Our data presented here suggest that lower BCR levels which presumably result in lower tonic BCR signalling affect BAFF-R manifestation levels in immature as well as mature splenic B lymphocytes. were indistinguishable between WT and IgHE-GFP/E-GFP FO and MZ cells. It has been explained that treatment with BAFF raises B-cell size (Patke et al, 2006). We consequently compared the increase of B-cell size by BAFF treatment upon activation with LPS. Amazingly, IgM production by IgHE-GFP/E-GFP B cells was undetectable (Number 7C) despite apparently normal activation and proliferation of HC-hypomorphic cells (Number 7D). B cells from IgHE-GFP/E-GFP mice showed improved cell size 3 days after activation with LPS, indicating regular activation (Number 7D). Staining for cytoplasmic HC, DGKD however, revealed almost absent cytoplasmic HC protein in LPS blasts from IgHE-GFP/E-GFP mice (Number 7D), which was also confirmed by western blot analysis (Supplementary Number S6). These findings led us to hypothesize that during plasma cell differentiation of mutant B cells AGK2 the pA machinery is preferentially directed to the strong SV40 pA sites downstream of the gfp-cassette, which have been launched by gene focusing on and therefore creating truncated HCs. It has recently been shown AGK2 that enhanced loading of the transcription elongation element ELL2 and the polyadenylation element CstF-64 on RNA polymerase II upon plasma cell differentiation causes enhanced use of the proximal pA site of the secretory form of IgH (Martincic et al, 2009). In case of the IgHE-GFP allele, the SV40 pA site of the gfp-cassette is the most proximal pA site, providing a possible explanation for seriously reduced cytoplasmic HC levels during plasma cell differentiation. Indeed, QPCR analysis exposed an 12-collapse reduction of JH2-C1 mRNA levels in LPS blasts from IgHE-GFP/E-GFP mice as compared with WT mice (Supplementary Number S7), whereas JH2-C1 mRNA levels were only modestly reduced in unstimulated splenic B cells (Table I). The mRNA levels of -LCs were even enhanced in mutant LPS blasts (Supplementary Number S7). We, consequently, propose that the preferential usage of the launched pA site is mainly responsible for strongly impaired antibody secretion in IgHE-GFP/E-GFP mice. Conversation It is widely accepted that development and positive selection of B lymphocytes depends on the quality and quantity of signals generated by their pre-BCRs and BCRs and that signalling strength is also involved in B-lymphocyte lineage decisions (Niiro and Clark, 2002; Casola et al, 2004). These notions developed from plenty of studies dealing with gene-targeted mice, which lack positive or bad regulators of BCR signalling or on the other hand overexpress particular signalling parts. As most signalling components of the BCR have diverse functions and are involved in additional signalling pathways, as exemplified for the syk tyrosine kinase (Kulathu et al, 2009) the reported phenotypes can hardly ever be attributed to modified signalling strength of the BCR only. This AGK2 is definitely even more complicated in downstream components of the signalling machinery, for example, for the NF-B signalling pathway (Derudder et al, 2009). Here, we describe a unique and novel mouse mutant in which attenuated receptor signals in both developing and adult B-cell subsets resulted only from your global reduction of HC mRNA and hence HC protein levels. Any receptors and regulators involved in pre-BCR and BCR signalling pathways remained intact. The 1st checkpoint in B-cell development in which signals downstream of the HC are involved in positive selection happens when pre-BCR signalling induces a burst of proliferation, which increases the quantity of cells that have successfully recombined their IgH genes (examined in Herzog et al (2009)). The 5- to 6-fold reduction of mutant large pre-B cells in competitive BM chimeras supports the notion that the strength of pre-BCR signalling quantitatively affects the proliferation of pre-B cells. This is in accordance with the suggested repertoire selection model (Melchers, 2005; Vettermann et al, 2006), in which the pairing capacity of HCs with the SLC settings the denseness of AGK2 surface pre-BCR levels and therefore the magnitude of signals for proliferation and clonal growth. B-cell development in the BM of IgHE-GFP/E-GFP mice was partially blocked in the transition from pre-B to immature B cells accompanied by enforced secondary LC rearrangements. Relating to a hypothetical receptor editing model (Nemazee.