Mixed xenogeneic chimerism (rat -> mouse) following sublethal conditioning: Targeting of recipient T- and NK-cells prevents rejection of xenogeneic bone marrow cells

Background: Bone marrow chimerism induces donor-specific tolerance and prev ents the rejection of concordant xenografts. We have previously presented t hat sublethal conditioning with 700 cGy TBI followed by a single dose of Cy clophosphamide (CyP) on day + 2 allows for stable engraftment of rat bone m arrow in mice. Aim of this study was to further reduce the TBI dose require d for engraftment by specific targeting of radio-resistant host cells that mediate rejection of xenogeneic bone marrow. Methods: B10 mouse recipients were treated with 600 cGy TBI and transplante d with 40 X 10(6) bone marrow cells from F344 rats. On day + 2 a single dos e of 50 mg/kg CyP wa:; injected intraperitoneally. Experimental recipients were pretreated with anti-CD4, anti-CD8 or anti-NK1.1 mAb on day - 3 and - 1. Chimerism was assessed by flow cytometry on day 28 and monthly thereafte r. Results: Conditioning with TBI and CyP alone resulted in detectable chimeri sm in 25% (3/12) of transplanted animals. Engraftment of rat bone marrow wa s achieved in 100% of animals (4/4) when CD4(+) and CD8(+) cells were speci fically targeted in the host prior to TBI using mAbs. The reduction of NKcells using anti-NK1.1 mAb was almost as effective (75%). Pretreatement wit h anti-CD4 or anti-CD8 mAb alone did not enhance engraftment of rat bone ma rrow. Chimeras presented stable mixed xenogeneic chimerism throughout the f ollow up of 6 months. Conclusion: Specific targeting of radioresistant host cells namely T- and N K-cells that mediate the rejection of xenogeneic bone marrow cells allows f or a reduction of the TBI dose required for engraftment.