Inhibitors of haemopoietic cell proliferation: reversibility of action

Effect of granulocyte chalone on acute and chronic granulocytic leukaemia in man. Report of seven cases

This study reports results from the first clinical tests in which 7 patients with myeloid leukaemia (5 acute and 2 chronic leukaemias in metamorphosis) were treated from 4 to 45 days with granulocyte chalone, the tissue-specific endogenous inhibitor of granulopoiesis. It was observed that i.v. injection of partially purified chalone inhibits proliferation of leukaemic, and presumably also normal granulocytic cells, leaving all other cell types unaffected. Inhibition of leukaemic growth was distinct in 6 of the 7 patients; in 5 cases the inhibition was followed by acutal regression of the leukaemia, lasting up to several months in the absence of any maintenance therapy, and in one case the treatment led to a complete remission. Chalone treatment also resulted in an enhancement of erythropoiesis and megakaryopoiesis, and in phenomena some of which were totally unexpected, such as immunostimulation and a remarkable resistance to bacterial infections in the presence of extreme granulocytopenia. This study shows that granulocyte chalone is biologically active against myeloid leukaemia in man, but not that the therapeutic value of the impure chalone is superior to modern cytostatic drugs. Long-term therapeutic trials were not possible with the partially purified preparations available, mainly because of side-effects which prevented adequate dosing.

Controls on the cell cycle

The control of cell proliferation, in physiological terms, depends not so much on our understanding of the sequence of biochemical events unfolding as a cell progresses through its proliferation cycle, as upon the recognition by a tissue of the demands for functional cells of a particular type. After considering the modes of control possible, i.e. by recruitment of resting G0-state cells into cycle or by modifying the proliferative behaviour of already proliferating cells, haemopoietic tissue is used as a model to illustrate how the principles of proliferation control in specific cell lineages can be effected. Although the mode of stem cell control is different from that in the maturing populations, all depend on a co-ordination of negative feedback loops for inhibitor and stimulator which are specific to that cell population. The concept of a 'quantal' cell cycle is considered but its application to control in an adult steady-state tissue must be modified to take account of microenvironmental influences which are shown, by their cellular organization, to be an important feature in haemopoietic and probably all other tissues.

Investigations into the in vitro sensitivity to erythropoietin of haematopoietic tissue from ex-hypoxic, erythrocytotic mice

If 'sensitivity' is defined as the ratio of haem synthesis in erythropoietin (Ep)-stimulated cultures to that in cultures without added Ep, then haematopoietic tissue of ex-hypoxic mice is less sensitive to Ep in vitro than similar tissue from normal donors. This appears to be due primarily to the loss of erythropoietin responsive cells from the marrow and spleen. In addition, saline extracts of mature erythrocytes inhibited erythropoiesis in normal bone marrow cultures in a manner directly related to the 'haematocrits' used during their preparation. These data support recent suggestions that the preference of erythrocytotic mice over normal animals for Ep assay probably can be attributed to their low baseline level of erythropoiesis rather than an increased sensitivity to Ep.

Modulative effect of endogenous granuloid inhibitors on cell proliferation

The particle-free crude extract of differentiated granulocytes (GCE) and the GI-1, GI-2, GI-3 proliferation inhibitory fractions (M.W. larger than or equal to 70,000 approximately 11,500 and less than or equal to 4000) were studied for the effects they exert on cultured cells. As shown by the curve, in bone marrow suspension cultures, in the dose range of 0.01-10.0 microng/ml, GCE maximally inhibits 3H-TdR incorporation into the DNA for 5.5 hours. In agar-gel cultures, 1.0-5.0 microng/ml of GCE reduces both the number (by 50.6-54.8%) and the size of the colonies formed. The manifestation of the inhibitory effect of GI-1, GI-2 and GI-3 fractions requires 2.5-3.5 hours. The inhibitors designated GI-2 and GI-2 are target-cell specific but they do not reduce 3H-TdR incorporation either in thymocyte suspension or HeLa monolayer cultures. On the other hand GI-1 inhibis the proliferation in HeLa cultures, as well. These endogenous inhibitors exert their optimum effect, even in a partially purified state at 2-4 orders or magnitude lower concentrations than does 1,2/5,6-dianhydrogalactitol (DAD), a cell-aspecific inhibitor used for comparison in the same system.

