Unusually early and high rate of spontaneous occurrence of nonthymic leukemias in SL/Kh mice, a subline of SL strain.

Extremely early and high rate of spontaneous occurrence of nonthymic leukemias was observed in SL/Kh mice, a subline of SL mice. In a cohort of 68 females and 21 males, the cumulative incidence at 6 months of age was 88% in females and 48% in males, and at 12 months it was 100% in females and 94% in males. There were two types of leukemias with distinct in vivo homing behavior: the major type (72/86) proliferated in lymph nodes and the spleen, and the minor type (14/86) proliferated predominantly in the bone marrow cavity. In the minor type, paraplegia due to spinal compression by growing leukemia cells in vertebrae was a characteristic clinical manifestation and this was faithfully reproduced by intravenous injection of the leukemia cells from bone marrow. Both types of leukemias were histologically poorly differentiated lymphoblastic leukemias of null cell type, since none of 8 immunocytological and 4 cytochemical differentiation markers allowed either positive identification of their normal derivation or distinction between them. In contrast to AKR mice with a high incidence of thymic leukemias, spontaneous expression of xenotropic virus was observed not only in the thymus but also in the spleen and bone marrow. In addition to the high level of expression of eco- and xenotropic viruses, viruses with dual-tropic host range were isolated sporadically from preleukemic and leukemic lymphoid tissues. Possible factors determining the type of leukemias in SL/Kh mice are discussed.