Background COVID-19 is associated with a dysregulated immune response but it is unclear how immune dysfunction contributes to the chronic morbidity persisting in many COVID-19 patients during convalescence (long COVID).
Methods We assessed phenotypical and functional changes of monocytes in COVID-19 patients during hospitalization and up to 9 months of convalescence following COVID-19, respiratory syncytial virus (RSV) or influenza A (flu). Progressive fibrosing interstitial lung disease (PFILD) patients were included a positive control for severe, ongoing lung injury.
Results Monocyte alterations in acute COVID-19 patients included aberrant expression of leucocyte migration molecules, continuing into convalescence (n=142) and corresponding to specific symptoms of long COVID. Long COVID patients with unresolved lung injury, indicated by sustained shortness of breath and abnormal chest radiology, were defined by high monocyte expression of chemokine receptor CXCR6 (p<0.0001) and adhesion molecule PSGL-1 (p<0.01), alongside preferential migration of monocytes towards CXCR6 ligand CXCL16 (p<0.05) which is abundantly expressed in the lung. Monocyte CXCR6 and lung CXCL16 were heightened in PFILD patients (p<0.001) confirming a role for the CXCR6-CXCL16 axis in ongoing lung injury. Conversely, monocytes from long COVID patients with ongoing fatigue exhibited sustained reduction of the prostaglandin-generating enzyme COX-2 (p<0.01) and CXCR2 expression (p<0.05). These monocyte changes were not present in RSV or flu convalescence.