The chronic nature of neuropathic pain can dramatically reduce productivity and quality of life, and is notoriously difficult to manage using currently available therapeutic regimens. The principal immunomodulatory activity of IL-10 is to inhibit the production of proinflammatory cytokines . We have previously demonstrated that IL-10 mediated by the HSV vectors reversed formalin-induced inflammatory pain . Recent studies show that animals inoculated with the HSV vectors expressing IL10 reduces mechanical allodynia induced by the spinal cord injury . Previous studies have also demonstrated that the application of gp120 onto sciatic nerve, systemic ddC, or the combination of these treatments, resulted in NP characterized by mechanical allodynia and upregulation of inflammatory factors in the spinal cord and/or the DRG [4, 15, 29–31]. Moreover, these effects can be inhibited by intrathecal anti-inflammatory chemicals or by HSV-mediated TNF soluble receptor [14, 17]. The current study demonstrates that gp120 combined with ddC induced neuropathic pain, and that IL-10 mediated by the HSV vectors resulted in a significant elevation of mechanical threshold that was apparent one week post vector inoculation. Specifically, AUC of the mechanical threshold response in the HSV vectors expressing IL-10 was increased compared with the control vectors, indicating the markedly anti-allodynic effect of IL-10. The HSV vectors expressing IL-10 reversed the upregulation of p-p38, TNFα, SDF1α, and CXCR4 induced by gp120 with ddC in the lumbar SDH and the DRG at 2 and/or 4 weeks.
MAPKs such as p38 are important for intracellular signal transduction and play critical roles in regulating neural plasticity and inflammatory responses . The signaling of p38 is critical upon exposure to HIV gp120 for the neurotoxic phenotype of monocytic cells [35, 36]. In in vivo studies, Milligan and colleague have reported that the systemic p38 inhibitor CNI-1493 blocks intrathecal gp120-induced mechanical allodynia . Our unpublished data show that systemic ddC induced the upregulation of p-p38 in the spinal cord dorsal horn and the DRG.
HIV virus infection is able to increase the production of several cytokines . It is reported that there is an increased level of TNFα in the CSF , blood plasma , spinal cord , and brain  in patients with HIV. We and others have reported that the application of the recombinant gp120 to the sciatic nerve increases TNFα in the DRG and the spinal cord [4, 28]. Furthermore, intrathecal TNFα siRNA or TNF soluble receptor (TNFSR) reduces the gp120 application-induced mechanical allodynia, indicating that TNFα in the spinal cord and/or the DRG are involved in neuropathic pain induced by HIV gp120 . TNF soluble receptor mediated by the HSV vectors suppresses gp120-induced neuropathic pain and reduces TNFα . Taken together, these data highlight the importance of TNFα in the development of the exaggerated pain state related to HIV gp120.
HAART has dramatically reduced the HIV/AIDS associated morbidity and mortality . Although the incidence of most neurological complications of HIV has fallen with HAART, rate of HIV-SN has been rising . One of the reasons is that NRTIs are neurotoxic and can cause a dose-dependent painful peripheral neuropathy . Our previous studies demonstrate that TNFα is involved in the mechanisms of ddC-induced neuropathic pain . Knockdown of TNFα with siRNA blocks the mechanical allodynia induced by ddC; intrathecal administration of recombinant TNFSR, reverses mechanical allodynia induced by ddC, suggesting that TNFα is involved in ddC-induced neuropathic pain . Using HSV vectors expressing TNFSR, we extended our previous studies and found that it suppressed mechanical allodynia and decreased TNFα induced by ddC .
Evidence shows that chemokines and their receptors play an important role in inducing and maintaining neuropathic pain [14–16]. Chemokine receptors, in particular CXCR4 and CCR5, mediate HIV infection of immunocompetent cells as well as microglia . The interplay of TNFα and HIV-1 leads to the enhanced expression of toxic chemokines . CXCR4 and its ligand SDF1α are important factors in the neuropathogenesis of HIV/AIDS . HIV gp120 may bind to and activate CXCR4 expressed in the DRG neurons in a CD-4-independent manner [47, 48], suggesting the direct neurotoxic effects of gp120 on the neurons . Our recent studies have shown that HIV gp120 induces the upregulation of SDF1 and CXCR4 in the spinal cord and the DRG . White and colleagues reported that systemic ddC induces the expression of CXCR4 mRNA in glia and neurons, and SDF1 mRNA in glia . Pain hypersensitivity produced by ddC is inhibited by systemic CXCR4 antagonist, AMD3100, suggesting that NRTIs produce painful hypersensitivity through the CXCR4 signaling in the DRG . We report that ddC induces the overexpression of SDF1α and CXCR4 in the protein level in the spinal cord and the DRG, and that intrathecal administration of AMD3100 reverses the mechanical allodynia induced by ddC . In in vitro studies, SDF1 is produced under the control of inflammatory factors, such as TNFα . TNFα significantly enhances expression of CXCR4, which facilitates the chemotactic invasiveness of human mesenchymal stem cells toward SDF1α . Our studies demonstrate that blockage of TNFα by HSV-mediated TNFSR reverses the upregulation of SDF1α and CXCR4, suggesting that SDF1α/CXCR4 system is involved in the mechanisms of TNFα in neuropathic pain induced by gp120 or ddC [14, 17].
