A sensitized genetic system for the analysis of murine B lymphocyte signaltransduction pathways dependent on Bruton's tyrosine kinase

Modifier screens have been powerful genetic tools to define signaling pathw ays in lower organisms. The identification of modifier loci in mice has beg un to allow a similar dissection of mammalian signaling pathways. Transgeni c mice (Btk(lo)) expressing 25% of endogenous levels of Bruton's tyrosine k inase (Btk) have B cell functional responses between those of wild-type and Btk(-/-) mice. We asked whether reduced dosage or complete deficiency of g enes previously implicated as Btk regulators would modify the Btk(lo) pheno type. We used two independent assays of Btk-dependent B cell function. Prol iferative response to B cell antigen receptor cross-linking in vitro was ch osen as an example of a relatively simple, well-defined signaling system. I n vivo response to type II T-independent antigens (TI-ll) measures complex interactions among multiple cell types over time and may identify additiona l Btk pathways. All modifiers identified differentially affected these two assays, indicating that Btk mediates these processes via distinct mechanism s. Loss of Lyn, PTEN (phosphatase and tensin homolog), or SH2-containing in ositol phosphatase suppressed the Btk(lo) phenotype in vitro but not in viv o, whereas CD19 and the p85 alpha form of phosphoinositide 3-kinase behaved as Btk(lo) enhancers in vivo but not in vitro. Effects of Lyn, PTEN, or p8 5 alpha haploinsufficiency were observed. Haploinsufficiency or complete de ficiency of protein kinase C beta, Fyn, CD22, G alpha q, or G alpha 11 had no detectable effect on the function of Btk(lo) B cells. A transgenic syste m creating a reduction in dosage of Btk can therefore be used to identify m odifier loci that affect B cell responses and quantitatively rank their con tribution to Btk-mediated processes.