Table 1 Functionally different FLS subsets in RA

2016 [25]

PDPN⁺

Mainly in lining.

Increased in RA synovium.

Capable of vascular transmigration from primary site to distant cartilage sites where they attach and invade cartilage. TNF-α or IL-1 stimulation in vitro increased PDPN expression.

Invade and degrade cartilage in the SCID mouse model of cartilage degradation. Migrate to secondary site at early time.

CD248⁺

Restricted to sublining

TGFβ1 stimulation in vitro increased CD248 expression. Did not invade the implants of cartilage in vivo.

Migrate to secondary site at later time.

2018 [26]

CD90^-CD34^-

Lining

High expression of MMP1, MMP3, PRG4, HAS1 and CD55.

Coculture with peripheral blood monocytes led to increased number of TRAP-positive osteoclastic cells in vitro.

CD34⁺

Lining and sublining

Higher in swollen joints

Characterized by the expression of inflammatory cytokine genes IL6, CXCL12, and CCL2; increased expression of genes involved in fibroblast migratory response (CTHRC1, TWIST1, POSTN, LOXL2, PDGFRB and MMP14).

Exhibited enhanced in vitro invasion and migration in response to PDGFBB in vitro; recruit more peripheral blood monocytes in a transwell leukocyte recruitment assay.

CD90⁺CD34^-

Sublining (localize to perivascular zone)

Expanded in RA, higher in swollen joints.

CDH-11⁺. Positively correlated with the proportion of infiltrated leukocytes in synovium. Enrichment of mitotic and proliferative genes; increased expression of genes involved in fibroblast migratory response (CTHRC1, TWIST1, POSTN, LOXL2, PDGFRB and MMP14); high expression of RANKL and low expression of TNFRSF11B;

Exhibited enhanced in vitro invasion and migration in response to PDGFBB in a transwell matrix invasion assay; coculture with peripheral blood monocytes led to increased osteoclastic cells.

2018 [27] 

CD55⁺

Lining

Enrichment in expression of hyaluronan synthase 1 (HAS1); Enrichment in genes involved in pathways associated with endothelial cell proliferation and regulation of reactive oxygen species responses.

ND

CD90⁺

Sublining

Enrichment in genes involved in pathways associated with MMP activity and organization of the extracellular matrix.

ND

2019 [28] 

CD55⁺

Lining

High expression of lubricin (encoded by PRG4); High expression of DNASE1L3, a gene whose loss of function is associated with RA;

ND

CD90⁺CD34⁺

Sublining

Express genes related to the extracellular matrix

ND

CD90⁺HLA-DRA^high

Sublining

Expanded > 15-fold in leukocyte rich RA compared with OA.

CD34^-. Express genes related to the extracellular matrix; Express genes related to MHC class II presentation and the IFNγ-mediated signaling pathway (such as IFI30); high expression of CXCL12, CXCL9; may contribute the main source of IL-6 production.

ND

CD90⁺DKK3⁺

Sublining

Express genes related to the extracellular matrix; High expression of DKK3, CADM1 and COL8A2

ND

2019 [29] 

PDPN⁺FAPα⁺CD90^-

Lining

Bone-effector profile; High expression of CCL9 and TNFSF11, potent inducers of osteoclast activity; High expression of MMP3, MMP9 and MMP13; Surface expression of RANKL; Secrete high levels of RANKL; Have an increased RANKL: osteoprotegerin ratio; correlated with cartilage damage.

Stimulate osteoclast differentiation and activation in vitro; Mediate bone and cartilage damage and promote osteoclast activity when transferred into the joints of mice with STIR

PDPN⁺FAPα⁺CD90⁺

Sublining

Expanded in RA

Immune-effector profile; High expression of genes encoding cytokines and chemokines (including IL6, LIF, IL33 and IL34); correlated positively with severity of joint inflammation and bone erosion.

Transfer of these cells into mice with CIA increases CD4⁺ T cell, neutrophil and macrophage infiltration and reduces FOXP3⁺ regulatory T cells. Transfer of these cells into mice with STIR exacerbates disease.

