Propiombacterium Acnes Accelerates Intervertebral Disc Degeneration By Inducing The Pyroptosis Of Nucleus Pulposus Cells Via ROS/NLRP3 Pathway

In recent years,multiple studies have reported Propionibacterium acnes(P.acnes),a low-virulence anaerobic bacterium,in disc material.While the existence and mechanism of P.acnes in degenerated intervertebral discs(IVDs)remains controversial.In this study,we firstly examined the prevalence of P.acnes in degenerated IVDs and the correlation between bacterial infection and clinical symptoms.Then we also explored whether P.acnes induces nucleus pulposus cell pyroptosis,a more dangerous cell-death process than apoptosis,and accelerates IVD degeneration via the pyroptotic products of interleukin(IL)-1β and IL-18.Finally,we further explored whether P.acnes can activate nucleus pulposus cell pyroptosis through reactive oxygen species(ROS)/NOD-like receptor 3(NLRP3)signaling pathway.Our study verified the occurrence of P.acnes,latently residing in degenerated IVDs.Of the 80 cultured discs,23 showed the presence of bacteria:P.acnes(20 cases)and coagulase-negative staphylococci(3 cases).MRI revealed that the presence of P.acnes was significantly associated with Modic Changes(MCs)(P<0.001).We also verified that after co-culturing with P.acnes,human nucleus pulposus cells showed significant up-regulation of NLRP3,cleaved IL-1β,cleaved caspase1,and cleaved GSDMD in response to the over-expression of IL-1β and IL-18 in a time-and dose-dependent manner.In addition,the gene expression of inflammatory factors and catabolic enzymes significantly increased in normal nucleus pulposus cells when co-cultured with pyroptotic nucleus pulposus cells in a trans-well system,and the adverse effects were inhibited when nucleus pulposus cells pyroptosis was suppressed.Furthermore,inoculation of P.acnes into the IVDs of rats caused significant pyroptosis of nucleus pulposus cells and remarkable IVD degeneration.Finally,co-culture of nucleus pulposus cells with P.acnes induced the over-expression of ROS and NLRP3,while inhibition of both factors reduced nucleus pulposus cells pyroptosis.Overall,our findings further validated the presence of low-virulence anaerobic bacteria in degenerated IVDs,and P.acnes was the most frequent bacterium.In addition,the latent infection of bacteria in IVDs was associated with Modic changes.P.acnes may play a crucial role in the degenerated IVDs and it may induce nucleus pulposus cells pyroptosis via the ROS/NLRP3 signaling pathway,and the pyroptotic nucleus pulposus cells cause an IVD degeneration cascade.Targeting the P.acnes-induced pyroptosis of nucleus pulposus cells may become an alternative treatment strategy for IVD degeneration in the future.