Sites of switch recombination in IgG2b- and IgG2a-producing hybridomas

The technique of genomic Southern blot analysis was employed to map the sites of class switch rearrangement in five hybridoma cell lines, four of which produce γ2b, whereas the fifth secretes γ2a. Two of the class switch recombinations we have mapped represent secondary switches to γ2b with a γ1 intermediate. We also observe isotype specificity in class switch recombination, with both alleles rearranging to the same isotype in the three lines when an unexpressed chromosome is retained. Every 5' break occurred outside of and immediately 5' to the tandem repeats associated with Cμ, a result consistent with earlier studies on myelomas but in contrast to the pattern seen in lymphomas. Seven of the eight 3' switch sites, however, were within tandem repeats. This disparity in the distribution of 5' and 3' break points may reflect a unique role for the μ-associated tandem repeats in the switch process.