Ptld in Miniature Swine Following Novel Low Intensity Conditioning for Haploidentical Hct: the Mgh Experience

Haploidentical HCT (haplo-HCT) is a potent and potentially curative therapy for a number of lymphohematopoietic neoplasms. Haplo-HCT has been limited by toxic conditioning regimens and the development of lethal graft-versus-host disease (GVHD). Similar to what has been observed in the clinic, myeloablative HCT outcomes in the pig have a high incidence of GVHD. Less toxic, non-myeloablative haplo-HCTs developed in our laboratory resulted in a greatly reduced incidence and intensity of GvHD. However, post-transplantation lymphoproliferative disease (PTLD) was often observed following these novel reduced intensity conditioning protocols. Haplo-HCT conditioning with 700-1000 cGy thymic irradiation (TI), CD3 immunotoxin and 30-60 days of cyclosporine had a 6% incidence of GVHD (2 of 32) but was complicated by a 33% incidence of PTLD (11 of 32). Of these 11 pigs, 2 resolved their PTLD after cyclosporine was discontinued but then developed GVHD. When thymic irradiation was eliminated from the protocol, no PTLD was observed and GVHD was only observed in 2 of 24 (8.3%) transplanted pigs. Unfortunately this very mild preparatory regimen without TI failed to consistently achieve successful long term engraftment. In an attempt to decrease the incidence of PTLD, avoid GVHD and increase haplo-HCT engraftment outcomes, 100 cGy of total body irradiation (TBI) was introduced to the protocol. Of 46 haplo-HCT recipients, only 4 (<10%) developed PTLD and 2/46 animals developed GVHD, one of which recovered spontaneously. We also attempted to identify serum markers for diagnosing PTLD in our pigs. As it is observed in humans, we identified lactate dehydrogenase (LDH), to be elevated in pigs prior to onset of PTLD. In support of these findings, three porcine PTLD tumor cell lines harvested from animals and passaged in vitro demonstrate higher LDH levels compared to naïve PBMCs. We conclude that: (1) our current haplo-HCT protocol utilizing 100 cGy TBI, porcine CD3 immunotoxin and 45 days of cyclosporine is relatively safe and significantly reduces the incidence of PTLD; (2) stable engraftment and multilineage chimerism can be achieved with minimal GVHD; and (3) LDH is a clinically relevant serum marker for the diagnosis of PTLD in the pig. These findings reinforce the pig model as a valuable tool for HCT translational studies.