Do RA associated HLA-DR molecules bind citrullinated peptides or peptides from PAD4 to help the development of RA specific antibodies to citrullinated proteins?

Peptidylarginine deiminase (PAD): A promising target for chronic diseases treatment

In 1958, the presence of citrulline in the structure of the proteins was discovered for the first time. Several years later they found that Arginine converted to citrulline during a post-translational modification process by PAD enzyme. Each PAD is expressed in a certain tissue developing a series of diseases such as inflammation and cancers. Among these, PAD2 and PAD4 play a role in the development of rheumatoid arthritis (RA) by producing citrullinated autoantigens and increasing the production of inflammatory cytokines. PAD4 is also associated with the formation of NET structures and thrombosis. In the crystallographic structure, PAD has several calcium binding sites, and the active site of the enzyme consists of different amino acids. Various PAD inhibitors have been developed divided into pan-PAD and selective PAD inhibitors. F-amidine, Cl-amidine, and BB-Cl-amidine are some of pan-PAD inhibitors. AFM-30a and JBI589 are selective for PAD2 and PAD4, respectively. There is a need to evaluate the effectiveness of existing inhibitors more accurately in the coming years, as well as design and production of novel inhibitors targeting highly specific isoforms.

The role of anti-citrullinated protein antibody in pathogenesis of RA

Rheumatoid arthritis (RA) is a common autoimmune rheumatic disease that causes chronic synovitis, bone erosion, and joint destruction. The autoantigens in RA include a wide array of posttranslational modified proteins, such as citrullinated proteins catalyzed by peptidyl arginine deiminase4a. Pathogenic anti-citrullinated protein antibodies (ACPAs) directed against a variety of citrullinated epitopes are abundant both in plasma and synovial fluid of RA patients. ACPAs play an important role in the onset and progression of RA. Intensive and extensive studies are being conducted to unveil the mechanisms of RA pathogenesis and evaluate the efficacy of some investigative drugs. In this review, we focus on the formation and pathogenic function of ACPAs.

Diagnostic values, association with disease activity and possible risk factors of anti-PAD4 in rheumatoid arthritis: a meta-analysis

OBJECTIVE

Anti-peptidyl arginine deaminase 4 (anti-PAD4) antibody has been a subject of investigation in RA in the last two decades. This meta-analysis investigated the diagnostic values, association with disease activity and possible risk factors of anti-PAD4 antibody in rheumatoid arthritis.

METHOD

We searched studies from five databases up to 1 December 2022. Bivariate mixed-effect models were used to pool the diagnostic accuracy indexes, and the summary receiver operating characteristics (SROC) curve was plotted. The quality of diagnostic studies was assessed using QUADAS-2. Non-diagnostic meta-analyses were conducted using the random-effects model. Sensitivity analysis, meta-regression, subgroup analyses and Deeks' funnel plot asymmetry test were used to address heterogeneity.

RESULT

Finally, 24 journal articles and one letter were included. Anti-PAD4 antibody had a good diagnostic value between RA and healthy individuals, but it might be lower between RA and other rheumatic diseases. Moreover, anti-PAD4 could slightly enhance RA diagnostic sensitivity with a combination of ACPA or ACPA/RF. Anti-PAD4 antibody was positively correlated with HLA-SE and negatively correlated with ever or current smoking in patients with RA. RA patients with anti-PAD4 antibody had higher DAS28, ESR, swollen joint count (SJC) and the possibility of having interstitial lung disease (ILD) and pulmonary fibrosis compared with those without.

CONCLUSION

Our study suggests that anti-PAD4 antibody is a potentially useful diagnostic biomarker and clinical indicator for RA. Further mechanistic studies are required to understand the impact of HLA-SE and smoking on the production of anti-PAD4 antibody.

Role of the PADI family in inflammatory autoimmune diseases and cancers: A systematic review

The peptidyl arginine deiminase (PADI) family is a calcium ion-dependent group of isozymes with sequence similarity that catalyze the citrullination of proteins. Histones can serve as the target substrate of PADI family isozymes, and therefore, the PADI family is involved in NETosis and the secretion of inflammatory cytokines. Thus, the PADI family is associated with the development of inflammatory autoimmune diseases and cancer, reproductive development, and other related diseases. In this review, we systematically discuss the role of the PADI family in the pathogenesis of various diseases based on studies from the past decade to provide a reference for future research.

