Alpha v beta 5 and related receptors in human B lymphocyte development

Abstract

CD23 is a multi-functional protein which exists in membrane-bound and soluble forms. Its functions include acting as the low affinity receptor for IgE and generating pro-inflammatory cytokine release in monocytes. CD23 has been found to interact with αvβ5 and this interaction greatly enhances growth of the B cell precursor cell line SMS-SB. This interaction may have a role in the development of normal human B cells and in cancer as the integrin is expressed on both precursor and ALL cells but not on normal mature B cells. One of the aims of this investigation was to expand on the finding that CD23 peptides containing an RKC motif had the same positive growth effect on SMS-SB cells as CD23. Other B cell lines – representative of both precursor and mature stages – were studied to ascertain whether this proliferative effect was dependent upon cell differentiation stage and/or presence of the αvβ5 integrin. It was found that peptides containing the basic RKC motif were mitogenic only for precursor B cells which were expressing αvβ5. Details of these peptides and their varying effects on the different cell lines are in Chapter 4. Stimulation of SMS-SB cells, presumably via the αvβ5, results in signalling through PI3K and subsequent phosphorylation of Akt. The growth of SMS-SB cells observed following stimulation with peptides containing the RKC motif was abrogated by the PI3K inhibitor LY294002 and western blotting revealed that phosphorylation of Akt was enhanced by stimulation with RKS containing peptides. Among CD23’s receptors is the integrin αvβ3. This integrin can form a signalling complex with CD47. Ligation of CD47 by anti-CD47 antibodies induces apoptosis in some cell lines. To determine whether a pattern exists between response to this stimulation and expression of αvβ3 integrin, cell lines with and without the integrin were tested. It was found that the myeloma cell lines KMS11 and H929 were responsive to this stimulus. Since these cell lines differ in their expression of αvβ3 (H929 cells express αvβ3 whereas KMS11 do not) it does not appear that any connection between the presence of the integrin and response via CD47 exists and therefore this signalling mechanism would appear to occur independently of the complex formed by CD47 and αvβ3