Treatment naive 14 Febrile Controls CXCL9/10 IL-27 M-CSF Spectral Flow Cytometry Upregulation of Conventional Dendritic Cells type 1 (cDC1) implicates antigen cross presentation in Multisystem Inflammatory Syndrome (MIS-C) cytokines MIS-C pa ents as compared to febrile controls Background: Multisystem inflammatory syndrome in children (MIS-C) is an acute, febrile, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-associated syndrome, often with cardiohemodynamic dysfunction. Insight into mechanism of disease is still incomplete. Objective: Our objective was to analyze immunologic features of MIS-C patients compared to febrile controls (FC). Methods: MIS-C patients were defined by narrow criteria, including having evidence of cardiohemodynamic involvement and no macrophage activation syndrome. Samples were collected from 8 completely treatment-naive patients with MIS-C (SARS-CoV-2 serology positive), 3 patients with unclassified MIS-C-like disease (serology negative), 14 FC, and 5 MIS-C recovery (RCV). Three healthy controls (HCs) were used for comparisons of normal range. Using spectral flow cytometry, we assessed 36 parameters in antigen-presenting cells (APCs) and 29 in T cells. We used biaxial analysis and uniform manifold approximation and projection (UMAP).Results: Significant elevations in cytokines including CXCL9, M-CSF, and IL-27 were found in MIS-C compared to FC. Classic monocytes and type 2 dendritic cells (DCs) were downregulated (decreased CD86, HLA-DR) versus HCs; however, type 1 DCs (CD11c 1 CD141 1 CLEC9A 1 ) were highly activated in MIS-C patients versus FC, expressing higher levels of CD86, CD275, and atypical conventional DC markers such as CD64, CD115, and CX3CR1. CD169 and CD38 were upregulated in multiple monocyte subtypes. CD56 dim /CD57 2 / KLRG hi /CD161 1 /CD38 2 natural killer (NK) cells were a unique subset in MIS-C versus FC without macrophage activation syndrome.