A New Approach to Evaluate the Total Reserve of Hematopoietic Progenitors after Acute Irradiation

Acute and late effects of combined internal and external radiation exposures on the hematopoietic system

Purpose: Incidents, such as nuclear facility accidents and the release of a 'dirty bomb', might result in not only external irradiation of personnel, but additional internal exposures through concomitant inhalation and/or ingestion of radioactive particulates. The purpose of this study was to define the impact of such a combination of radiation injuries on the hematopoietic niche.Material and methods: To assess changes in the murine hematopoietic system, we used a combined exposure of total body irradiation (TBI, 6 Gy) followed immediately by an internal (intraperitoneal) administration of 100 µCi of soluble ¹³⁷Cs. We then evaluated acute survival in combined versus single modality exposure groups, as well as assessing hematopoietic function at 12 and 26 week time points.Results: Acutely, the combination of external and internal exposures led to an unexpected delay in excretion of ¹³⁷Cs, increasing the absorbed dose in the combined exposure group and leading to mortality from an acute hematopoietic syndrome. At 12 weeks, all exposure paradigms resulted in decreased numbers of phenotypic hematopoietic stem cells (HSCs), particularly the short-term HSCs (ST-HSC); long-term HSCs (LT-HSC) were depleted only in the internal and combined exposure groups. At 26 weeks, there was significant anemia in both the TBI alone and combined exposure groups. There were decreased numbers in both the LT- and ST-HSCs and decreased functionality, as measured by competitive repopulation, was seen in all radiation groups, with the greatest effects seen in the internal and combined exposure groups.Conclusions: Our data indicate that a combined injury of sublethal external irradiation with internal contamination induces significant and persistent changes in the hematopoietic system, as may have been predicted from the literature and our own group's findings. However, a novel observation was that the combined exposure led to an alteration in the excretion kinetics of the internal contamination, increasing the acute effects beyond those anticipated. As a result, we believe that a combined exposure poses a unique challenge to the medical community during both the acute and, possibly, delayed recovery stages.

Biodistribution of (137)Cs in a mouse model of chronic contamination by ingestion and effects on the hematopoietic system

The aim of this work was to define the possible occurrence of hematological changes during the course of a chronic ingestion of (137)Cs. A mouse model was used, with ingestion through drinking water with a cesium concentration of 20 kBq l(-1). Ingestion started in parent animals before mating, and (137)Cs intake and its effect on the hematopoietic system was studied in offspring at various ages between birth and 20 weeks. (137)Cs content was measured in various organs, indicating that (137)Cs was distributed throughout the organism including lympho-hematopoietic organs, i.e., femurs, spleen and thymus. However, we did not observe any effect on the hematopoietic system, whatever the parameter used. In fact, blood cell counts, mononuclear cell counts and progenitor frequency in bone marrow and spleen, and Flt3-ligand, Erythropoietin, G-CSF and SDF-1 concentration in plasma remained unchanged when compared to control animals. Moreover, phenotypic analysis did not show any change in the proportions of bone marrow cell populations. These results indicate that, although (137)Cs was found in all organs implicated in the hematopoietic system, this did not induce any changes in bone marrow function.

Fanca-/- hematopoietic stem cells demonstrate a mobilization defect which can be overcome by administration of the Rac inhibitor NSC23766

Fanconi anemia is a severe bone marrow failure syndrome resulting from inactivating mutations of Fanconi anemia pathway genes. Gene and cell therapy trials using hematopoietic stem cells and progenitors have been hampered by poor mobilization of HSC to peripheral blood in response to G-CSF. Using a murine model of Fanconi anemia (Fanca(-/-) mice), we found that the Fanca deficiency was associated with a profound defect in hematopoietic stem cells and progenitors mobilization in response to G-CSF in absence of bone marrow failure, which correlates with the findings of clinical trials in Fanconi anemia patients. This mobilization defect was overcome by co-administration of the Rac inhibitor NSC23766, suggesting that Rac signaling is implicated in the retention of Fanca(-/-) hematopoietic stem cells and progenitors in the bone marrow. In view of these data, we propose that targeting Rac signaling may enhance G-CSF-induced HSC mobilization in Fanconi anemia.

