Imaging Granzyme B Activity Assesses Immune-Mediated Myocarditis.

RATIONALE The development of molecular imaging approaches that assess specific immunopathologic mechanisms can advance the study of myocarditis. OBJECTIVE This study validates a novel molecular imaging tool that enables the in vivo visualization of granzyme B activity, a major effector of cytotoxic CD8+ T lymphocytes. METHODS AND RESULTS We synthesized and optimized a fluorogenic substrate capable of reporting on granzyme B activity and examined its specificity ex vivo in mice hearts with experimental cytotoxic CD8+ T lymphocyte-mediated myocarditis using fluorescence reflectance imaging, validated by histological examination. In vivo experiments localized granzyme B activity in hearts with acute myocarditis monitored by fluorescent molecular tomography in conjunction with coregistered computed tomography imaging. A model anti-inflammatory intervention (dexamethasone administration) in vivo reduced granzyme B activity (vehicle versus dexamethasone: 504±263 versus 194±77 fluorescence intensities in hearts; P=0.002). CONCLUSIONS Molecular imaging of granzyme B activity can visualize T cell-mediated myocardial injury and monitor the response to an anti-inflammatory intervention.