Combined host-conditioning with CTLA4-Ig, tacrolimus, anti-lymphocyte serum, and low-dose radiation leads to stable mixed hematopoietic chimerism

Objective. The toxic dose of irradiation required to achieve stable mixed h ematopoietic chimerism is the major limitation to its clinical application in transplantation and other nonmalignant conditions such as hemoglobinopat hies. This study examines the additive effect of costimulatory. blockage, t o our previously described tacrolimus-based conditioning regimen, in furthe r reducing the dose of total-body irradiation to achieve stable mixed chime rism in rats. Methods. Fully mismatched, 4- to 6-week-old ACI and Wistar Furth rats were used as donors and recipients, respectively, Recipients were administered C TLA4-Ig 2mg/kg/day (alternate days) in combination with tacrolimus 1 mg/kg/ day (daily) from day 0 through day +10, antilymphocyte serum 10 mg at day 10 (single dose!, and total-body irradiation ranging from 100-600 cGy, prio r to bone marrow transplantation (day 0) with 100 X 10(6) of T-cell-deplete d bone marrow cells, Levels of donor chimerism were determined over a perio d of 12 months, Results. The short course of CTLA4-Ig, tacrolimus, and ALS led to dramatic engraftments at reduced doses of irradiation: 100% (5/5) and 93% (13/14) of the animals developed mixed chimerism at 400 cGy and 300 cGy, respectively . Att 300 cGy, recipients exhibited durable, multilineage mixed chimerism a t 365 days with donor cells ranging from 19-42% (mean 23.4%) with no eviden ce of graft-vs-host disease. These mixed chimeras exhibited in vitro (mixed lymphocyte reaction) and in vivo (skin grafts) donor-specific, tolerance. Conclusion. This study suggests that addition of costimulatory blockade to a tacrolimus-based conditioning regimen reduces the dose of irradiation req uired to achieve stable multilineage chimerism in rats. (C) 2001 Internatio nal Society for Experimental Hematology. Published by Elsevier Science Inc.