Abstract
PurposeFormation of terminal complement complex (TCC), a downstream complement system activation product inducing inflammatory processes and cell lysis, has been identified in degenerated discs. However, it remains unclear which molecular factors regulate complement activation during disc degeneration (DD). This study investigated a possible involvement of the pro-inflammatory cytokine interleukin-1β (IL-1β) and the lysosomal protease cathepsin D (CTSD).MethodsDisc biopsies were collected from patients suffering from DD (n = 43) and adolescent idiopathic scoliosis (AIS, n = 13). Standardized tissue punches and isolated cells from nucleus pulposus (NP), annulus fibrosus (AF) and endplate (EP) were stimulated with 5% human serum (HS) alone or in combination with IL-1β, CTSD or zymosan. TCC formation and modulation by the complement regulatory proteins CD46, CD55 and CD59 were analysed.ResultsIn DD tissue cultures, IL-1β stimulation decreased the percentage of TCC + cells in AF and EP (P < 0.05), whereas CTSD stimulation significantly increased TCC deposition in NP (P < 0.01) and zymosan in EP (P < 0.05). Overall, the expression of CD46, CD55 and CD59 significantly increased in all isolated cells during culture (P < 0.05). Moreover, cellular TCC deposition was HS concentration dependent but unaffected by IL-1β, CTSD or zymosan.ConclusionThese results suggest a functional relevance of IL-1β and CTSD in modulating TCC formation in DD, with differences between tissue regions. Although strong TCC deposition may represent a degeneration-associated event, IL-1β may inhibit it. In contrast, TCC formation was shown to be triggered by CTSD, indicating a multifunctional involvement in disc pathophysiology.
Highlights
Intervertebral disc (IVD) degeneration is perceived as the major cause of back pain [1]
A significantly higher percentage of terminal complement complex (TCC) + cells was found in the presence of complementcompetent human serum (HS) in annulus fibrosus (AF) (2.4-fold, P < 0.01), nucleus pulposus (NP) (1.7-fold, P < 0.05) and EP (4.9-fold, P < 0.05) cells compared to serum-free medium (SF), with the highest increase observed for EP (Fig. 1a; representative staining images in Supplemental Fig. S1)
Addition of IL-1β resulted in lower TCC formation than in the HS group, which was significant for AF and EP (P < 0.05)
Summary
Intervertebral disc (IVD) degeneration is perceived as the major cause of back pain [1]. IVD degeneration is characterized by a catabolic cell phenotype, with matrix degeneration, pro-inflammatory response, senescence and apoptosis [5, 6]. The inflammatory response comprises an increased production of soluble mediators, such as interleukin (IL)-1β, and increased recruitment of immune cells [8,9,10]. The complement system is a crucial arm of fluidphase innate immunity and our body’s first line of defence. It modulates and amplifies inflammation and can significantly contribute to inflammation-mediated tissue damage [11]. We have recently observed that CD59 (protectin), a membrane-bound protein known to control TCC formation [15], was likewise increased in degenerated human discs [14]
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