Fc receptors and immunoglobulins in polyarthritis : A matter of function, supply and demand?

Fc receptors (FcR) and immunoglobulins (Ig) play important roles in the defence against pathogens. However, altered interactions of these may promote chronic inflammation in rheumatic diseases. An excess of Igs forming immune complexes (IC) could lead to continuous FcR activation and spreading of autoimmune inflammation to other tissues.This work focuses on the evaluation of the FcR status and function in the two most common polyarthritides - psoriatic arthritis (PsA) and rheumatoid arthritis (RA) – in relation to various Igs, joint and skin disease activity, and effect of anti-rheumatic treatment (determined by the EULAR response criteria for RA). Monocyte subpopulations (defined by surface CD14 and CD16 expressions) in patients and HC were also characterised, since different monocyte subsets may have opposing functions in inflammatory conditions. In addition, the effect and safety of long-term B-cell suppression in advanced RA was studied. In PsA, elevated serum levels of IgG1, 2, and 3 were noted while the early naïve RA patients - besides being positive for autoantibodies like IgMRF, IgARF, IgGRF, and ACPA IgG - were distinguished by high levels of IgG1 and IgG3. Monocytes of PsA and RA patients were heavily loaded with IgG and expressed more CD64 (i.e. the high affinity FcγR) than HC. An increase in CD64 turnover was specific for early RA, while a higher CD16a (i.e. a low affinity FcγR) turnover was seen in both RA and PsA compared with HC. The FcγR function was impaired in both polyarthritides compared to HC, but the RA monocytes were more affected of this than the PsA monocytes. RA non-responders had a much lower capacity of IC binding compared with RA good responders. Alterations of the FcR status and function reflected joint disease activity markers in both polyarthritides but not the skin disease activity in PsA. I therefore conclude that the observed FcR statuses in both diseases were specific for joint inflammation. In addition, RA patients with high levels and the occurrence of several simultaneously appearing RF isotypes presented a minor FcγR function, while patients experiencing a good treatment effect were more likely to show low levels of Igs. This suggests that RFs/IC in excess could be important promotors of the ongoing inflammation in RA. However, for ACPA IgG no associations with the rheumatoid monocytic FcR status/function were noted. CD16negative classical monocytes were elevated in early naïve RA, especially in the non-responders - while PsA patients showed an increase in CD16low expressing cells compared to HC. These observations indicate that different monocyte subpopulations could be important in the two polyarthritides. In a cohort of RA patients with advanced disease, long-term B-cell suppression resulted in conversion to RF negativity which indicated a good treatment response but not an increased risk of infection. FcRs and Igs are important players that promote chronicity of inflammation in polyarthritis, especially in RA. An impaired FcγR function following an excess of Igs/IC reflects this state of the immune system. This work has identified an increased monocytic CD64 turnover as a RA specific feature. Future treatment options in RA might include supporting/normalizing the FcγR function. Today suppressing B-cell activity is an effective and relatively safe way to tackle the problem from the opposite side