Evidence for the cellular origin of Gross virus-induced leukemia in the rat: description of a unique LDH isozyme sub-band in leukemic lymphoid cells and lymphohemopoietic precursor cells.

Neoplastic thymocytes from rat thymic lymphoma-leukemias induced by the rat-adapted Gross-leukemia virus (RAGV) were analyzed for a variety of differentiation markers to define their differentiation state and possible cellular origin. A majority of thymocytes from leukemic rats had the phenotypic characteristics of subcapsular cortical thymocytes that are the most ancestral of the thymocytes. These cells exhibited readily detectable levels of Thy-1 and histocompatibility antigens on their surfaces, they contained terminal deoxynucleotidyl transferase (TdT) and they contained low adenosine deaminase (ADA) and high purine nucleoside phosphorylase (PNP) specific activity. The leukemic thymocytes also contained a sub-band of the LDH-5 isozyme (LDH-5') that was not detected in normal thymocytes but that was present in lymphocyte-rich fractions of postnatal bone marrow, fetal and prepubertal spleen, and fetal and neonatal liver. The tissue distribution and ontogeny of LDH-5'-containing cells is similar to prethymic TdT+ cells in the rat and both TdT and LDH-5' are enriched in a subset of bone marrow "null" cells. These results suggest that TdT+ thymocyte progenitors or their precursors are the targets of leukemic transformation of RAGV.