PAD2 immunization induces ACPA in wild-type and HLA-DR4 humanized mice

Rheumatoid arthritis (RA) is associated with HLA‐DRB1 alleles expressing the "shared epitope." RA is usually preceded by the emergence of anti‐citrullinated protein autoantibodies (ACPAs). ACPAs recognize citrulline residues on numerous proteins. Conversion of arginine into citrulline is performed by enzymes called peptidyl arginine deiminases (PADs). We have previously demonstrated that C3H mice immunized with PADs can produce ACPAs by a hapten‐carrier mechanism.

Here, we address the influence of HLA‐DR alleles in this model in mice expressing RA‐associated HLA‐DRB1*04:01 (KO/KI*04:01), HLA‐DRB1*04:04 (KO/KI*04:04), or non‐RA‐associated HLA‐DRB1*04:02 (KO/KI*04:02) after murine PAD2 immunization.

Immunization with mPAD2 triggers production of ACPAs in wild‐type (WT) and HLA‐DR4 C57BL/6 mice. Both I‐A^b and HLA‐DR are involved in the activation of mPAD2‐specific T lymphocytes. Among HLA‐DR4 mice, mice expressing RA‐associated HLA‐DRB1*04:01 are the best responders to mPAD2 and the best anti‐citrullinated peptide antibody producers.

How RA Associated HLA-DR Molecules Contribute to the Development of Antibodies to Citrullinated Proteins: The Hapten Carrier Model

The risk to develop ACPA positive rheumatoid arthritis (RA), the most destructive type of autoimmune arthritis, is carried by HLA-DRB1 alleles containing a 5 amino acid motif: the shared epitope (SE). RA is preceded by the emergence of disease specific anti citrullinated protein antibodies (ACPA). SE positive HLA-DRB1 alleles are associated with ACPA and ACPA positive RA, not with ACPA negative RA, suggesting that ACPA contribute to the pathogenesis of RA. Understanding how HLA-DRB1 genotypes influence ACPA could lead to a curative or preventive treatment of RA. The “Shared epitope binds citrullinated peptides “ hypothesis suggests that RA associated HLA-DR alleles present citrullinated peptides to T cells that help ACPA producing B cells. The “Hapten carrier model” suggests that PAD4 is the target of the T cells which help ACPA specific B cells through a hapten carrier mechanism in which PAD4 is the carrier and citrullinated peptides are the haptens. Direct binding assay of citrullinated peptides to purified HLA-DR molecules does not support the “shared epitope binds citrullinated peptides” hypothesis. The Odds Ratios to develop ACPA positive RA associated with each of 12 common HLA-DRB1 genotypes match the probability that the two HLA-DR molecules they encode can bind at least one peptide from PAD4, not from citrullinated fibrinogen. Thus, PAD4 tolerization might stop the carrier effect and switch off production of ACPA.

Autoantibodies in Rheumatoid Arthritis: Historical Background and Novel Findings

Autoantibodies represent a hallmark of rheumatoid arthritis (RA), with the rheumatoid factor (RF) and antibodies against citrullinated proteins (ACPA) being the most acknowledged ones. RA patients who are positive for RF and/or ACPA ("seropositive") in general display a different etiology and disease course compared to so-called "seronegative" patients. Still, the seronegative patient population is very heterogeneous and not well characterized. Due to the identification of new autoantibodies and advancements in the diagnosis of rheumatic diseases in the last years, the group of seronegative patients is constantly shrinking. Aside from antibodies towards various post-translational modifications, recent studies describe autoantibodies targeting some native proteins, further broadening the spectrum of recognized antigens. Next to the detection of new autoantibody groups, much research has been done to answer the question if and how autoantibodies contribute to the pathogenesis of RA. Since autoantibodies can be detected years prior to RA onset, it is a matter of debate whether their presence alone is sufficient to trigger the disease. Nevertheless, there is gathering evidence of direct autoantibody effector functions, such as stimulation of osteoclastogenesis and synovial fibroblast migration in in vitro experiments. In addition, autoantibody positive patients display a worse clinical course and stronger radiographic progression. In this review, we discuss current findings regarding different autoantibody types, the underlying disease-driving mechanisms, the role of Fab and Fc glycosylation and clinical implications.

