Abstract

The mononuclear phagocyte system (MPS), which includes monocytes/macrophages and dendritic cells (DCs), is notably involved in the modulation of immune-inflammatory processes. While the former act as the first natural barrier against any pathogenic noxa, the latter are the must regulators of adaptive immunity. There is evidence that the MPS is a key player in Coronavirus disease-2019 (COVID-19) pathogenesis.

We analyzed by multi-parametric flow cytometry the frequency distribution of peripheral blood monocytes expressing the type I interferon-inducible receptor CD169 and of conventional CD1c+ and CD141+ (namely cDC2 and cDC1) and plasmacytoid CD303+ DCs in 40 patients (M= 30; mean age: 68-yrs) with COVID-19 pneumonia on hospital admission. Thirty age- and sex-matched healthy controls were enrolled for comparison.

Our preliminary results show that the median frequency of CD169+ monocytes were significantly higher in patients than in controls  (3.89 vs. 1.11; p=0.01).  Conversely, all DC subsets were markedly depleted in the former (p<0.0001 in all instances), with no apparent association with disease severity (i.e., inflammation markers and radiological extent of lung abnormalities). Both high frequencies of CD169+ monocytes (> than the median value of 3.89 %) and low frequencies of cDC2 cells (less than the median value of 0.09%) were significantly associated with in-hospital mortality.

Our findings suggest that the interplay between the different components of the MPS is dysregulated in acute COVID-19 patients. This may explain, at least in part, the imbalance between innate and adaptive immunity and its impact on disease outcome.