Clinical and Serologic Factors Associated with Lupus Pleuritis

MATERIALS AND METHODS

Study population

The 1000 Faces of Lupus Cohort (1000 Faces) is a prospective multicenter observational cohort of incident and prevalent cases of adult and pediatric SLE. Its purpose is to characterize ethnic differences in the clinical manifestations, disease course, and outcome of SLE across Canada. The study has been approved by the Ethics Review Board of each participating site. Patients were enrolled at 14 sites across Canada, including 10 adult and 4 pediatric rheumatology clinics. Only adult patients are included in the current analysis.

Patients were eligible if they were identified by the site investigator(s) as having a clinical diagnosis of SLE.

Study variables

At the baseline visit, all available medical records were reviewed by the site investigators; clinical data were abstracted and entered onto a comprehensive database form. Clinical manifestations of SLE, i.e., those satisfying the American College of Rheumatology (ACR) criteria and those included in the revised Systemic Lupus Activity Measure (SLAM-2) and the revised SLE Disease Activity Index (SLEDAI-2K), were recorded^1,17–19. Disease activity was measured at baseline and annually using the SLAM-2 and the SLEDAI-2K validated lupus activity scales^17–19. Next, we modified the ACR criteria (m-ACR) and SLEDAI-2K (m-SLEDAI-2K) by subtracting the number attributed to pulmonary involvement from the total score. In addition, at cohort entry, components of the Systemic Lupus International Collaborating Clinics/ACR Damage Index (SDI) were abstracted from the medical record, and reviewed with the patients during the interview²⁰.

Detailed sociodemographic data were recorded at the first visit and updated yearly, including age and sex. Self-reported ethnicity was categorized based on the format used by Statistics Canada²¹ and coded into the following categories: Aboriginal (First Nations, Métis, Inuit, other), Asian (east Asian, southeast Asian, south Asian, central Asian, north African, Arabian peninsula, eastern Middle East, northern Middle East, other Middle Eastern/Arab), Black (African, Caribbean, North American), and Caucasian (European, North American, Australian/New Zealand, French Canadian, Acadian). Age of SLE onset was calculated from date of birth until the date of physician diagnosis of lupus according to the ACR criteria. Disease duration at enrollment was defined as duration from the date of physician diagnosis until the date at first study visit. Patients were also questioned about smoking habits. Ever smokers were defined as having smoked regularly, at least one cigarette per day for ≥ 3 months over their lifetime; nonsmokers were those who did not smoke regularly for ≥ 3 months over their lifetime.

Autoantibodies were screened at the baseline visit and annually thereafter, including antinuclear antibody (ANA), anti-Ro, anti-La, anti-Sm, anti-RNP, anti-dsDNA, and aCL or LAC (aPL). ANA were measured using immunofluorescence; antibodies to dsDNA were measured by ELISA or Farr assay. Testing for extractable nuclear antigens (anti-Ro, anti-La, anti-Sm, and anti-RNP) was by ELISA followed by a confirmatory immunoblot; seropositivity was defined as one or more positive assays for that particular antibody.

Outcome measures

Pleuritis was defined as either serositis according to the ACR criteria, or physician-recorded pleuritis on the SLEDAI-2K index^1,17. Notably, pleuritis was defined by the ACR criteria as the presence of a “convincing history of pleuritic pain or rub heard by a physician or evidence of pleural effusion” at any time during the disease course¹. SLEDAI-2K, also a valid measure of SLE activity, weights the severity of each manifestation and relies primarily on objective evidence of SLE activity within the preceding 10-day period¹⁷. Pleurisy by SLEDAI-2K is present if a subject has pleuritic chest pain with a pleural rub or effusion, or pleural thickening¹⁷.

