Endotoxin Protection and Colony-Forming Units

Bacterial endotoxin is known to increase survival of irradiated animals and to be a potent stimulator of the hematopoietic system. The purpose of the present experiment was (1) to study the effect of endotoxin on hematopoietic stem cells, as evaluated by colony-forming units (CFU's), and (2) to determine the extent to which changes in the numbers of CFU's could be correlated with the survival-promoting effects of endotoxin. The greatest effects on survival and CFU content of the spleen occurred when endotoxin was given 24 hours before irradiation. Animals treated in this manner showed at least a 25-fold increase in spleen CFU's, but no alteration in the CFU D₃₇ of about 90 R. In general, the largest increases in spleen CFU's were produced by procedures (dose, route, number of time of injection) which produced the greatest survival. On the other hand, by varying the time of endotoxin injection relative to irradiation, it was shown that the numbers of spleen CFU's were increased ...