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Phenotypic characterization of mice of thymus target cells susceptible to productive infection by the radiation leukemia virus.

The spread of virus replication was studied by electron microscopy in the thymuses of inbred C57BL/Ka mice after intrathymic inoculation of the radiation leukemia virus (RadLV). The first type C-budding virus particles appeared in scarce blast cells of the subcapsular zone. Most of these blast cells were "X-cells," i.e., the thymus lymphoid cells most actively engaged in DNA synthesis. Virus replication spread to the entire cortical blast cell population and, from day 7 on, to the small cortical lymphocytes. The first virus-producing cells were derived from a very few target cells (approximately 0.001-0.003% of thymocytes) susceptible to RadLV infection. For determination of the phenotypes of these target cells, various thymocyte subpopulations obtained through a battery of cell separation methods were tested for their ability to support the replication of RadLV/VL3 virus in short-term culture. Most of these target cells were sensitive to the lytic effect of hydrocortisone and migrated in the fastest fraction of a 1Xg sedimentation gradient, together with the majority of [3H] thymidine-incorporating blast cells. They exhibited an intermediate density and expressed H-2 and Thy 1,2 cell surface antigens, although they were not found preferentially among the high Thy 1,2 population to which most of the cortical blast cells belonged. The spread of RadLV within the thymus and the surface phenotype characteristics of target cells indicate that these cells correspond to the thymocyte subset at the earliest stage of thymic lymphopoiesis and may be transitional between the prothymocytes and the subcapsular blast cell population.