The effects of red blood cell extracts on the proliferation of erythrocyte precursor cells, in vivo

Saline incubation extracts of mature erythrocytes were assayed in vivo by a variety of techniques in order to study their ability to modify the proliferation of maturing erythroid cells. Using comparable extracts from granulocytes and lymphocytes, the specificity of the effect of the red cell extract for erythroid cells was confirmed by measurement of autoradiographic labelling indices, radio-iron incorporation and spleen colony growth. The erythroid cells were found to be very sensitive to the effects of the extract, as little as 10 microgram per mouse producing a maximum effect on iron incorporation. It was found that the extract does not block erythroid cells proliferation completely but simply lengthens the cell cycle, mainly by increasing the GI phase of the cycle. There was no effect on the committed erythroid precursor cells. The in vivo activity, specificity and non-toxicity to the cells, together with the cells' sensitivity to red cell extract suggest, therefore, that this inhibitor may play a physiological role in the control of red cell production.

Inhibitor of stem cell proliferation in normal bone marrow

A saline extract from normal bone marrow cells having a molecular weight in the range 50000-1000000 daltons has been found to protect rapidly proliferating haemopoietic spleen colony forming cells (CFUs) from the lethal effects of large doses of tritiated thymidine. This extract is non-toxic to the cells. It is not found in regenerating marrow where the CFUs population is rapidly proliferating. Its effect appears to be specific for the CFUs since it has no effect on the proliferation of its close descendant, the granulocytic precursor cell (CFUc), and no effect on the average cytoplasmic structuredness of the whole bone marrow cell population. The active material is probably protein since it is degraded by trypsin.

The solubility properties of granulopoiesis inhibiting factor

The solubility of granulopoiesis inhibiting factor (GIF) in various aqueous and organic solvents was investigated. GIF is soluble in water, 10% acetic acid, and 10% pyridine. It is not extractable by 1-butanol at low and high pH. A high solubility was found in polar organic solvents (ethanol-acetone 9:1, and chloroform-methanol 1:1), whereas GIF seems to be insoluble in pure chloroform.

The influence of a granulocytic inhibitor(s) on hematopoiesis in an in vivo culture system

The in vivo diffusion chamber (DC) technique for mouse marrow culture was used to determine the effect of a granulocyte inhibitor on the proliferation of the pluripotent stem cell(CFU-s) and the granulocyte progenitor cell (CFU-c). Granulocyte conditioned medium was injected intraperitoneally into mice bearing DCs during the initial 48 hr of culture. The early injections of inhibitor resulted in a significantly reduced number of granulocytic progeny formed within the DCs while there was no growth inhibition of mouse fibroblasts cultured under identical conditions. The reduced cell production was due in part to a significant reduction in the self-renewal rate of the CFU-c while no apparent direct effect was observed upon the growth of the CFU-s within the same cultures. These data suggest that the granulocytic inhibitor(s) acted to reduce the proliferation within the CFU-c population and thereby diminished the amplification potential inherent in the initial cell inoculum.

Control of granulopoiesis in man.III. inhibition of colony formation by dense leukocytes

Cellular feeder layers, prepared from normal blood leukocytes, usually stimulate human marrow to form colonies. A significant increase in the stimulating activity of unseparated leukocyte feeder layers is brought about following the removal of dense leukocytes in a manner which avoids enrichment of any remaining cell type. Restoration of dense leukocytes to a dense leukocyte depleted leukocyte feeder layer results in the reduction of stimulating activity to that of an unseparated leukocyte feeder; however, addition of dense leukocytes to unseparated leukocyte feeder layers has no effect on the stimulatory activity, over the range of concentrations used in this study.