RA Synovial Carboxypeptidase and Clinical Responses to Rituximab, Infliximab

Although rituximab and infliximab are considered highly effective therapies for rheumatoid arthritis (RA), a considerable number of patients have an incomplete response or do not respond, and when response occurs, it can take a few weeks to several months for patients to achieve an optimal response. 

Investigators from Canada sought to determine whether expression levels of carboxypeptidase B (CPB) and its substrates in RA synovium would be altered by these agents and be predictive of a clinical response.¹ They found that CPB and its substrates all decreased following treatment with rituximab and infliximab but that these decreases were not necessarily associated with a robust clinical response.

“The importance of CPB and its substrates, particularly complement C5a, in RA was highlighted in a 2011 paper by Robinson et al.^[2] This study found that CPB-deficient mice developed worse inflammatory arthritis and that C5-deficient mice were resistant to the development of inflammatory arthritis. It was hypothesized that CPB acted in an anti-inflammatory manner by deactivating proinflammotory C5a,” said the study’s corresponding author Stefan Edginton, BSc, Department of Internal Medicine, University of Manitoba, Winnipeg, Manitoba, Canada, in an interview with Rheumatology Advisor. “We wanted to explore this relationship in RA to further understand where CPB is expressed in the synovium and how it changes in relation to C5a in response to biologic treatment,” he added.

Edginton and colleagues’ study included 14 patients who had failed at least 1 conventional disease-modifying antirheumatic drug before receiving infliximab (n=9) or rituximab (n=5), which were administered according to standard clinical practice. All patients underwent an arthroscopic synovial biopsy of their most inflamed knee joint before starting treatment and then at 12 to 16 weeks after treatment initiation. 

Patients were followed for 1 year and clinical response was calculated at the time of the second biopsy (approximately 3 months), at 6 months, and at 1 year based on the European League Against Rheumatism (EULAR) response criteria for DAS28 scores.

Baseline synovial biopsy samples were found to be similar between treatment groups, with expression levels of CPB, C5a, and osteopontin (OPN) being significantly higher in RA synovium samples compared with osteoarthritis (OA) synovial tissue samples randomly selected from an OA tissue bank to serve as controls. CPB expression also correlated with several inflammatory markers, including CD3, CD20, and CD68, with the strongest association found with CD68 (r = 0.758; P = .002).

In the second biopsy sample, expression levels of CPB, C5a, and OPN decreased in parallel with decreases in CD3, CD20, and CD68 for both treatment groups, with no significant differences observed in the magnitude of these changes between groups. The decrease in C5a was particularly surprising to Dr Edginton and colleagues.

“Seeing that one of CPB’s functions is to cleave C5a, we would have expected a drop in CPB to lead to an increase in C5a expression, but the opposite occurred, and C5a levels dropped in tandem with CPB,” said Dr Edginton. “This result highlights the complex interactions between these molecules in the synovial environment, and there are a number of factors, including the production and breakdown of C5a, that affect its levels.”

When grouping patients as responders and nonresponders per EULAR criteria, 1 patient had a good response and 6 had a moderate response at 3 months (time of biopsy), which increased to 3 good responses and 5 moderate responses at 6 months. At 1 year, the number of moderate responders decreased to 4. 

Although the difference in percent reduction between responders and nonresponders was more evident at 6 months than at 3 months, the change in biomarker expression between responders and nonresponders was not statistically significant. Furthermore, although C5a expression in the synovium at 16 weeks was correlated to clinical response at 12 weeks and 1 year, the overall change in expression was not correlated to clinical response.

Summary and Clinical Applicability

Although levels of CPB and its substrates were not found to correlate to clinical response with rituximab and infliximab, Dr Edginton and colleagues’ study shows that this pathway is mediated by complex proinflammatory and anti-inflammatory effects that warrant further investigation. “Once the relationship between CPB and RA has been more thoroughly characterized, CPB or one of its substrates could potentially represent a future aim for targeted therapies,” said Dr Edginton.

Future studies would ideally address some of the limitations of Dr Edginton and colleagues’ study, including a small study population, potential sampling bias, and use of an aggregate index (ie, the DAS28) to assess clinical efficacy. “In particular, future studies should examine CPB and C5a in a larger cohort of patients with RA, which might show a stronger link between synovial expression and clinical response,” said Dr Edginton. 

 “In addition, the use of dual immunofluorescent staining would aid in co-localizing CPB expression with specific cell markers, which could identify with more certainty which cells are expressing CPB in the synovium and could target further research,” he added. “Finally, along the lines of C5a and its role in RA, a study looking at the effects of RA therapy on leukocyte-derived microparticles could explore an important mechanism of synovial inflammation.”