Expansion and activation of Vβ21.3+ T cells in MIS-C. (A) Frequency of total CD3+ T cells expressing the indicated Vβ chains in MIS-C PBMCs, as measured by flow cytometry. Bubbles represent individual Vβ frequency shown as the standard deviation (SD) from age-dependent reference mean (m) value for each Vβ subset in the general healthy pediatric population. Expansion is defined by a frequency greater than the reference age-dependent mean plus 2 SD in healthy population. (B) Upper: representative plot of Vβ21.3 staining on CD3+ T cells of a MIS-C patient. Middle and bottom: representative plots of flow cytometric analyses of Vβ21.3+ versus Vβ21.3− subsets within CD3+ T cells of the same MIS-C patient, respectively. Expression of HLA-DR, CD38, PD-1, and TIM-3 as well as CD45RA and CCR7 expression defining naïve and memory subsets within CD3+ T cells are shown.