Chapter 4 Assembly of immunoglobulin variable region gene segments

Multitasking of Ig-alpha and Ig-beta to regulate B cell antigen receptor function

Since their discovery as signaling subunits of the B cell antigen receptor (BCR), Ig-alpha and Ig-beta are discussed to serve either a redundant or distinct function for B cell development, maintenance, and activation. Dependent upon the experimental system that has been used to address this issue, evidence could be provided to support both possibilities. Only recently has it become clear that Ig-alpha and Ig-beta possess a unique signaling identity but that both together are required to orchestrate proper B cell function in vivo. Here we discuss some of the underlying mechanisms that may involve direct coupling to discrete subsets of BCR effector proteins, such as protein tyrosine kinases or the intracellular adaptor SLP-65/BLNK.

Biased VH Gene Usage in Early Lineage Human B Cells: Evidence for Preferential Ig Gene Rearrangement in the Absence of Selection

Certain VH genes are predominantly expressed in mature B cells. We hypothesized that several, mutually nonexclusive VH-dependent mechanisms operating at distinct stages during B cell development may be responsible for overrepresentation of these VH genes. In the present study, we have assessed whether one of the mechanisms involves preferential rearrangement at the pro-B cell stage. The frequency of individual VH4 and VH3 genes in rearrangement libraries from FACS-purified human CD34+/CD19+ pro-B and CD34−/CD19+ pre-B cells was assessed. The in-frame and out-of-frame rearrangements from both cell populations were analyzed using a high resolution PAGE system. The frequencies of individual VH gene segments among out-of-frame rearrangements from pro-B cells were determined, because these frequencies should reflect only processes before the translation of the μ-heavy chain and should not be biased by selection mechanisms. Our results demonstrate that, at the pro-B cell stage, the V4–34, V4–39, and V4–59 gene segments are the most frequently rearranged VH4 family genes, and the V3–23 and V3–30 gene segments are the most frequently rearranged VH3 family genes. This finding suggests that the predominant expression of these VH genes in peripheral mature B cells is determined to a significant degree by their preferential rearrangement during V-DJ recombination.

A survey of protein-DNA interaction sites within the murine immunoglobulin heavy chain locus reveals a particularly complex pattern around the DQ52 element

The expression of immunoglobulin (Ig) genes is regulated at two levels: rearrangement of individual gene segments and transcription of continuous genes. To find transacting factors involved in mediating locus- and segment specific gene activation and expression, we surveyed a 3600-bp genomic region of the murine Ig heavy chain locus, spanning from the DQ52 element to the Ig heavy chain intron enhancer. We discovered nine, previously undescribed, protein-DNA complexes and estimated their individual binding-affinity preferences (Kr) by quantitative gel shift measurements. We observed one novel protein DNA interaction at the enhancer, two in the JH1 region and six within a 300-bp region immediately 5' to the DQ52 locus. The latter show a complex and specific binding pattern when comparing nuclear extracts derived from pre-B cells and fibroblasts. Further characterization of the interactions at the DQ52 locus by electron microscopy revealed the preferential formation of a protein complex binding to the DQ52 locus with pre B cell extracts. This behavior and the clustering of interaction sites 5' to the DQ52 element suggest that this region is involved in the regulation of heavy chain gene assembly.

Deletion mapping of the mouse ornithine decarboxylase-related locus Odc-rs8 within Igh-V

The Odc-rs8 locus belongs to a family of mouse DNA sequences related to the gene encoding ornithine decarboxylase (ODC). Odc-rs8 was mapped by recombinant inbred (RI) strain analysis to the region of Chromosome (Chr) 12 occupied by the variable region genes of the immunoglobulin heavy chain (Igh) complex. In the present study, alleles at Odc-rs8 were shown to cosegregate with those for Igh variable region (Igh-V or VH) genes among 37 inbred mouse strains that had been characterized previously for their haplotypes at Igh. For a more precise definition of the location of Odc-rs8 relative to Igh-V, DNAs from 17 Abelson murine leukemia virus (A-MuLV)-transformed pre-B cell lines cultured from mice heterozygous at Igh and Odc-rs8 were analyzed for the presence of DNA restriction fragments (RFs) derived from each parental Odc-rs8 allele. These cell lines, each of which has rearranged one or both Igh genes, previously were employed in mapping members of nine VH gene families by deletion analysis (Brodeur et al. 1988). Comparing the deletion profiles of the cell lines for Odc-rs8 with those for the VH gene families has located Odc-rs8b within the VHJ558/VH3609 gene cluster and Odc-rs8c either within or upstream of the 5'-most 9% of VHJ558, identifying Odc-rs8 as a potentially useful marker for the 5' end of the Igh complex.