The dysregulation of monocyte subpopulations in individuals at risk of developing rheumatoid arthritis

Objectives. Individuals carrying antibodies against citrullinated proteins (ACPA) are at high risk of developing RA. EULAR provided a clinical definition of individuals with arthralgia suspicious for progression to RA (clinically suspect arthralgia, CSA). The alteration of monocyte subpopulations in patients with established RA has been previously described. We analysed peripheral blood monocyte subpopulations in individuals with arthralgia at risk of RA. Methods. We included 70 at-risk individuals, defined as having arthralgia without arthritis and being either ACPA(+) or meeting the clinical CSA definition, 23 patients with early RA (ERA) and 19 healthy controls (HCs). Monocytes classified as classical (CD14(++)CD16(-)), intermediate (CD14(++)CD16(+/++)) and nonclassical (CD14(-/+)CD16(++)) were analysed by flow cytometry. Results. Of the 70 at-risk individuals, 46 were ACPA(+) and 45 met the CSA definition. The at-risk individuals and, especially, ERA patients had a lower percentage of classical monocytes and a higher percentage of nonclassical monocytes than the HCs. ACPA positivity had no effect on the difference in the distribution of the monocyte subsets between at-risk individuals and ERA patients, but a difference was determined in those reaching the ERA phase. However, when compared with HCs, the shift of monocyte subsets was more significant in ACPA(+) than in ACPA(-) individuals with arthralgia. This trend was observed in individuals who did not meet the CSA definition. This finding was, however, determined by a selection bias, as these individuals were solely ACPA(+). Conclusion. The shift from classical to nonclassical monocyte subpopulations was observed already in individuals at risk of developing RA.