Fig. 1.

The involvement of MDSCs in COVID-19-associated lymphopenia. MDSCs play a crucial role in lymphopenia. Mechanisms include suppression of the proliferation of T cells by Arg-1 and IDO (A); promotion of apoptosis of T cells via iNOS and ROS (B); induction of PD-1/PD-L1 and Fas/FasL interaction through IL-10 and TGF-β, leading to T-cell exhaustion(C). MDSCs: myeloid-derived suppressor cells; Arg-1: arginase 1; PD-L1: programmed cell death ligand 1; PD-1: programmed cell death protein 1; FasL: Fas ligand; ROS: reactive oxygen species; NOX2: NADPH oxidase 2; iNOS: inducible nitric oxide synthase; IDO: indoleamine 2,3-dioxygenase; TGF-β: transforming growth factor-β; IL-10: interleukin-10.