Monocyte-derived fibrocytes contribute to nucleus pulposus fibrosis and intervertebral disc degeneration

Fibrotic remodeling of nucleus pulposus (NP) leads to structural and mechanical anomalies of intervertebral discs that prone to degeneration, leading to low back pain incidence and disability. Emergence of fibroblastic cells in disc degeneration has been reported, yet their nature and origin remain elusive. In this study, we performed an integrative analysis of multiple single-cell RNA sequencing datasets to interrogate the cellular heterogeneity and fibroblast-like entities in degenerative human NP specimens. We found that disc degeneration severity is associated with an enrichment of fibrocyte-like phenotype, characterized by CD45 and collagen I dual positivity, and expression of myofibroblast marker -smooth muscle actin. Refined clustering and classification distinguished the fibrocyte-like populations as subtypes in the NP cells - and immunocytes-clusters, expressing disc degeneration markers HTRA1 and ANGPTL4 and genes related to response to TGF-{beta}. In injury-induced mouse disc degeneration model, fibrocyte-like cells were found recruited into the NP undergoing fibrosis and adopted a myofibroblast phenotype. Depleting the fibrocyte-like cells in CD11b-DTR mice in which monocytic lineages were ablated by diphtheria toxin could markedly attenuate fibrous modeling and myofibroblast formation in the NP of the degenerative discs, and prevent disc height loss and histomorphological abnormalities. Marker analysis supports that disc degeneration progression is dependent on a function of CD45+COL1A1+ and SMA+ cells. Our findings reveal that fibrocyte-like cells play a pivotal role in NP fibrosis and may therefore be a target for modifying disc degeneration and promoting its repair.