2020 [30] 

PRG4⁺

Lining

Constitute the majority of stromal cells in OA.

ND

CD90⁺

Sublining (perivascular zone, close to endothelium)

Expanded in RA.

Enriched in Notch activation signature. Expansion in RA was driven by endothelium derived Notch3 ligands.

Notch inhibitor DAPT blocked the differentiation of CD90^high FLS in vitro. Genetic deletion of Notch3 or the blockade of NOTCH3 signaling attenuates inflammation and prevents joint damage in mice with inflammatory arthritis.

2020 [31] 

-

Lining

High expression of MMPs in active RA. Expression of MMPs was reduced and mediators of tissue repair and resolution (for example, IGFBP5/6, AXL) were increased in RA in remission.

ND

HLA^high

Sublining

ND

ND

CD90^high

Sublining

ND

ND

CD90⁺CXCL14⁺

Sublining

Express GAS6, and this expression was increased in RA in remission. GAS6 derived from this subset may contribute to the homeostatic regulatory functions of lining-layer MerTK⁺ macrophages.

ND

CD90⁺CD34⁺

Sublining

ND

ND

2022 [14] 

Smoc2/Col15a1⁺

Sublining

Homeostatic state: CD90⁺. Marked by the expression of Smoc2, Thbs1, Vwa1, Col15a1 and genes that encode matricellular proteins and the BMP coreceptor Rgma, along with BMP/SMAD signaling pathways; expression of genes associated with steroid metabolism, including the cortisone-conversion enzyme Hsd11d1, suggesting an anti-inflammatory role.

During arthritis: Positively regulate fibroblast migration and apoptotic processes. disappearance of characteristic steroid biosynthetic process, BMP signaling, and chondrogenesis.

ND

Comp/Sfrp1⁺

directly adjacent to the lining,

Homeostatic state: CD90⁺. High expression of WNT modulators Dkk2 and Sfrp1, enrichment in WNT-mediated responses, TGF activity, and osteogenesis. the specific expression of Ecrg4 indicates a role in regulating tissue repair processes.

During arthritis: exhibit phosphatidylinositol 3-kinase signaling; positive regulation of developmental growth, Wnt regulation, epithelial-to-mesenchymal transition; androgen receptor signaling are gradually reduced.

ND

Osr1/Nr2f2⁺

adjacent to the lining

Homeostatic state: CD90⁺. Gene expression is linked to joint morphogenesis and reparative processes.

ND

Meox1/Clu⁺

sublining

Homeostatic state: characterized by BMP signaling pathway activation and osteoblast and myoblast differentiation. Characteristic gene expression involves the Klf5, Clu, Id1, and Meox1 genes.

During arthritis: exhibit phosphatidylinositol 3-kinase signaling. participate in osteoclast differentiation.

ND

Dkk3/Lrrc15⁺

both sublining and lining

specific in arthritis

CD90⁺Prg4^high, most expanded in the hTNFtg joints. Express highly important genes for joint pathology including the ECM component Fbln7, Thbs4, Cthrc1, lrrc15, TF Runx1. Expressed genes suggesting multiple biological processes including regulation of immune and redox response, cell fate determination, and ECM remodeling, which indicate a multipotent transcriptional signature.

ND

Dpp4/Pi16⁺

sublining

Homeostatic state: The gene expression signature indicates that these SFs drive processes relative to vasculogenesis and regulation of type 2 immune responses and myeloid lineage differentiation and homeostasis. Characterized by the expression of Pi16, Sema3c, Efemp1, and Dpp4.

During arthritis: exhibit phosphatidylinositol 3-kinase signaling. engage in blood vessel remodeling. responses to hypoxia, the regulation of TGFβR signaling, the development of cartilage, and the type 2 immune responses are absent.

ND

Prg4^high/Tspan15⁺

lining

Homeostatic state: Prg4^highCD90^-, high express genes as markers of lining SFs such as Tspan15, Hbegf, and Htra4.

