INTRATHYMIC TOLERANCE IN THE LEWIS-TO-F344 CHRONIC CARDIAC ALLOGRAFT-REJECTION MODEL

Successful induction of donor-specific unresponsiveness by intrathymic inoculation of alloantigen in several experimental acute rejection mo dels has led us to hypothesize that similar immune manipulations can p revent chronic rejection and development of graft arteriosclerosis in the Lewis-to-F344 rat chronic cardiac allograft rejection model. Recip ient F344 rats were treated with donor (Lewis) splenocytes by intrathy mic injection (i.t.) alone (10x10(6) cells/lobe); with donor splenocyt es i.t. plus a one-time dose of ALS (1 mg) by intraperitoneal injectio n (i.p.); or with ALS i.p. (1 mg) alone 2 and 6 weeks prior to heterot opic Lewis heart transplantation. Control F344 recipients received sal ine i.t. Allografts were monitored by daily palpation, and long-term s urviving grafts were harvested on day 90 for histopathologic analysis. Control allografts had 28.6% long-term survival (>90 days) with mean graft survival of 46.7 +/- 12.2 days. At day 90 the surviving control allografts were enlarged and fibrotic with barely palpable heartbeat ( mean heartbeat grade 0.29 +/- 0.18), and histologically showed diffuse moderate mononuclear cell infiltrates and advanced graft arterioscler osis (mean vessel score 3.57 +/- 0.10 and 89 +/- 1% vessels diseased). Recipient treatment with intrathymic donor splenocytes alone signific antly prolonged graft survival (89% long-term survival; mean 83.8 +/- 6.2 days, P<0.04), but did not significantly inhibit the development o f graft arteriosclerosis (score 2.98 +/- 0.53 and 79 +/- 8% diseased, P=NS). By contrast, treatment with i.t. donor splenocytes plus ALS 2 w eeks prior to transplantation prolonged graft survival (100% long-term ; mean 90.0 +/- 0.0 days, P<0.04), and markedly inhibited graft arteri osclerosis (score 0.80 +/- 0.14, P<0.05; 21 +/- 4% diseased, P<0.05). ALS alone given two weeks prior to transplantation also prolonged graf t survival (100% long-term; mean 90.0 +/- 0.0 days, P<0.04), and inhib ited graft arteriosclerosis (score 0.89 +/- 0.31, P<0.05; 25 +/- 7% di seased, P<0.05). However, when ALS was given 6 weeks prior to heart tr ansplantation the beneficial effect of ALS alone was abolished, sugges ting that lymphocyte depletion may have been responsible for the obser ved effects when ALS was administered at 2 weeks. Interestingly, intra thymic donor splenocytes plus ALS given 6 weeks prior to transplantati on, on the other hand, showed significant prolongation of allograft su rvival (100% long-term, mean 90.0 +/- 0.0 days, P<0.04), and inhibited graft arteriosclerosis (score 0.41 +/- 0.02, P<0.05; 16 +/- 2% diseas ed, P<0.05). Our results indicate the importance of the cell-mediated immune response in initiating and mediating development of chronic rej ection and graft arteriosclerosis, and suggest that strategies aimed a t induction of tolerance may be effective in their prevention.