Differential effects of everolimus and cyclosporine A on intimal α-actin-positive cell dynamics of carotid allografts in mice

Background. We recently demonstrated that donor medial cells are replaced by smooth muscle α-actin (SMA)-expressing host cells in murine carotid allografts. In this study, we investigated neointimal cell dynamics including effects of cyclosporine A (CsA) and everolimus (SDZ RAD, Certican). To obtain a functional readout, we measured the effects of these treatments on cardiac allografts. Methods. Heterotopic heart allotransplantation was performed between C57BL/6 (B6) and BALB/c (B/c) mice. Orthotopic carotid artery allotransplantation was performed between B6 and B/c mice expressing green fluorescent protein (GFP). Both strains served as donors and recipients. Groups of mice were treated for 4 and 8 weeks (heart and carotid recipients, respectively) with placebo, CsA 20 mg/kg per day, or everolimus 1.0 mg/kg per day using Alzet minipumps. At 2, 4, and 8 weeks, carotid grafts were harvested for histology. Results. In the GFP-B/c to B6 strain combination, everolimus but not CsA significantly reduced neointima formation and accumulation of SMA-positive host (non-GFP) cells at doses that did not fully suppress heart rejection. In the B6 to GFP-B/c strain combination, everolimus and CsA strongly prevented heart rejection under treatment, partly suppressed neointima formation, and completely prevented SMA-positive host (GFP) cell accumulation. Conclusions. Donor-derived cells expressing SMA never appear in the neointima, and such cells are completely recipient derived. Adequate immunosuppression delays or prevents the disappearance of donor cells and the appearance of host cells in the graft, a phenomenon apparently associated with the neointima formation. Even immunosuppression sufficient to prevent heart allograft rejection under treatment completely, diminishes neointima formation only by approximately 50%.