title: Autocrine and paracrine growth mechanisms in human B lymphocytes
Creators: name: Bradshaw, Clare Louise
Publisher: name: University of Glasgow
Year: 1999

description: The SMS-SB cell line is an acute lymphoblastic leukaemia cell line with pre-B lymphocyte-like characteristics. Once removed from the patient, the cells could be grown in the absence of all exogenous mitogens, and were density-dependent for growth thus demonstrating the synthesis and secretion of an autocrine growth factor. Previous studies have found that this autocrine factor (SB-AF) can rescue SMS-SB cells from death, but have not been able to identify a cytokine able to substitute for its activity. The original aim of this thesis was to identify and characterise SB-AF and determine its anti-apoptotic mechanism. SB-AF was found to sustain the expression levels of the anti-apoptotic protein Bcl-2 and promote cell cycle progression. In addition, SB-AF seems to be a multi-component factor, whose anti-apoptotic activity results from the synergistic action of all its components. During these investigations, SMS-SB cells were found to express high levels of Bcl-2, which is unusual for lymphocytes at the pre-B developmental stage. In addition to the autocrine factor, previous work has demonstrated that soluble CD23 (sCD23) can act in a paracrine fashion to rescue low cell density cultures of SMS-SB cells from apoptotic death. This pleiotropic cytokine is a cleavage product of the 45kDa type II transmembrane CD23 antigen. As SMS-SB cells do not express the known receptors for CD23, namely, CD21, CD11b, and CD11c, the anti-apoptotic signals of sCD23 are mediated by a novel CD23 receptor. The work in this thesis shows that unlike sCD23, 45kDa membrane-associated CD23 does not seem to elicit the same response in SMS-SB cells, thus suggesting that only the soluble forms of CD23 can mediate anti-apoptotic effects via the novel receptor. However, the monolayer cell system used to present 45kDa CD23 appeared to be affecting SMS-SB cultures possibly influencing their response to CD23. Investigations were also undertaken to determine whether the novel receptor for CD23 was unique to the SMS-SB cell line. Preliminary data obtained from BIAcore surface plasmon resonance technology, demonstrate that the leukaemic pre-B cell lines Blin-1 and Nalm-6, and the mature B cell line 1E8, are able to bind sCD23 in the absence of the known CD23 receptors. Therefore, in addition to SMS-SB these cell lines also express a novel CD23 receptor. Further studies are required to determine whether all the cell lines express the same molecular receptor species, and whether signalling via these receptor(s) can prevent the apoptosis of Blin-1, Nalm-6 and 1E8 cells. The discovery of a novel CD23 binding receptor on a mature B cell line implies that, in addition to potentially having a role in precursor B cell development, sCD23 may also influence the growth and survival of more mature cell types away from the bone marrow microenvironment. It will therefore be interesting to investigate the role of CD23, and its novel receptor, throughout all stages of B cell development, and ultimately in all haematopoietic cells.