Shared epitope-antagonistic ligands: a new therapeutic strategy in mice with erosive arthritis

Development of Clarstatin, a Novel Drug Lead for the Therapy of Autoimmune Uveitis

We describe the design, synthesis, and activity of a potent thiourea-bridged backbone cyclic peptidomimetic known as Clarstatin, comprising a 5-amino acid sequence (Q/D)1-(R/K)2-X3-X4-A5-(Gln/Asp)1-(Arg/Lys)2-AA3-AA4-Ala5-based on a motif called "shared epitope (SE)", specifically present in specific alleles of the HLA-DRB1 gene. This SE binds to a particular site within the proline reach domain (P-domain) of the cell surface-calreticulin (CS-CRT). CS-CRT is a multifunctional endoplasmic reticulum (ER) calcium-binding protein that is located on the cell surface of T cells and triggers innate immune signaling, leading to the development of inflammatory autoimmune diseases. The development of Clarstatin was based on the parent peptide W-G-D1-K2-S3-G4-A5- derived from the active region of the SE. Following the design based on the cycloscan method, the synthesis of Clarstatin was performed by the Fmoc solid phase peptide synthesis (SPPS) method, purified by HPLC to 96% homogeneity, and its structure was confirmed by LC-MS. Clarstatin reduced calcium levels in Jurkat lymphocyte cultures, ameliorated uveitis in vivo in the experimental autoimmune uveitis (EAU) mice model, and was safe upon acute toxicity evaluation. These findings identify Clarstatin as a promising lead compound for future drug development as a novel class of therapeutic agents in the therapy of uveitis.

HLA-G and the MHC Cusp Theory

Human leukocyte antigens (HLA) are significant genetic risk factors in a long list of diseases. However, the mechanisms underlying these associations remain elusive in many cases. The best-characterized function of classical major histocompatibility complex (MHC) antigens is to allow safe presentation of antigenic peptides via a self/non-self-discrimination process. Therefore, most hypotheses to date have posited that the observed associations between certain HLA molecules and human diseases involve antigen presentation (AP). However, these hypotheses often represent inconsistencies with current knowledge. To offer answers to the inconsistencies, a decade ago we have invoked the MHC Cusp theory, postulating that in addition to its main role in AP, the MHC codes for allele-specific molecules that act as ligands in a conformationally-conserved cusp-like fold, which upon interaction with cognate receptors can trigger MHC-associated diseases. In the ensuing years, we have provided empirical evidence that substantiates the theory in several HLA-Class II-associated autoimmune diseases. Notably, in a recent study we have demonstrated that HLA-DRB1 alleles known to protect against several autoimmune diseases encode a protective epitope at the cusp region, which activates anti-inflammatory signaling leading to transcriptional and functional modulatory effects. Relevant to the topic of this session, cusp ligands demonstrate several similarities to the functional effects of HLA-G. The overall goal of this opinion article is to delineate the parallels and distinctive features of the MHC Cusp theory with structural and functional aspects of HLA-G molecules.

HLA-DRB1 allelic epitopes that associate with autoimmune disease risk or protection activate reciprocal macrophage polarization

Associations between particular human leukocyte antigen (HLA) alleles and susceptibility to-or protection from-autoimmune diseases have been long observed. Allele-specific antigen presentation (AP) has been widely proposed as a culprit, but it is unclear whether HLA molecules might also have non-AP, disease-modulating effects. Here we demonstrate differential macrophage activation by HLA-DRB1 alleles known to associate with autoimmune disease risk or protection with resultant polarization of pro-inflammatory ("M1") versus anti-inflammatory ("M2") macrophages, respectively. RNA-sequencing analyses of in vitro-polarized macrophages in the presence of AP-incompetent short synthetic peptides corresponding to the third allelic hypervariable regions coded by those two HLA-DRB1 alleles showed reciprocal activation of pro- versus anti-inflammatory transcriptomes, with implication of corresponding gene ontologies and upstream regulators. These results identify a previously unrecognized mechanism of differential immune modulation by short HLA-DRB1-coded allelic epitopes independent of AP, and could shed new light on the mechanistic basis of HLA-disease association.