IL-10 inhibits the production of proinflammatory cytokines [21, 22]. In the in vitro studies, IL-10 diminishes TNF mRNA after the onset of stimulation of polymorphonuclear leukocytes with LPS, identifying the biological action of IL-10 as a suppressor of the inflammatory response . We have shown that IL-10 reduces the p-p38 and decreases the expression of full-length membrane spanning TNFα following lipopolysaccharide stimulation of microglia in vitro; IL-10 also reduces intracellular cleavage of membrane TNFα . Hypoxia-mediated increases in CXCR4 expression and cell survival are lower in IL-10-deficient othelial progenitor cell . IL-10 also downregulates CXCR4 mRNA expression in CD4+ T lymphocytes . In the in vivo studies, IL-10 inhibits the writhing response induced by acetic acid or zymosan in mice, and the knee joint incapacitation induced by zymosan in rats; IL-10 inhibits the release of TNFα from mice peritoneal macrophages obtained after local injection of zymosan . Acute intrathecal administration of rat IL-10 protein itself briefly reverses CCI-induced mechanical allodynia . Hyperalgesic responses to TNFα or carrageenan are inhibited by intraplantar administration of IL-10 . In the present studies, we report for the first time that IL-10 suppresses TNFα and SDF1/CXCR4 in the neuropathic pain state induced by gp120 with ddC.
To produce a long-term analgesic effect, non-viral plasmids or viral vectors expressing IL-10 may represent a promising approach in a variety of pain states. Intrathecal delivery of plasmid DNA encoding IL-10 gene prevents, and progressively reverses the allodynic state induced by paclitaxel (a chemotherapy drug), and markedly decreases paclitaxel-induced expression of TNF mRNA in the lumbar DRG . Repeated intrathecal delivery of the plasmid DNA vectors encoding IL-10 gene abolishes neuropathic pain induced by sciatic chronic constriction injury (CCI) . Adenoviral vectors encoding human IL-10 gene prevent and reverse thermal hyperalgesia and mechanical allodynia in the CCI model . Gene transfer to the primary sensory neurons of the DRG with self-complementary recombinant adeno-associated virus serotype 8 expressing IL-10, leads to significant reversal of mechanical allodynia in chronic neuropathic pain induced by L5 spinal nerve ligation . We have found that transduction of the DRG neurons in vivo achieved by subcutaneous inoculation of the HSV vectors in the foot results in production of transgene-coded IL-10 in the DRG neurons and transport of the gene product to terminals in the spinal cord, suppresses the formalin-induced nociceptive effect and reduces TNFα and p-p38 expression . IL-10 mediated by HSV almost totally reversed the upregulation of mRNA of TNFα in the spinal cord in the formalin pain model . Recent studies show that animals inoculated with the HSV vectors expressing IL10 reduces mechanical allodynia induced by the spinal cord injury, which correlates with a significant decrease in spinal TNFα . In the current studies, we extend our previous results showing that IL-10 expressed by the HSV vectors reduced neuropathic pain induced by HIV gp120 combined with ddC, and reversed the upregulation of p-p38, TNFα, SDF1α, and CXCR4 in the neuropathic state in the lumbar SDH and the DRG at 2 and/or 4 weeks. The mechanisms by which IL-10 reduces neuropathic pain are not clear. Previous studies suggest that TNFα mediated SDF1α/ CXCR4 pathway in the gp120 and ddC induced neuropathic pain models [14, 17]. It is possible that in this study, IL-10 suppressed SDF1α/ CXCR4 through reduced TNFα signaling in the gp120 combined with ddC state. Local application of gp120 to sciatic nerve induces wide neurochemical changes in both the DRG and the spinal cord. Meanwhile, IL-10 mediated by HSV reduced those inflammatory factors in both the DRG and the spinal cord. It is possible that IL10 may protect against HIV-induced pain by preserving integrity of gene expression in DRG and thus preventing abnormal release of nociceptive peptides from DRG neurons into the dorsal horn. Future work will study the exact molecular mechanisms/pathways by which IL-10 suppresses those inflammatory factors.