Blocking of up-regulated ICAM-1 does not prevent macrophage infiltration during Wallerian degeneration of peripheral nerve

Circulating blood monocytes infiltrate into distal degenerating nerve and differentiate into activated macrophages that remove degenerating axonal and myelin debris and promote axonal regeneration. The cellular and molecular mechanisms responsible for this monocyte-macrophage recruitment remain largely unknown. Cell adhesion molecules which mediate monocyte and endothelial cell interactions, such as the endothelial cell adhesion molecule intercellular adhesion molecule-1 (ICAM-1) interaction with the monocyte adhesion molecules Mac-1 (complement receptor type 3) and LFA-1 (lymphocyte function-associated antigen-1), have been shown to play a critical role in mediating the transendothelial migration of circulating monocytes into nonneural tissues following various types of injury. This study investigated whether these cell adhesion molecules also play a critical role in mediating monocyte-macrophage infiltration during Wallerian degeneration of peripheral nerve. Following sciatic nerve transection, Mac-1- and LFA-1-positive macrophages in distal degenerating nerve increased in number at 2 days and peaked at 14 days before declining. The number of ICAM-1-immunostained blood vessels increased maximally at 1 day before declining to baseline levels by 14 days. Three days following nerve transection, the intensity of ICAM-1 immunostaining on intraneural blood vessels was maximal and then decreased to baseline levels by 14 days. To test the role of ICAM-1 in mediating monocyte-macrophage recruitment, we used two complementary experimental strategies following a sciatic nerve transection: (1) intravenous administration of a rat ICAM-1-blocking monoclonal antibody and (2) ICAM-1 knockout mice. In both cases, the number of infiltrating monocytes-macrophages was above controls, which is opposite to what has been shown to occur in other tissues following injury.