Changes in Soluble CD18 in Murine Autoimmune Arthritis and Rheumatoid Arthritis Reflect Disease Establishment and Treatment Response

Abstract

Introduction: In rheumatoid arthritis (RA) immune activation and presence of autoantibodies may precede clinical onset of disease, and joint destruction can progress despite remission. However, the underlying temporal changes of such immune system abnormalities in the inflammatory response during treat-to-target strategies remain poorly understood. We have previously reported low levels of the soluble form of CD18 (sCD18) in plasma from patients with chronic RA and spondyloarthritis. Here, we study the changes of sCD18 before and during treatment of early RA and following arthritis induction in murine models of rheumatoid arthritis.

Methods: The level of sCD18 was analyzed with a time-resolved immunoflourometric assay in 1) plasma from early treatment naïve RA patients during a treat-to-target strategy (the OPERA cohort), 2) plasma from chronic RA patients, 3) serum from SKG and CIA mice following arthritis induction, and 4) supernatants from synovial fluid mononuclear cells (SFMCs) and peripheral blood mononuclear cells (PBMCs) from 6 RA patients cultured with TNFα or adalimumab.

Results: Plasma levels of sCD18 were decreased in chronic RA patients compared with early RA patients and in early RA patients compared with healthy controls. After 12 months of treatment the levels in early RA patients were similar to healthy controls. This normalization of plasma sCD18 levels was more pronounced in patients with very early disease who achieved an early ACR response. Plasma sCD18 levels were associated with radiographic progression. Correspondingly, the serum level of sCD18 was decreased in SKG mice 6 weeks after arthritis induction compared with healthy littermates. The sCD18 levels in both SKG and CIA mice exhibited a biphasic course after arthritis induction with an initial increase above baseline followed by a decline. Shedding of CD18 from RA SFMC and RA PBMC cultures was increased by TNFα and decreased by adalimumab.

Conclusions: The plasma sCD18 levels were altered in patients with RA, in mice with autoimmune arthritis and in cell cultures treated with TNFα and adalimumab. Decreased levels of plasma sCD18 could reflect autoimmunity in transition from early to chronic disease and normalization in response to treatment could reflect autoimmunity in remission.

Fig 1. Plasma levels of sCD18 in 152 early treatment naïve RA (eRA) patients during 12 months of treatment, 30 chronic RA (cRA) patients, and 88 healthy controls (HC).

(A) Plasma levels of sCD18 in early RA patients at the time of inclusion, in chronic RA patients, and in HCs. Lines indicate median and whiskers indicate IQR. Data were analyzed using the student’s t-test on log-transformed data. (B) Plasma levels of sCD18 in RA patients during 12 months of treatment. Symbols and lines indicate median and IQR. DAS28CRP score serves as a measure of clinical disease (symbols and lines indicate median and IQR). Data were analyzed with the paired t-test on log-transformed data. (C) Association between ratio of the change in plasma sCD18 levels from baseline to 12 months after treatment and disease duration. Data were analyzed using the Spearman correlation. (D) Ratio of the change in plasma sCD18 levels from 3 to 12 months after treatment in ACR non-responders (NR) and ACR responders (R) after 3 months of treatment. The mean increase in plasma levels of sCD18 from 3 to 12 months after treatment was greater in ACR responders compared with ACR non-responders. Boxes and error bars indicate mean and 95% CI. Data were analyzed using the student’s t-test. Months indicate time after inclusion (treatment initiation). * P < 0.05, *** P < 0.001, and **** P < 0.0001.

Fig 2. Serum levels of sCD18 in SKG and CIA mice.

(A) The median value of serum sCD18 in healthy SKG mice was lower compared with mice 14 days after arthritis induction with mannan but higher compared with mice 42 days after arthritis induction with zymosan. n = 9. (B) The median value of serum sCD18 in CIA mice was increased after each collagen injection followed by a secondary decrease. n = 4. Lines indicate median and whiskers indicate IQR. Data were analyzed using the student’s t-test on log-transformed data. Arthritis score serves as a measure of clinical disease (symbols and lines indicate the median and IQR). * P < 0.05.

Fig 3. In vitro CD18 shedding from SFMC and PBMC from 6 chronic RA patients with disease flare after stimulation with TNFα or inhibition with adalimumab (ADA).

The concentration of sCD18 from RA SFMC and PBMC was increased by TNFα and decreased by adalimumab compared with untreated cultures (UT). n = 6. (A) The concentration of sCD18 measured as mU/ml. (B) The relative change in sCD18 compared with untreated cultures. Boxes and error bars indicate median and interquartile range. Data were analyzed using non-parametric statistics. * P < 0.05.