Functional studies of anti-gpi monoclonal antibodies and anti-citrullinated protein antibodies (ACPAS)

Abstract

RA is an autoimmune disease that primarily impacts joints throughout the body. The disease can result in pain, stiffness, and reduced mobility. It is a chronic disease, meaning that it lasts a long time and can worsen over time if left untreated. As the disease progresses, it can lead to joint deformity and severe pain, which can significantly impact the patient's daily life. While there are treatments available to manage the symptoms of RA, there is currently no cure for the condition.

Glucose-6-phosphate isomerase (GPI) is an enzyme that plays a key role in glycolysis, the metabolic pathway that breaks down glucose to produce energy. Establishment of two GPI-dependent arthritis mouse models (K/BxN and GPI protein-induced arthritis mouse models), successfully identifying GPI as one of the RA-associated autoantigens. In human RA, elevated levels of free GPI protein and anti-GPI autoantibodies were found in the sera and synovial fluid of RA patients. Despite extensive research on the pathogenicity of GPI in RA, many questions remain unanswered, including what are the major pathogenic B-cell epitopes of GPI? What is the prevalence of anti-GPI antibodies in RA patients? Are anti-GPI antibodies specific for RA disease?

Cartilage Oligomeric Matrix Protein (COMP) is a glycoprotein that helps in the formation and maintenance of cartilage. It exists in the synovial fluid and serum of RA patients and the level of COMP have been found to be positively correlated with the severity of the disease. Studies have indicated that COMP might contribute to the breakdown of cartilage in RA by stimulating the production of inflammatory cytokines and activating enzymes known as matrix metalloproteinases.

Anti-citrullinated protein antibodies (ACPAs) are autoantibodies that have highest specificity for RA. During the autoimmune stage of RA, which occurs years before clinical symptoms appear, the immune system already begins to produce ACPAs. Studies have shown that the production of ACPAs is highly associated with the HLA-DRB1*0401 allele, which is one of the highest genetic risk factor for RA. Some studies believed that HLADRB1* 0401 allele increases the likelihood of citrullinated peptides being presented to T cells, which can result in the production of ACPA by B cells. However, subsequent scientific research has disproved this hypothesis, indicating that HLA-DRB1*0401 does not exhibit a preference for presenting citrullinated peptides over non-citrullinated peptides. The current hypotheses and associated conclusions are either based solely on statistical analysis, so further experimental data must be obtained to test these hypotheses.

In the first study, we identified the arthritogenic B cell epitopes of GPI in mice. We showed that this pathogenic GPI B cell epitope is exposed exclusively on structurally modified GPI on the cartilage surface. Identification of these specific B cells by tetramer technology showed that these cells were not negatively selected and present in naïve mice. These naturally autoreactive cells escaped tolerance mechanisms and could potentially play a pathogenic role in disease as shown by the association of the antibody response against this epitope with clinical disease parameters in human RA cohorts.

In the second study, we investigated the pathogenic role of anti-COMP antibodies in murine arthritis models in conjunction with studies on the autoantibody response to COMP in human RA cohorts.

In the third study, our findings demonstrate a link between the induction of ACPAs and the presence of HLA-DRB1*0401, as well as between the development of arthritis and this HLA allele. Interestingly, we found that the function of ACPAs is variable. ACPAs cross-reactive with cartilage can be pathogenic, whereas other promiscuous ACPAs can be protective or have no detectable impact on arthritis.