During arthritis: expansion markers of inflammatory response (Ccl2, Ccl5, Hmox1 Saa3), class I antigen presentation (H2-K1, B2m, H2-Q7), and ECM remodeling (Mmp3, Timp1, Cd44)

ND

Birc5/Aqp1⁺

both sublining and lining

specific in arthritis

CD90⁺Prg4^high, increase in the hTNFtg joints. express Mki67, PDGFα, Birc5, Aqp1, Acta2, the C1qtnf3 adipokine, and other chemokines such as Cxcl5, as well as several adhesion molecules. The functional annotation related to increased proliferating capacity, adhesion, and peptidase activity.

ND

Pxt3/Notch3⁺

Sublining, around the vascular cells

Homeostatic state: activation of cytokines and chemokine pathways (Ccl7, Cxcl10, IL6, and Ptx3) and are associated with immune-regulatory functions including response to IFN-beta/gamma and LIF. Notch3 mainly in this cluster in normal state.

During arthritis: exhibit PI3K signaling, participate in osteoclast differentiation, regulate monocyte differentiation and uniquely present activation of protein kinase B activity, positive regulation of stress-activated MAPK cascade, and positive regulation of response to hepatocyte growth factor, regulation of several cytokine responses and tissue regeneration were lost.

ND

2022 [32]

PARC⁺COL3A1⁺

Perivascular

Vascular-interacting subsets. Closely resemble the CD90⁺NOTCH3-activated FLS. Marker genes were enriched in pathways centered around ECM binding (includes COL11A1, SPARC, and LRRC15) and disassembly (includes MMP13, MMP11, and FAP) and developmental pathways (COL3A1, COL1A1, COL5A1, and TGFB1), may play a role in vascular remodeling. Notch signaling is specific to this subset.

ND

CXCL10⁺CCL19⁺

Next to CD3⁺ T-cell-regions

Immune-interacting subsets. Enriched in genes for pathways involved in direct interaction with immune cells, including lymphocyte chemotaxis (CCL19, CCL2, and CCL13), antigen presentation (CD74, HLA-DRA, and HLA-DRB1), and positive regulation of T-cell proliferation (TNFSF13B, VCAM1, and CCL5). Show broad evidence of response to the pro-inflammatory cytokines interferon (IFN) g, IFN a, TNF-a, IL-1 and IL-12. Overlapped significantly with CD90⁺HLA-DRA^high FLS.

ND

2022 [34]

CD90^-PRG4⁺

Lining

Highest in the myeloid pathotype

Express high level of CD55, MMP3, and FN1. Enriched for pathways such as ‘focal adhesion’, ‘ECM-receptor interaction’ and ‘mineral absorption’; correlated positively with disease severity parameters in the fibroid pathotype.

ND

CD90^low CXCL12⁺

Sublining

Most prominent in fibroid pathotype.

Express high level of CXCL12, CCL2 and ADAMTS1, enriched for pathways associated with proinflammatory states (TNF signaling pathway, MAPK signaling pathway).

ND

CD90^intermediate POSTN⁺

Sublining

Most prominent in fibroid pathotype.

Express high level of collagen genes. Pathways unique for this subset are “human papillomavirus infection” (included genes: COL1A1, COL3A1, COL1A2, ACTN1, LAMB1) and “regulation of actin cytoskeleton” (included genes: ITGA10, MYLK, ACTN1, MYH10, PDGFRB, MYL9, ENAH, ITGA11, ITGB5). Positively related with tender and swollen joint counts and CRP levels in the lymphoid pathotype.

ND

CD90^highCXCL14⁺

Sublining

Express high level of CXCL14, C3, CD34, ASPN; enriched in pathways of “phagosome”, “gap-junction” and “arachidonic acid metabolism”; negatively associated with disease severity in all pathotypes.

ND

2024 [33]

CD200⁺ DKK3⁺

Sublining

Express high level of Cdh11, DKk3 but not Lrrc15

Conjunction with ILC2s and play a pivotal role in establishing a pro-resolving network.

MMP3⁺/IL6⁺

sublining

Express high level of MMP3 or IL-6

Colocalized with pro-inflammatory immune cells in regions with an active inflammatory phenotype