Monocyte-driven atypical cytokine storm and aberrant neutrophil activation as key mediators of COVID-19 disease severity.

Epidemiological and clinical reports indicate that SARS-CoV-2 virulence hinges upon the triggering of an aberrant host immune response, more so than on direct virus-induced cellular damage. To elucidate the immunopathology underlying COVID-19 severity, we perform cytokine and multiplex immune profiling in COVID-19 patients. We show that hypercytokinemia in COVID-19 differs from the interferon-gamma-driven cytokine storm in macrophage activation syndrome, and is more pronounced in critical versus mild-moderate COVID-19. Systems modelling of cytokine levels paired with deep-immune profiling shows that classical monocytes drive this hyper-inflammatory phenotype and that a reduction in T-lymphocytes correlates with disease severity, with CD8+ cells being disproportionately affected. Antigen presenting machinery expression is also reduced in critical disease. Furthermore, we report that neutrophils contribute to disease severity and local tissue damage by amplification of hypercytokinemia and the formation of neutrophil extracellular traps. Together our findings suggest a myeloid-driven immunopathology, in which hyperactivated neutrophils and an ineffective adaptive immune system act as mediators of COVID-19 disease severity.

Monocytes from patients with Primary Ciliary Dyskinesia show enhanced inflammatory properties and produce higher levels of pro-inflammatory cytokines.

Patients with Primary Ciliary Dyskinesia (PCD) suffer from recurrent upper and lower airway infections due to defects in the cilia present on the respiratory epithelium. Since chronic inflammatory conditions can cause changes in innate immune responses, we investigated whether monocytes isolated from the peripheral blood of pediatric PCD patients respond differently to inflammatory stimuli, compared to monocytes from healthy children and adults. The receptor for C5a (C5aR) was upregulated in PCD, whereas expression levels of the leukocyte chemoattractant receptors CCR1, CCR2, CCR5, BLT1 and FPR1 on PCD monocytes were similar to those on monocytes from healthy individuals. Also in vitro migration of PCD monocytes towards the ligands of those receptors (CCL2, fMLP, C5a and LTB4) was normal. Compared to healthy children, PCD patients had a higher percentage of the non-classic monocyte subset (CD14+CD16++) in circulation. Finally, PCD monocytes produced higher levels of pro-inflammatory cytokines (IL-1ß and TNF-α) and chemokines (CCL3, CCL5, CCL18 and CCL22) in response to LPS, peptidoglycan and/or dsRNA stimulation. These data suggest that monocytes might exacerbate inflammatory reactions in PCD patients and might maintain a positive feedback-loop feeding the inflammatory process.

Gene cloning of a new plasma CC chemokine, activating and attracting myeloid cells in synergy with other chemoattractants.

Chemokines are important mediators of cell migration during inflammation and normal leukocyte trafficking. Inflammatory chemokines are induced in multiple cell types at sites of infection. Here, we describe a novel bovine CC chemokine, designated regakine-1, that is constitutively present at high concentrations in plasma. Cloning of its gene revealed an expected two intron/three exon organization, with a rather long first intron. In addition to a 21-residue signal peptide, the coding sequence corresponded to a 71-residue secreted protein. However, the natural regakine-1 protein missed the COOH-terminal lysine residue. Regakine-1 has only weak sequence similarity (<50% identical residues) with other animal or human chemokines. Northern blot analysis demonstrated regakine-1 RNA expression in spleen and lung. At physiological concentrations (30-100 ng/mL), natural 7.5 kDa regakine-1 stimulated gelatinase B release from neutrophils and chemoattracted immature myeloid HL-60 cells, as well as mature granulocytes. Regakine-1 was more potent on human myeloid cells than the human plasma CC chemokine hemofiltrate CC chemokine-1 (HCC-1). Moreover, regakine-1 synergized with the bacterial peptide N-formylmethionylleucylphenylalanine (fMLP), yielding a 10-fold increase in neutrophil chemotactic response above their additive effect. Regakine-1 did not compete with interleukin-8 (IL-8) for binding to neutrophils, nor did it affect fMLP-induced calcium signaling, suggesting that regakine-1 recognizes a different receptor. In view of its high constitutive plasma concentration, regakine-1 is believed to recruit myeloid cells into the circulation, whereas its synergy with other neutrophil chemoattractants suggests that it also enhances the inflammatory response to infection.