SARS-CoV-2 plasma viral load polarizes the systemic myeloid cell compartment towards an immunosuppressive/regulatory phenotype in severe COVID-19 patients

Severe COVID-19 determines high death rates and health resources overwhelming. Aim of the present study was to shed light on the immune-pathogenesis of severe COVID-19, evaluating the effects of detectable plasma viral load on circulating myeloid cell phenotype and activation. Adult COVID-19 patients with pneumonia stratified according to the degree of respiratory insufficiency were longitudinally followed. Flow cytometry data of monocytes (Mo) and dendritic cells (DCs) subsets were analysed by supervised and unsupervised approaches, in parallel with SARS-CoV-2 plasma viral load by digital droplet PCR and serum cytokines and chemokines. All patients featured systemic immune-suppressive myeloid cell responses, characterized by a reduction of CD14lowCD16+ Mo and reduced expression of CD80, CD86, and SLAN. All DC subset were significantly reduced and both Mo and DC showed increased expression of CD163, CD204, CD206 and PD-L1 immune-regulatory markers. These changes correlated with an unbalanced cytokine/chemokine response, in which G-CSF, M-CSF, IL-6 increased similarly in all patient groups, while low levels of IFN-alpha, beta, IL-1a and IL-1b persisted, associated with increased concentrations of immune-regulatory cytokines as IL-1RA and IL-10. A significantly greater expansion of a PD-L1 positive M2-like population was seen in classical monocytes from viremic patients. Our data favour the conclusion that massive replication of SARS-CoV-2, leading to release in blood, polarize the systemic myeloid cell compartment response towards a more differentiated immunosuppressive/regulatory phenotype.