Cytofluorometric analysis of thymic interdigitating cells from C57BL/6 mice prior and after leukemogenic X-irradiation

The thymus is populated by various Ia⁺ cell populations, including epithelial cells, macrophages and dendritic cells. Thymic cell suspensions were stained with an anti-Ia antibody and shown by cytofluorometry to contain a small number of strongly Ia⁺ cells characterized by a large diameter. The cell population was separated with the aid of the fluorescence-activated cell sorter (FACS) and characterized. They were shown to express high levels of membranal Ia antigens; they demonstrated ATPase activity and displayed the ultrastructural features characteristic of the previously described thymic interdigitating cells. C57BL/6 mice were submitted to various regimens of X-irradiation. Whereas exposure to a single dose of X-irradiation was followed by an increase in the percentage of strongly Ia⁺ cells, exposure to a leukemogenic regimen of fractionated X-irradiation led to a decrease in the percentage and absolute numbers of these cells in the thymus. Of the C57BL/6 mice that were irradiated with fractionated X-irradiation, 77% developed leukemia. Intravenous injection of syngeneic bone marrow one day following the last irradiation or protection of the femur during irradiation prevented both the appearance of leukemia and the disappearance of interdigitating cells. Therefore an inverse correlation between the presence of thymic dendritic cells and the incidence of leukemia in C57BL/6 mice could be demonstrated. These findings are discussed in relation to the putative role of dendritic cells in the thymus.