This research focuses on the maturational sequence of the B lymphocytes which express lambda light chain proteins, and the mechanism by which the expression of lambda light chain genes is controlled. Immunoglobulins secreted from plasma cells contain either kappa or lambda light chains, but not both. That is, in these cells productive kappa and lambda genes are mutually exclusive. This phenomenon has been termed kappa/lambda isotypic exclusion. At the level of the genes, kappa-producing cells have rearranged kappa genes, but the lambda genes are in germline configuration. In most lambda-producing cells, however, the kappa genes are either nonproductively rearranged or deleted. These observations have led to a "sequential" model which postulates that kappa genes rearrange first and, only if both kappa alleles are aberrantly rearranged or deleted, lambda genes become accessible for rearrangement. The studies proposed here are designed to test an alternative hypothesis, that cells which express lambda light chains constitute a unique developmental lineage of B lymphocytes. This lineage is characterized by an intermediate cell type that coproduces kappa and lambda light chains and fails to turn off immunoglobulin gene rearrangement. First, fluorescent-labelled antibodies and a fluorescence activated cell sorter will be used to determine whether B cells which coexpress kappa and lambda are present in normal mice and to determine whether the expression of lambda positive cells is restricted to any previously defined B lymphocyte population. Second, a rearrangement-dependent drug resistance gene, pHRD, will be tested in lambda and kappa hybridomas to determine whether the B cells that express lambda have an increased ability to rearrange immunoglobulin genes compared to B cells that express the kappa light chain. The results of this research should provide significant new information on the mechanisms which control immunoglobulin light chain gene expression and, more generally, on the mechanism of regulation of gene expression.
