Reply to Chen and Vitetta

To the Editor—We recently addressed an ill-defined facet of immune responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), that is, untouched circulating leukocytes, with the aim of an unbiased hierarchical approach to immune responses to SARS-CoV-2 [1]. By doing this, we focused attention on granulocytes, the first line of defense during infection shaping downstream responses. As mentioned by Chen and Vitetta [2], we showed upregulation of PD-L1 on eosinophils and basophils from severe forms of coronavirus disease 2019 (COVID-19) as compared to mild forms. PD-L1 is expressed by an array of hematopoietic and nonhematopoietic cell types, either constitutively or upon stimulation [3]. PD-L1 and its opposing counterpart PD-L2 are ligands of PD-1, a co-inhibitory receptor (“immune checkpoint”) expressed by antigen-stimulated T cells and, thus, a gatekeeper for broad suppression of adaptive immune responses. PD-1 ligation results in T-cell receptor (TCR) down-regulation by...