A Transgenic Marker for Mouse B Lymphoid Precursors

Three lines of transgenic mice have been generated which express human CD25 under the control of the 722-base pair region located immediately 5′ of the precursor (pre)–B cell–specific λ5 gene. All three strains express human CD25 in parallel to endogenous λ5 on pre–B cells, but not on mature B lymphocytes or other blood cell lineages. High expression of human CD25 on B lineage cells of transgenic mice has allowed the identification of a new B220⁺CD19⁻λ5⁺ precursor of the B220⁺CD19⁺λ5⁺ c-kit⁺ pre-BI cells. Both types of precursors are clonable on stromal cells in the presence of interleukin-7. The CD19⁻ precursors have a sizeable part of their immunoglobulin heavy chain gene loci in germline configuration, while the CD19⁺ pre–BI cells are predominantly DJ_H rearranged. The results indicate that random integration of the 722-bp 5′ region of the λ5 gene into the mouse genome confers tissue and differentiation stage–specific expression of a transgene.