Further evidence on a role of chemokines in injury-related pain hypersensitivity: commentary on a paper by Saika et al. (2012, this issue).

Injury of the body induces immune responses that involve release of inflammatory mediators and recruitment of immune cells. Among cascades of events, the small chemotactic cytokines, chemokines, play a crucial role in conveying/integrating information between inflammatory cytokines and immune cells. Evidence has been cumulating to support the view that chemokines also simultaneously interact with nociceptive pathways to facilitate the development of pain hypersensitivity following injury (White and Miller, 2010). In this issue, Saika et al. (2012) present evidence indicating that activity of CC-chemokine ligand 4 [CCL4, macrophage inflammatory protein (MIP)-1β] is correlated with neuropathic pain behaviour after partial ligation injury of the sciatic nerve in mice. They show that CCL4 mRNA was significantly upregulated in the injured nerve. Neutralizing CCL4 in the injured nerve with anti-CCL4 antibodies attenuated mechanical allodynia. On the other hand, injection of recombinant CCL4 into the sciatic nerve produced tactile allodynia. The effect of CCL4 is receptor mediated as shown by up-regulation of its receptor CCR5 and attenuation of behavioural hyperalgesia by a CCR5 antagonist. These findings argue for a role of the chemokine CCL4 and associated signalling in neuropathic pain. While up-regulation of CCL4 and its receptor CCR5 in the injured nerve and infiltrating macrophages indicates a role in initial inflammatory responses to injury, the CCL4–CCR5 signalling may also play a role in peripheral nociceptive sensitization. Although neuropathic pain after sciatic nerve injury lasts for months, it is noted that significant up-regulation of CCL4 lasted for only 6 h to 3 days after nerve injury. The authors correctly recognize that CCL4 is important for the initiation but not maintenance of injury-induced pain hypersensitivity. Behavioural results are consistent with this view as neutralizing CCL4 before 7 days, but not on 14–20 days after injury, attenuated hyperalgesia. The up-regulation of two other macrophage-related chemokines, CXCL2 (MIP-2α) and CCL3 (MIP-1α), has also been shown to be relatively short, ranged from 1 to 7 days after injury (Kiguchi et al., 2010, 2012). Thus, the induction of these chemokines is correlated with initial immune responses to injury and likely to further contribute to immune cell infiltration and Wallerian degeneration. The localization of CCL4 in Schwann cells and macrophages supports this. It is yet to be determined, however, whether the induced CCL4 chemokine directly sensitize peripheral nociceptors. Despite a short lasting up-regulation after nerve injury, the authors show that a single perineural injection of recombinant CCL4 induced tactile allodynia that lasted for at least 14 days. A reasonable explanation for this result is that CCL4 was able to initiate hyperalgesia that was maintained by other factors. Immune responses are coordinated events involving multiple intermingling pathways. In fact, the authors show that neutralizing CCL4 attenuated up-regulation of key proinflammatory cytokines interleukin (IL)-1β, IL-6 and tumour necrosis factor (TNF)-α. Apparently, this suggests a positive interrelationship of CCL4 with other inflammatory cytokines, although the interactions are more complex. In mice with chronic constriction injury of the sciatic nerve, up-regulation of IL-1β and TNF-α occurs almost immediately at 1 h (Uceyler et al., 2007), yet the up-regulation of CCL4 did not reach statistical significance until 6 h. This temporal relationship of cytokine/chemokine response to nerve injury fits general inflammatory process that chemokines are stimulated by cytokines released from tissue macrophages. However, CCL4 is likely to provide a positive feedback to inflammatory cytokines, which is important for maintained cytokine activity. As a possible complement to the earlier CCL4 response, another more extensively studied chemokine CCL2 (monocyte chemoattractant protein-1) is up-regulated for up to 4 weeks at multiple levels of pain pathways after injury (Abbadie et al., 2003; Zhang and De Koninck, 2006; Thacker et al., 2009; Guo et al., 2012), suggesting a role in the persistent phase of inflammation and pain. The interrelationship between pain-related chemokine/cytokines and their receptors in regulating nociception needs to be further studied. A puzzling observation by Saika et al. (2012) is that tactile allodynia, but not thermal hyperalgesia, was attenuated by anti-CCL4 treatment. Similar differential effects of chemokine on mechanical and thermal nociception have been reported previously. Thacker et al. (2009) show that intraspinal injection of CCL2 induces mechanical allodynia but not thermal hyperalgesia in rats. In mice lacking the receptor for CCL2, there was a trend for a reduction of mechanical allodynia but not an effect on thermal hyperalgesia after the adjuvant treatment (Abbadie et al., 2003). From a neural point of view, mechanical and thermal pain may be mediated by different primary afferent fibres that express distinct sets of mediators and opioid receptors (Scherrer et al., 2009). The research on chemokines indicates that immune regulation also plays a role. Saika et al. (2012) also show that CCR5 antagonist nevertheless blocked both tactile allodynia and thermal hyperalgesia. This could be explained by the fact that CCR5 also accepts binding from other chemokines such as CCL3 and CCL5. Neutralizing CCL3 attenuated thermal hyperalgesia in mice (Kiguchi et al., 2010). As a final note, the effect of nerve injury lingers. For example, macrophage activity lasts for months after initial nerve injury (Frisen et al., 1993). Further studies should address differential involvement of the immune system in the initiation and persistency of injury-related pain, as well as the events that lead to the transition to persistent pain.