Fatigue after COVID-19 infection is associated with peripheral immunometabolic alterations affecting neuroimmune responses in the hippocampus.

Fatigue is a common and disabling symptom reported following SARS-CoV-2 infection, yet the underlying biological mechanisms remain poorly understood. In this study, we investigated whether fatigue severity in individuals previously infected with SARS-CoV-2 is associated with immune and metabolic alterations in serum and whether these peripheral changes can influence hippocampal cell function in vitro. Serum cytokines, kynurenine pathway, and tryptophan-derived and monoamine-related metabolites were measured in a total of 38 individuals with past COVID-19 infection. Human hippocampal progenitor cells were exposed to 1% patient serum during proliferation and differentiation, with readouts including cytokine release, metabolite production, and markers of neurogenesis (doublecortin, DCX) and astrocytic reactivity (glial fibrillary acidic protein, GFAP; aquaporin-4, AQP4). Results show that fatigue severity correlates with lower serum levels of interleukin-8 (IL-8) and with lower levels of metabolites of the kynurenine pathway and tryptophan-derived and monoamine-related metabolites, including kynurenine (KYN) and quinolinic acid (QUIN), and 5-hydroxyindoleacetic acid (5HIAA). Exposure of hippocampal cells to serum from individuals with higher fatigue was associated with increased endogenous production of interleukin-13 (IL-13) and the kynurenine metabolite anthranilic acid (ANA) in the cell supernatant, as well as with increased neurogenesis (increased DCX expression) and enhanced astrocytic reactivity (increased GFAP expression). Notably, serum IL-8 level was inversely correlated with both cellular outcomes. Likewise, serum 5-HIAA levels were negatively correlated with IL-13 release, with mediation analysis indicating that 5-HIAA significantly mediated the association between fatigue severity and IL-13 production (71% explained). Overall, our results suggest that fatigue after COVID-19 infection is associated with neuroimmune and metabolic changes in hippocampal cells, involving peripheral serotonin metabolism (5-HIAA) and cytokine signalling (IL-13).