Expression of ICAM-1 and CD11b after experimental spinal cord injury in rats.

We have performed an immunohistochemical study on the expression of the adhesion molecules ICAM-1 and CD11b 1 h to 1 week following a compression injury to the rat spinal cord. The spinal cord of control animals showed ICAM-1 expression in some vessels and in the leptomeninges. Mechanical compression of the spinal cord induced an endothelial upregulation of ICAM-1 that was maximal in rats surviving 1-2 days after injury. This reaction was seen at the center of the lesion as well as in the perifocal zones. Apart from the endothelial upregulation, increased ICAM-1 expression also was found in leptomeningeal and ependymal cells of traumatized animals. In control animals resting microglial cells were moderately CD11b immunoreactive. Trauma induced a rapid microglial upregulation of CD11b in the white matter that was evident even at 1 h after injury. By 1 day to 1 week posttrauma conformational changes consistent with microglial activation, i.e., transformation into phagocytic microglial cells, were seen in the white matter. In the gray matter, CD11b immunohistochemistry revealed massive infiltration of phagocytic microglial cells and macrophages in animals surviving 1 day to 1 week. Intravascular and infiltrating leukocytes were intensely CD11b immunopositive. As reflected by CD11b immunohistochemistry, the maximal infiltration of polymorphonuclear leukocytes occurred at 2 days after the insult. Endothelial upregulation of ICAM-1 facilitates adhesion and extravasation of leukocytes by binding to the counterreceptor CD11b. Knowledge regarding the expression and cellular distribution of such molecules after central nervous system trauma is important since inflammatory mechanisms have been suggested to be involved in secondary neurological damage and thus constitute potential targets of therapy.