Marked T cell activation, senescence, exhaustion and skewing towards TH17 in patients with Covid-19 pneumonia.

We provide an in-depth investigation of the T cell compartment and functionality, cytokine production and plasma levels in a total of 39 patients affected by Covid-19 pneumonia. At admission, patients were lymphopenic; for all, SARS-CoV-2 was detected in a nasopharyngeal swab specimen by real-time RT-PCR, and pneumonia was subsequently confirmed by X-rays. Detailed 18-parameter flow cytometry coupled with unsupervised data analysis revealed that patients showed similar percentages of CD4+ and CD8+ T cells, but a decreased absolute number in both populations. For CD4+ T lymphocytes, we found a significant decrease in the number of naïve, central and effector memory cells and an increased percentage of terminally differentiated cells, regulatory T cells, and of those that were activated or that were expressing PD1 and CD57 markers. Studies on chemokine receptors and lineage-specifying transcription factors revealed that, among CD4+ T cells, patients displayed a lower percentage of cells expressing CCR6 or CXCR3, and of those co-expressing CCR6 and CD161, but higher percentages of 62 CXCR4+ or CCR4+ cells. No differences were noted in the expression of T-bet or GATA-3. Analyses of patients' CD8+ T cells showed decreased numbers of naïve and central memory and increased amounts of activated cells, accompanied by increased percentages of activated cells and of lymphocytes expressing CD57, PD1, or both. CD8+ T cells expressed lower percentages of CCR6+, CXCR3+ or T-bet+ cells and of CXCR3+,T-bet+ or CCR6+,CD161+ lymphocytes. We also found higher percentages of cells expressing CCR4+, CXCR4 or GATA-3. Analyses of lymphocyte proliferation revealed that terminally differentiated CD4+ and CD8+ T cell from patients had a lower proliferative index than controls, whereas cellular bioenergetics, measured by the quantification of mitochondrial oxygen consumption and extracellular acidification rate, was similar in CD4+ T cells from both groups. We measured plasma level of 31 cytokines linked to inflammation, including T helper (TH)type-1 and TH2 cytokines, chemokines, galectins, pro- and anti-inflammatory mediators, finding that most were dramatically increased in Covid-19 patients, confirming the presence of a massive cytokine storm. Analysis of the production of different cytokines after stimulation by anti-CD3/CD28 monoclonal antibodies revealed that patients not only had a high capacity to produce tumour necrosis factor (TNF)-α, interferon (IFN)-γ and interleukin (IL)-2, but also showed a significant skewing of CD4+ T cells towards the TH17 phenotype. A therapeutic approach now exists based on the administration of drugs that block IL-6pathway, and seems to improve the disease. IL-17 is crucial in recruiting and activating neutrophils, cells that can migrate to the lung and are heavily involved in the pathogenesis of Covid-19. We show here that a skewing of activated T cells towards the TH17 functional phenotype exists in Covid-19 patients. We therefore suggest that blocking the IL-17 pathway by biological drugs that are already used to treat different pathologies could provide a novel, additional strategy to improve the health of patients infected by SARS-CoV-2.