Abstract
Immune cell dysregulation and lymphopenia characterize COVID-19
pathology in moderate to severe disease. While underlying inflammatory
factors have been extensively studied, homeostatic and mucosal migratory
signatures remain largely unexplored as causative factors. In this study
we evaluated the association of circulating IL-6, soluble mucosal
addressin cell adhesion molecule (sMAdCAM) and IL-15 with cellular
dysfunction characterizing mild and hypoxemic stages of COVID-19. A
cohort of SARS-CoV-2 infected individuals (n=125) at various stages of
disease progression together with healthy controls (n=16) were recruited
from COVID Care Centres (CCCs) across Mumbai, India. Multiparametric
flow cytometry was used to perform in-depth immune subset
characterization and to measure plasma IL-6 levels. sMAdCAM, IL-15
levels were quantified using ELISA. Distinct depletion profiles, with
relative sparing of CD8 effector memory and CD4+ regulatory T cells were
observed in hypoxemic disease within the lymphocyte compartment. An
apparent increase in the frequency of intermediate monocytes
characterized both mild as well as hypoxemic disease. IL-6 levels
inversely correlated with those of sMAdCAM and both markers showed
converse associations with observed lympho-depletion suggesting opposing
roles in pathogenesis. Interestingly, IL-15, a key cytokine involved in
lymphocyte activation and homeostasis, was detected in symptomatic
individuals but not in healthy controls or asymptomatic cases. Further,
negative association of plasma IL-15 with depleted T, B and NK subsets
suggested a compensatory production of this cytokine in response to the
profound lymphopenia. Finally, higher levels of plasma IL-15 and IL-6,
but not sMAdCAM, were associated with longer duration of
hospitalization.