During immune responses, B lymphocytes may switch from the expression of immunoglobulin M (IgM) to the expression of another isotype (e.g., IgG, IgE, IgA). In stable hybridomas and myelomas expressing a "switched" (S) isotype, DNA deletions between S mu and a "downstream" S region (S region recombination) have been found. In primary B cells, studies of the molecular basis of switching have been limited by the ability to sensitively quantitate the amount of DNA deletion; such studies would be of interest because other nondeletional mechanisms (trans-splicing, alternative processing of a long transcript) have been proposed to account for isotype switching in certain circumstances. We have applied the digestion-circularization polymerase chain reaction (DC-PCR) technique to measure the amount of S region recombination that occurs in the course of class switching in primary B lymphocytes. Resting B cells were cultured in lipopolysaccharide (LPS) and interleukin 4 (IL-4) to stimulate switching to IgG1. These cells begin to express membrane IgG1 at day 2.5 of culture and reach maximum expression by day 4.5. DNA was prepared from cultured cells and analyzed for S mu-S gamma 1 rearrangement by DC-PCR. Chimeric switch regions, indicating S mu-S gamma 1 recombination, were detected in amounts that, in most cases, correlated with surface expression. Furthermore, when cells were sorted on the basis of surface IgG1 expression, a mean of at least one S mu-S gamma 1 rearrangement per cell was seen in five out of seven experiments. In general, the IgG1+ cells obtained at 4.5 and 5.5 d of culture had close to 2 S mu-S gamma 1 rearrangements per cell. In IgG1- cells, S mu-S gamma 1 rearrangements were detectable, but at frequencies substantially lower that in IgG1+ cells. Thus, these results indicate that DNA deletion accompanies class switching in normal B cells stimulated with LPS and IL-4.

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