Thymosin alpha 1 (Tα1) reduces the mortality of severe COVID-19 by restoration of lymphocytopenia and reversion of exhausted T cells

Abstract

Background

We previously reported that lymphocytopenia and T cell exhaustion is notable in acute COVID19 patients, especially in aged and severe cases. Thymosin alpha 1 (Tα1) had been used in the treatment of viral infections as an immune response modifier for many years. However, clinical benefits and mechanism of Tα1 supplement to COVID-19 are still unclear.

Methods

We retrospectively reviewed the clinical outcomes of 76 severe cases with COVID-19 admitted into two hospitals in Wuhan from December 2019 to March 2020. The thymus output in peripheral blood mononuclear cells (PBMCs) from COVID-19 patients was measured by T cell receptor excision circles (TREC). The levels of T cell exhaustion markers PD-1 and Tim-3 on CD8⁺ T cells were detected by flow cytometry.

Results

Compared with untreated group, Tα1 treatment significantly reduces mortality of severe COVID-19 patients (11.11% vs. 30.00%, p=0.044). Tα1 timely enhances blood T cell numbers in COVID-19 patients with severe lymphocytopenia (the counts of CD8⁺ T cells or CD4⁺ T cells in circulation lower than 400/μL or 650/μL, respectively). Under such conditions, Tα1 also successfully restores CD8⁺ and CD4⁺ T cell numbers in aged patients. Meanwhile, Tα1 reduces PD-1 and Tim-3 expression on CD8⁺ T cells from severe COVID-19 patients in comparison with untreated cases. It is of note that restoration of lymphocytopenia and acute exhaustion of T cells are roughly parallel to the rise of TRECs.

Conclusions

Tα1 supplement significantly reduce mortality of severe COVID-19 patients. COVID-19 patients with the counts of CD8⁺ T cells or CD4⁺ T cells in circulation lower than 400/μL or 650/μL, respectively, gain more benefits from Tα1. Tα1 reverses T cell exhaustion and recovers immune reconstitution through promoting thymus output during SARS-CoV-2 infection.