Committed T lymphocyte stem cells of rats. Characterization by surface W3/13 antigen and radiosensitivity.

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Abstract
The existence of stem cells committed to the T lymphoid lineage was deduced from studying how rat T and B stem cells differ in their expression of membrane W3/13 antigen and in their susceptibility in vivo to .gamma.-irradiation. Stem cell activity of rat bone marrow and fetal liver was measured in long-term radiation chimeras using B and T cell alloantigenic surface markers to identify the progeny of donor cells. Monoclonal mouse anti-rat thymocyte antibody W3/13 labeled .apprx. 40% of fetal liver cells and 60-70% of young rat bone marrow cells (40% brightly, 25% dimly). Bright, dim and negative cells were separated on a fluorescence-activated cell sorter. All B and T lymphoid stem cells in fetal liver were W3/13 bright, as were B lymphoid stem cells in bone marrow. W3/13 dim bone marrow had > 1/2 the T cell repopulating activity of unseparated marrow but gave virtually no B cell repopulation. The radiosensitivity of endogenous B and T lymphoid stem cells was determined by exposing host rats to between 4.5 and 10 Gy of .gamma.-irradiation before repopulation with genetically marked marrow. The results depended on whether chimerism was assayed before day 50 or after day 100. At early times, a radioresistant T stem cell was indicated, whose activity waned later. Thus, committed T stem cells of rats carry moderate amounts of W3/13 antigen and are more radioresistant but less permanently chimeragenic than the stem cells that regenerate B lymphocytes.