Myeloid-derived suppressor cells and T cell populations in children with Multisystem Inflammatory Syndrome.

Background: Multisystem inflammatory syndrome in children (MIS-C) represents a hyperinflammatory state that can result in multi-organ dysfunction and death. Myeloid-derived suppressor cells (MDSC) are an immunosuppressive cell population that expands under inflammatory conditions and suppresses T cell function. We hypothesized that MDSC would be increased in children with MIS-C and that MDSC expansion would be associated with T cell lymphopenia.

Methods: We conducted a prospective, observational study. Initial blood samples were collected within 48 h of admission. Age-matched healthy controls underwent sampling once. MDSC and T cell populations were identified by flow cytometric methods.

Results: We enrolled 22 children with MIS-C (12 ICU, 10 ward) and 21 healthy controls (HC). Children with MIS-C demonstrated significantly higher MDSC compared to HC, and MDSC expansion persisted for >3 weeks in the ICU group. Children with MIS-C admitted to the ICU demonstrated significantly lower absolute numbers of T cells and natural killer cells. There were no significant associations between MDSC and cardiac dysfunction, duration of hospitalization, or vasoactive inotrope score.

Conclusions: Our study suggests that children critically ill with MIS-C have expansion of MDSC and associated decreased T cell and NK cell populations. Our results did not demonstrate associations between MDSC and clinical outcomes.

Impact: Multisystem inflammatory syndrome in children (MIS-C) is a dysregulated immune response occurring several weeks after SARS-CoV-2 infection that can result in multi-organ dysfunction and death. Children severely ill with MIS-C demonstrated increased myeloid-derived suppressor cells and decreased absolute numbers of CD4+ and CD8 + T cells and NK cells compared to healthy controls. There was no significant association between MDSC numbers and clinical outcomes; including cardiac dysfunction, length of stay, or requirement of vasoactive support, in children with MIS-C.