| Abstract: | The aim of this study was to expand the primitive and committed hematopoietic cell compartments in vivo in order to confer resistance of the blood cell forming system against the cytotoxic, cell cycle specific drug 5-fluorouracil (5-FU). Possible chemoprotective effects of such a pretreatment could result from increased numbers of hematopoietic cells, present before 5-FU administration. In addition, we hypothesized that an enhanced number of primitive and progenitor calls would result in a reduced cycling activity, ie, 5-FU sensitivity, of these same cells, due to normal physiological feedback loops. Administration of stem cell factor (SCF) plus interleukin-11 (IL-11) to mice was shown to result in expansion of the various immature cell compartments in marrow and, in particular, spleen. The total body content of the primitive cobblestone area forming cells (CAFC)-day 28 was increased to 140%, whereas the more committed cells (CAFC-day 7, erythroid and granuloid progenitors) were increased to 500%. This in vivo expansion resulted in a decreased 5-FU sensitivity of the hematopoietic system. In particular, mice that had received 5-FU 24 hours after discontinuation of growth factor pretreatment showed significantly less toxicity of committed cell stages. Compared with mice not pretreated, it appeared that in pretreated mice, 24 hours after 5-FU administration, the absolute number, but also the fraction of surviving CAFC, was much higher in both marrow and spleen. This was caused by a decrease in the cycling activity of all primitive cell subsets. To explore the possible use of this finding in a chemotherapeutic setting, we determined the interval between two subsequent doses of 5-FU (160 mg/kg) that was required to prevent drug- induced mortality. When control mice received a second dose of 5-FU 7, 10, or 14 days after the first, respectively 0%, 20%, and 80% survived. In contrast, 40% and 100% of mice that received SCF + IL-11 before the first dose of 5-FU, survived a second dose of 5-FU given respectively after 7 or 10 days. To assess whether chemoprotection in this setting could be ascribed to protection of the hematopoietic system, we transplanted a high number of normal bone marrow cells (sufficient to compensate for any hematopoietic deficiency) to normal and pretreated mice after they had been administered 2 doses of 5-FU, given 7 days apart. Bone marrow transplantation (BMT) could only rescue 50% of mice not pretreated, showing that a significant part of the mortality was because of nonhematologic toxicity. However, a BMT given to growth factor pretreated mice saved all mice, indicating that in this setting SCF + IL-11 had additional protective effects on cell systems other than hematopoiesis. In conclusion, our study showed fundamental knowledge about the behavior of primitive cells in vivo and has shown that manipulation of these and other cell compartments with appropriate growth factors may confer resistance against cytotoxic drugs. |
