Macrophages are important immune effector cells in both innate and adaptive immune responses. Injury to peripheral nerves triggers activation of resident macrophages and infiltration of haematogenous macrophages, which they play critical roles in Wallerian degeneration and neuropathic pain. As macrophages are able to change their phenotypes in response to environment cues, we attempt to identify distinct phenotypes of macrophages in injured nerves and to understand the potential contribution of each macrophage subpopulation to the genesis of neuropathic pain associated with nerve injury.

Rat mental nerves (terminal branches of trigeminal nerve) were loosely ligated. Sensitivity to mechanical stimuli at the lower lip area was monitored using calibrated von Frey Hairs. We examined the expression pattern of Iba-1, MAC1 and ED1 which allow us to reveal the immunophenotypes of macrophages at different time points post-injury. Functional status of each macrophage subpopulation was further investigated by colocalization with cytokines/chemokines, myelin basic protein and MHC II antigen, which reflect respectively secretory, phagocytic and antigen presentation properties of activated macrophages.

Following nerve injury, a burst of Iba-1⁺ macrophages was found in injured mental nerves. Among them, we detected two major immunophenotypes: MAC1⁺ cytokines/chemokines secreting macrophages and ED1⁺ phagocytic macrophages. Small, round shaped MAC1⁺ macrophages were distributed essentially around the lesion site and existed only at early time points. Large, irregular and foamy ED1⁺ macrophages were found among damaged nerve fibers and they persisted for at least 3months post-injury. Although ED1⁺ macrophages did not secrete inflammatory mediators, they were able to express neurotransmitter CGRP and MHC II at later time points. In parallel, we observed that mechanical allodynia developed after the nerve ligation was at its lowest level within 1month. Although slightly increased afterwards, the head escape threshold maintained significantly lower than before injury until 3months. We suggest that MAC1⁺ macrophages contribute to the initiation of neuropathic pain by releasing cytokines/chemokines, and ED1⁺ macrophages may contribute in maintaining the hypersensitivity under other mechanisms.

Our results highlighted the heterogeneity and the plasticity of macrophages in response to the injury and provided further information on their potential involvement in neuropathic pain. Exploring the full spectrum of macrophage phenotypes in injured nerve is necessary. Individual macrophage population may be selectively targeted by cell-specific intervention for an effective treatment of neuropathic pain.