A previous trial of intravenous immunoglobulin therapy (IVIg) added to guideline-directed medical therapy (GDMT) in patients with dilated cardiomyopathy (DCM) and cardiac parvovirus-B19 (B19V) persistence showed no improvement of cardiac function at six months follow-up. We investigated whether IVIg confers long-term clinical benefits and analyzed its molecular and cellular effects in cardiac tissue to elucidate the immunomodulatory mechanisms underlying the observed long-term clinical improvement.