Dear Editor,rnThe outbreak of the new coronavirus SARS-CoV-2 has resulted in a global pandemic.Due to the lack of a specific drug against this virus, the current clinical management of this disease mainly depends on supportive care to reduce inflammatory responses and to keep the lung functioning.1 Understanding the underlying immunopathology of coronavirus disease 2019 (COVID-19) is therefore of paramount importance for improving the current treatment.In this study, we found a distinct feature of adaptive immunity in severely affected patients, the coincidence of impaired cellular and enhanced humoral immune responses, suggesting that dysregulated adaptive immune responses advanced severe COVID-19.Interestingly, expression of Prothymosin alpha (PTMA), the proprotein of Thymosin alpha-1 (Tα1), was increased in a group of CD8 T memory stem cells accumulated during severe disease.We further showed that Tα1 slightly reduced T cell activation in vitro and promoted proliferation of effector T cells.Moreover, Tcα1 treatment relieved the lymphopenia in COVID-19 patients.Our data suggest that early intervention of adaptive immune response might be critical for the prevention of severe COVID-19.