Key Points CD1c+ DC but not BDCA-3+ DC or other antigen-presenting cells secrete high amounts of bioactive IL-12. CD1c+ DC efficiently cross-present antigens, prime CD8+ T cells, and induce the highest levels of cytotoxic molecules.

Background. COVID-19 patients develop pneumonia generally associated to lymphopenia and severe inflammatory response due to uncontrolled cytokine release. These mediators are transcriptionally regulated by the JAK-STAT signaling pathways, which can be disabled by small molecules. Methods. A group of subjects (n = 20) was treated with baricitinib according to an offlabel use of the drug. The study was designed as an observational longitudinal trial and approved by the local ethical committee. The patients were treated with baricitinib 4 mg twice daily for 2 days, followed by 4 mg per day for the remaining 7 days. Changes in the immune phenotype and expression of pSTAT3 in blood cells were evaluated and correlated with serum-derived cytokine levels and antibodies anti-SARS-CoV-2. In a single treated patient, we evaluated also the alteration of myeloid cell functional activity. Results. We provided evidence that baricitinib-treated patients have a marked reduction in serum levels of interleukin (IL)-6, IL-1β and tumor necrosis factor (TNF)-a, a rapid recovery in circulating T and B cell frequencies, and increased antibody production against SARS-CoV-2 spike protein, which were clinically associated with a reduction in oxygen flow need and progressive increase in the P/F. Conclusion. These data suggest that Baricitinib prevented the progression towards a severe/extreme form of the viral disease by modulating the patients' immune landscape and these changes were associated with a safer and favorable clinical outcome of patients with COVID-19 pneumonia. Trial registration. The ClinicalTrials.gov identifier of this project is protocol NCT04438629.

Alteration of the gut microbiota has been associated with different gastrointestinal disorders. Normobiosis restoration by faecal microbiota transplantation (FMT) is considered a promising therapeutic approach, even if the mechanisms underlying its efficacy are at present largely unknown. Here we sought to elucidate the functional effects of therapeutic FMT administration during experimental colitis on innate and adaptive immune responses in the intestinal mucosa. We show that therapeutic FMT reduces colonic inflammation and initiates the restoration of intestinal homeostasis through the simultaneous activation of different immune-mediated pathways, ultimately leading to IL-10 production by innate and adaptive immune cells, including CD4+ T cells, iNKT cells and Antigen Presenting Cells (APC), and reduces the ability of dendritic cells, monocytes and macrophages to present MHCII-dependent bacterial antigens to colonic T cells. These results demonstrate the capability of FMT to therapeutically control intestinal experimental colitis and poses FMT as a valuable therapeutic option in immune-related pathologies.

Background&Aims:The Milan metropolitan area in Northern Italy was among the most severely hit by the SARS-CoV-2 outbreak. The aim of this study was to examine the seroprevalence trends of SARS-CoV-2 in healthy asymptomatic adults, the risk factors, and laboratory correlates. Methods:We conducted a cross-sectional study in a random sample of blood donors since the start of the outbreak (February 24 th to April 8 th 2020, n=789). Presence of IgM/IgG antibodies against the SARS-CoV-2-Nucleocapsid protein was assessed by a lateral flow immunoassay. Results:The test had a 100/98.3 sensitivity/specificity, and for IgG+ was validated in a subset by an independent ELISA against the Spike protein (N=34, P<0.001). At the outbreak start, the overall adjusted seroprevalence of SARS-CoV-2 was 2.7%, 95% c.i. 0.3-6% (P<0.0001 vs. 120 historical controls). During the study period characterized by a gradual implementation of social distancing measures, there was a progressive increase in adjusted seroprevalence to 5.2%, 95% c.i. 2.4-9.0, due to a rise in IgG+ tests to 5%, 95%CI 2.8-8.2 (P=0.004 for trend, adjusted weekly increase 2.7±1.3%), but not of IgM+ (P=NS). At multivariate logistic regression analysis, seroconversion to IgG+ was more frequent in younger (P=0.043), while recent infections (IgM+) in older individuals (P=0.002). IgM+ was independently associated with higher triglycerides, eosinophils, and lymphocytes (P<0.05).Conclusions: SARS-CoV-2 infection was already circulating in Milan at the outbreak start. Social distancing may have been more effective in younger individuals, and by the end of April 2.4-9.0% of healthy adults had evidence of seroconversion. Asymptomatic infection may affect lipid profile and blood count.

