Background Undifferentiated arthritis (UA) is defined as an inflammatory oligo-or polyarthritis that does not fulfil criteria for a definitive diagnosis. Earlier diagnosis would permits a better functional prognosis. Synovial tissue (ST) macrophages have been associated with disease activity, radiographic erosion and response to therapy in RA. Furthermore, it has been reported that M1 polarised macrophages predominate in RA synovitis, whereas M2 predominate in SpA synovitis.¹

Objectives To analyse polarised macrophages (M1 proinflammatory and M2 anti-inflammatory) in ST of patients with UA which evolved to RA or PsA, after a long follow-up, to explore their diagnostic value.

Methods To determine the polarisation state of macrophages in ST obtained by arthroscopy from patients with UA that evolved to RA (UA-RA=8) or PsA (UA-PsA=9), the expression of proteins associated to GM-CSF-driven polarisation M1(INHBA, TNFα and MMP12) and M-CSF-driven polarisation M2(CD209) was assessed in ST CD163⁺macrophages. Patients with established RA(n=12) or PsA(n=10) were included as control. 4 µm cryosections of ST in OCT were blocked for 10 min with 1% human immunoglobulins and incubated with primary (1–5 µg/ml) and secondary antibodies. Imaging was performed with an inverted confocal microscope (SPE, Leica Microsystems) and glycerol immersion objective (ACS-APO 20x/NA 0.60). Single cell mean fluorescence intensity(MFI) was assessed using ImageJ software (National Institutes of Health, Bethesda, MD, USA). At least three random fields were evaluated for each type of ST, quantifying the expression of INHBA, TNF-α, MMP12 and CD209 in all segmented CD163⁺macrophages. Macrophages density was normalised based on selected tissue area(mm²). After background subtraction, data were plotted using GraphPad software (GraphPad Software, La Jolla, CA, USA).

Results CD163⁺ sublining (SL) macrophages from UA-RA expressed abundantly the INHBA-encoded activin A, whereas TNFα and MMP12 were variably detected. Regarding the M-CSF-associated marker CD209, 2 populations of CD163⁺ macrophages were found in the SL of UA-RA, CD163⁺CD209⁺ and the other CD163⁺CD209^-, with higher than 100 arbitrary units (au) for CD163⁺CD209⁺ and lower than 100 au for CD163⁺CD209^-. Similarly, INHBA, MMP12 and TNFα expression and 2 populations of CD209 were detected in CD163⁺ macrophages from UA-PsA. Macrophage density was also found comparable between UA, with 650/mm² in UA-RA and 649/mm² in UA-PsA. Quantification of the above indicated markers in CD163 +ST macrophages from established RA and PsA revealed similar levels of INHBA, TNFα, MMP12 and CD209 than those from UA-RA and UA-PsA.

Conclusions This study shows for the first time that the polarisation state of ST CD163⁺ macrophages in UA progressing to RA and PsA is similar to that of established RA and PsA in terms of INHBA, MMP12, TNFα and CD209 expression. Therefore, the inflammatory polarisation state of macrophages is similar in RA and PsA and it is already detected at the earlier steps.

Reference [1] Vandooren B, et al. Arthritis Rheum2009;60(4):966–75.

Acknowledgements Financed “Fondo de Investigación Sanitaria”(PI14/00785.JD Cañete)Instituto de Salud Carlos III. Cofinanced BECA FER-2015.

Disclosure of Interest None declared