We recently described a non-viral gene therapy paradigm offering long-term resolution of established neuropathic pain in several animal models. Here, the requirements for long-term therapeutic effects are described, and evidence is provided for a mechanism of action based on immunological priming of the intrathecal (i.t.) space. Long-term pain reversal was achieved when two i.t. injections of various naked plasmid DNA doses were separated by 5 h to 3 days. We show that an initial DNA injection, regardless of whether a transgene is included, leads to an accumulation of phagocytic innate immune cells. This accumulation coincides with the time in which subsequent DNA injection efficacy is potentiated. We show the ability of non-coding DNA to induce short-term pain reversal that is dependent on endogenous interleukin-10 (IL-10) signaling. Long-term efficacy requires the inclusion of an IL-10(F129S) transgene in the second injection. Blockade of IL-10, by a neutralizing antibody, either between the two injections or after the second injection induces therapeutic failure. These results show that this gene therapy paradigm uses an initial 'priming' injection of DNA to induce accumulation of phagocytic immune cells, allowing for potentiated efficacy of a subsequent 'therapeutic' DNA injection in a time- and dose-dependent manner.

The present experiments examined the role of spinal proinflammatory cytokines [interleukin-1β (IL-1)] and chemokines (fractalkine) in acute analgesia and in the development of analgesic tolerance, thermal hyperalgesia, and tactile allodynia in response to chronic intrathecal morphine. Chronic (5 d), but not acute (1 d), intrathecal morphine was associated with a rapid increase in proinflammatory cytokine protein and/or mRNA in dorsal spinal cord and lumbosacral CSF. To determine whether IL-1 release modulates the effects of morphine, intrathecal morphine was coadministered with intrathecal IL-1 receptor antagonist (IL-1ra). This regimen potentiated acute morphine analgesia and inhibited the development of hyperalgesia, allodynia, and analgesic tolerance. Similarly, intrathecal IL-1ra administered after the establishment of morphine tolerance reversed hyperalgesia and prevented the additional development of tolerance and allodynia. Fractalkine also appears to modulate the effects of intrathecal morphine because coadministration of morphine with intrathecal neutralizing antibody against the fractalkine receptor (CX3CR1) potentiated acute morphine analgesia and attenuated the development of tolerance, hyperalgesia, and allodynia. Fractalkine may be exerting these effects via IL-1 because fractalkine (CX3CL1) induced the release of IL-1 from acutely isolated dorsal spinal cord in vitro. Finally, gene therapy with an adenoviral vector encoding for the release of the anti-inflammatory cytokine IL-10 also potentiated acute morphine analgesia and attenuated the development of tolerance, hyperalgesia, and allodynia. Taken together, these results suggest that IL-1 and fractalkine are endogenous regulators of morphine analgesia and are involved in the increases in pain sensitivity that occur after chronic opiates.