To address the goals of the study, the 1000 Faces database was screened for eligible patients from July 2005 to November 2007. Inclusion criteria required age ≥ 18 years and satisfaction of the revised 1997 ACR classification criteria for SLE, and completion at the first visit of an SDI and SLEDAI-2K score^1,17,20. From a total of 1417 patients identified in this calendar period, we excluded children (n = 138), patients who did not meet ACR criteria or had missing data (n = 170), patients who had only one extractable nuclear antigen screened (n = 181), and patients missing an SDI index (n = 50) or SLEDAI-2K index (n = 2). Thus, 876 Canadian patients with lupus were studied, each of whom had at least 2 extractable nuclear antigens screened (anti-Sm, anti-Ro, anti-La, anti-RNP).

Statistical analysis

Descriptive statistics including means and standard deviations were computed. The association of demographic and clinical measures with presence (vs absence) of pleuritis was compared using the Student t test for continuous variables and chi-square test for categorical variables. Disease duration and age at diagnosis were dichotomized at the median value. Next, measures of disease activity and damage were compared between the 2 groups, including examination of modified scores for the m-ACR and m-SLEDAI-2K index. Odds ratios were calculated using logistic regression analysis to estimate the strength of association of particular demographic, clinical, and serologic features with the presence of pleuritis. Parameters initially identified as being statistically significant (p ≤ 0.05) in the univariate analyses were entered into the multivariate regression models, to assess the independent contribution of the predictor (e.g., autoantibody) with the outcome of interest. Analyses were performed with Stata statistical software release 9 (Stata Corp., 2005, College Station, TX, USA) and Statistical Package for the Social Sciences 15 (SPSS Inc., 2006, Chicago, IL, USA).

RESULTS

Among the 876 adult patients with lupus in our study population, the mean age was 46.8 ± 13.5 years and mean disease duration 12.1 ± 9.9 years. The majority of the cohort was female (n = 795; 91%). Ethnic distribution was as follows: 569 (64.9%) were Caucasian, 103 (11.8%) Asian, 75 (8.6%) Afro-Caribbean, 50 (5.7%) Aboriginal, 42 (4.8%) were of other ethnic descent, and ethnicity was not reported in 37 (4.2%) participants. Three hundred sixty-seven patients (43%) were regular smokers.

In terms of the lupus-related profile of the study participants, the mean number of m-ACR criteria fulfilled was 5.8 ± 1.5. Further, 578 of the participants had complete SLAM-2 scores and 866 had complete SLEDAI-2K scores; the mean SLAM-2 score was 5.8 ± 4.0 and mean m-SLEDAI-2K score was 4.8 ± 4.4. Overall, about one-third of the 1000 Faces study cohort (n = 296; 34%) developed pleuritis.

In terms of autoantibody profile, 505 (58%) subjects were seropositive for anti-dsDNA antibody, 345 (39%) had anti-Ro, 250 (29%) had anti-Sm, 254 (29%) had an anti-RNP, 176 (20%) had an aPL, and 142 (16%) had anti-La antibodies.

We next focused on the predictors of pleuritis. Specifically, the association of demographic, clinical, and serologic measures with pleuritis, as well as measures of lupus disease activity and damage, was examined (Table 1). Interestingly, those with pleuritis were younger at disease presentation (mean age at SLE diagnosis 33.1 yrs vs 35.5 yrs; p = 0.009), and they experienced longer duration of disease (mean 13.6 yrs vs 11.4 yrs; p = 0.002) and greater cumulative organ system damage (mean 2.03 yrs vs 1.4 yrs; p ≤ 0.0001). Overall, the median age at SLE diagnosis was 32 years; patients with a diagnosis of SLE at ≤ 32 years of age were significantly more likely to develop pleuritis compared to those diagnosed with SLE after age 32 years (p = 0.001). The median disease duration for SLE was 10 years; patients with disease duration ≥ 10 years were significantly more likely to have pleuritis than those with a briefer disease course (p = 0.001). No significant differences by age, ethnicity, smoking status, total m-ACR, or m-SLEDAI scores were observed between SLE patients with and those without pleuritis.

