CD4 and COVID-19: We speculated that these cells might establish an unfavorable environment for T cells and, indeed, effector and helper T cells were heavily contracted while T regulatory lymphocytes (Tregs, identified as CD4+FoXP3+ cells) significantly expanded in the spleen of vFLIP mice (Supplementary information, Fig. S3E-F), unveiling a pronounced systemic lymphopenia that mirrors COVID-19-associated CRS.