Background: Nociceptive and neuropathic pain occurs as part of the disease process after traumatic brain injury (TBI) in humans. Central and peripheral inflammation, a major secondary injury process initiated by the traumatic brain injury event, has been implicated in the potentiation of peripheral nociceptive pain. We hypothesized that the inflammatory response to diffuse traumatic brain injury potentiates persistent pain through prolonged immune dysregulation.

Results: To test this, adult, male C57BL/6 mice were subjected to midline fluid percussion brain injury or to sham procedure. One cohort of mice was analyzed for inflammation-related cytokine levels in cortical biopsies and serum along an acute time course. In a second cohort, peripheral inflammation was induced seven days after surgery/injury with an intraplantar injection of carrageenan. This was followed by measurement of mechanical hyperalgesia, glial fibrillary acidic protein and Iba1 immunohistochemical analysis of neuroinflammation in the brain, and flow cytometric analysis of T-cell differentiation in mucosal lymph. Traumatic brain injury increased interleukin-6 and chemokine ligand 1 levels in the cortex and serum that peaked within 1–9 h and then resolved. Intraplantar carrageenan produced mechanical hyperalgesia that was potentiated by traumatic brain injury. Further, mucosal T cells from brain-injured mice showed a distinct deficiency in the ability to differentiate into inflammation-suppressing regulatory T cells (Tregs).

Conclusions: We conclude that traumatic brain injury increased the inflammatory pain associated with cutaneous inflammation by contributing to systemic immune dysregulation. Regulatory T cells are immune suppressors and failure of T cells to differentiate into regulatory T cells leads to unregulated cytokine production which may contribute to the potentiation of peripheral pain through the excitation of peripheral sensory neurons. In addition, regulatory T cells are identified as a potential target for therapeutic rebalancing of peripheral immune homeostasis to improve functional outcome and decrease the incidence of peripheral inflammatory pain following traumatic brain injury.

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Published in Molecular Pain, v. 12, p. 1-12.

© The Author(s) 2016

This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 3.0 License (http://www.creativecommons.org/licenses/by-nc/3.0/) which permits non-commercial use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access page (https://us.sagepub.com/en-us/nam/open-access-at-sage).

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This work is supported by NIH grants K02 DA19656 and R01 DA037621 (BKT) and R01 NS065052 (JL). Dr. Rowe is funded by a Bisgrove Scholar Award from Science Foundation Arizona. Mr. Harrison was partially supported by the Diane and Bruce Halle Foundation and NIH F31 NS09092. Flow cytometry was carried out at the University of Kentucky Flow Cytometry and Cell Sorting Core Facility, which is supported in part by the Office of the Vice President for Research, the Markey Cancer Center and a grant from the NIH Shared Instrument Program (S10 RR026827).