Background: Coronavirus disease 19 (COVID-19) is an emerging infectious disease caused by SARS-CoV-2. Anti-viral immune response is crucial to achieve pathogen clearance, however in some patients an excessive and aberrant host immune response can lead to an acute respiratory distress syndrome. The comprehension of the mechanisms that regulate pathogen elimination, immunity, and pathology is essential to better characterize disease progression and widen the spectrum of therapeutic options. Methods: We performed a flow cytometric characterization of immune cells subsets from 30 COVID-19 patients and correlated these data with clinical outcomes. Results: COVID-19 patients showed decreased numbers of circulating T, B and NK cells, and exhibited a skewing of CD8+ T cells towards a terminally differentiated/senescent phenotype. In agreement, T CD4+, T CD8+ but also NK cells displayed reduced anti-viral cytokine production capability. Moreover, a reduced cytotoxic potential was identified in COVID-19 patients, particularly in those that required intensive care. The latter group of patients showed also increased serum IL-6 levels, that correlated to the frequency of granzyme-expressing NK cells. Off-label treatment with tocilizumab restored the cytotoxic potential of NK cells. Conclusion: In conclusion, the association between IL-6 serum levels and the impairment of cytotoxic activity suggests the possibility that targeting this cytokine may restore anti-viral mechanisms.

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Resting T lymphocytes can internalize reduced cysteine (Cys) but not cystine, the oxidized form of the amino acid that predominates extracellularly. In vitro studies have shown that DC provide Cys to T cells during antigen presentation, allowing their activation. Here, we show that increased thiol production is a hallmark of immune response in vivo. Indeed, the thiol content of LN increases dramatically after antigen injection. Non-protein thiols co-distribute with DC and are highly abundant in germinal centers. In agreement, activated but not resting B lymphocytes and macrophages release free thiols. Increased thiol release following activation requires thioredoxin and is paralleled by increased thioredoxin expression. The T zones of LN are consistently less stained, and both resting and activated T cells are unable to release thiols. Interestingly, the cystine/glutamate transporter x c À is absent in resting T lymphocytes but is rapidly induced by TCR triggering in vitro, indicating that the release of T cells from the need of exogenous Cys occurs early after activation. These results indicate that a reducing microenvironment is essential to start the immune response but dispensable for its evolution, and support the emerging concept that extracellular redox is implicated in the control of crucial cellular functions.Key words: Lymphocytes Á Macrophages Á Non-protein thiols Á Redox Á Thioredoxin IntroductionThe free thiol (cysteine, Cys) and the disulfide bonded (cystine, CySS) form of the amino acid Cys exist in an equilibrium defined by redox conditions. In the oxidizing environment of the extracellular space, CySS prevails while Cys is almost absent [1]. Conversely, within the cytosol, Cys is present mainly as a free thiol. Interestingly, T lymphocytes can take up Cys but not CySS [2]. Since Cys is an essential amino acid for T lymphocytes, required for GSH synthesis [3], its reduced form must be made available in the microenvironment of T cells to sustain their activation and proliferation. In addition, pre-B and resting B lymphocytes require exogenous Cys to grow in vitro, but are released from this requirement upon differentiation [4].We have previously shown in vitro that, during antigen presentation, DC increase the uptake of CySS and release Cys, allowing T-cell activation. The intracellular conversion of CySS to Cys requires the activity of the redox-active enzyme, thioredoxin (Trx) [5].Although the interaction of T or B lymphocytes with antigenpulsed APC or mitogens may recapitulate lymphocyte activation in vitro, the generation of an immune response in vivo is the result of a much more complex cellular choreography. In secondary lymphoid organs, distinct subsets of immune cells are strategically [6,7]. GC develop a few days after arrival of the antigen from afferent lymphatic drainage, when antigen-activated B cells move to the center of B-cell follicles in secondary lymphoid organs and proliferate within the follicular DC network [6]. Many of the B cells generated within the GC die in situ and nuc...

Summary De novo expression of costimulatory molecules in tumours generally increases their immunogenicity, but does not always induce a protective response against the parental tumour. This issue was addressed in the mouse Sp6 hybridoma model, comparing different immunization routes (subcutaneous, intraperitoneal and intravenous) and doses (0·5 × 106 and 5 × 106 cells) of Sp6 cells expressing de novo B7‐1 (Sp6/B7). The results can be summarized as follows. First, de novo expression of B7‐1 rendered Sp6 immunogenic, as it significantly reduced the tumour incidence to ≤15% with all delivery routes and doses tested, whereas wild‐type Sp6 was invariably tumorigenic (100% tumour incidence). Second, long‐lasting protection against wild‐type Sp6 was mainly achieved when immunization with Sp6/B7 was subcutaneous: a dose of 0·5 × 106 Sp6/B7 cells elicited protection that was confined to sites in the same anatomical quarter as the immunizing injection. Repeated injections of the same dose extended protection against wild‐type Sp6 to other anatomical districts, as well as a single injection of a 10‐fold higher dose (5 × 106 cells). Finally, Sp6‐specific cytotoxic T‐lymphocyte activity was detected in draining lymph nodes, and the splenic expansion of Sp6‐specific cytotoxic T‐lymphocyte precursors quantitatively correlated with the dose of antigen. We conclude that activation of a protective immune response against Sp6 depends on the local environment where the immunogenic form of the ‘whole tumour cell antigen’ is delivered. The antigen dose regulates the anatomical extent of the protective response.