The distribution of positive autoantibodies among SLE subjects with pleuritis was similar: 174/296 (59%) were seropositive for anti-dsDNA, 117/294 (40%) for anti-Ro, 105/294 (36%) anti-RNP, 104/296 (35%) anti-Sm, 55/296 (19%) aPL, and 49/294 (17%) anti-La.

In univariate analysis, seropositivity for RNP was significantly associated with pleuritis (OR 1.57, 95% CI 1.14–2.15, p = 0.005; Table 2]. Further, younger age at diagnosis of SLE, longer disease duration, and higher SDI score were also related to prevalent pleuritis. Seropositivity for anti-Sm antibodies was similarly significantly related to an increase in risk of pleuritis (OR 1.40, 95% CI 1.02–1.91, p = 0.04; Table 2). Perhaps most noteworthy was that concomitant seropositivity for both RNP and Sm antibodies was identified with the greatest risk of prevalent pleuritis in these univariate analyses (OR 1.92, 95% CI 1.31–2.82, p = 0.001).

We therefore investigated in multivariate analyses which serologic profile retained a statistically significant relationship to the outcome of pleuritis. We categorized the serologic profile of the 874 patients screened for both RNP and Sm antibodies into one of 4 mutually exclusive serologic categories. Specifically, in terms of respective serologic profile, 128 (15%) patients were Sm-negative but RNP-positive; 124 (14%) were Sm-positive but RNP-negative; 126 (14%) were concomitantly Sm- and RNP-positive; and 496 (57%) patients were seronegative for both Sm and RNP antibodies (Table 3).

Most important, in these multivariate analyses, concomitant seropositivity for both RNP and Sm was related to a nearly 2-fold increase in risk of pleuritis (OR 1.98, 95% CI 1.31–3.00). In contrast, seropositivity for RNP only and for Sm only no longer retained a significant relationship with prevalent pleuritis. In addition, the other clinical and disease activity variables each remained related to prevalent pleuritis. Therefore, age at SLE diagnosis ≤ 32 years (OR 1.39, 95% CI 1.03–1.87, p = 0.03), higher SDI score (OR 1.14, 95% CI 1.06–1.23, p = 0.001), and disease duration ≥ 10 years (OR 1.38, 95% CI 1.0–1.90, p = 0.04) were each significantly associated with pleuritis (Table 2).

DISCUSSION

Pleuritis has been recognized to be a prominent manifestation that contributes to substantial morbidity and heightened mortality in patients with SLE²². In our multiethnic Canadian study, one-third of the cohort manifested pleuritis. Further, concomitant seropositivity for both RNP and Sm autoantibodies, as well as longer disease duration, greater cumulative organ damage, and younger age at onset of SLE were each significantly associated with having pleuritis in multivariate analyses.

Previously, the frequency of SLE pleuritis across Canada was unknown. We identified prevalent pleuritis in one-third of the 1000 Faces Study, an estimate higher than that reported in Europe (16%), Spain (29%), and among symptomatic patients in China (12%)^23–25. However, the estimated prevalence of 34% is quite similar to that reported in a pooled analysis of over 5000 SLE patients²⁶. This latter study also found late-onset SLE was associated with a higher frequency of pleuritis.

Reports have previously identified that anti-RNP antibody is associated with pulmonary disease involvement, specifically pleuritis, in SLE and in other connective tissue diseases^27–30. A report of 330 SLE patients followed in Manitoba, Canada, that overlapped in composition with some of the patients comprising our cohort, described that anti-RNP was associated with renal disease, central nervous system (CNS) involvement, and pleuritis³¹.

In terms of serologic profile, anti-Sm antibodies are reported as being associated with an increase in risk of renal disease, CNS disease, and mortality in SLE^9,31,32. In a study by Yasuma, et al the presence of anti-Sm antibodies was significantly associated with pulmonary fibrosis and pericarditis³³. Finally, Jurencak, et al showed that patients with pediatric SLE seropositive for anti-RNP, anti-Sm, anti-dsDNA, antichromatin antiribosomal P, anti-Ro, and anti-La autoantibodies are at risk for serositis³⁴.

