Hematopoietic target cells

G-CSF is a relatively poor stimulus for progenitor cell development - only a few colonies are found (in comparison with IL-3, GM-CSF or M-CSF) and these consist predominantly of mature neutrophils, although occasional monocytes and macrophages can be found. Generally, the colonies are small and contain 50-150 cells (cf. GM-CSF or IL-3 where colonies containing 104-105 cells can routinely be observed). However, G-CSF can work also in synergy with other cytokines: a combination of GM-CSF plus G-CSF or of M-CSF plus G-CSF can recruit proliferation and development of more primitive 'target' cells than either factor used alone.

In addition to its mitogenic effects on progenitor cells, G-CSF can prolong the survival time of mature neutrophils, and mature cells exposed to G-CSF demonstrate enhanced antibody-dependent cellular cytotoxicity (ADCC), and enhanced response to the chemotactic peptide N-formyl-Met-Leu-Phe in terms of the generation of the superoxide anion. These responses are presumably related to the role of G-CSF in the activation and recruitment of neutrophils in response to infectious agents.

Radioiodinated G-CSF has been used to determine its receptor-binding properties. It binds to (some) blast cells, promyelocytes, myelocytes, metamyelocytes and mature neutrophils, with the numbers of receptors increasing from around 300 (on blast cells) up to 3000 on mature blood neutrophils. A small proportion of promonocytes and monocytes also have G-CSF receptors, but cells of other maturing hematopoietic lineages are uniformly negative. Thus, the receptor data are in broad agreement with the biological effects elicited by this cytokine. Analysis of leukemic cell populations has shown that receptors are present on most myeloid, but absent from lymphoid, malignancies. The receptor for G-CSF present on murine cells has recently been molecularly cloned and has some structural features shared by receptors for other CSFs and human interleukins.

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