The role of FLT3L and BAFFin B cell development and homeostasis

The maintenance of homeostasis in the immune system is a complex process. The control of B cell numbers occurs at many different stages in the developmental pathway, and numerous factors are involved. In this thesis, certain aspects of FLT3L, BAFF-R and BAFF on B cells were analyzed. In the first part (chapter 3), the effect of increased availability of FLT3L on EPLMs, which have been found previously to be B cell progenitors, was investigated. Upon FLT3L administration in mice, the number of EPLMs increased in a dose-dependent manner, but the number of B cells decreased. This decrease was reversible as B cell numbers recovered after cessation of FLT3L treatment. In vitro studies revealed, that EPLMs from FLT3L treated mice had a reduction of about 20-fold in B-lineage potential, while the potential for the T cell lineage was only slightly reduced and myeloid potential was unchanged. Transplantation of EPLMs from FLT3L treated mice into lymphopenic hosts showed, that these cells are able to give rise to B and T cells, but not to myeloid cells. From these experiments we concluded, that FLT3L is able to increase the number of EPLMs, and that these progenitor cells can reconstitute B and T cell compartments in vivo. In the second part (chapter 4), novel sets of monoclonal antibodies specific for mouse BAFF-R or human BAFF are described. Among the anti-mBAFF-R antibodies, several different clones that were able to block the binding of ligand to BAFF-R could be identified. With the anti-hBAFF antibodies, a sensitive ELISA for the detection of hBAFF in human sera was developed. In the third part (chapter 5), the role of BAFF-R signaling in the in vivo maintenance of mature B cells was investigated with the use of the new monoclonal anti-mBAFF-R antibodies. We showed that 14 days following a single injection of a monoclonal antimBAFF-R antibody that prevents BAFF binding, both follicular and marginal zone B cell numbers were drastically reduced, whereas B-1 cells were not affected. Injection of control, isotype-matched but non-blocking anti-mBAFF-R monoclonal antibodies did not result in B cell depletion. We also showed that this depletion is neither due to antibody-dependent cellular cytotoxicity nor to complement-mediated lysis. Moreover, prevention of BAFF binding lead to a decrease in the size of the B cell follicles and to a strong impairment of T cell dependent humoral immune response. These results establish a central role for BAFF - BAFF-R signaling in the in vivo survival and maintenance of both follicular and marginal zone B cell pools. In the fourth part (chapter 6), the precise expression of BAFF-R on murine BM B cells was investigated with the use of the new monoclonal anti-mBAFF-R antibodies. We found that expression of BAFF-R is first detectable by flow cytometry on a fraction of CD19+ CD93+ CD23- bone marrow (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 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 last part (chapter 7), serum levels of hBAFF from patients with diseases connected with disturbed B cell homeostasis were analyzed. We found, that in a subset of patients with SLE, Sjögrens syndrome or autoimmune thyroid diseases, hBAFF levels were elevated. Also, hBAFF levels were elevated in most patients suffering from Hodgkin or non-Hodgkin lymphomas. In patients with chronic hepatitis C, hBAFF levels were elevated in a subgroup of patients. Furthermore, we found no correlation between hBAFF levels and anti-dsDNA autoantibody titers in SLE patients. In addition, hBAFF levels did not correlate with age or gender. Taken together, this analysis suggests, that hBAFF might be involved in the genesis and/or maintenance of autoimmune and chronic infectious diseases only in a subgroup of patients, while its role in B cell leukemias might be more pronounced.