Identification of osteoblast stimulating factor 5 as a negative regulator in the B-lymphopoietic niche

Recent studies have revealed the crucial role of the niche which supports B-lymphocyte differentiation from hematopoietic stem cells. In this study, we aimed to identify a novel regulator of B lymphopoiesis secreted in the specific niche using the signal sequence trap method. Among the identified proteins from MS5 stromal cells, expression of pleiotrophin, placental proliferin 2, and osteoblast stimulating factor 5 (OSF-5) was dominantly high in several stromal cell lines. We found that OSF-5 suppressed early B lymphopoiesis in transgenic mice producing the target protein. The number of pre-B and immature B cells was reduced by more than half compared with control in the transgenic mice. In vitro studies showed that a secreted variant of OSF-5 inhibited the proliferation and colony formation of pre-B cells, whereas cell-intrinsic form had no influence on B lymphopoiesis. The main components of the B-lymphopoietic niche, osteoblasts in mice and mesenchymal cells in humans, are primary producers of OSF-5. These results define a novel mechanism of B lymphopoiesis in bone marrow. In the specific niche, B-lymphocyte differentiation is fine-tuned by negative regulators as well as supportive factors.