Alloimmune-mediated apoptosis - Comparison in mouse models of acute and chronic cardiac rejection

Background. The purpose of the present study was (1) to compare apoptotic a ctivity in models of acute and chronic rejection and (2) to study the cellu lar distribution of parenchymal versus inflammatory cell apoptosis, Methods. Heterotopic cardiac mouse transplantation (CBA into C57BL/6) was u sed to produce allografts undergoing acute (day 7, untreated recipients, n= 6) or chronic (day 55, anti-CD4/8 for 28 days, n=6) rejection. As reference s, we used 55-day isograft controls (n=5) and native hearts (n=6), To asses s apoptotic activity, we quantified DNA laddering ((32)p incorporation), DN A fragmentation (antinucleosome ELISA), and caspase-1 transcript levels (P- 32-reverse transcriptase-polymerase chain reaction). To localize apoptosis, we performed terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. Results, DNA laddering and nucleosome levels were increased in allografts u ndergoing acute or chronic rejection when compared with both controls. Both parameters were twofold higher in acutely compared with chronically reject ing hearts. Caspase-1 transcript levels were increased in acutely (P<0.0001 ) and chronically rejecting hearts (P=0,004), Acutely rejecting grafts had more terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling- positive nuclei (53+/-3 nuclei/high-powered field) than chronically rejecti ng grafts (9+/-1 nuclei/high-powered field, P<0.0001), but the distribution between graft-infiltrating inflammatory cells and myocytes was similar. Va scular cells undergoing apoptosis were infrequent in both forms. Conclusion. Using four separate indices, apoptotic activity is more pronoun ced in cardiac allografts undergoing acute compared with chronic rejection. This reflects, in part, the degree of alloimmune response. However, we spe culate that the contributions of apoptosis to various forms of rejection ar e multifactorial, The long-term outcome to the graft may depend upon the ma gnitude, timing, and target of programmed cell death.