A novel interaction between CXCR and CCR signalling in monocytes constitutes an underlying mechanism for persistent vincristine-induced pain.

BACKGROUND

A dose-limiting side effect of chemotherapeutic agents such as vincristine (VCR) is neuropathic pain, which is poorly managed at present. Chemokine-mediated immune cell/neuron communication in preclinical VCR-induced pain forms an intriguing basis for the development of analgesics. In a murine VCR model, CXCR receptor-mediated signalling in monocytes/macrophages in the sciatic nerve orchestrates the development of mechanical hypersensitivity (allodynia). CXCR-deficient mice however still develop allodynia, albeit delayed; thus, additional underlying mechanisms emerge as VCR accumulates. Whilst both patrolling and inflammatory monocytes express CXCR, only inflammatory monocytes express CCR receptors. We therefore assessed the role of CCR in monocytes in later stages of VCR-induced allodynia.

METHODS

Mechanically evoked hypersensitivity was assessed in VCR-treated CCR- or CXCR-deficient mice. In CXCR-deficient mice, the CCR antagonist, RS-102895, was also administered. Immunohistochemistry and Western blot analysis were employed to determine monocyte/macrophage infiltration into the sciatic nerve as well as neuronal activation in lumbar DRG, whilst flow cytometry was used to characterise monocytes in CXCR-deficient mice. In addition, THP-1 cells were used to assess CXCR-CCR receptor interactions in vitro, with Western blot analysis and ELISA being used to assess expression of CCR and proinflammatory cytokines.

RESULTS

We show that CCR signalling plays a mechanistic role in allodynia that develops in CXCR-deficient mice with increasing VCR exposure. Indeed, the CCR antagonist, RS-102895, proves ineffective in mice possessing functional CXCR receptors but reduces VCR-induced allodynia in CXCR-deficient mice, in which CCR monocytes are elevated by VCR. We suggest that a novel interaction between CXCR and CCR receptors in monocytes accounts for the therapeutic effect of RS-102895 in CXCR-deficient mice. Indeed, we observe that CCR, along with its ligand, CCL, is elevated in the sciatic nerve in CXCR-deficient mice, whilst in THP-1 cells (human monocytes), downregulating CXCR upregulates CCR expression via p38 MAP kinase signalling. We also show that the CXCR-CCR interaction in vitro regulates the release of pronociceptive cytokines TNF-α and IL1β.

CONCLUSIONS

Our data suggests that CCL/CCR signalling plays a crucial role in VCR-induced allodynia in CXCR-deficient mice, which arises as a result of an interaction between CXCR and CCR in monocytes.