title: Growth regulation of human B lymphocyte progenitor cells
Creators: name: White, Lindsey Jane
Publisher: name: University of Glasgow
Year: 1995
Subjects: subject: Immunology.

description: This thesis describes an investigation into the growth characteristics of a human pre-B cell acute lymphocytic leukaemia cell line, SMS-SB. Most lymphocytic tumours are difficult to adapt to tissue culture and enter a crisis after a few rounds of cell division where the majority of cells die. To sustain proliferation of the remaining cells, addition of exogenous mitogens is usually required. SMS-SB was an unusual leukaemia because the cells did not go through a crisis phase and grew indefinitely, in the absence of exogenous mitogens. This sustained proliferation in tissue culture appears to reflect the synthesis and secretion of an autocrine growth factor (s); the cells are density-dependent for growth, and proliferation can occur in media completely devoid of protein. The original aim of this work was to identify and characterise the autocrine growth factor, termed SB-AF. During investigations to identify cytokines with the ability to substitute for the autocrine growth factor activity, platelet-derived growth factor (PDGF) was shown to stimulate the growth of SMS-SB cells under low cell density conditions; SMS-SB cells are known to secrete PDGF and express PDGF receptors. However, antibody inhibition experiments suggest that PDGF cannot account for all the autocrine activity of SB-AF, thus other cytokine components of SB-AF were sought. CD23 is a 45kDa type-II transmembrane glycoprotein and a member of the C-type lectin superfamily. There is a soluble form of CD23 (sCD23) which is released by cleavage from the surface of cells into the extracellular fluid, and this form has been attributed multiple cytokine activities. It was discovered that sCD23 dramatically promotes thymidine incorporation by SMS-SB cells. The work of this thesis has shown that SMS-SB cells undergo apoptosis when cultured at low cell density; sCD23 is the only cytokine tested with the ability to prevent SMS-SB cell apoptosis. Apoptotic SMS-SB cells have low levels of the proto-oncogene bcl-2 but sCD23 can sustain bcl-2 levels in the cells. The investigations have shown that SMS-SB cells do not express CD23, negating the hypothesis that CD23 is acting in an autocrine fashion. The most interesting discovery made during these investigations was that SMS-SB cells bind CD23-containing liposomes specifically but they do not express the known receptors for CD23, namely CD21, CD11a and CD11b; SMS-SB cells express a novel CD23 receptor. Thus, SMS-SB cells express a novel receptor for CD23 and signalling via this receptor prevent apoptosis of the cells. Preliminary data is presented from CD23 affinity columns used to isolate and characterise the novel CD23 receptor. A protein of 85kDa has been identified as a candidate receptor, but further characterisation is required. SMS-SB cells will provide a good model to examine the role of autocrine growth factors in early B cell development, moreover, the discovery of a novel CD23 receptor on pre-B cells, implies a role for sCD23 in early B cell development. Since sCD23 has previously been shown to promote the growth and maturation of early T cells and myeloid progenitors, it will be interesting to investigate the role of CD23, and the novel receptor, in all aspects of haematopoiesis.