Induction of Light Chain Replacement in Human Plasma Cells by Caffeine Is Independent from Both the Upregulation of RAG Protein Expression and Germ Line Transcription

Abstract

When some human plasma cell lines are cultured with concanavalin A, the original light chain is replaced with another light chain which results from secondary VJ recombination (light chain shifting). We examined various intracellular factors involved in the induction of light chain shifting. Light chain shifting can be induced upon treatment with agents with phosphatase inhibitory activity such as caffeine and okadaic acid. Although the plasma cells used express both RAG-1 and RAG-2, the expression level of these proteins was not affected by caffeine or okadaic acid. Transcription of the germ line locus, which correlates to the locus activation for rearrangement, is also not influenced by phosphatase inhibition. However, the amount of signal broken-ended DNA intermediates generated during V(D)J rearrangement was shown to increase upon caffeine or okadaic acid treatment. The inhibitory activity of caffeine on phosphatase was the same as okadaic acid. However, caffeine exhibited much higher activity for VJ coding joint formation than okadaic acid. Therefore, although phosphatase inhibition might act, in part, on a mechanism by which V(D)J recombinase activity is regulated within the human plasma cells, other factor(s) are probably also involved in the process.Immunoglobulin genes are assembled during B cell development through a series of site-specific recombination events collectively termed V(D)J recombination (1). The V(D)J recombination reaction is initiated by the recombination activating gene products RAG-1 and RAG-2 and is completed by a set of proteins employed in most cell types for DNA double-stranded break repair such as the Ku-80 antigen, a large catalytic subunit of DNA-dependent protein kinase and XRCC4 (2-6). Although many of the VJ recombinations at the light chain loci occur in pre-B cells, recent findings suggest that light chain gene rearrangements often continue in immature-B cells and germinal center B cells (7-11). We also have shown that secondary VJ recombination can occur in some human plasma cell lines when stimulated with concanavalin A (ConA)