Differential response of macrophage subpopulations to myelin degradation in the injured rat sciatic nerve

Molecular mechanisms of myelin removal by macrophages were explored by exam ining the immunophenotypes of macrophages following injury of rat sciatic n erve, using a combined method of immunohistochemistry and confocal laser mi croscopy. In the crush injury model, the involvement in myelin clearance of a cytoplasmic antigen specific for monocytes/macrophages, ED1, was evident . The obvious recruitment of ED1-immunoreactive (-ir) cells was detected fi rst at the crush injury site and then in the distal stump within which Wall erian degeneration had occurred. Double labelling revealed that the ED1-ir cells, except for monocyte-like round cells, always phagocytosed myelin bas ic protein-ir myelin debris. On the other hand, the expression of ED2, a su rface antigen specific for resident macrophages, was significantly differen t; ED2-ir cells also increased while myelin removal was progressing from da y 3 to day 7, but only some of the cells were engaged in myelin phagocytosi s. The poor capacity of myelin phagocytosis by ED2-ir cells was supported b y the transection model, in which the proximal stump was ligated to suppres s regeneration. ED2 may be involved in events other than myelin removal, pr oviding a local environment conducive to axonal regeneration. Our findings thus seem to suggest that ED1 is one of the most reliable markers for cells carrying out myelin phagocytosis, whereas ED2 may participate in entirely different functions. The expression of complement receptor type 3, OX42, wa s similar to that of ED1 in terms of the swift recruitment of immunopositiv e cells, their distribution with close association to myelin debris and the ir high phagocytotic capacity. This supports previously reported in vitro e vidence that myelin phagocytosis by macrophages may be complement-mediated.