Investigation of pulmonary immune cell signature showed that in contrast with the minor infiltration of CD4- or CD8-positive T lymphocytes or CD20-positive B lymphocytes in alveolar space or the lung interstitial compartment (Fig. 2C), extensive infiltration of CD68-positive macrophages was found in the areas with diffuse alveolar damage in COVID-19 patients (Fig. 2C). Here, CD4 is linked to COVID-19.