Abstract
Coronavirus disease 2019 (COVID-19), represents an enormous new threat to our healthcare system and particularly to the health of older adults. Although the respiratory symptoms of COVID-19 are well recognized, the neurological manifestations, and their underlying cellular and molecular mechanisms, have not been extensively studied yet. Our study is the first one to test the direct effect of serum from hospitalised COVID-19 patients on human hippocampal neurogenesis using a unique in vitro experimental assay with human hippocampal progenitor cells (HPC0A07/03 C). We identify the different molecular pathways activated by serum from COVID-19 patients with and without neurological symptoms (i.e., delirium), and their effects on neuronal proliferation, neurogenesis, and apoptosis. We collected serum sample twice, at time of hospital admission and approximately 5 days after hospitalization. We found that treatment with serum samples from COVID-19 patients with delirium (n = 18) decreased cell proliferation and neurogenesis, and increases apoptosis, when compared with serum samples of sex- and age-matched COVID-19 patients without delirium (n = 18). This effect was due to a higher concentration of interleukin 6 (IL6) in serum samples of patients with delirium (mean ± SD: 229.9 ± 79.1 pg/ml, vs. 32.5 ± 9.5 pg/ml in patients without delirium). Indeed, treatment of cells with an antibody against IL6 prevented the decreased cell proliferation and neurogenesis and the increased apoptosis. Moreover, increased concentration of IL6 in serum samples from delirium patients stimulated the hippocampal cells to produce IL12 and IL13, and treatment with an antibody against IL12 or IL13 also prevented the decreased cell proliferation and neurogenesis, and the increased apoptosis. Interestingly, treatment with the compounds commonly administered to acute COVID-19 patients (the Janus kinase inhibitors, baricitinib, ruxolitinib and tofacitinib) were able to restore normal cell viability, proliferation and neurogenesis by targeting the effects of IL12 and IL13. Overall, our results show that serum from COVID-19 patients with delirium can negatively affect hippocampal-dependent neurogenic processes, and that this effect is mediated by IL6-induced production of the downstream inflammatory cytokines IL12 and IL13, which are ultimately responsible for the detrimental cellular outcomes.