Brief cyclosporine treatment prevents intrathymic (IT) tolerance induction and precipitates acute rejection in an IT rat cardiac allograft model

Background. Intrathymic (IT) alloantigen combined with administration of rabbit anti-rat anti-lymphocyte serum (ALS) intraperitoneally induces donor- specific tolerance to rat cardiac transplants. The purpose of this study was to examine the effect of a brief course (4 days) of cyclosporine (CsA) on the development of IT tolerance. Methods. Buffalo (BUF) (RT1(b)) rats were given 25 x 10⁶ fully MHC-mismatched Lewis (LEW) (RT1¹) splenocytes by IT injection plus 1.0 ml of ALS intraperitoneally. Twenty-one days later, IT donor-specific LEW (group 1) or third-party (ACI, RT1(a)) (group 2) hearts were heterotopically transplanted to the abdominal aorta. A third group of BUF (group 3) were given daily CsA (10 mg/kg) by oral gavage for 4 days before administration of IT LEW cells and ALS. Rejection as defined by the cessation of a palpable heartbeat was confirmed by histology. Cytokine profiles of allografts from all groups were then analyzed using a multi-probe RNase protection assay. Results. Sixty-seven percent of IT/ALS-treated BUF recipients not pretreated with CsA accepted LEW heart grafts for greater than 90 days. However, 86% of animals treated with CsA for 4 days before IT injection and ALS rejected allografts at 10.7±3.2 days. Third-party allografts (ACI) were uniformly rejected (7.0±0.0 days). Histology confirmed cellular rejection in CsA-treated allografts and cytokine analysis detected increased interleukin (IL)-3, IL-5, and tumor necrosis factor-α when compared to increased IL-2 and interferon-γ in rejecting untreated controls. Conclusions. CsA can prevent the induction of intrathymic alloantigen tolerance. These results support the development of a CsA-sensitive, but IL- 2-independent, active regulatory mechanism after intrathymic exposure to donor-specific alloantigen and depletion of mature peripheral T cells.