NK Cell and Monocyte Dysfunction in Multisystem Inflammatory Syndrome in Children

To investigate the cellular mechanisms contributing to the immune dysregulation observed in multisystem inflammatory syndrome in children (MIS-C), particularly focusing on antibody-mediated effector functions. Despite robust neutralizing antibody production in MIS-C, children often remain SARS-CoV-2 polymerase chain reaction positive for weeks after infection. This study aimed to determine whether impaired Fc receptor–mediated cellular responses, specifically monocyte and natural killer (NK) cell functions, underlie the persistent inflammation and delayed viral clearance seen in MIS-C. The study also explored whether functional restoration of NK cell activity could be achieved using engineered bispecific and trispecific immune cell engagers.The study prospectively enrolled 66 SARS-CoV-2–infected individuals and 21 uninfected controls between May 2020 and May 2021. Pediatric participants (aged <21 years) were recruited from Children’s Hospitals and Clinics of Minnesota and Indiana University. Samples were collected prior to intravenous immunoglobulin (IVIG) or immunomodulatory therapy. Additional genomic data were obtained from 20 patients with MIS-C at the Karolinska Institute. Participants with immunodeficiency, malignancy, or poor sample quality were excluded.This prospective observational study analyzed peripheral blood samples from pediatric and adult participants categorized by MIS-C and COVID-19 status. Samples were collected within 72 hours of hospitalization and prior to administration of IVIG or other immunomodulatory therapies. Peripheral blood mononuclear cells, plasma, and red blood cells were isolated and cryopreserved. Researchers conducted flow cytometry–based immunophenotyping and ex vivo functional assays to assess antibody-dependent cellular phagocytosis (ADCP), antibody-dependent cellular cytotoxicity, natural cytotoxicity, and cytokine production. Recombinant bispecific (BiKE) and trispecific (TriKE) killer engagers were used to stimulate or rescue NK cell function. Multiplex cytokine analysis and enzyme-linked immunosorbent assay were employed to quantify systemic cytokine levels. Genetic analysis of Fc receptor polymorphisms was performed using whole-genome sequencing data from a Swedish MIS-C cohort and compared with public databases. Statistical analyses included nonparametric tests (eg, Kruskal-Wallis) and principal component analysis for genetic variation.Children with MIS-C demonstrated dysregulated antibody-mediated cellular immune responses. Despite strong neutralizing antibody levels, they exhibited persistent inflammation and SARS-CoV-2 positivity. Monocytes in MIS-C showed increased ADCP and produced higher levels of proinflammatory cytokines (interleukin [IL]-6, IL-1β, IL-32, tumor necrosis factor α) compared with controls. This hyperfunction was not linked to differences in Fc receptor expression or genetic polymorphisms. NK cells in MIS-C were hypofunctional, with reduced cytokine production (eg, interferon γ), lower degranulation, and decreased perforin expression. These defects correlated with elevated systemic IL-6 and changes in NK cell markers of expression, (programmed cell death protein 1 and T cell immunoglobulin and ITIM domain) correlating with NK exhaustion. NK cell function was partially restored in vitro using novel CD16-targeting bispecific and trispecific killer engagers (BiKEs/TriKEs), highlighting potential therapeutic approaches. Overall, the study identified a pattern of hyperactive monocytes and exhausted NK cells in MIS-C, suggesting that misregulated antibody-effector cell interactions contribute to disease pathology.The study demonstrates that children with MIS-C exhibit a distinct pattern of immune dysregulation, characterized by exaggerated monocyte activation and impaired NK cell function, despite the presence of strong neutralizing antibodies. This imbalance in antibody-mediated effector responses likely contributes to prolonged inflammation and tissue damage. Restoration of NK cell activity using CD16-engaging immune engagers in vitro suggests a potential therapeutic avenue. These findings provide critical insight into MIS-C pathogenesis and offer a foundation for more targeted diagnostic and immunomodulatory strategies.This study highlights distinct innate immune dysregulation in MIS-C, characterized by hyperfunctional monocytes and hypofunctional, exhausted NK cells, despite robust antibody responses. Although the study does not directly propose specific clinical interventions, these findings imply that immune monitoring in MIS-C may need to extend beyond antibody titers to include functional assessment of innate immune cells. Additionally, NK cell exhaustion emerges as a potential therapeutic target, with CD16-engaging agents like BiKEs and TriKEs showing promise in restoring NK cell function in vitro.