Background
Although anti-citrullinated protein antibodies (ACPA) are the most important risk factor for joint destruction in rheumatoid arthritis (RA), the underlying pathophysiological process is unclear. Traditionally, it is hypothesized that ACPAs can enhance inflammation [
1] (for instance via immune complexes that stimulate macrophages to secrete pro-inflammatory cytokines) and that inflammation is required for destruction, resulting in e.g. visible bone erosions on radiographs. Recent in vitro studies and mouse models have generated a new concept in which ACPA can directly induce osteoclast activation, followed by autocrine enhancement of osteoclast maturation and activation [
2,
3]. This may subsequently lead to bone loss (and pain) as observed in studies performed in vivo following injection of ACPA [
2‐
5]. The finding that ACPA can be present long before synovitis is clinically detectable [
6] and that sensitive imaging techniques have detected small erosions in patients with arthralgia [
7] fit with the hypothesis that joint inflammation is not necessary to generate erosions [
5]. Despite observations made in vitro and in vivo in mice [
2,
3], there is presently little information available on ACPA-positive patients in the absence of local inflammation. Hence, it is not known if ACPA can lead to bone erosions only with concurrent presence of inflammation, or that ACPA induces direct osteoclast activation (leading to erosions without requiring concomitant inflammation) in humans as well. By performing association studies in patients that are in the disease phase of arthralgia without the presence of clinical synovitis, information on these relationships can be obtained as only a proportion of the patients with arthralgia display subclinical inflammation. Therefore, the arthralgia setting provides the possibility to study associations between ACPA, (local) inflammation and erosions.
Likewise, this setting can also be used to answer whether the effect of ACPA - if any - is dependent on the presence of rheumatoid factor (RF). Studies within early (rheumatoid) arthritis, using high-resolution computed tomography (CT), have shown that combined presence of ACPA and RF is associated with the number and size of erosions rather than ACPA alone [
8]. In addition, it has been shown that patients with early arthritis harboring both ACPA and RF display increased osteitis scores as detected by magnetic resonance imaging (MRI), in contrast to ACPA single-positive patients [
9].
With the aim to find supporting evidence that ACPAs themselves are directly linked to bone erosions in humans, this study in patients with Clinically Suspect Arthralgia evaluated whether (1) ACPA were associated with higher erosion scores (detected by MRI) independent of the presence of inflammation, and (2) whether higher erosion scores were associated with ACPA alone or with ACPA and RF combined.
Discussion
This study evaluated associations between ACPA, RF, (local) subclinical joint inflammation and erosions in patients with arthralgia at risk of RA. Presence of ACPA alone, without inflammation, was not associated with higher erosion scores, in contrast to the combined presence of ACPA and inflammation. Mediation analyses revealed that local inflammation was intermediary in the causal path to erosions. These results indicate that joint inflammation has a role in the development of erosions in ACPA-positive individuals, and suggest that findings in vitro or mouse models on the independent effect of ACPAs on erosions are in contrast to findings in humans.
Furthermore, the combination of ACPA and RF, rather than presence of ACPA alone, was associated with erosions in patients with arthralgia. These results align with those obtained in patients with early rheumatoid arthritis [
8,
9] and fuel the hypothesis that ACPAs alone are not the main and/or single pathogenic factor contributing to joint erosions. Although one can speculate how - or if - ACPAs contribute to joint erosions together with inflammation, results from association studies do not allow conclusions on biological mechanisms.
Our results suggest that in addition to ACPA, local joint inflammation is required for more severe erosive disease. Based on the mediation analysis we cannot definitely differentiate between full or partial mediation; the significance for ACPA from step 1 was lost in step 3 suggesting full mediation. However, as the beta was not zero, partial mediation cannot be excluded. Nonetheless, results of the mediation analyses supported the notion that erosions in ACPA-positive arthralgia rarely occurred without concomitant inflammation. This finding is in line with a previous study that showed that increased levels of CD19+ B cells and CXCL13 were observed in ACPA-positive RA and were associated with erosive disease [
18].
In this study, the use of sensitive high-quality MRI data allowed us to detect erosions in a population in which the total burden of erosions is relatively low. In contrast to the setting of early inflammatory arthritis where all patients have current or recent joint inflammation, the arthralgia setting allows comparison of patients with and without inflammation.
Not all patients considered at risk of RA will develop arthritis over time, even though ACPA or (subclinical) inflammation might be present. However, because we addressed whether ACPA can directly mediate bone loss with/without concurrent inflammation, the study could be performed independent of the final clinical diagnosis.
The subgroups obtained after stratification were small in some cases (especially the ACPA+RF−MRI− subgroup after triple stratification), which could lead to underpowered analyses and the possibility of not identifying statistically significant differences. However, all analyses showed that erosion scores are highest when both ACPA and inflammation are present simultaneously, which strengthens the overall findings. Finally, our study cannot address the question as to whether the results are different for specific ACPA reactivity, as the presence of ACPA was evaluated using the commercially available CCP2 test.
We studied erosions in humans because a direct effect of ACPA on erosions has been suggested [
5]. Although loss of trabecular bone as observed in mice may be dissimilar from periarticular-located erosions in humans, including the underlying mechanisms, our results indicate that ACPAs do not directly contribute to the formation of bone erosions, one of the hallmarks of RA.
Conclusions
In conclusion, the present data in patients with arthralgia showed that erosions are associated with the combined presence of ACPA and RF, rather than with ACPA alone, and preferentially occur in patients with joint inflammation.