Imbalance of Circulating Monocyte Subsets in Subjects with Newly Emerged and Recurrent Hospital-Acquired Pneumonia

Hospital-acquired pneumonia (HAP) is one of the most common diseases in the intensive care unit, where the development of disease is closely related with the host immune response. Monocytes play an important role in both innate and adaptive immune system. We aimed to investigate the changes of circulating monocyte subsets in subjects with HAP to explore its value in monitoring HAP.

In total, 60 HAP patients and 18 healthy individuals were enrolled in this study. Human monocyte subsets are classified into 3 groups: nonclassical (NC), intermediate (ITM), and classical (CL). Also, programmed death ligand 1 (PD-L1) expression on circulating monocyte subsets was measured by flow cytometry.

Data showed that the ratio of NC, ITM, and CL among monocytes was comparable between HAP patients and healthy controls (P > .05). There was a remarkable imbalance of NC and CL in newly emerged HAP compared to healthy controls (P < .05), subsequently reaching normalization in recurrent HAP (P > .05). Furthermore, although PD-L1 was seemly constitutively expressed by NC, ITM, and CL groups regardless of disease status, it was noted that PD-L1 was dominantly expressed in the CL group (P < .05).

Given distinct PD-L1 expression, a shift of CL/NC in newly emerged HAP would constitute an inhibitory anti-pathogen immune response. Normalization of circulating monocyte subsets on recurrence of HAP might be the consequence of immune memory of bacterial infection.