Phenotypic and functional properties of murine thymocytes. II. Quantitation of host- and donor-derived cytolytic T lymphocyte precursors in regenerating radiation bone marrow chimeras

Thymocytes from radiation bone marrow chimeras, in which donor bone marrow and irradiated recipient differed at the Thy-1 locus, were stained by indirect immunofluorescence with monoclonal anti-Thy-1 antibodies and analyzed by flow microfluorometry (FMF). Kinetic studies indicated an early appearance of host-derived (CBA, Thy-1.2⁺) thymocytes, which reached maximum numbers of 10 to 20 x 10⁶ cells at 12 to 16 days after bone marrow reconstitution. Donor-derived (AKR, Thy-1.1⁺) cells were not detectable until 10 to 12 days after reconstitution; subsequently, they increased exponentially in number until 28 days, when they accounted for essentially all cells in the thymus (50 x 10⁶). Concomitant with the appearance and disappearance of host-derived cells was a change in their Thy-1 surface phenotype. In particular, the proportion of host cells having a 'mature' phenotype (weakly Thy-1.2 staining) increased progressively with time after irradiation. Functional studies using a sensitive mixed leykocyte microculture system to quantitate cytolytic T lymphocyte precursors (CTL-P) were also carried out in regenerating chimeric thymuses. Initially, the regenerating thymus contained few CTL-P, but by 4 wk after reconstitution, frequencies similar to control adult thymuses were obtained. Analysis of the CTL-P content of host and donor-derived subpopulations, separated either by appropriate anti-Thy-1 antibody plus complement or by direct cell sorting, indicated that both host- and donor-derived cells contained appreciable numbers of CTL-P. Furthermore, increases in CTL-P frequency of both host and donor subpopulations correlated with changes in their surface Thy-1 phenotype. These phenotypic and functional results are discussed in the context of the utility of the radiation bone marrow chimera model for analyzing the role of nonlymphoid thymic components in the acquisition of the T cell repertoire.