Regulatory T (T-reg) cells offer new therapeutic options for controlling undesired systemic and local immune responses. The aim of the current study was to determine the impact of therapeutic T-reg administration on systemic and cardiac inflammation and remodeling in coxsackievirus B3 (CVB3) -induced myocarditis. Therefore, syngeneic T-reg cells were applied intravenously in CVB3-infected mice 3 d after infection. Compared with CVB3 + PBS mice, CVB3 + T-reg mice exhibited lower left ventricle (LV) chemokine expression, accompanied by reduced cardiac presence of proinflammatory Ly6C(high)CCR2(high)Cx3Cr1(low) monocytes and higher retention of proinflammatory Ly6C(mid)CCR2(high)Cx3Cr1(low) monocytes in the spleen. In addition, splenic myelopoiesis was reduced in CVB3 + T-reg compared with CVB3 + PBS mice. Coculture of T-reg cells with splenocytes isolated from mice 3 d post-CVB3 infection further demonstrated the ability of T-reg cells to modulate monocyte differentiation in favor of the anti-inflammatory Ly6C(low)CCR2(low)Cx3Cr1(high) subset. T-reg-mediated immunomodulation was paralleled by lower collagen 1 protein expression and decreased levels of soluble and insoluble collagen in LV of CVB3 + T-reg compared with CVB3 + PBS mice. In agreement with these findings, LV systolic and diastolic function was improved in CVB3 + T-reg mice compared with CVB3 + PBS mice. In summary, adoptive T-reg transfer in the inflammatory phase of viral-induced myocarditis protects the heart against inflammatory damage and fibrosis via modulation of monocyte subsets.Pappritz, K., Savvatis, K., Miteva, K., Kerim, B., Dong, F., Fechner, H., Muller, I., Brandt, C., Lopez, B., Gonzalez, A., Ravassa, S., Klingel, K., Diez, J., Reinke, P., Volk, H.-D., Van Linthout, S., Tschope, C. Immunomodulation by adoptive regulatory T-cell transfer improves Coxsackievirus B3-induced myocarditis.