IL21 and viral myocarditis: Adaptive immunity predominates in the autoimmune myocarditis stage, and CD4+ T cells differentiate into various Th subsets, among which Th1 and Th17 cell subsets secrete cytokines IL-17, IFN-γ, TNF-α, and IL-21 in order to promote viral myocarditis progression, whereas differentiation to Th2 and Treg cell subsets improves viral myocarditis.