INTRODUCTION

Pulmonary involvement in rheumatoid arthritis (RA) is a major contributor to morbidity and mortality (Koduri et al. 2010; Young et al. 2007). Although pulmonary manifestations encompass the main airway, parenchyma, vasculature and pleura, interstitial lung disease (ILD) in particular, is associated with reduced survival (Bongartz et al. 2010). Up to 10% of RA patients suffer from clinically significant ILD while a substantial proportion have abnormal computed tomography (CT) chest findings despite being asymptomatic (Dawson et al. 2001; Fujii et al. 1993).

There are various biochemical and serological markers to predict the severity of the joints in RA (Sakthiswary et al. 2016). However, the clinical and laboratory determinants of RA related ILD (RA-ILD) are not well defined owing to the paucity of research data in this regard. In some patients, ILD preceded the onset of articular manifestations. The underlying pathogenesis is poorly understood. It has been postulated that RA-ILD is linked to the presence of citrullinated proteins (Bongartz et al. 2007). These proteins however, were not specific for RA-ILD, as they were found in the lung tissues of patients with idiopathic pulmonary fibrosis and in the broncho-alveolar lavage of heavy smokers (Makrygiannakis et al. 2008).

RA patients who test positive for rheumatoid factor (RF); which is an autoantibody directed against the Fc portion of immunoglobulin G are more prone to develop ILD. In clinical practice, immunoglobulin M (IgM) RF is used to establish the diagnosis of RA. Studies have pointed out that IgA RF may be a more specific prognostic marker than IgM RF in early RA (Teitsson et al. 1984). However, results have been conflicting on the relationship between the individual RF serotype and extra-articular manifestations. A recent population based study revealed that RF-positive subjects without clinical evidence of RA had a lower forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) than RF-negative subjects (Hwang et al. 2016). The presence of RF may contribute to the pathogenesis of RA-ILD via the antigen-antibody interactions which interferes with the physiological functions of the targeted proteins.

High titres of RF was associated with more aggressive joint disease. The levels of RF may influence the degree of systemic inflammation and organ-specific damage (Giles et al. 2014). Thus, we sought to explore the correlation between the titres of the individual RF serotypes and the severity of the structural damage in RA-ILD based on CT scan. 

MATERIALS AND METHODS

Study Population

We randomly selected 100 patients with RA under our Rheumatology Clinic follow-up, to undergo high resolution computer tomography (HRCT) of the chest in addition to RF serotypes testing.  Participants were aged above 18 years, met the 2010 ACR/EULAR RA criteria (van der Helm-van Mil & Huizinga 2012), had RA for more than 6 months, were non-smokers, not pregnant and had no known chronic lung disease or lung malignancy based on their medical records. This study was approved by the Universiti Kebangsaan Malaysia Medical Centre Institutional Review Board and informed consent was obtained from all study subjects. 

Outcome Measures 

HRCT protocol and scoring system

HRCT of the chest was performed with 1.5mm thick axial sections at 1cm intervals throughout the thorax. Between 25 to 30 images were obtained for each subject. The images were reviewed independently by 2 Radiologists who were blinded to the patients’ identities, clinical and laboratory parameters. Each lobe of the lung was scored based on a scoring system proposed by Kazerooni et al. (Kazerooni et al. 1997). Table 1 shows the scoring scheme of each lobe for both alveolar and interstitial abnormalities. Ground glass opacities represented the alveolar findings whereas honeycombing and septal thickening were the interstitial findings. The ground glass and fibrosis scores were on a scale of 0-5, with higher scores for greater involvement of the lobes. For each subject, the maximum ground glass and fibrosis scores were 25, respectively. The scores from both the radiologists were averaged for the purpose of data analysis. 

RF serotypes

The different RF isotypes (IgM, IgA and IgG) were measured by using an indirect solid-phase enzyme-linked immunosobent assay (ELISA; Orgentec Diagnostika, Mainz, Germany)  involving the highly purified Fc fragment of human IgG which binds to the relevant autoantibody in the test serum. The quantitative analysis for IgM, IgG and IgA RF were calibrated as reported in the kit manual. The procedure was carried out in triplicate. Due to the lack of international reference values, the IgA and IgG RFs were calibrated in arbitrary units (U/ml). For IgM RF, the assay was calibrated based on the international WHO standards. Hence, the following cutoff values were given in the manufacturer’s instructions; 18 U/ml for IgG RF, 18 IU/ml for IgM RF and 15 U/ml for IgA RF.

Other measures

Demographics, past medical and RA disease history were gathered by interviewing the subjects and going through the medical records. The cumulative dose of methotrexate (MTX) was calculated for all subjects given the association between MTX and ILD (Barrera et al. 1994; Cottin et al. 1996). Twenty-eight joints were examined for swelling and tenderness by a single rheumatologist and RA disease activity was calculated using the Disease Activity Score for 28 joints with erythrocyte sedimentation rate (DAS28-ESR) (Inoue et al. 2007). The Stanford Health Assessment Questionnaire (HAQ) (Maska et al. 2011) was used to determine the functional capacity. Radiographs of the hands and feet were scored using the modified Sharp score (MSS) (Pincus et al. 1997) by a single experienced radiologist who was blinded to the study data. Serum samples from all subjects were obtained for the testing of anti-citrulinated cyclic peptide (CCP) using a commercial kit. 

Statistical Analysis

All data were analysed using Statistical Package for Social Sciences (SPSS) version 21.0. The continuous variables were tested for normality using Kolmogorov Smirnov test. Data with normal distribution were expressed as mean ± standard deviation (SD) whereas the data which were skewed or with no particular distribution, were presented as median range. The relationship between two continuous variables were determined using bivariate correlation analysis. Multivariable linear regression was used to model the association of CT ground glass and fibrosis scores with the panel of RF serotypes and the other clinical covariates while adjusting for confounders. A p value of less than 0.05 was considered to be significant.

