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B cell differentiation: I. Development and functional analysis of murine B cells immortalized by a recombinant retrovirus.

B lymphoblast cells were immortalized by infection with a v-myc and v-raf/mil containing recombinant retrovirus. The immortalized B cells do not require exogenous growth factors or mitogens for growth. These cells express characteristic mature B cell phenotypic markers including IgM and IgD. Northern blot analysis detected mu and delta mRNA, and Southern blot analysis revealed rearrangements of Ig genes but not TCR genes in these cells. The immortalized B cells were found to respond to B cell mitogens by proliferation and IgM secretion, and to respond to IL-4 by an increase in la expression. Furthermore, they were found to differentiate to secrete IgM and to switch to IgG1 production following interaction with Staphylococcal enterotoxin B- activated Th2 cells. These cell lines are thus valuable tools for analyzing the molecular events involved in B lymphocyte growth and differentiation.