This suggests that transgenic H-chain constructs containing the genomic region including Eμ and Cμ, ideally of endogenous origin, can initiate normal antigen-independent B-cell differentiation events (Kurosaki et al., 2010 and Dunnick et al., 2011b). The rat 3′RR containing hs3a, hs1,2 and hs3b is similar to the mouse but it is unclear if there is an equivalent region to hs4 in the rat (Sepulveda et al., 2005). In our constructs either the potentially complete rat 3′RR, including hs3a, hs1,2 and hs3b located downstream of Cα (Bruggemann et al., 1986), or a minimal 3′RR sequence Raf inhibitor with hs1,2 (Pettersson et al., 1990) was used. The 3′RR hs1,2 sequence has also been used in other, fully
human, constructs (Harding and Lonberg, 1995) but no previous constructs
contained the large 3′RR accommodating multiple transcriptional enhancer elements. It has been reported that a minimal 3′RR sequence, www.selleckchem.com/products/Vorinostat-saha.html accommodating only one or possibly two hs regions, reduces germline transcription and class-switch recombination (Pinaud et al., 2001 and Dunnick et al., 2011b), which agrees with our findings. The constructs Hu-Rat Belinda (HC13) and Hu-Rat Frieda (HC17) are identical except the former has only a 3′RR hs1,2, which is replaced later with the complete region including Cα and the 3′RR. Animals expressing HC13 switched very inefficiently, while HC17 rats switched and underwent hypermutation normally. Separately derived animals, but carrying the same translocus, produced very similar results. This implies that the functionality of the full 3′RR appears to comprehensively mediate or control downstream expression events; from the transitional B-cell stage onwards when IgM+ lymphocytes exit the bone marrow and enter the blood
to reach other lymphoid organs, such as spleen and lymph nodes, where they mature further (Kurosaki et al., 2010). Maturation is accompanied by class-switch recombination and somatic hypermutation, which leads to antigen-dependent cell expansions with differentiation into plasma or memory B-cells. This is supported by very recent results, which showed that the removal of the whole 3′RR in the mouse abrogated class-switch recombination and abolished somatic hypermutation in germinal centers (Vincent-Fabert G protein-coupled receptor kinase et al., 2010 and Rouaud et al., 2013). A summary of these events in our different transgenic lines is shown in Table 1. In three of the chimeric constructs the ~ 30 kb 3′RR is present, but despite this, in the Hu-Rat Emma line, the first made, little switching occurs with only a few Cγ2b(Hu CH1) transcripts being isolated. Here Cγ2b is immediately downstream of the γ2c germline promoter and I-exon, taking the position of Cγ2c. In wt rats the expression of this isotype is reduced compared to other IgGs (Bazin et al., 1974), which may to some extent explain the low levels we find.