Osteoblasts Support Early B Lymphoiesis as well as Stem Cell Proliferation and Myelopoiesis: Identification of the Mammalian Cellular Analog of the Bursa of Fabricius.

The self-renewal, survival and differentiation of hematopoietic stem cells (HSC) are greatly influenced by the activities of neighboring osteoblasts and non-osteogenic bone marrow (BM) stromal cells such as fibroblasts, endothelial cells and adipocytes. Previously, we showed that osteoblasts from human long bones support the in vitro self-renewal as well as myeloid differentiation of human CD34⁺ cord blood cells. Recently, Li’s and Scadden’s groups provided in vivo evidence indicating a primary role of trabecular osteoblasts as a major component of HSC niche and of stromal osteoblastic cells in facilitating the self-renewal of HSCs. We have now asked whether osteoblasts contribute to early lymphopoiesis as well as myelopoiesis, by measuring the cellular outpout of purified HSCs on isolated osteoblasts alone, or with added non-osteoblast stromal cytokines as well. We prepared mature osteoblasts, as monitored and confirmed by homogeneous OPN and CD61 expression, by pretreating osteoblastic cells isolated from neonatal calvaria of C57BL/6 mice (CD45.2) with 1X10⁻⁷ M PTH. Purified OB were then co-cultured for 6 days with Lin⁻ BM cells (CD45.1⁺) isolated from congenic B6 mice(CD45.1) and labeled with CFSE. Osteoblast coculture stimulated the proliferation of Lin⁻ CD45.1⁺ BM cells 50-fold during culture, with most cells (87%) remaining tightly adherent to the osteoblast monolayer; no live cells were recovered from Lin⁻ BM cell culture without osteoblasts. In addition to mature granulocytes/monocytes, a substantial amount of CD45.1⁺B220⁺ B lymphocytes (about 10% of small size cells gated by forward and side scatter), were detected. In contrast, very few CD45.1⁺Lin-Sca-1⁺c-Kit⁺ (LSK) cells or CD45.1⁺Lin⁻Sca-1⁻c-Kit⁺ (CMP) cells were detected under these conditions. Most B220⁺ cells attached to osteoblasts were found to be CD43⁺CD24⁺ pre-B cells undergoing division. In contrast to the cells recovered attached to the osteoblasts, the pre-B lymphocytes found in suspension were more mature with phenotype of B220⁺CD43⁻CD24⁺. Prevention of direct contact of Lin⁻ BM cells with osteoblasts by Transwell co-culture abrogated the production of pre-B cells in both adherent and suspension compartments, indicating that physical contact is required for the interaction. Interestingly, when 20ng/ml of SCF, 6ng/ml of IL-3, 10ng/ml of IL-6 and 25ng/ml TPO were added to osteoblast/Lin⁻ cell co-culture, B lymphpoiesis was repressed, while the production of CD45.1⁺LSK HSCs and CMPs was significantly enhanced. These data demonstrate a direct role of osteoblasts in inducing and supporting the early development of B lymphocytes from HSCs or/and common lymphoid progenitors. Additional cytokines, perhaps provided in specific in vivo niches by non-osteogenic stromal cells, cooperate with the stimulatory signals from osteoblasts to promote the survival and expansion of HSCs. Taken together, these results suggest that osteoblasts may be the mammalian analog of the avian Bursa of Fabricius, and that their local degree of proximity to non-osteogenic stromal cells may define specific microniches for stem cell survival, myelopoiesis and/or B lymphopoiesis.