Single-cell sequencing reveals a senescent immune landscape in bone marrow lesions inducing articular cartilage damage in osteoarthritis.

Bone marrow lesions (BML) are early signs of osteoarthritis (OA) and are strongly correlated with the deterioration of cartilage lesions. Single-cell RNA sequencing (scRNA-seq) analyses were performed on BM from non-BML and BML areas and articular cartilage from intact and damaged areas to explore BML landscape and BML-cartilage crosstalk. We revealed the immune landscape of BM in non-BML and BML, and the transition to pro-inflammatory states of clusters in BMLs, such as classical monocytes and non-classical monocytes. Non-classical monocytes have high inflammation, OA gene signatures, and senescence scores, and are potential primary clusters promoting OA progression. Histological signs of OA related to the cellular landscape in damaged cartilage were identified, including PreFC exhaustion. The BM-cartilage crosstalk at the cell-cell interaction (CCIs) level and the TNF signal transmitted by non-classical monocytes are the critical CCIs in BML-induced cartilage damage, and PreFC is one of the primary receivers of the signal. We further validated the higher senescence level of non-classical monocyte and FC-2 in OA mice, compared with classical monocyte and PreFC, respectively. Transcription factor 7 like 2 (TCF7L2) was identified as a shared transcription factor in the senescence of monocytes and chondrocytes, facilitating the development of the senescence-associated secretory phenotype (SASP). Therefore, senescent non-classical monocytes promote BMLs and inflammation and senescence of chondrocytes by modulating BML-cartilage crosstalk in OA, with TCF7L2 serving as a regulator.