Soluble Markers of Antibody Secreting Cell Function as Predictors of Infection Risk in Rheumatoid Arthritis

Abstract

Background: Rheumatoid arthritis (RA) is a systemic autoimmune disease associated with immune dysregulation and increased risk of infections. The presence of autoantibodies and immunoglobulin abnormalities indicates B-cell and antibody-secreting cell (ASC) dysfunction. We hypothesize that soluble factors associated with B-cell and ASC activity are decreased in RA patients and that this is linked to higher susceptibility to infections.

Methods: Using the Johns Hopkins Arthritis Cohort and Biorepository, we contrasted serum protein levels of soluble factors involved in B-cell activation (CD40, CD40L) and B-cell/ASC homing (CXCL10, CXCL11, and CXCL13) or survival (BAFF, APRIL, TACI, and BCMA) in 10 healthy subjects and 23 adult RA patients (aged 24-65 years). We subdivided RA patients into those with (n = 17) and those without infections (n = 6) within a 2-year period. In order to reduce the effect of RA treatment, we only included patients receiving methotrexate monotherapy or no RA treatments at baseline. Soluble serum protein levels of B-cell/ASC factors were quantified by multiplex immunoassays.

Results: We identified that (1) serum levels of soluble BCMA, APRIL, CD40, and CD40L were significantly decreased in RA patients relative to healthy individuals; (2) serum soluble BCMA, predominantly released by ASC, correlated with serum concentrations of class-switched immunoglobulins, IgG and IgA; and (3) RA patients with a history of infections had significantly lower soluble BCMA levels compared with healthy donors and with RA patients without infections.

Conclusions: Our study using soluble factors linked to B-cell/ASC activation and survival suggests that there is a paucity of ASC in a subset of RA patients and that this may be linked to altered antibody production and increased risk of infections. Further delineating the link between ASC and infection susceptibility in RA may optimize disease management and provide novel insights into disease pathogenesis that are susceptible to intervention.

Figure 1

Flow diagram shows selection of patients included in our study. A total of 23 RA patients from the JHAC were included in the final analysis. Abbreviations: RA = rheumatoid arthritis; JHAC = Johns Hopkins Arthritis Cohort; DMARDs = disease-modifying antirheumatic drugs.

Figure 2

Box plots comparing serum B-cell factor protein levels in RA patients and healthy controls. Levels of each protein are indicated for healthy controls (HC, top yellow box; n = 10) and RA patients (RA, bottom red box; n = 23). The median value is indicated with a vertical line, with the box comprising the IQR, and whiskers representing the maximum and minimum values within the fences (±1.5 IQR). ^∗∗ p < 0.01 and ^∗ p < 0.05. Serum levels of B-cell activation factors (CD40, CD40L) and B-cell/ASC survival elements (sBCMA, APRIL) were significantly decreased in RA patients compared with healthy controls.

Figure 3

Correlation of B-cell factors and immunoglobulins in RA patients. (a) Spearman correlation matrix of soluble B-cell factors and Ig levels in RA. (b-d) Scatter plots demonstrating individual correlations between sBCMA and IgA (b), IgG (c), and IgM (d). Serum sBCMA (but not CD40, CD40L, or APRIL) had a positive correlation with serum IgG (rho = 0.6, p < 0.01) and IgA (rho = 0.6, p < 0.001), both ASC-derived class-switched antibodies, but not with IgM levels which is produced without class-switching. Moderate correlations were also seen between B-cell activation factors (CD40, CD40L) and sBCMA levels. ^∗ p < 0.05, ^∗∗ p < 0.01, and^∗∗∗ p < 0.001.

Figure 4

Box plots comparing serum sBCMA protein levels among RA patients with and without infections and healthy donors. Levels of sBCMA in healthy controls (HC, left white box; n = 10), RA patients without infections (middle gray box; n = 17), and RA patients with infections (right gray box; n = 6) are shown. Patients with RA with a history of infections had significantly lower sBCMA levels compared to RA subjects without a history of infections (p < 0.05) and to healthy controls (p < 0.01). ^∗∗ p < 0.01 and^∗ p < 0.05.

Figure 5

Box plots and analyses comparing serum immunoglobulins (IgG, IgA, or IgM) in RA patients with infections and without infections. The median value is indicated with a horizontal line, with the box comprising the IQR and whiskers representing the maximum and minimum values within the fences (±1.5 IQR). Comparisons between groups were performed using Wilcoxon rank-sum tests. No statistically significant differences were found in serum IgG, IgA, or IgM between the two patient groups.