Expression and function of the αVβ5 integrin during human B lymphopoiesis

The integrin αVβ5 is a receptor for sCD23 molecule and the αVβ5-CD23 interaction sustains proliferation of the pre-B cell line SMS-SB. This thesis describes further investigation into the role of αVβ5 integrin during human B cell development. B cell development in the bone marrow involves stepwise maturation of progenitor cells through different defined stages. A tight regulation of proliferation and differentiation mediates the progression of progenitor cells through this developmental pathway. A variety of signals from soluble molecules and adhesive interactions regulate this balance of proliferation and differentiation. The main aim of this work was to assess the importance of the integrin αVβ5 during B cell development by defining its expression and function during specific stages of B cell development in the bone marrow. The αVβ5 integrin was expressed by B cell precursors in the bone marrow, by different pre-B cell lines and the αVβ5-CD23 interaction sustained proliferation of some pre-B cell lines. The pre-B cell lines SMS-SB, RS4;11 and 697 showed a significant proliferative response to αVβ5 stimulation by sCD23, a sCD23-derived long peptide and anti-αVβ5 MAb 15F11. Transitional and more mature B cell lines down-regulated αVβ5 expression and did not show a proliferative response. Both αVβ5 and αVβ3 integrin could be detected on normal bone marrow B cell precursor populations, though αVβ5 was the more highly expressed integrin. In preliminary functional experiments, stimulation of CD19+/κ- cells with sCD23 induced cell proliferation whereas equivalent treatment of CD19+/κ+ cells did not. The data are consistent with the interpretation that αVβ5 integrin is expressed in B cell precursors but that its ability to sustain growth of these cells wanes as the cells mature towards a membrane immunoglobulin-positive state. The αVβ5-mediated proliferation was enhanced by the chemokine SDF-1 and PDGF but not by the cytokines IL-3, IL-4, IL-7 or IL-11. This effect was apparently restricted to earlier B cell precursors and was independent of levels of expression of both αVβ5 and CXCR4. Stimulation of SMS-SB cells by αVβ5 ligands provoked ERK phosphorylation and co-stimulation with SDF-1 promoted a more rapid and sustained ERK activation. PDGF induced a similar effect on αVβ5-mediated activation of ERKphosphorylation. These data suggest that ligation of αVβ5 by soluble, adhesion-independent stimuli activates ERK phosphorylation and this pathway can be modulated by inputs from G-protein-coupled and tyrosine kinase receptors. The murine pro-B cell line BAF03 also displayed αVβ5-mediated proliferation in response to human CD23. Preliminary experiments showed that human CD23 sustained growth of murine bone marrow B cell precursors. Therefore, these data suggest that murine B cells can also use αVβ5 integrin to sustain their growth and the studies described here in human cell lines could be translated into in vivo murine models. Further work is needed to confirm the proliferative response due to the αVβ5-CD23 interaction in normal B cell precursors in the bone marrow and to define the exact stage of development where this interaction is critical. In addition to its expression in the bone marrow B cell precursors, previous work has also demonstrated the expression of αVβ5 integrin in B cells from patients with ALL. Therefore, the αVβ5-CD23 interaction could have important implications not only in proliferation of normal B cell precursors but also in proliferation of neoplastic B cells. These data identify the αVβ5-CD23 interaction as a potentially important interaction during early B cell development, as αVβ5 expression and function is stage-specific, regulated by other molecules and can be demonstrated in both human and murine cell lines