Background/Purpose: Male C57Bl/6 (WT) mice develop transient inflammation in response to K/BxN serum transfer and show a corresponding pain state, which persists beyond the resolution of inflammation. The current studies first examined the role of spinal cytokines, specifically TNF and IFNβ in the transition to a persistent, post-inflammatory pain state, which is mediated through Toll-like receptor (TLR) 4 signaling. Importantly, the contribution of TLR4 to pain states has been found to differ between male and female animals. Thus, our second aim was to examine whether female WT mice respond similarly to K/BxN serum transfer, and to examine whether TLR4 signaling associated spinal cytokines were involved in the development of tactile allodynia (TA) in female animals.

Methods: K/BxN sera (100μl) was injected into male and female WT, Tnf^-/–, Tlr4^-/–, and Ifnar1^-/- mice on Days 0 and 2. Ankle width and withdrawal thresholds were examined over 28 days and then analyzed in the peak inflammatory (d3-10) and post-inflammatory (d13-18) phases. Spinal cords were collected from WT and Tlr4^-/- arthritic mice and changes in gene expression were measured using qPCR.

Results: We examined spinal cords for differences in TNF and IFNβ gene transcripts on day 10 post K/BxN serum transfer, relative to day 0. IFNβ transcripts decreased in WT mice (average fold change: 0.41) and were increased in Tlr4^-/- mice (average fold change: 18.84). In contrast, TNF transcripts increased in WT mice (average fold change 1.33), and remained unchanged in Tlr4^-/- mice (average fold change 0.96). Next, we assessed the development of TA in male Ifnar1^-/- and Tnf^-/- mice. The early, inflammatory phase of pain is attenuated in Ifnar1^-/- mice (1.18g threshold relative to 0.5g in WT males, p < .05); however these mice develop persistent pain while the late phase of pain is reduced in Tnf^-/- mice (1.29g threshold, relative to 0.74g in WT males, p < .05).

 Next, we examined female WT mice after K/BxN serum transfer. To our surprise, female mice developed an initial tactile allodynia that is indistinguishable from males (0.72g relative to 1.97g at baseline), but do not develop a persistent pain state (post-inflammatory threshold: 1.36g, p < .05). The behavioral phenotype of female WT mice resembled that of Tlr4^-/- males, and we assessed whether similar cytokine signaling contributed to the K/BxN pain phenotype. Both female Ifnar1^-/- and Tnf^-/- mice were indistinguishable from their male counterparts in terms of both ankle inflammation and pain (p > .05). We then determined whether TNF and IFNβ transcripts change over time (days 0, 3, 10, 18, and 28) by qPCR in male and female WT mice. Males showed a general increase in spinal TNF mRNA expression, and a decrease in IFNβ. Although the female mice also showed an increase in TNF transcripts, they had a transient decrease in IFNβ and then recovered to initial levels.

Conclusion: These results show persistent pain in male WT animals is associated with differential modulation by TNF and type I IFN. Female WT animals, however, fail to develop persistent pain, and show a recovery of IFNβ transcription indicating that co-modulation is important to prevent the development of persistent, post-inflammatory TA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/contribution-of-tnf-and-type-i-interferon-to-the-development-of-persistent-post-inflammatory-mechanical-allodynia-in-arthritic-mice/