Proinflammatory Cytokines IL-1β and TNF-α Influence Human Annulus Cell Signaling Cues for Neurite Growth: In Vitro Coculture Studies.

STUDY DESIGN: Institutional review board-approved research using human annulus cells cocultured with F11 nerve cells.

OBJECTIVE: To perform functional, kinetic assays of neurite dynamics and media neurotrophin measurements to test whether proinflammatory cytokines influence annulus cells' signaling cues for neurite growth/repulsion.

SUMMARY OF BACKGROUND DATA: Nerves grow in response to signaling molecules called neurotrophins, which disc cells produce (e.g., brain-derived neurotrophic factor [BDNF], glial cell line-derived neurotrophic factor [GDNF], and neurotrophin 3 [NT3]) and which influence neuron survival, differentiation, and migration. How proinflammatory cytokines influence disc signaling cues for neurite growth/repulsion is poorly understood.

METHODS: Studies used our previous model of 4-day human annulus cell-F11 nerve cell coculture to assess effects of added proinflammatory cytokines interleukin 1 beta (IL-1β; 10 pmol/L) or tumor necrosis factor alpha (TNF-α) (10 pmol/L). Annulus cells were cultured from 6 Thompson grade I, 9 grade II, 8 grade III, 11 grade IV, and 7 grade V discs. Neurite lengths were measured following control conditions or with added IL-1β or TNF-α, and conditioned media assayed with RayBiotech Growth Factor Arrays. Standard statistical methods used analysis of variance and Spearman correlation coefficient testing associations of neurite length with neurotrophin levels.

RESULTS: IL-1-β or TNF-α significantly increased neurite lengths (P < 0.001) and BDNF, NT3, and GDNF media levels (P ≤ 0.01) versus controls. Significant positive correlations were present between media neurotrophin levels for BDNF, NT3, and GDNF and neurite lengths under control conditions, following addition of IL-1β, and following addition of TNF-α. Novel data showed production of the neurotrophin amphiregulin.

CONCLUSION: In vitro data supported the hypothesis that nerve-disc cell interactions may be influenced by the heightened proinflammatory milieu present in degenerating discs, leading to increased nerve migration. Data may have direct clinical relevance/implications for nerve ingrowth and pain in the outer annulus (where disc cell numbers are high), and in regions where nerves penetrate into the disc via annular tears.

LEVEL OF EVIDENCE: N/A.