Hyperinflammation contributes to lung injury and subsequent acute respiratory distress syndrome (ARDS) with high mortality in patients with severe coronavirus disease 2019 (COVID-19). To understand the underlying mechanisms involved in lung pathology, we investigated the role of the lung-specific immune response. We profiled immune cells in bronchoalveolar lavage fluid and blood collected from COVID-19 patients with severe disease and bacterial pneumonia patients not associated with viral infection. By tracking T cell clones across tissues, we identified clonally expanded tissue-resident memory-like Th17 cells (Trm17 cells) in the lungs even after viral clearance. These Trm17 cells were characterized by a a potentially pathogenic cytokine expression profile of IL17A and CSF2 (GM-CSF). Interactome analysis suggests that Trm17 cells can interact with lung macrophages and cytotoxic CD8+ T cells, which have been associated with disease severity and lung damage. High IL-17A and GM-CSF protein levels in the serum of COVID-19 patients were associated with a more severe clinical course. Collectively, our study suggests that pulmonary Trm17 cells are one potential orchestrator of the hyperinflammation in severe COVID-19.

NK cells are the main cells of the innate immune system that produce IFN-γ, and they express this cytokine at early stages of maturation in response to cytokine stimulation. Conversely, acquisition of IFN-γ-competence in CD4 + T helper cells requires a differentiation process from naïve toward type 1 (Th1) cells, which is associated with epigenetic remodeling at the IFNG locus. In the present study, we show that the ability of NK cells to produce IFN-γ in response to activating receptor (actR) engagement is gradually acquired during terminal differentiation and is accompanied by progressively higher NF-κB activation in response to actR triggering. Moreover, during the differentiation process NK cells gradually display increasing expression of IFNG and TBX21 (encoding T-bet) transcripts and demethylation at the IFNG promoter. This study provides new insights in the molecular mechanisms underlying NK-cell ability to express IFN-γ upon actR engagement. Thus, we propose that in order to efficiently produce IFN-γ in response to infected or transformed cells, NK cells gain Th1-like features, such as higher IFN-γ competence and epigenetic remodeling of the IFNG promoter, during their terminal differentiation.Keywords: Cell differentiation r Chromatin remodeling r IFN-γ r NK cells Additional supporting information may be found in the online version of this article at the publisher's web-site Introduction IFN-γ is a cytokine critically involved in protection against intracellular pathogens and is expressed primarily by T-cell and NK-cell Correspondence: Dr. Chiara Romagnani e-mail: romagnani@drfz.de lineages. Naïve CD4 + T helper (Th) cells must endure a detailed program of proliferation and differentiation in the periphery toward type 1 (Th1) cells before they are competent to transcribe the IFNG gene [1,2]. Differentiation of naïve T cells toward a Th1 phenotype requires at least two phases: an early TCR/IFN-γ/ STAT1-dependent programing, which induces low levels of the T-box transcription factor T-bet (encoded by TBX21) and upregulation of IL-12Rβ2, followed by an IL-12/STAT4/T-bet-dependent C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2014. 44: 2074-2084 Innate immunity 2075 commitment, during which IL-12 further enhances T-bet expression and heightens IFNG gene expression [3][4][5]. Once T-bet and IL-12Rβ2 are expressed, IFNG transcriptional activity is acutely promoted by TCR triggering or combined stimulation with IL-12 and IL-18. Both stimuli induce the activation of NF-κB p65 (RelA), which can couple either with STAT4 (selectively induced by or with TCR-induced NFAT. These complexes bind to the correspondent accessible sites of the IFNG locus, thus enhancing IFN-γ transcription in Th1 cells [6,7]. A key mechanism by which this transcription factor network stabilizes Th1-lineage commitment is epigenetic imprinting, resulting in heritable DNA and histone modifications of IFNG cis-elements, such as the promoter and several conserved noncoding sequences. Similarly, effector/...

It is well accepted that Th17 cells are a highly plastic cell subset that can be easily directed toward the Th1 phenotype in vitro and also in vivo during inflammation. However, there is an ongoing debate regarding the reverse plasticity (conversion from Th1 to Th17). We show here that ectopic ROR-γt expression can restore or initiate IL-17 expression by non-classic or classic Th1 cells, respectively, while common pro-Th17 cytokine cocktails are ineffective. This stability of the Th1 phenotype is at least partially due to the presence of a molecular machinery governed by the transcription factor Eomes, which promotes IFN-γ secretion while inhibiting the expression of ROR-γt and IL-17. By using a mouse model of T cell-dependent colitis we demonstrate that Eomes controls non-classic Th1 cell development also in vivo and promotes their pathogenic potential. Eomes expression associates to a highly inflammatory phenotype also in patients with juvenile idiopathic arthritis. Indeed, it favors the acquisition of a cytotoxic signature, and promotes the development of IFN-γ GM-CSF cells that have been described to be pathogenic in chronic inflammatory disorders.

At present, it is not clear how memory B lymphocytes are maintained over time, and whether only as circulating cells or also residing in particular tissues. Here we describe distinct populations of isotype-switched memory B lymphocytes (Bsm) of murine spleen and bone marrow, identified according to individual transcriptional signature and B cell receptor repertoire. A population of marginal zone-like cells is located exclusively in the spleen, while a population of quiescent Bsm is found only in the bone marrow. Three further resident populations, present in spleen and bone marrow, represent transitional and follicular B cells and B1 cells, respectively. A population representing 10-20% of spleen and bone marrow memory B cells is the only one qualifying as circulating. In the bone marrow, all cells individually dock onto VCAM1 + stromal cells and, reminiscent of resident memory T and plasma cells, are void of activation, proliferation and mobility.