Effect of macrophage suppression with silica on the proliferation of Schwann cells during Wallerian degeneration

Prostaglandin E(2) and 6-keto-prostaglandin F(1alpha) production is elevated following traumatic injury to sciatic nerve

Sciatic nerve explants cultured either alone or in the presence of peritoneal macrophages were used to study prostaglandin E(2) (PGE(2)) and 6-keto-PGF(1alpha) production following traumatic peripheral nerve injury. Although barely detectable at early time points (1-3 h in vitro), the production of PGE(2) and 6-keto-PGF(1alpha) by sciatic nerve explants increased significantly after 18 h and remained elevated for up to 96 h. The cyclooxygenase-2 (COX-2) selective inhibitor, NS-398, inhibited PGE(2) and 6-keto-PGF(1alpha) production by injured sciatic nerve in a dose-dependent manner. Consistent with the observed effect of NS-398, peripheral nerve explants, as well as Schwann cells and perineural fibroblasts cultured from neonatal sciatic nerve, each contained COX-2 immunoreactivity after 24 h in vitro. Both Schwann cells and perineural fibroblasts produced significant amounts of PGE(2) and 6-keto-PGF(1alpha); but only in the presence of arachidonic acid. As observed for injured sciatic nerve, the production of PGE(2) and 6-keto-PGF(1alpha) by primary Schwann cells and perineural fibroblasts was completely inhibited by NS-398. Compared to macrophages cultured alone, macrophages cultured in the presence of sciatic nerve explants produced large amounts of PGE(2), whereas the level of 6-keto-PGF(1alpha) was unchanged. In contrast, macrophages treated with adult sciatic nerve homogenate did not produce significant amounts of either PGE(2) or 6-keto-PGF(1alpha) during the entire course of treatment. We conclude that injured sciatic nerves produce PGE(2) and 6-keto-PGF(1alpha) by a mechanism involving COX-2 activity and that macrophages produce large amounts of PGE(2) in response to soluble factors produced by injured nerve but not during the phagocytosis of peripheral nerve debris.

Effect of liposome-mediated macrophage depletion on Schwann cell proliferation during Wallerian degeneration

The axolemma is considered a well-established mitogen, responsible for Schwann cell proliferation during Wallerian degeneration in the peripheral nerve. However, very little is known about the role of macrophages in Schwann cell proliferation. To test the possible influence of macrophages on Schwann cell proliferation during Wallerian degeneration, macrophages were depleted by dichloromethylene diphosphonate (CI2MDP)-containing liposomes in two-month old C57BL/6J mice. CI2MDP-containing liposomes were injected into the mice intravenously prior to inducing Wallerian degeneration. The injection was repeated every other day to maintain macrophage depletion. Physiologic saline was injected into the control mice. To assess macrophage depletion in vitro, cells were isolated from sciatic nerves at 1, 2, 3, 5, and 7 days post-transection (DPT) and Mac-1 positive cells attached to coverslips were counted. In an in vivo study, Mac-1 positive cells were counted on sciatic nerve sections at the same time points. Throughout the course, the number of Mac-1 positive cells in macrophage-depleted mice was less than that in the control mice both in vivo and in vitro. Schwann cell proliferation was assessed by an in vitro system that reflects in vivo status at the time of cell isolation. Schwann cells were isolated from sciatic nerves at the same time points and proliferation rate was measured by thymidine autoradiography. The proliferation rate was mildly suppressed in macrophage-depleted mice, especially for the initial 3 DPT; however, the pattern of proliferation was not significantly different from controls. These results suggest that macrophages contribute to Schwann cell proliferation during Wallerian degeneration however, their contribution may be relatively limited.