The interaction of the cell surface antigen Fas with Fas ligand (FasL) serves as a signaling pathway for apoptosis. Recent evidence that Fas related apoptosis plays a role in T cell cytotoxicity suggests that this process may be important in allograft rejection. Because ICAM-1 is involved as an accessory molecule in the process of T cell adherence/activation, we used reverse transcription polymerase chain reaction (RT-PCR) to examine the expression of Fas and FasL, as well as the lymphocyte surface molecules CD4 and CD8, in normal allografts (+/+) and allografts from mice genetically deficient in ICAM-1(-/-). Methods: Heterotopic abdominal cardiac transplants (Tx) were performed between H-2 disparate mice. Allografts harvested at time of peak rejection, as well as non-Tx control (C) hearts, were snap frozen. Samples analyzed included: +/+ C (n=2), -/- C (n=3), -/- D from +/+ R (n=3), +/+ D from +/+ R (n=3), and +/+ D from -/- R (n=3). After RNA extraction, samples were analyzed for Fas/FasL mRNA by oligo(dT) priming and RT, followed by PCR of cDNA using specific primer pairs. All samples were run simultaneously with β-actin primers to compare relative abundance of PCR products. Products were confirmed by direct sequencing.Results: Myocardium from all 5 C mice (2 +/+; 3-/-) had constitutive expression of Fas, however transcripts for FasL were not detected. In contrast, both FasL and Fas expression was confirmed by RT-PCR in 3/3 -/- and 3/3 +/+ allografts transplanted to normal recipients. Interestingly, neither Fas nor FasL was detected in 2 of the 3 normal allografts which had been transplanted to recipients genetically deficient in ICAM-1. Both CD4 and CD8 mRNA was detected in 8/9 allografts.Conclusions: (1) As has previously been described, Fas is constitutively expressed in murine myocardium, and FasL is up-regulated in rejecting allografts. (2) Deficiency of ICAM-1 in D heart does not appear to alter the expression of Fas/FasL during allograft rejection, although this preliminary data suggests that Fas/FasL expression may be altered in mice genetically deficient in ICAM-1. (3) CD4 and CD8 mRNA was detected in both normal and mutant allografts, suggesting that graft infiltration by these lymphocyte subsets is unaffected in this setting.