Leukemic transformation in hrs mice occurs in an immature-nonactivated thymocyte subpopulation

The murine leukemic strain HRS/J has an autosomal-recessive, mutant gene, hr, with homozygotes (hr/hr) having a 72% incidence of thymic leukemia at 18 months of age compared to 20% in heterozygotes (hr/+). This study was done to (a) determine if expression of thymocyte differentiation and murine leukemia virus (MuLV) antigens during leukemic transformation were different in hr/hr compared to hr/+ mice, (b) define the subpopulations that were targets for leukemic transformation, and (c) compare the results to reports in other leukemic strains. Flow cytometry analysis of thymus cell suspensions was done with anti-T-cell and anti-H-2 monoclonal antibodies, peanut agglutinin (PNA), and heteroantisera to MuLV antigens. Thymocytes of 1- to 3-month-old HRS/J mice were Thy 1.2+, Lyt 1+2+, H-2Kk-, and MuLV- with an immature-nonactivated phenotype, i.e., PNA+, and Iak-. Preleukemic and leukemic thymocytes showed diversity in expression of Thy 1.2 and Ly antigens with increased H-2Kk and MuLV expression. No differences in phenotype patterns were noted between hr/+ and hr/hr mice during the time course of leukemogenesis. Persistently high PNA/low Iak expression of preleukemic and leukemic thymocytes indicated that the target for HRS leukemic transformation was an immature-nonactivated thymocyte subpopulation in contrast to AKR/J mice in which leukemic transformation involves a mature-activated thymocyte subpopulation. These findings suggest that spontaneously generated leukemogenic viruses in HRS mice have tropism for thymocytes of an immature-nonactivated phenotype.