Selective irradiation of the vascular endothelium has no effect on the survival of murine intestinal crypt stem cells

Abstract

The possible role of vascular endothelial cell damage in the loss of intestinal crypt stem cells and the subsequent development of the gastrointestinal (GI) syndrome is addressed. Mice received whole-body epithermal neutron irradiation at a dose rate of 0.57 +/- 0.04 Gy x min(-1). An additional dose was selectively targeted to endothelial cells from the short-ranged (5-9 microm) particles released from neutron capture reactions in 10B confined to the blood by incorporation into liposomes 70-90 nm in diameter. Different liposome formulations produced 45 +/- 7 or 118 +/- 12 microg/g 10B in the blood at the time of neutron irradiation, which resulted in total absorbed dose rates in the endothelial cells of 1.08 +/- 0.09 or 1.90 +/- 0.16 Gy x min(-1), respectively. At 3.5 d after irradiation, the intestinal crypt microcolony assay showed that the 2- to 3-fold increased doses to the microvasculature, relative to the nonspecific whole-body neutron beam doses, caused no additional crypt stem cell loss beyond that produced by the neutron beam alone. The threshold dose for death from the GI syndrome after neutron-beam-only irradiation was 9.0 +/- 0.6 Gy. There were no deaths from the GI syndrome, despite calculated absorbed doses to endothelial cells as high as 27.7 Gy, in the groups that received neutron beam doses of <9.0 Gy with boronated liposomes in the blood. These data indicate that endothelial cell damage is not causative in the loss of intestinal crypt stem cells and the eventual development of the GI syndrome.

Fig. 1.

¹⁰B concentrations in blood and tissues as a function of time after injection of 0.2 ml of the MAC liposome preparation into the retroorbital sinus. Points represent the mean ± SD (n = 5 mice per time point).

Fig. 2.

Number of regenerating crypts per intestinal circumference as a function of neutron beam dose 84 ± 1 h after whole-body irradiation in the presence or absence of boronated liposomes in the blood or with a uniform ¹⁰B distribution using BPA. The triangles and squares represent irradiation conditions where the dose to the microvasculature was increased relative to the rest of the mouse by a factor of 2 or 3, respectively. Points represent the mean ± SD of at least 16 jejunal cross sections, 4 sections from each of 4 mice.

Fig. 3.

Survival experiments. (A) Mouse survival after whole-body, neutron-beam-only irradiations. (B) Mouse survival after whole-body neutron irradiation with boronated liposomes present in the blood. The microvascular absorbed doses were 10.8 ± 0.9 (MAC), 25.0 ± 2.0 (MAC+TAC), and 27.7 ± 2.2 Gy (MAC+TAC), respectively.

Fig. 4.

Histopathology in small intestine and bone marrow after whole-body, neutron-beam-only absorbed doses of 8.3 ± 0.6 or 9.0 ± 0.6 Gy. Also shown are tissues from a mouse in the 8.3-Gy beam plus (MAC+TAC) liposomes group in which the calculated absorbed dose to the endothelial cells was 27.7 ± 2.2 Gy.