Differential V region mutation of two transfected Ig genes and their interaction in cultured B cell lines

We have established B cell culture systems in which transfected and stably integrated Ig constructs spontaneously undergo high rates of variable (V) region mutation. Mutation rates were determined using reversion analysis of an Ig V region nonsense codon (Vn). A construct (Vn/γ2a) in which a Vn was associated with the γ2a constant region and its intervening and immediate flanking sequences mutated at a high rate of 2.2 x 10^-4/bp/generation in the NSO myeloma cell line. This same Vn, when associated with the μ constant region (Vn/μ), mutated at a 1000-fold lower rate in NSO. The Vn/γ2a construct also mutated at high rates in the 18.81 pre-B and the S107 myeloma cell lines and at a low rate in the J558 myeloma cell line. In NSO, the presence of the γ2a construct raised the mutation rate of the μ construct and the μ decreased the mutation rate of γ2a. These results suggest that there is both positive and negative regulation of V region mutation and that different cell lines express different combinations and/or amounts of trans-acting factors that are involved in the mutational process.