Chimeric Thymus Rudiments: A Strategy for the Study of Tolerance Induction in Vitro

The techniques of fetal thymus organ culture have been widely utilized for the study of thymocyte differentiation under carefully controlled conditions. Recent results suggesting a role for dendritic cells (DC) in selection of a competent T-cell repertoire have prompted attempts to construct chimeric thymus rudiments in vitro. Here, we describe a novel approach based on the migratory properties of mature lymphoid DC. Purified DC from adult thymus or spleen were found to migrate into fetal thymus rudiments in culture and localize specifically within the medulla. This distribution closely mirrors the situation in vivo, underlining the physiological relevance of the resulting microenvironment. Recolonization was shown to be selective, excluding cell types not normally represented in the thymus. To assess the extent of repertoire selection in recolonized fetal thymi, chimeric rudiments in which I-E determinants were expressed exclusively on the surface of immigrant DC were constructed. The failure of such rudiments to recruit a population of CD4⁺8⁻Vβ6⁺ thymocytes, restricted to antigen recognition in the context of I-E, argued against a role for DC in positive selection, in contrast, the widespread deletion of potentially autoreactive CD4⁺8⁻Vβ17a⁺ cells suggested an active role for DC in negative selection of the T-cell repertoire. These conclusions are consistent with the findings of various in vivo studies, endorsing the suitability of such a model for the study of tolerance induction in vitro.