BONE-MARROW TRANSPLANTATION IN MINIATURE SWINE .4. DEVELOPMENT OF MYELOABLATIVE REGIMENS THAT ALLOW ENGRAFTMENT ACROSS MAJOR HISTOCOMPATIBILITY BARRIERS

Studies of the myeloablative regimens capable of permitting successful BMT across MHC barriers in miniature swine have been performed. To mi nimize graft-versus-host disease (GVHD), engraftment was studied in th e Fl-->P combination (i.e., MHC homozygous [''parental''] swine receiv ing bone marrow from one-haplotype matched MHC heterozygous [''F1''] d onors). Animals given total body irradiation (TBI) up to 1100 cGy, 10 cGy/min, in a single dose failed to engraft. Increasing the dose rate led to unacceptable extramedullary toxicity without improving engraftm ent. Eleven different fractionated TBI regimens were tested in this F1 -->parent model. At all of the dose rates tested, a total dose of less than 1000 cGy was insufficient for engraftment, and a total dose of 1 400 cGy led to unacceptable toxicity. Between these extremes, a window was defined in which engraftment could be obtained without unacceptab le extramedullary toxicity utilizing 2 equally divided fractions of TB I delivered 24 hr apart. The addition of 50 mg/kg cyclophosphamide i.v . to fractionated TBI (1150 cGy total dose [500 + 650]) also permitted engraftment, with decreased incidence of interstitial pneumonitis as compared to fractionated TBI (1300 cGy total dose [650x2]). Both of th ese regimens were also confirmed to permit engraftment between heteroz ygous donors and recipients sharing a single common haplotype (''F1--> F1''). The regimen of 1300 cGy (650x2) also permitted engraftment in c ompletely MHC mismatched BMT, but with subsequent death from GVHD. The se studies of the myeloablative regimens permitting engraftment across defined MHC barriers in miniature swine provide a basis for further s tudies of allogeneic BMT and GVHD in this large animal preclinical mod el.