Expansion and activation of Vβ21.3+ T cells in Danish MIS-A cohort. (A) Timelines for individual MIS-A patients indicating date of MIS-A onset defined by symptom debut, evidence of SARS-CoV-2 infection, IVIG administration, blood sampling for PBMC isolation, and vaccination. (B) Frequency of total CD3+, CD4+, and CD8+ T cells expressing the indicated Vβ chains in Danish MIS-A PBMCs, as measured by flow cytometry. Bubbles represent individual Vβ frequency shown as the standard deviation (SD) from reference mean (m) value for each Vβ subset in the general healthy adult population. Expansion is defined by a frequency greater than the reference mean plus 2SD in healthy population. (C) Representative plots and graphical summaries of flow cytometric analyses of Vβ21.3+ versus Vβ21.3− subsets within CD3+ T cells of MIS-A patients. Upper: HLA-DR and CD38 expression on CD3+Vβ21.3+ and CD3+Vβ21.3− T cells. Middle: PD-1 and TIM-3 expression on CD38hiHLA−DR+ subset within CD3+Vβ21.3+ and CD3+Vβ21.3− T cells. Bottom: CD45RA and CCR7 expression defining naïve and memory subset distribution within CD3+Vβ21.3+ and CD3+Vβ21.3− T cells (naïve T cells are CD45RA+CCR7+, central memory T cells [TCM] are CD45RA−CCR7+, EM T cells [TEM] are CD45RA−CCR7−, and EM re-expressing CD45RA T cells [TEMRA] are CD45RA+CCR7−). Graphical summaries of CD45RA−CCR7− TEM frequencies are shown. Plots on the left (solid lines) correspond to MIS-A patients with Vβ21.3 expansion (MIS-A 1, MIS-A 3, and MIS-A 4), and plots on the right (dashed lines) correspond to MIS-A patients without Vβ21.3 expansion (MIS-A 2 and MIS-A 5). Representative contour plots correspond to patient MIS-A 1 (Vβ21.3 expanded) and MIS-A 5 (Vβ21.3 non-expanded).