2003). Correspondingly, the attenuated up‐regulation of cardiac MMP‐9 expression in APN‐KO mice subjected to CVB3 myocarditis was accompanied by significantly diminished ICTP serum levels compared to CVB3‐infected WT mice (Fig. 6D). Taken together, diminished MMP‐9 release from both resident cardiac and infiltrated immune cells seems to contribute to attenuated up‐regulation of cardiac MMP‐9 expression as well as decreased collagen type I cleavage in CVB3‐infected APN‐KO mice compared to their WT littermates (Fig. 7). Here, MMP9 is linked to myocarditis.