Microchimerism in thymus is associated with UP-regulated helper type 1 cytokine transcription during cardiac allograft rejection in rats

Background. Intrathymic microchimerism (MC) is thought to be responsible fo r inducing allograft tolerance. However, the role of MC in the thymus gland after transplantation, particularly in the rejection response, is unknown. We investigated serial changes in intrathymic cytokine production associat ed with MC and allograft rejection. Methods. Donor-specific cell injection (DSI) and heterotopic heart transpla ntation (HTx) were performed in the fully allogeneic combination using DA r ats (RT1(a)) as donors and WS rats (RT1(k))as recipients. MC was checked by polymerase chain reaction (PCR) using a donor RT1.B beta domain 1 region s equence-specific primers. Reverse transcription (RT)-PCR analysis of cytoki ne (interleukin [IL]-2, interferon-gamma, IL-4, and IL-10) profiles of the thymus was performed in animals given DSI, HTx, or DSI/HTx. Results. DSI alone resulted in an immediate development of MC, detected by PCR, in various organs including the thymus, spleen, liver, and blood, of m ost rats, lasting for over 2 months. However, DSI-induced MC selectively di sappeared in the thymus on day 7 after grafting, several days before the re jection of cardiac allograft, RT-PCR analysis of cytokine profiles showed t hat the levels of Th1 (IL-2 and interferon-gamma) cytokines transcribed in the thymus were higher than in the spleen. MC reappeared in the thymus on d ay 21 after grafting, but was not associated with elevation of Th1 cytokine transcription when allograft was replaced by fibrosis. Conclusions. Intrathymic MC does not always confer unresponsiveness to allo antigen, but can be eliminated after anti-donor response.