Immunohistochemical localization of host and donor-derived cells in the regenerating thymus of radiation bone marrow chimeras.

The anatomical distribution of CBA (Thy-1.2) host and AKR (Thy-1.1) donor-derived cells in the regenerating thymus of AKR----CBA radiation bone marrow chimeras was investigated. Cryostat sections of chimeric thymuses were incubated with biotin-conjugated monoclonal anti-Thy-1 antibodies specific for host and donor-derived cells and the distribution of the corresponding Thy-1 antigen revealed by the immunoperoxidase staining technique. The thymus was initially repopulated by Thy-1.2+ host-derived cells, but by 28 days following bone marrow reconstitution the few remaining host cells were found mostly in the thymus medulla. However, occasional Thy-1.2+ cells were still present in extramedullary, primarily cortical, sites. Donor-derived (Thy-1.1+) cells were first seen in the 11-day chimeric thymus as single cells frequently closely associated with blood vessels in medullary areas. By 17 days, the cortex contained many Thy-1.1+ cells, although occasional single positive cells were still present in the medulla. Changes in the anatomical distribution of host and donor-derived cells in the regenerating chimeric thymus appeared to correlate with changes in their Thy-1 fluorescence profile as determined by flow microfluorometry.