ILC1-like cells pulmonary fibrosis and that this is associated with a Notch3-IL-18 signalling pathway.