Background: immune responses underlie vascular insult in rheumatoid arthritis (RA), hence accounting for cardiovascular risk excess. However, candidate mechanisms are yet to be identified. GPR55 is a cannabinoid receptor expressed in a variety of haematopoietic and stromal tissues. GPR55 signalling has been reported to modulate atherosclerosis progression and vascular remodelling, especially in knock-out animal models. No evidence in human patients is available.

Objectives: to evaluate the GPR55 expression in leukocyte populations in RA patients and their potential role as biomarker in atherosclerosis.

Methods: GPR55 expression on monocytes (CD14+ and CD14low subsets) and B-cells (CD19+) was quantified by flow cytometry in Peripheral Blood Mononuclear Cells (PBMCs) samples from 63 RA patients (2010 EULAR/ACR classification criteria; 69% RF, 67% ACPA; mean DAS28: 5.32±1.14), 11 individuals with clinically-suspect arthralgia (EULAR definition; 57% RF, 50% ACPA) and 36 matched healthy controls (HC). All patients were recruited at onset (untreated). Lipoprotein profiles were evaluated by conventional and NMR-based approaches. Atherosclerosis occurrence was measured by Doppler-ultrasound. Serum levels of proinflammatory cytokines were evaluated by immunoassays. Serum proteins were evaluated through a pre-defined panel by using high-throughput targeted proteomics.

Results: GPR55 was detected on human monocytes and B-cells by flow cytometry. RA patients exhibited lower GPR55 expression (measured as mean fluorescence intensity) in monocyte subsets (CD14+: p=0.002, and CD14low: p<0.001) and B-cells (p=0.002) compared to HC. Equivalent results were obtained when the number of GPR55+ cells was computed for each subset (p=0.052, p=0.010 and p=0.010, respectively). No differences were observed between CSA individuals and HC (all p>0.050).

GPR55 expression was unrelated to disease activity, joint counts, symptom duration or traditional cardiovascular risk factors in RA patients (all p>0.050). No associations were retrieved for the lipoprotein profile (all p>0.050). However, smoking was associated to lower expression on B-cells (p=0.047). This effect was dose-dependent (cigarettes/day: r=-0.249, p=0.039) and restricted to patients carrying the Shared Epitope (p=0.050, and r=-0.375, p=0.027; respectively). Atherosclerosis occurrence or cIMT were not associated with GPR55 expression on B-cells (p=0.174, and r=0.027, p=0.841, respectively), neither on monocyte subsets (all p>0.050).

GPR55 expression on B-cells was correlated with serum levels of proinflammatory cytokines (IL6: r=0.236, p=0.062; IL-18: r=0.410, p<0.001, and IL-8: r=0.317, p=0.011) in RA patients. Furthermore, GPR55 expression on B-cells was associated with several proteins related to vascular remodelling (PGF, CXCL1, IL-18, SERPINA12, MMP7, THBS2 and MMP12) and B-cell activation/differentiation (SLAMF7, IL-6, TNFRSF13B or VSIG2). These proteins informed signatures (protein-protein interaction: p=5.99·10 ^-11 ) related to humoral adaptive responses, cytokine production and regulation of defence response. However, these associations were atherosclerosis-dependent, and were only restricted to patients with atherosclerosis, where specific proteomic pathways involved in the positive regulation of cytokine production and cellular response to lipopolysaccharide predominated. Equivalent findings were found for the associations with proinflammatory cytokines.

Conclusion: this is the first study assessing GPR55 expression in patients with rheumatic and musculoskeletal conditions. Reduced GPR55 expression hallmarked monocyte and B-cell subsets in RA patients at onset, whereas no differences were found in earliest stages. GPR55 expression was linked to B-cell activation-related pathways, especially in patients with atherosclerosis. GPR55 may be a novel hub to understand the links between immune (humoral) responses and atherosclerosis.

REFERENCES: NIL.

Acknowledgements: ISCIII (PI21/00054 and FI22/00148).

Disclosure of Interests: None declared.