Reduction of Leaky Lymphocyte Clones Producing Immunoglobulins and Thymic Lymphocytic Leukemia by Selective Inbreeding of SCID (Severe Combined Immunodeficiency) Mice

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Abstract
Selective inbreeding of C.B17-scid/scid mouse pairs showing undetectable IgG and IgM has been carried out in order to reduce the mortality of mice by early occurrence of thymic lymphocytic leukemia and abnormal lymphocyte clones producing immunoglobulins, both of which inhibit the successful heterotransplantation of normal and neoplastic human tissues. Although the majority of C.B17-scid/scid mice showed undetectable (< 1 microgram/ml) or low level (< or = 25 micrograms/ml) of serum IgG and IgM, some produced abnormally high concentrations of IgG and IgM (> 25 micrograms/ml). The incidence of such mice showing higher levels of IgG was very high at F1 and F2 generation (10/55, 18.2%), but significantly low after the F3 generation (18/446, 4.0%, p < 0.001). Although leukemia incidence was very high at F4 to F5 generations (8/40, 20.0%), death from leukemia was not observed early in life (4-6 months after birth) at F7 to F10 generations (0/36, 0%, p < 0.01) and was very low during the age of 6-10 months after the F8 generation (11/66, 16.7% at F4 and F5 vs 4/93, 4.3% at F8-10), p < 0.01). Scid mice improved by the selective inbreeding will provide an invaluable experimental system for the heterotransplantation of normal and neoplastic human tissues.