Distribution and persistence of antigen-presenting cells after intrathymic injection

Intrathymic injection of donor immune cells has been shown by previous studies to prolong survival of rat allogeneic tissues. The aim of this pilot study was to assess the distribution and the persistence of intrathymically (i.t.) injected purified antigen presenting cells (APC) over a period of time in the rat model DA-to-WAG (RT1av to RT1u) using a specific monoclonal antibody (Mab) with RT1Aa class I polymorphic specificity (R3/13 clone). Purified non-parenchymal cells (NPC) or dendritic cells (DC) were prepared from liver and spleen of DA rats with a purity of greater than 60% and 90%, respectively, shown by selected Mab staining methods. DA NPC (1 x 10(6)) or DC (5 x 10(5)) in 20 microl were injected into both lobes of the thymus of WAG rats with or without 1 ml antilymphocyte serum (ALS) intraperitoneally. Thymus tissue was removed on days 3, 5, 10, 20 and 30, and processed for frozen sections and immunohistochemical staining with R3/13. Numerous DA-positive cells were detected in the first 3-10 days post-i.t. inoculation in both NPC- and DC-treated rats, with or without ALS. After day 10, the proportion of positive cells decreased in all cases except in rats given NPC and ALS, where similar numbers of R3/13-positive cells were seen throughout. These DA-positive cells were mostly found in the medullary portion of the thymus at the corticomedullary junction in close proximity to thymic dendritic interdigitating cells. We concluded from this pilot study that the injected cells remained in the thymus for a limited period. However, the immunosuppressive effect of ALS promoted some degree of persistence of donor APC in the thymus beyond 30 days. Further studies are in progress to reveal the specificity of these cells.