Prediction of Lympho-Hematopoietic Regulatory Responses to Different Agents Using a 7-Population In Vitro Proliferation Assay.

The organizational hierarchy and the different population components of the lympho-hematopoietic system have been developed over five decades by studying the response of the system to a variety of perturbations from different sources, including radiation and chemical and drug administration. Indeed, the stem cell compartment hierarchy as we know it today would not have been possible without such studies. Many of these have utilized the in vitro functional ability of the cells to differentiate into colonies of cells in semi-solid medium using the colony-forming assay. However, without initial proliferation of the cells, differentiation would not occur. Therefore, the colony-forming assay implies proliferation, but does not directly measure it. In this communication we describe the proliferative response of 7-lympho-hematopoietic in vitro populations measured directly and simultaneously in mouse and human bone marrow to a number of different compounds, including chemicals, antibiotics, anti-inflammatory and anti-cancer drugs. The majority of these agents are part of 347 compounds listed in the Registry of Cytotoxicity (RC) for which the IC50 values (detected by neutral red uptake in mouse 3T3 and normal human keritinocyte cells) and LD50 (oral rat or mouse values) are known. Each of these compounds were tested over a dose range from 1pM to 10μM on 2 stem cell (HPP-SP and CFC-GEMM), 3 hematopoietic (BFU-E, GM-CFC and MK-CFC) and 2 lymphopoietic (T-CFC and B-CFC) populations simultaneously. Combinations of growth factors used routinely in the colony-forming assay were used to induce proliferation in the absence (controls) or presence of the test compounds. Each compound dose response was tested on a single human or mouse bone marrow population by suspending the cells in serum components, methyl cellulose and the respective growth factors and dispensing 8 replicates of 100μl for each dose into a 96-well plate. During the 5 (mouse) or 7 day (human) incubation period after which proliferation, with little or no differentiation, is measured, there is an increase in intracellular ATP (iATP) concentrations that is directly proportional to the proliferative status of the cells. Intracellular ATP is then released after lysing the cells and drives a luciferin/luciferase reaction to produce luminescence that is detected in a plate luminometer. The results indicate a variety of responses from no effect on any of the populations over the dose range measured to toxicity for every population with IC50 values in the pM range. From these results and the form of the dose response curves produced, a 5-point prediction model has been established that, when used in conjunction with the 7-population assay, can describe the global response of the lympho-hematopoietic system to virtually any compound. For cadmium chloride, ibuprofen, propanolol HCl, sodium dichromate, 5-fluorouracil, busulfan, chloramphenicol, doxorubicin and donaurubicin, the IC50 values were plotted against the LD50 values provide in the RC list. A positive correlation coefficient of R=0.91 with a probability (P) of <0.001 was obtained. The strong correlation between the 7-population assay and the RC provides validation for the assay and even allows it to be used to predict initial dosing for animal experiments by converting IC50 to in vivo values. In conclusion, a rapid, non-subjective and powerful tool is described that can provide response, regulatory and predictive information on the proliferative status of the lympho-hematopoietic system.