Use of flt3 ligand to evaluate residual hematopoiesis after heterogeneous irradiation in mice

We evaluated the possibility of using plasma Flt3 ligand (FL) concentration as a biological indicator of bone marrow function after heterogeneous irradiation. Mice were irradiated with 4, 7.5 or 11 Gy with 25, 50, 75 or 100% of the bone marrow in the field of irradiation. This model of irradiation resulted in graded and controlled damage to the bone marrow. Mice exhibited a pancytopenia correlated with both the radiation dose and the percentage of bone marrow irradiated. The FL concentration in the blood increased with the severity of bone marrow aplasia. Nonlinear regression analysis showed that the FL concentration was strongly correlated with the total number of residual colony-forming cells 3 days after irradiation, allowing a precise estimate of residual hematopoiesis. Moreover, the FL concentration on day 3 postirradiation was correlated with the duration and severity of subsequent pancytopenia, suggesting that variations in FL concentrations might be used as a predictive indicator of bone marrow aplasia, especially by the use of linear regression equations describing these correlations. Our results provide a rationale for the use of FL concentration as a biological indicator of residual hematopoiesis after heterogeneous irradiation.

The mobilization of hematopoietic progenitors to peripheral blood is predictive of the hematopoietic syndrome after total or partial body irradiation of mice

PURPOSE

In previous studies we showed that administration of mobilizing growth factors (MGFs) to mice previously exposed to total body irradiation mobilizes to peripheral blood (PB) a number of progenitors that correlates with the total reserve of progenitors surviving the exposure. Now we have tested whether this finding is independent of the radiosensitivity of the mice and of the homogeneity of the radiation exposure. Also we have investigated whether numbers of mobilized progenitors predict the hematopoietic syndrome after irradiation.

METHODS AND MATERIALS

Mice were subjected to partial or total body irradiation and treated with MGFs. Thereafter, the number of colony-forming units granulocyte-macrophage progenitors in PB was correlated with the total reserve of surviving progenitors and with the nadir of leukocytes after the irradiation.

RESULTS

The number of progenitors mobilized to PB after irradiation of normal and radiosensitive mice showed the same correlation with respect to the reserve of bone marrow progenitors surviving the exposure. Additionally, the number of mobilized progenitors correlated with the leukocytes' nadir after the irradiation, regardless of homogeneous or inhomogeneous exposures.

CONCLUSIONS

In a mouse experimental model, the number of hematopoietic progenitors mobilized to PB by MGFs is a good predictor of the hematopoietic syndrome occurring after total or partial body irradiation.

Comparison of autologous cell therapy and granulocyte-colony stimulating factor (G-CSF) injection vs. G-CSF injection alone for the treatment of acute radiation syndrome in a non-human primate model

PURPOSE

To compare the efficacy of autologous cell therapy after irradiation combined with granulocyte-colony stimulating factor (G-CSF) injections with G-CSF treatment alone in a heterogeneous model of irradiation representative of an accidental situation.

MATERIAL AND METHODS

Non-human primates were irradiated at 8.7 Gy whole-body dose with the right arm shielded to receive 4.8 Gy. The first group of animals received G-CSF (lenograstim) injections starting 6 h after irradiation, and a second group received a combination of G-CSF (lenograstim) injections and autologous expanded hematopoietic cells. Animals were followed up for blood cell counts, circulating progenitors, and bone marrow cellularity.

RESULTS

No significant differences were seen between the two treatment groups, whatever the parameter observed: time to leukocyte or platelet recovery and duration and severity of aplasia.

CONCLUSION

Our results indicated that identical recovery kinetic was observed when irradiated animals are treated with G-CSF independently of the reinjection of ex vivo expanded autologous hematopoietic cells. Thus G-CSF injections might be chosen as a first-line therapeutic strategy in the treatment of accidental acute radiation victims.

Application of autologous hematopoietic cell therapy to a nonhuman primate model of heterogeneous high-dose irradiation

We developed a model of heterogeneous irradiation in a nonhuman primate to test the feasibility of autologous hematopoietic cell therapy for the treatment of radiation accident victims. Animals were irradiated either with 8 Gy to the body with the right arm shielded to obtain 3.4 Gy irradiation or with 10 Gy total body and 4.4 Gy to the arm. Bone marrow mononuclear cells were harvested either before irradiation or after irradiation from an underexposed area of the arm and were expanded in previously defined culture conditions. We showed that hematopoietic cells harvested after irradiation were able to expand and to engraft when reinjected 7 days after irradiation. Recovery was observed in all 8-Gy-irradiated animals, and evidence for a partial recovery was observed in 10-Gy-irradiated animals. However, in 10-Gy-irradiated animals, digestive disease was observed from day 16 and resulted in the death of two animals. Immunohistological examinations showed damage to the intestine, lungs, liver and kidneys and suggested radiation damage to endothelial cells. Overall, our results provide evidence that such an in vivo model of heterogeneous irradiation may be representative of accidental radiation exposures and may help to define the efficacy of therapeutic interventions such as autologous cell therapy in radiation accident victims.