Background Targeting molecules involved in monocyte activation is an important treatment strategy for RA. In this study we aimed to determine monocyte maturation and activation from bone marrow (BM) via blood into synovial fluid (SF) by investigating monocytes transcriptomes and by cytometric profiling of classical (CD14⁺⁺CD16^-), intermediated (CD14⁺⁺CD16⁺) and non-classical (CD14⁺CD16⁺) monocytes.

Materials and methods CD14⁺ cells from BM and blood of RA and osteoarthritis (OA) patients were profiled with Affymetrix microarrays. A detailed functional analysis was performed with reference transcriptomes of BM precursors, monocyte blood subsets, monocyte activation and egress from BM induced by G-CSF (granulocyte colony-stimulating factor). Cytometric profiling of CD14, CD16, HLA-DR and CD163 expression were used to determine monocyte subsets and to follow their activation and differentiation in BM, blood and SF.

Results Transcriptomes of RA-BM monocytes exhibited i) pronounce gene pattern of early myeloid precursors from BM and ii) weak gene pattern of late myeloid precursors from BM. Transcriptomes of RA blood monocytes demonstrated i) pattern of late myeloid precursors from BM and ii) reduced pattern of terminally differentiated CD14⁺CD16⁺ monocytes from blood.

Cytometric profiling of BM, blood and SF monocytes in RA and OA showed that all three body compartments have their own distribution of monocyte subsets. BM was characterised with classical and intermediate subsets and both subsets showed decreased CD16 expression in RA when compared to OA. As expected, blood was characterised with three subsets, and RA blood showed decreased CD14 and HLA-DR expression on classical monocytes and reduced frequency of non-classical subset. In RA-SF, classical monocytes were absent, intermediate were most dominant and cell-phenotype with low CD16 expression but similar to non-classical monocytes was related to macrophages. Cell frequency of intermediate subset in SF positively correlated with inflammation (ESR; R>0.85) and showed the highest expression of HLA-DR, CD14, CD163.

Conclusions Monocyte turnover is increased in RA and characterised with accelerated monocytopoiesis, faster BM egress and migration into inflamed joints. Permanent monocyte activation in the joint and their role in linking innate and adaptive immunity, which is targeted by biologics, emphasises their high diagnostic value and relevance for therapeutic stratification.