Shared epitope-aryl hydrocarbon receptor crosstalk underlies the mechanism of gene-environment interaction in autoimmune arthritis

The susceptibility to autoimmune diseases is affected by genetic and environmental factors. In rheumatoid arthritis (RA), the shared epitope (SE), a five-amino acid sequence motif encoded by RA-associated HLA-DRB1 alleles, is the single most significant genetic risk factor. The risk conferred by the SE is increased in a multiplicative way by exposure to various environmental pollutants, such as cigarette smoke. The mechanism of this synergistic interaction is unknown. It is worth noting that the SE has recently been found to act as a signal transduction ligand that facilitates differentiation of Th17 cells and osteoclasts in vitro and in vivo. Intriguingly, the aryl hydrocarbon receptor (AhR), a transcription factor that mediates the xenobiotic effects of many pollutants, including tobacco combustion products, has been found to activate similar biologic effects. Prompted by these similarities, we sought to determine whether the SE and AhR signaling pathways interact in autoimmune arthritis. Here we uncovered a nuclear factor kappa B-mediated synergistic interaction between the SE and AhR pathways that leads to markedly enhanced osteoclast differentiation and Th17 polarization in vitro. Administration of AhR pathway agonists to transgenic mice carrying human SE-coding alleles resulted in a robust increase in arthritis severity, bone destruction, overabundance of osteoclasts, and IL17-expressing cells in the inflamed joints and draining lymph nodes of arthritic mice. Thus, this study identifies a previously unrecognized mechanism of gene-environment interaction that could provide insights into the well-described but poorly understood amplification of the genetic risk for RA upon exposure to environmental pollutants.

Heightened autoantibody immune response to citrullinated calreticulin in bronchiectasis: Implications for rheumatoid arthritis

Calreticulin (CRT) and citrullinated (citCRT) are implicated in rheumatoid arthritis (RA) pathology. citCRT binds to RA shared epitopes (SE) on HLA-DR molecules with high affinity and triggers pro-inflammatory events in adjacent cells. The aim of the study was to detect the presence of citCRT prior to developing RA and evaluate if citCT is a target for autoantibodies in RA cohorts with and without lung disease. Antibodies were assessed by ELISA against native CRT, citCRT and general protein citrullination, in sera from 50 RA patients without lung disease, 122 bronchiectasis (BR) patients, 52 bronchiectasis patients with RA (BRRA), 87 asthma patients and 77 healthy controls (HC). Serum citCRT was detected by immunoblotting and mass spectrometry. Genomic DNA was genotyped for HLA-DRB1 alleles. Patients were assessed for DAS28, rheumatoid factor, and anti-cyclic citrullinated peptide antibodies. Extracellular citCRT was detected in BR patients sera prior to them developing RA. A citCRT SE binding peptide GEWKPR_261citQIDNPDYK was identified. Anti-CRT antibodies were observed in 18% of BR patients with or without RA. Anti-citCRT antibodies were observed in ∼35% of BR or RA patients, increasing to 58% in BRRA patients. In the RA alone patients 7/20 (35%) who were negative for RF and anti-CCP were anti-CRT antibody positive and had higher DAS28 scores than triple negative RA alone patients. Three of the four BR patients who developed RA over 18 months were anti-citCRT+ve SE^+ve. The detection of citCRT in BR and development of anti-citCRT in BR patients suggests citCRT antigens are early targets of antigenicity in these patients, especially in SE^+ve patients prior to the onset of RA.

Spontaneous destructive periodontitis and skeletal bone damage in transgenic mice carrying a human shared epitope-coding HLA-DRB1 allele

Objective

Shared epitope (SE)-coding DRB1 alleles are associated with bone erosion in several diseases, including rheumatoid arthritis (RA) and periodontal disease (PD), but the underlying mechanism is unknown. We have recently identified the SE as an osteoclast-activating ligand. To better understand the biological effects of the SE in vivo, here we sought to determine whether it can facilitate spontaneous bone damage in naïve mice.

Methods

3-month old naïve transgenic mice that carry the human SE-coding allele DRB1*04:01, or a SE-negative allele DRB1*04:02 were studied. Bone tissues were analysed by micro-CT, and the tooth-supporting tissues were studied by histology, immunohistochemistry and immunofluorescence. Serum biomarkers were determined by ELISA.

Results

Transgenic mice expressing the SE-coding DRB1*04:01 allele, but not mice carrying the SE-negative allele DRB1*04:02, showed spontaneous PD associated with interleukin (IL)-17 overabundance and periostin disruption. Mandibular bone volumetric and mineralisation parameters were significantly lower in SE-positive mice, and alveolar bone resorption was significantly increased in these mice. SE-positive mice also had more slender tibiae, and their marrow, cortical and total areas were lower than those of SE-negative mice. Additionally, significantly increased serum IL-17, tumour necrosis factor-α and osteoprotegrin levels were found in SE-positive mice, while their receptor activator of nuclear factor κ-B ligand levels were significantly lower.

Conclusions

A human SE-coding allele increases the propensity to spontaneous bone-destructive periodontal inflammation and skeletal bone damage in transgenic mice. These findings provide new insights into the previously documented but poorly understood association of the SE with accelerated bone erosion in RA and several other human diseases.