Cellular repopulation of human vein allograft bypass grafts

Vein allografts are an alternative conduit for patients lacking available autogenous vein. The ability to develop a neoendothelium is a potential advantage of allografts over other nonautogenous grafts, because endothelial cells have been shown to play numerous essential roles in vessel survival. However, repopulation by endothelial cells has not previously been demonstrated or characterized in human subjects.

In our prospective trial, 40 patients (20 men, 20 women) underwent cryo- preserved saphenous vein bypass grafting procedures for limb salvage. Several patients underwent multiple grafting procedures. All grafts were sampled at implantation. During the 31 month follow-up interval, 22 allografts were explanted at the time of revision or subsequent surgical procedure. All grafts (22 of 22) demonstrated intact endothelium at implantation and explantation. Seventeen explantation biopsy samples (seven from men, 10 from women) from 16 patients (seven men, nine women) were adequate for further histologic and immunofluorescent analysis. Explants were stained with hematoxylin-eosin and immunohistochemical markers to quantitate rejection and also underwent fluorescence in-situ hybridization, with probes for X and Y chromosomes and counterstain for nuclear envelope. Cells were counted as XX, XY, XO, YO, or unstained. The endothelium and vessel walls were analyzed for origin of cells based on sex-mismatched transplants, with sex-matched transplants serving as controls.

Evidence of cellular damage was noted in all explanted allografts, and moderate or severe rejection (lymphocyte infiltrate, +CD3, +CD8, +CR3, cytotoxic granules) was noted in six explanted allografts (29%). All allografts demonstrated intact endothelium (complete or partial), at the time of both implantation and explantation. Sex-matched (male to male) control explants showed only male cells, as expected. Male donor-female recipient transplants showed complete repopulation by recipient (female) cells in nine of 10 cases (90%), whereas one case (10%) demonstrated partial repopulation (a mosaic of male and female cells). One patient's slides were unreadable. Findings in cells of the allograft wall were identical to those of the endothelium (nine recipient-only cells and one mosaic). Complete absence of donor cells was noted as early as 1 week after implantation, but mosaicism was demonstrated in one patient 3 months after grafting. No relationship could be demonstrated between repopulation and time (P >.05), quantity of rejection (P >.05), or donor age (P >.05).

Both the endothelial lining and vessel wall of venous allografts repopulate with cells of recipient origin, resulting in either a completely novel cellular constituency or a mosaic of host and donor cells. The loss of donor cells may be mediated by apoptosis or rejection, and the rate of migration of repopulating host cells is, at this point, unclear. Although the development of a completely endothelial-lined conduit offers a potential advantage over other alternative conduits, the functional status of the neoendothelium and repopulated vessel wall and their role in maintenance of allograft patency require further investigation.