Persistent T cell phenotypic alterations and early innate immune dysregulation as potential biomarkers of Long COVID.

Long COVID is a complex condition characterized by a broad spectrum of persistent, multisystemic symptoms that often impair daily activities and cause disability with significant socioeconomic impact. Although several hypotheses have been proposed to explain its pathophysiology-including immune dysregulation-the underlying mechanisms remain poorly understood. Indeed, there are no current treatments for this condition. Therefore, we aimed to characterize immune alterations induced by SARS-CoV-2 during the acute phase of infection and three months post-hospital discharge in a cohort of patients who later developed Long COVID, referred to a matched cohort who did not. We performed high-dimensional immune profiling of peripheral blood mononuclear cells using a 40-marker mass cytometry panel. Data analysis combined classical hierarchical gating strategies with unsupervised computational approaches. Our results revealed distinct immune signatures between Long COVID and non-Long COVID patients, affecting both the innate and adaptive immune compartments. Notably, individuals who subsequently developed Long COVID exhibited early abnormalities at infection in innate immunity, particularly involving dendritic cells and γδ T cells. Moreover, three months post-discharge, persistent alterations were observed in several adaptive immune cell subsets in patients who next developed Long COVID over time. In summary, our findings suggest that early immune dysregulation following viral infection may predispose individuals to persistent post-viral pathology.