Virus - transformed Pre - B Cells Show Ordered Activation but Not Inactivation of Immunoglobulin

Virus-transformed pre-B cells undergo ordered immunoglobulin (Ig) gene rearrangements during culture. We devised a series of highly sensitive polymerase chain reaction assays for Ig gene rearrangement and unrearranged Ig gene segment transcription to study both the possible relationship between these processes in cultured pre-B cells and the role played by heavy (H) chain (,u) protein in regulating gene rearrangement. Our analysis of pre-B cell cultures representing various stages of maturity revealed that transcription of each germline Ig locus precedes or is coincident with its rearrangement. Cell lines containing one functional rearranged H chain allele, however, continue to transcribe and to rearrange the allelic, unrearranged H chain locus. These cell lines appear to initiate but not terminate rearrangement events and therefore provide information about the requirements for activating rearrangement but not about allelic exclusion mechanisms. T he mature cells of the body arise by differentiation from immature precursors through many stages that together form a lineage. Generally, these stages are difficult to define because pure cell populations representing individual stages cannot be prepared. In the B lymphocyte lineage, virally transformed tumor cells of immature phenotype have provided extremely useful models of individual developmental stages. These models have limitations, however. Being tumor cells, they might not represent normal cells with fidelity. Furthermore , they often display a mixed phenotype because individual cells appear to slowly progress through the lineage, whereas a stem line of relatively fixed phenotype persists. Characterization of these pre-B cell transformants has been extensive (1, 2). Ig gene rearrangements can be used as markers for successive developmental stages since the various H and L chain gene rearrangements are ordered during B cell development (2, 3). The most immature are lines derived by Palacios et al. (4), which have germline H and L chain loci but can be induced to differentiate under special conditions. LyD9 and BaF3, which display the B220 pan-B cell surface marker, are representatives. Pre-B cells that are mainly germline at the H chain loci but carry out the earliest stages of H chain gene rearrangement (D to Jjoining) are typified by HAFTL cell lines (5), fetal liver cells transformed by Harvey leukemia virus. Abelson virus-transformed fetal liver and bone marrow pre-B cells are somewhat more mature. These include early lineage cells such as 300-19 (6), which carry out predominantly H chain V to DJ rearrangements, and more mature cell lines like PD31, which have functional H …