In vivo effect of interleukin-6 on cycling status of hematopoietic progenitors from adults and neonates.

In vitro, IL-6 can induce hematopoietic progenitors to progress from G0 into cycle, but a role for IL-6 in regulating cycling status of progenitors in vivo has not been established. In our studies, groups of five to six adult and newborn rats received i.v. injections of either IL-6 (1 ng/g body wt) or the vehicle (control), after which cycling of hematopoietic progenitors was evaluated by tritiated thymidine suicide. Progenitors from adult rats injected with the control had thymidine suicide rates of 7 +/- 1% (mean +/- SEM), compared with 23 +/- 7% in the IL-6 recipients (p less than 0.02). Progenitors from newborn rats injected with the control had thymidine suicide rates of 19 +/- 2%, compared with 29 +/- 1% in the IL-6 recipients (p less than 0.003). In addition, IL-6 administration resulted in release of cells from the neutrophil storage pool into the circulation, as evidenced by fewer polymorphonuclear cells flushed from the long bones (neonates, p less than 0.001; adults, p less than 0.003), a rise in blood neutrophil concentration (neonates, p less than 0.001; adults, p less than 0.05), and a leukocyte "left shift" (neonates, p less than 0.001; adults, p less than 0.01). Thus, the effects of IL-6 in vivo in newborn and adult rats include cycle induction of hematopoietic progenitors and release of neutrophils from the storage pool into the circulation.