We compared the effect of different immunosuppressive drug regimens on
both mean and long-term allograft survival and the histologic appeara
nce of donor/recipient chimeras in the ACI to Lewis rat heterotopic ca
rdiac transplant model. Intraabdominal cardiac transplantation was per
formed in standard fashion and microchimerism was assessed using immun
ohistochemical staining of cut sections of recipient spleen and lymph
nodes. All of the drug regimens studied resulted in excellent mean all
ograft survival. Furthermore, long-term (>120 day) allograft survival
was consistently observed in each group. In fact, many of the recipien
ts continue to have functioning heterotopic grafts even as they approa
ch the end of the natural life span of a Lewis rat. Mixed allogeneic c
himerism, however, was not observed with 100% frequency despite the co
nsistent induction of graft tolerance. This finding may be related to
the immunosuppressive agents used or the sensitivity of the immunohist
ochemical assay. Therefore, the exact role, if any, of chimerism in th
e induction of graft tolerance remains unanswered. (C) 1996 Academic P
ress, Inc.