We focused on the VH7183 family because it represents a manageable component of the active repertoire, because we and others had previously established patterns of VH7183 utilization during ontogeny and development in BALB/c mice, and because VH7183 gene segments have been shown to be components of antibodies with both self and nonself reactivities (reviewed in [8]). A total of 577 unique, in-frame, open transcript sequences were obtained including 72 from B (pro-B), 133 from C (early pre-B), 75 from D (late pre-B), 78 from this website E (immature B), and 219 from

F (mature, recirculating B). The C57BL/6 mouse genome contains only nine VH7183 family gene segments with open-reading frames (Fig. 1), or approximately half that of the BALB/c mouse genome.

Of these nine, only seven were identified in our sample of bone marrow transcripts (Fig. 2). As in the case of BALB/c mice, the usage of the C57BL/6 VH81X (IGHV05–2, IMGT) gene segment declined fourfold during early B-cell development (28% in B versus 7% in D, p = 0.0008). However, unlike BALB/c mice where there was a further fivefold late-stage reduction between fractions D or E to F (p < 0.02), in C57BL/6 mice the prevalence of VH81X usage did not change between fractions D, E, and F (Fig. 2). The most JH distal VH, IGHV05–17 (IMGT), exhibited a doubling of usage in the transition from pro-B-cell to immature B cell and beyond (BF, p < 0.05), ultimately contributing to almost one-third of the VH7183-containing transcripts from the mature XL765 cell line B-cell pool (Fig. 2). The closest BALB/c VH7183 homologue to IGHV05–17, VH7183.18, exhibited a similar increase in usage with development, but contributed to only 10% of the final repertoire. Use of the remaining five C57BL/6 VH gene segments did not vary statistically with development, also following the same pattern as their BALB/c homologues (Fig. 1). However, the VH gene segment most commonly used in BALB/c mice at all stages of development, VH7183.10, has no C57BL/6 VH7183 homologue; and thus its structure and binding

activity is missing in C57BL/6 mice. Significant differences in the complement of DH gene segments were observed between C57BL/6 and BALB/c mice. The C57BL/6 genome has only one DFL family gene segment, two DST family gene segments, and six DSP family gene segments; whereas the BALB/c genome has two DFL family gene segments, Resminostat one DST family gene segment, and nine DSP family gene segments. Both strains of mice had a single DQ52 gene segment that was conserved in sequence. In total, therefore, the C57BL/6 genome contains three fewer functional D gene segments than the BALB/c genome (Fig. 1). If DH usage were primarily a function of gene number, one might expect C57BL/6 mice to halve their use of the DFL family and double the use of DST family. However, while use of the DST family did increase, use of the single DFL gene segment in C57BL/6 mice increased to match the combined usage of the two DFL gene segments in BALB/c mice.