Regulation of murine B cell development and function

Vertebrates have developed an effective and protective, yet complex and dynamic immune system to defend themselves against the many threats from microorganisms. In this thesis some of the questions regarding the regulation of murine B cell development and function have been addressed.
In the first part it was investigated whether the quality of the pairing interaction between the µH chain and the surrogate light (SL) chain of a given pre-B cell receptor may determine the size of the clonal expansion at the large pre-B II cell stage in the bone marrow (BM). VHDHJH-rearrangements derived from pre-B cells having proliferated to different extent in vitro in the absence of any added cytokines were isolated and analysed for their sequences and pairing capacities. To this end, the present study shows a tendency of a better pairing capacity for VHDHJH-rearrangements derived from cells, which have undergone more cell divisions.
In the second part the effect of TSLP in lymphocyte development was investigated. It could be shown that increased TSLP availability through transgene expression fully restored lymphopoiesis in IL-7 deficient mice. It rescued B-cell development, increased the thymic cellularity, and rescued the thymic architecture in IL-7-/- mice. Adult wt bone marrow cells differentiated normally into B and T lineages and restored peripheral compartments when adoptively transferred into lethally irradiated IL-7-/- TSLP Tg recipients. Moreover, it could be shown that B cells generated in IL-7-/- TSLP Tg mice originated from adult precursors as these B cells contained N nucleotide additions in their IgH junctions.
In the third part the expression of BAFF-R on murine BM B cells was investigated with the use of novel monoclonal anti-mBAFF-R antibodies. We found that expression of BAFF-R is first detectable by flow cytometry (FACS) on a fraction of CD19+ CD93+ IgM+ CD23- BM B cells. This BAFF-R+ BM B cell population showed higher levels of surface IgM expression and decreased recombination-activating gene 2 (RAG-2) transcripts than BAFF-R- immature B cells. When cultured in vitro, BAFF-R+ immature B cells did not undergo further B cell receptor rearrangement, while BAFF-R- immature B cells did. However, when cultured in the presence of an anti-kappa light chain antibody, BAFF-R+ immature B cells could be induced to undergo receptor editing and this correlated with the upregulation of RAG-2 and the downregulation of both surface IgM and BAFF-R expression. Addition of BAFF did not inhibit this induced receptor editing. We concluded, that expression of BAFF-R can be used as a marker to identify immature B cells, which under normal conditions no longer undergo BCR editing, but can still be induced to do so by BCR engagement.
In the fourth part a putative new B cell population was phenotypically and functionally characterized. These cells, in this study termed ‘newB’ cells, were defined by the expression of CD19+ CD93- CD21-/low CD23-/low CD5-. The collected data so far supports the idea that newB cells could represent an additional intermediate cell population in the transition of immature to mature B cells. Alternatively, newB cells could represent cells, which for as yet, unknown reasons get selected out from the pool of mature, reactive B cells later, because they do not fulfill criteria to remain and therefore are rendered anergic and regain a more immature phenotype.
In the last part the differential response of splenic mature B cells to the mitogen lipopolysaccharide (LPS) was investigated. It was known that frequencies of LPS-reactive B cells in C57BL/6 mice is higher than in BALB/c mice. In this study, it could be shown that actually the FOB cells of C57BL/6 respond stronger to LPS in vitro than FOB cells of the BALB/c strain. In addition, MZB cells of both mouse strains showed a stronger response to LPS than the FOB cells. However, in contrast to the observation in FOB cells, MZB cells of both tested strains responded equally strong. A genetic approach did not lead to the identification of a responsible locus for the observed differential response in FOB cells, but indicated that most probably multiple genes control the response in a differential fashion in FOB cells in the tested mouse strains. Furthermore, the results suggest that the stronger response of FOB cells from C57BL/6 mice either involves components within the MyD88-dependent pathway or components, which can modulate the MyD88-dependent signaling pathway. In addition and in contrast to another study, we could show that the observed differential response is not determined by the MHC class II haplotype in these strains.