Abstract
SIGNALING THROUGH STAT4 OR STAT6 IS NOT REQUIRED FOR ACUTE ALLOGRAFT REJECTION OR FOR THE INDUCTION OF TOLERANCE BY CTLA4-IG TREATMENT IN A MURINE CARDIAC TRANSPLANT MODEL.
Ping Zhou, Greg L. Szot, Christine Guo, Gang He, Jun Wang, Kenneth A. Newell, J. Richard Thistlethwaite, Jeffrey A. Bluestone, Maria-Luisa Alegre. University of Chicago, Chicago, IL.
To address the roles of Th1 differentiation in the induction of acute cardiac allograft rejection and of Th2 differentiation in the induction of tolerance following costimulation blockade, we have utilized STAT4- and STAT6-deficient mice. STAT4-/- mice lack IL-12 signaling and have impaired Th1 differentiation, whereas STAT6-/- mice are devoid of IL-4 signaling and fail to generate Th2 cells. Fully allogeneic hearts from C3H/HEN mice were vascularized to the abdominal cavity of wild-type, STAT4-/- and STAT6-/- recipients on a BALB/c background. All recipients rejected the grafts promptly (9+1 days). When the CD28/B7 costimulatory pathway was blocked by the administration of CTLA4-Ig, cardiac graft survival was prolonged. Interestingly, although 100% of the CTLA4-Ig-treated wild-type and STAT4-/- mice retained their grafts for >100 days, approximately 30% of the CTLA4-Ig-treated STAT6-/- mice rejected their grafts between 20 and 100 days. CTLA4-Ig appeared to induce tolerance in most of the animals, inasmuch as all mice retaining the first graft for >100 days accepted a second C3H/HEN cardiac graft vascularized to the cervical vessels. Similarly, when splenocytes from CTLA4-Ig-treated STAT6-/- mice having retained a cardiac allograft for >100 days were adoptively transferred into syngeneic BALB/c/SCID recipients, abdominal C3H/HEN cardiac allografts were accepted whereas third party C57BL/6 hearts were rejected. In contrast, both C3H/HEN and C57BL/6 hearts were rejected in recipients of splenocytes from untreated STAT6-/- mice. Analysis of in situ cytokine gene expression by RT-PCR in grafts harvested at the time of rejection in untreated animals revealed decreased levels of IFN-g in STAT4-/- recipients and undetectable levels of IL-4 and IL-5 in STAT6-/- mice. In animals treated with CTLA4-Ig at >100 days post-transplant, levels of IFN-g were significantly reduced in wild-type and STAT6-/- mice. In contrast, levels of IL-4 were not modified by CTLA4-Ig treatment in all treated groups. We conclude that neither STAT4 nor STAT6 signaling is required for effective rejection of cardiac allografts in mice. In addition, prolongation of cardiac allograft survival by blockade of CD28/B7 pathway is also STAT4- and, at least in part, STAT6-independent. These data indicate that impaired Th1 differentiation does not prevent allograft rejection whereas lack of Th2 differentiation does not totally preclude tolerance induction.