Expression of a novel type of immunoglobulin Cλ transcripts in human mature B lymphocytes producing χ light chains

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Abstract
Ordered rearrangement of immunoglobulin genes (heavy chain then χ and eventually λ genes) is observed during B lymphocyte ontogeny. Unexpectedly, we found that human mature B cells producing χ chains and having germ-line λ genes contain λ mRNA consisting of an invariant 5′ region (herein termed X) and of one of the classical Cλ exons. The X region of these transcripts originates from a unique exon located 5 kb upstream of the J-Cλ1 gene segment. X-Cλ mRNA expression occurs without somatic DNA rearrangement. The use of the X DNA fragment as a probe allows definition of a family of human genes that comprises at least four members and includes the first exon of the λ14.1 gene. The latter is selectively transcribed in pre-B lymphocytes and directs the synthesis of a λ-like chain. In contrast, the X-Cλ transcripts do not appear to encode a Cλ-related polypeptide in mature B cells. Thus, despite a 73% homology extending far beyond the exon sequences, the X and λ14.1 genes are expressed at different stages of B cell development and might serve different functions.