Coronavirus disease 2019 (COVID-19) has caused more than 40,000 deaths worldwide. Approximately 14% of patients with COVID-19 experienced severe disease and 5% were critically ill. Studies have shown that dysregulation of the COVID-19 patients' immune system may lead to inflammatory storm and cause severe illness and even death. Tocilizumab treatment targeting interleukin 6 receptor has shown inspiring clinical results of severe COVID-19 patients. However, the immune network with Tocilizumab treatment at single cell resolution has not been uncovered. Here, we profiled the single-cell transcriptomes of 13,289 peripheral blood mononuclear cells isolated at three longitudinal stages from two severe COVID-19 patients treated with Tocilizumab. We identified a severe stage-specific monocyte subpopulation and these cells centric immune cell interaction network connected by the inflammatory cytokines and their receptors. The over-activated inflammatory immune response was attenuated after Tocilizumab treatment, yet immune cells including plasma B cells and CD8+ T cells still exhibited an intense humoral and cell-mediated anti-virus immune response in recovered COVID-19 patients. These results provided critical insights into the immunopathogenesis of severe COVID-19 and revealed fundamentals of effectiveness in Tocilizumab treatment.### Competing Interest Statement

Tocilizumab treatment in severe COVID-19 patients attenuates the inflammatory storm incited by monocyte centric immune interactions revealed by single-cell analysis