Prolyl Endopeptidase contributes to early neutrophilic inflammation in acute cardiac transplant rejection

Altered inflammation and tissue remodeling are cardinal features of cardiovascular disease and cardiac transplant rejection. Neutrophils have increasingly been understood to play a critical role in acute rejection and early allograft failure; however, discrete mechanisms that drive this damage remain poorly understood. Herein, we demonstrate that early acute cardiac rejection increases allograft prolyl endopeptidase (PE) in association with de novo production of the neutrophil pro-inflammatory matrikine proline-glycine-proline (PGP). In a heterotopic murine heart transplant model, PGP production and PE activity were associated with early neutrophil allograft invasion and allograft failure. Pharmacologic inhibition of PE with Z-Pro-Prolinal reduced PGP, attenuated early neutrophil graft invasion, and reduced pro-inflammatory cytokine expression. Importantly, these changes helped preserve allograft rejection-free survival and function. Notably, within two independent patient cohorts, both PGP and PE activity were increased among patients with biopsy-proven rejection. The observed induction of PE and matrikine generation provides a novel link between neutrophilic inflammation and cardiovascular injury, represents a potentially new target to reduce allogenic immune responses, and uncovers a previously unrecognized mechanism of cardiovascular disease.