RESULTS

Subject Characteristics

Out of the 100 patients, only 72 had ILD changes on HRCT of the chest and were finally included in this study and the statistical analysis. Subject characteristics were summarised in Table 2. The mean age was 53.5 years with 62% females. The median disease duration was 8.5 years, and the majority were anti-CCP antibody positive. The frequency of RF positivity was comparable across the 3 serotypes (83.33-84.72%). The mean RA disease activity was moderate. All subjects in this study had radiographic joint erosions. However, the mean HAQ DI was less than 1; which indicated no significant functional disability.  

As MTX is the anchor drug in the treatment of RA, more than half of the subjects were on this form of therapy either as monotherapy or in combination with other DMARDs (disease modifying anti-rheumatic drugs). The above subjects were on oral MTX. Apart from MTX, the other conventional synthetic DMARDs used in this study included leflunomide, sulfasalazine and hydroxychloroquine. Fourteen of the subjects were on advanced therapies i.e. tumour necrosis factor inhibitors, interleukin 6 receptor inhibitor or janus kinase inhibitors. 

Correlation between RF Serotypes and CT Scores of the Chest

The correlation between the clinical variables and the CT scores are listed in Table 3. The CT scores for ground glass showed significant positive correlation with disease duration and IgA RF levels. Based on the Pearson’s r value, the strength of the relationship was weak for disease duration but moderate for IgA RF levels. The CT scores for fibrosis, on the hand, demonstrated significant correlation with multiple clinical covariates i.e age, disease duration, IgA RF levels, IgG RF levels and anti-CCP levels. The strength of the relationship between the fibrosis scores and each of the above mentioned factors was weak except for IgA RF levels which was moderate. On multivariate analysis, only IgA RF levels remained significantly (p<0.05, standardized beta coefficient = 0.604) associated with the ground glass scores. As for the fibrosis scores, IgA RF levels and age were independent predictors based on multivariate analysis after adjusting for confounders, with p scores of <0.05 and 0.02, respectively. The IgA RF was the only serotype which showed significant association with both the ground glass and fibrosis scores in adjusted and unadjusted analyses. 

DISCUSSION

This was one of the few studies which investigated the association between CT findings in RA related ILD and the RF serotypes. We found that IgA RF levels had a significant relationship with the severity of ILD in RA. Higher IgA RF levels were associated with higher ground glass and fibrosis scores. The severity of the lung involvement did not parallel the extent of the radiographic joint damage based on the total MSS. This finding suggests that the pathogenesis of ILD in RA in terms of cytokine signalling, citrulination of proteins and antigen-antibody interactions may differ from the joints. Although RA is a systemic inflammatory disease, there maybe a disparity between the lungs and joints with regard to the rate of progression. The lung involvement in RA has the tendency to progress with time based on our study results which demonstrated a significant correlation between disease duration and the CT scores for ground glass and fibrosis. This finding, however was insignificant on adjusted analyses. 

In the last decade, there were several reports (Sakthiswary et al. 2014; Teitsson et al. 1984) on the association between IgA RF and severe manifestations of RA. IgA RF is more specific for RA than IgM and IgG RFs as it is the least frequently found serotype in asymptomatic individuals (Silman et al. 1991). Although the precise role of IgA RF in the pathogenesis of RA remains unclear, IgA RFs form immune complexes (Aho et al. 1995) that are believed to activate an alternative complement pathway with a sequential release of cytokines. This leads to tissue damage which contributes to a vicious cycle of more autoantibodies production through a positive feedback loop mechanism. Our findings were consistent with the results of the study published by Jonsson et al. (Jonsson et al. 1995). Jonsson et al. reported that IgA RF had stronger association with extra-articular manifestations in RA compared to IgM and IgG serotypes. Up to 80% of RA patients who tested positive for IgA RF had one or more extra-articular manifestations in the above mentioned study.

Apart from IgA RF, we found that higher circulating levels of IgG RF and anti CCP autoantibodies correlated with more severe lung fibrosis. Seropositivity in RA has a well established link with the presence of extra-articular manifestations. However, the relationship between the quantitative measurements of the autoantibodies and the severity of the RA-related ILD lacks convincing evidence. In terms of prognosis, Solomon et al. (Solomon et al. 2016) disclosed that IgM RF and not anti CCP was a predictor of mortality in RA-related ILD. However, the finding was based on univariate analyses.  

The fibrosis scores were more correlated with the levels of autoantibodies i.e. IgA RF, IgG RF and anti-CCP compared to the ground glass scores. This can be partially explained by the fact that ground glass pattern on CT imaging is nonspecific. Ground glass opacification encompasses filling of the alveolar spaces and thickening of the interstitium or alveolar walls (Lynch 1996). The predominant pattern of RA-related ILD is usual interstitial pneumonia which is characterised by honeycombing (Kelly et al. 2014) which was measured by the fibrosis scores.

We acknowledge the limitations of our study. Autoantibodies levels tend to fluctuate with time and treatment in RA. The ideal way of testing would be the average values of a few measurements of each autoantibody for every subject. Besides, pulmonary function testing was not performed as part of this study. The relationship between RF serotypes and pulmonary function test parameters would have provided a more comprehensive assessment of the clinical utility of the aforementioned autoantibodies.

CONCLUSION

In conclusion, the IgA RF was the only serotype which was independently associated with the severity of RA-ILD. High level of IgA RF should prompt clinicians to investigate for ILD and treat accordingly.