Suppression of Allograft Rejection By a Novel Protein ESAT-6.

Abstract

Achieving long-term allograft survival or tolerance without continuously global immunosuppression is a highly desirable goal in transplantation. The objective of this study is to induce long-term allograft survival without long-term conventional immunosuppression via short-term treatments with a novel protein ESAT-6, which is originally identified as a T cell Ag in the short-term culture filtrate of M. tuberculosis. Previous studies have shown that ESAT-6 is a virulence factor that promotes the growth and pathogenesis of M. tuberculosis while briefly weakening an individual's immunity. In this study, C57BL/6 mice were transplanted with Balb/C islets and treated with ESTA-6 (0.05mg i.p. on days 0, 2, 4 and 6) and/or low doses of rapamycin (0.2 mg/kg/day for 14 days). One-way MLRs were set up to determine alloreactive T cell proliferation by 3H-TdR uptakes and Th cell differentiation by ELISA while Graft-infiltrating T cell proliferation was measured by the BrdU labeling. We found that ESAT-6 significantly extended islet allograft survival (MST = 32 vs. 14 days, P<0.05). Importantly, treatments with both rapamycin and ESAT-6 further prolonged islet allograft survival [MST = 91 versus 30 days (rapamycin alone) or 32 days (ESAT-6 alone)], with 50% of recipient mice achieving long-term allograft survival (>100 days). ESAT-6 also suppressed graft-infiltrating T cell proliferation one week after transplantation (BrdU+: 29±3 vs. 41±4 %, P<0.05) while ESAT-6 plus rapamycin further inhibited T cell proliferation (BrdU+: 15±2 vs. 41±4 %, P<0.05). Moreover, ESAT-6 reduced IFN-gamma (6.8±0.7 vs 12.5±0.1.4, ng/ml, P<0.05) but increased IL-4 (5.8±0.7 vs. 3.1±0.4, ng/ml, P<0.05) and IL-10 (5.4±0.6 vs. 2.8±0.3, ng/ml, P<0.05) levels in the supernatant of MLRs while inhibiting T cell proliferation in vitro (CPM x1000: 8±1 vs. 18±3, P<0.05). However, it did not significantly alter IL-17A level and FoxP3+ cell numbers. Thus, for the first time, our studies demonstrate that ESAT-6 suppresses alloreactive T cell proliferation and promotes the induction of long-term allograft survival by tipping the balance of Th1/Th2 cell differentiation. Therefore, ESAT-6 could be utilized as a novel agent to induce long-term allograft survival in clinic.