Regulated expression of an MHC class II gene from a promoter-inducible retrovirus

Specific immune tolerance to fully allogeneic kidney grafts can be achieved in a miniature swine transplantation model by retrovirus-mediated transfer of allogeneic MHC class II genes into bone marrow cells (BMCs) of recipien t animals. Graft survival correlated with transient expression of the somat ic transgene (Tg) in the induction phase of tolerance, With the aim of inve stigating the effects of timing and threshold levels of Tg expression on in duction of hyporesponsiveness to the grafted tissues, two recombinant retro virus constructs containing the tetracycline binary regulatory system were used to achieve conditional expression of either the green fluorescent prot ein (tetGFP) as a control, or the porcine MHC class II DR beta chain (tetDR B), Effective downregulation of GFP gene transcription was demonstrated in transduced murine fibroblasts after doxycycline treatment, leading to a >90 % reduction of GFP fluorescence, Similar diminution of the DRB gene transcr iption was achieved in transduced pig endothelial cells (ECs). Drug-depende nt downregulation of DRBc gene expression in SLA(d) pig ECs coincided with complete inhibition of allogeneic activation of anti-class IIc-primed SLA(d ) T cells, These in vitro results suggest that the binary tetracycline retr ovirus system may also be adequate to regulate MHC class II Tg expression i n vivo.