Transgenic expression of the α heavy chain gene. (A) Expression of α germline transcripts. Germline transcripts from the transgene were distinguished from transcripts from the endogenous genes by digestion of the 333-bp IαCα PCR products with PstI and SstI, as illustrated at the top of the figure. The 263-bp endogenous fragment derives from an alternatively spliced version with a longer Iα exon. (B) Post-switch transcripts. The expression patterns of germline and post-switch transcripts for lines 231Δ and 231 were similar to those of lines 234Δ and 234 (not depicted). Cells from C57BL/6, 820Δ, and 820 were treated with CD40L, with or without TGFβ as indicated, instead of LPS. (C) HPRT transcripts. (D) Recombination between the transgenic VDJ and endogenous Cα in lines 774 and 774Δ. Transgenic and endogenous Cα sequences in mRNA, from B cells cultured in LPS+TGFβ, were distinguished by a SstI site found in the endogenous, but not the transgenic, Cα. Each experiment shown was repeated in one or two additional experiments.