COVID-19 infection induces readily detectable morphological and inflammation-related phenotypic changes in peripheral blood monocytes, the severity of which correlate with patient outcome

Dan Zhang; Rui Guo; Lei Lei; Hongjuan Liu; Yawen Wang; Yili Wang; Tongxin Dai; Tianxiao Zhang; Yanjun Lai; Jingya Wang; Zhiqiang Liu; Aili He; Michael O'Dwyer; Jinsong Hu

The best recognized hematologic abnormality is lymphopenia, which is seen in up to 85% of severe cases with the severity of lymphopenia linked to outcome. 5 Since regulatory T cells play an important role in dampening the immune response, excessive elimination of these cells could result in an unchecked innate immune responses, leading to an uncontrolled inflammatory response. 5 Indeed, there is growing evidence implicating excessive monocyte/macrophage activation and associated cytokine storm with the pathophysiology of severe SARS-CoV-2 disease related complications. 6 Despite this, there are few reports to date relating to abnormalities of monocytes in patients with COVID-19. Herein we describe novel observations in relation to changes in monocyte morphology and activation status, which correlate with the prognosis and severity of COVID-19 infection and which can be readily quantified by flow cytometry with the concurrent measurement of forward scatter (FSC) and (SSC), which measure cell size and complexity, respectively. Specifically, we have identified the presence of a population of monocytes with higher FSC than normal (FSC-high) not typically seen in other types of viral infection.