[Effects of Sidaxue, a Miao ethnomedicine recipe, on apoptosis and pyrolysis of human fibroblast-like synovial cells in rheumatoid arthritis]

OBJECTIVE

To investigate the effects of Sidaxue (SX), a recipe in Miao ethnomedicine, on apoptosis and pyrolysis of human fibroblast-like synovial cells in rheumatoid arthritis (RA-FLS).

METHODS

The target proteins related with RA and those involved in cell apoptosis and pyroptosis were searched in different online databases, and Venny software was used to obtain apoptosis- and pyroptosis-related proteins in RA. RA-apoptosis-pyroptosis protein interaction (PPI) network was constructed to identity the key target proteins related with apoptosis and pyroptosis in RA. Autodock vina software was used to perform molecular docking to verify the binding ability of the main active ingredients in SX with the apoptosis- and pyroptosis-related proteins. In the cell experiment, MH7A cells were treated with 5 mg/L TGT (positive control) or 5, 10, 20, and 40 mg/L SX, and the changes in cell migration and invasion abilities and expressions of apoptosis- and pyroptosis-related proteins were examined using wound healing assay, Transwell assay, ELISA and Western blotting.

RESULTS

We identified 9 RA-related apoptotic target proteins, 15 RA-related pyroptosis target proteins, and 4 overlapping target proteins related with RA, apoptosis and pyroptosis. The main active ingredients in SX had a high affinity with the target proteins including TNF-α, Fas, and Bax. In MH7A cells, SX treatment concentration-dependently increased the cell inhibition rate at 24, 48 and 72 h (P < 0.05), and significantly lowered the cell migration ability at 6, 12 and 24 h (P < 0.05); treatment with 20 and 40 mg/L SX for 24 h obviously suppressed MH7A cell invasion (P < 0.05). SX treatment (20 and 40 mg/L) and TGT treatment both significantly lowered the expression levels of TNF-α, IL-1β, and IL-18 in the cells (P < 0.05). The Bax/Bcl-2 ratio and Fas and FasL expressions were increased significantly in cells treated with 20 and 40 mg/L SX (P < 0.05), and caspase-1 expression was decreased significantly in cells treated with 5 and 40 mg/L SX (P < 0.05).

CONCLUSION

SX can induce apoptosis and pyroptosison in RA-FLSs possibly by down-regulating the expressions of TNF-α, IL-1β and IL-18, up-regulating the expressions of Bax, Fas, and FasL, and inhibiting Bcl-2 and caspase-1 protein expressions.

The role of posttranslational modifications in generating neo-epitopes that bind to rheumatoid arthritis-associated HLA-DR alleles and promote autoimmune T cell responses

While antibodies to citrullinated proteins have become a diagnostic hallmark in rheumatoid arthritis (RA), we still do not understand how the autoimmune T cell response is influenced by these citrullinated proteins. To investigate the role of citrullinated antigens in HLA-DR1- and DR4-restricted T cell responses, we utilized mouse models that express these MHC-II alleles to determine the relationship between citrullinated peptide affinity for these DR molecules and the ability of these peptides to induce a T cell response. Using a set of peptides from proteins thought to be targeted by the autoimmune T cell responses in RA, aggrecan, vimentin, fibrinogen, and type II collagen, we found that while citrullination can enhance the binding affinity for these DR alleles, it does not always do so, even when in the critical P4 position. Moreover, if peptide citrullination does enhance HLA-DR binding affinity, it does not necessarily predict the generation of a T cell response. Conversely, citrullinated peptides can stimulate T cells without changing the peptide binding affinity for HLA-DR1 or DR4. Furthermore, citrullination of an autoantigen, type II collagen, which enhances binding affinity to HLA-DR1 did not enhance the severity of autoimmune arthritis in HLA-DR1 transgenic mice. Additional analysis of clonal T cell populations stimulated by these peptides indicated cross recognition of citrullinated and wild type peptides can occur in some instances, while in others cases the citrullination generates a novel T cell epitope. Finally, cytokine profiles of the wild type and citrullinated peptide stimulated T cells unveiled a significant disconnect between proliferation and cytokine production. Altogether, these data demonstrate the lack of support for a simplified model with universal correlation between affinity for HLA-DR alleles, immunogenicity and arthritogenicity of citrullinated peptides. Additionally they highlight the complexity of both T cell receptor recognition of citrulline as well as its potential conformational effects on the peptide:HLA-DR complex as recognized by a self-reactive cell receptor.