In terms of potential pathophysiologic mechanisms to explain the reported serologic observations, anti-RNP antibody may play a role in the pathogenesis of tissue inflammation and injury through upregulation of inflammatory cytokine production of adhesion molecules at endothelial cells (intercellular adhesion molecule-1 and endothelial leukocyte adhesion molecule-1), via MHC class II molecule expression on endothelial cells, and by their interaction with Toll-like receptors³⁵. In a mouse model of mixed connective tissue disease, immunization with U1-70 kDa RNP led to lung disease^35,36. Finally, in the setting of mesenteric ischemic reperfusion, mice injected with anti-RNP antibodies developed pulmonary damage that occurred in a dose-dependent manner³⁷. In a recent study it was suggested that dendritic cells and their subsets play a role in directing RNP autoimmunity towards pulmonary end-organ damage³⁸.

In addition to the reported serologic associations with RNP and Sm, we note that greater disease duration was also related to risk of pleuritis. Longer disease duration may account for the association between pleuritis and cumulative damage, inasmuch as SLE patients accrue more damage over time, including pulmonary serosal damage^5,16,39. The lower prevalence of pleuritis reported among Chinese patients²³ compared with our study population may be explained in part by a lower mean disease duration of 7.2 years, compared to our study population mean disease duration of 12.1 years. It also seems plausible that patients with pleuritis may be more likely to receive immunosuppressive treatments, such as corticosteroids, which may lead to or be a marker for greater accrual of organ-system damage over time. We did not analyze the relationship of specific immunosuppressive medications with pleuritis in this cohort. Different genetic backgrounds and environmental exposures may explain in part the different frequencies of pleuritis found among independent cohorts.

We recognize several limitations in this study. Misclassification of pleuritis is possible. Serositis by ACR criteria includes both pericardial and pleural inflammation; thus, the prevalence of pleural inflammation may be overestimated. However, our definition of pleuritis does not differ from that of other large cohorts^16,40. While validated indices and criteria (ACR and SLEDAI-2K) do define pleuritis in an objective manner (either by physical examination or radiographic evaluation), these definitions may still not be optimal and reliable measures of pleuritis^1,17. The sensitivity and specificity for pleurisy defined by the ACR index are only 52% and 89%, respectively¹⁷. We may therefore have underestimated the amount of pleuritis in our population. Finally, medical record errors, physician or patient recall bias, and identification of more prevalent than incident cases of SLE might have had an effect on how serositis was recorded.

At the same time, the analytical approach we undertook deserves mention. We separated the cohort into 4 mutually exclusive serologic categories and were thus able to determine, in multivariate analyses, the risk of prevalent pleuritis associated with concomitant seropositivity for both RNP and Sm autoantibodies, compared to that associated with either autoantibody alone.

We found that pulmonary involvement is a common manifestation of SLE and occurred in one-third of this multiethnic Canadian cohort. Concomitant seropositivity for RNP and Sm antibodies as well as greater cumulative organ damage, longer disease duration, and younger age at onset of disease were each related to a heightened risk for prevalent lupus pleuritis. Establishing factors associated with pleuritis furnishes valuable information. We propose that seropositivity for both RNP and Sm antibodies is associated with a higher likelihood of developing pleuritis. Ultimately, we need to understand the clinical relevance of pleuritis beyond its serologic and clinical correlates and we could not do this directly in this cross-sectional study. Previous reports demonstrated a link between short-term mortality and pleuritis. Identifying SLE patients at risk for pleuritis and its consequences has been hampered in the past by a lack of predictors of pleuritis. Prospective studies are needed to determine the longterm relevance of identifying this subgroup of patients.