The development and maturation of semi-invariant natural killer T cells (iNKT cells) rely on the recognition of self antigens presented by CD1d restriction molecules in thymus. The nature of the stimulatory thymic self lipids remains elusive. We isolated lipids from thymocytes and found that ether-bonded mono-alkyl glycerophosphates and the precursors and degradation products of plasmalogens stimulated iNKT cells. Synthetic analogs showed high potency in activating thymic and peripheral iNKT cells. Mice deficient in the peroxisomal enzyme glyceronephosphate O-acyltransferase (GNPAT), essential for the synthesis of ether lipids, had significant alteration of the thymic maturation of iNKT cells and fewer iNKT cells in both thymus and peripheral organs, which confirmed the role of ether-bonded lipids as iNKT cell antigens. Thus, peroxisome-derived lipids are nonredundant self antigens required for the generation of a full iNKT cell repertoire.

Since the beginning of the SARS-CoV-2 pandemic, COVID-19 appeared as a unique disease with unconventional tissue and systemic immune features. Here we show a COVID-19 immune signature associated with severity by integrating single-cell RNA-seq analysis from blood samples and broncho-alveolar lavage fluids with clinical, immunological and functional ex vivo data. This signature is characterized by lung accumulation of naïve lymphoid cells associated with a systemic expansion and activation of myeloid cells. Myeloid-driven immune suppression is a hallmark of COVID-19 evolution, highlighting arginase-1 expression with immune regulatory features of monocytes. Monocyte-dependent and neutrophil-dependent immune suppression loss is associated with fatal clinical outcome in severe patients. Additionally, our analysis shows a lung CXCR6+ effector memory T cell subset is associated with better prognosis in patients with severe COVID-19. In summary, COVID-19-induced myeloid dysregulation and lymphoid impairment establish a condition of ‘immune silence’ in patients with critical COVID-19.

Whether human IL-10-producing regulatory T cells ("Tr1") represent a distinct differentiation lineage or an unstable activation stage remains a key unsolved issue. Here, we report that Eomesodermin (Eomes) acted as a lineage-defining transcription factor in human IFN-γ/IL-10 coproducing Tr1-like cells. In vivo occurring Tr1-like cells expressed Eomes, and were clearly distinct from all other CD4 + T-cell subsets, including conventional cytotoxic CD4 + T cells. They expressed Granzyme (Gzm) K, but had lost CD40L and IL-7R expression. Eomes antagonized the Th17 fate, and directly controlled IFN-γ and GzmK expression. However, Eomes binding to the IL-10 promoter was not detectable in human CD4 + T cells, presumably because critical Tbox binding sites of the mouse were not conserved. A precommitment to a Tr1-like fate, i.e. concominant induction of Eomes, GzmK, and IFN-γ, was promoted by IL-4 and IL-12-secreting myeloid dendritic cells. Consistently, Th1 effector memory cells contained precommitted Eomes + GzmK + T cells. Stimulation with T-cell receptor (TCR) agonists and IL-27 promoted the generation of Tr1-like effector cells by inducing switching from CD40L to IL-10. Importantly, CD4 + Eomes + T-cell subsets were present in lymphoid and nonlymphoid tissues, and their frequencies varied systemically in patients with inflammatory bowel disease and graft-versus-host disease. We propose that Eomes + Tr1-like cells are effector cells of a unique GzmK-expressing CD4 + T-cell subset.

Low-grade intestinal inflammation and alterations of gut barrier integrity are found in patients affected by extraintestinal autoimmune diseases such as type 1 diabetes (T1D), but a direct causal link between enteropathy and triggering of autoimmunity is yet to be established. Here, we found that onset of autoimmunity in preclinical models of T1D is associated with alterations of the mucus layer structure and loss of gut barrier integrity. Importantly, we showed that breakage of the gut barrier integrity in BDC2.5XNOD mice carrying a transgenic T cell receptor (TCR) specific for a beta cell autoantigen leads to activation of islet-reactive T cells within the gut mucosa and onset of T1D. The intestinal activation of islet-reactive T cells requires the presence of gut microbiota and is abolished when mice are depleted of endogenous commensal microbiota by antibiotic treatment. Our results indicate that loss of gut barrier continuity can lead to activation of islet-specific T cells within the intestinal mucosa and to autoimmune diabetes and provide a strong rationale to design innovative therapeutic interventions in “at-risk” individuals aimed at restoring gut barrier integrity to prevent T1D occurrence.