Medial smooth muscle cell loss in arterial allografts occurs by cytolytic cell induced apoptosis.
OBJECTIVE: Experimental arterial allografts, used as models of chronic rejection, undergo marked loss of smooth muscle cells (SMC) from their media prior to the development of occlusive, intimal proliferative lesions. Medial SMC loss has been described in human heart transplants, and may be related to the development of occlusive coronary lesions which are the hallmark of chronic rejection. This SMC loss does not exhibit the characteristics of necrotic cell death. We sought to determine whether medial SMC loss in arterial allografts occurs by apoptosis. We further investigated these allografts for cytolytic cell-derived inducers of apoptosis, Finally, we compared two different strain combinations to assess the impact of varying histoincompatability on medial SMC loss. METHODS: Evidence for internucleosomal DNA degradation, which is characteristic of apoptosis, was sought by the in situ terminal deoxynucleotidyl transferase nick end labelling (TUNEL) method carried out on Lewis to Fisher rat femoral artery transplants (disparate at minor loci only) and Brown Norway to Lewis aortic transplants (fully disparate at major and minor loci). Isografts (Lewis to Lewis) served as controls. In a separate series of experiments graft mRNA was extracted and analysed by reverse transcription-polymerase chain reaction (RT-PCR) with primers for molecular inducers of apoptosis (TNF-alpha, Fas ligand, perforin, and granzyme-B) which are derived from cytolytic cells known to be present in allografts. RESULTS: Allograft media contained large numbers of TUNEL stained nuclei in both strain combinations. Neither isografts nor ungrafted femoral artery segments stained positive for apoptosis. RT-PCR on whole allografts in both strain combinations revealed sustained upregulation of perforin, granzyme-B, Fas-ligand and TNF-alpha mRNA concomitant with medial SMC loss. Autografts demonstrated sustained up regulation of TNF-alpha, and perforin, but only brief upregulation of granzyme-B, and no upregulation of Fas-ligand. CONCLUSIONS: These data strongly suggest that medial SMC loss in allograft arteriopathy occurs by apoptosis. Further, RT-PCR data indicate that cytolytic cell-derived inducers of apoptosis are upregulated in these grafts and may be accountable for medial SMC apoptotic cell death. Finally, fully-disparate (Brown Norway to Lewis) and minor-only incompatible (Lewis to Fisher) strain combinations both resulted in marked intimal proliferation, medial SMC loss by apoptosis, and similar patterns of expression of cytolytic cell derived inducers of apoptosis. Insofar as intimal proliferative lesion-formation may be dependent on medial damage (as in arterial-injury models), understanding the mechanism of medial SMC loss may provide a novel therapeutic approach to human cardiac transplant arteriopathy.
Hirsch, GM; Kearsey, J; Burt, T; Karnovsky, MJ; Lee, T
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