In the current study, which is an ancillary study of the PICARD study [6], we aimed to assess, in a prospective cohort of patients with moderate to severe pneumonia-related ARDS: (1) the phenotype of alveolar and blood monocytes (i.e., levels of HLA-DR and PD-L1 expressions) and compare it to those of non ARDS patients; (2) the function of alveolar and blood monocytes; and (3) explore alveolar monocytes functions according to their level of HLA-DR expression. Here, CD274 is linked to pneumonia.