Identification of HLA functional motifs associated with severe systemic sclerosis and sclerotic-graft versus host disease: The shared epitopes of fibrosis

Background: The aim of our study was to identify the HLA class II functional motifs (shared epitopes) associated with severe skin fibrosis in SSc and scl-GVHD. Methods: Seventy consecutive SSc patients (15 with diffuse (dSSc) and 55 with limited (lSSc) cutaneous subset) fulfilling 2013 ACR/EULAR classification criteria and 43 sclGVHD patients with skin fibrosis involving >50 % of body surface area recruited from National Cancer Institute observational clinical trial on chronic GVHD (04-C-0281) were enrolled in the study. High resolution HLA class II typing, HLA data alignment and identification and qualitative and quantitative analysis of amino acids corresponding to β chain hypervariable regions of HLA-DP, HLA-DQ and HLA-DR molecules were performed for all patients. For the qualitative analysis amino acids were classified according the three classic categories (non-polar, polar-neutral, polar-charged) and two subcategories (hydrophobicity and hydrophilicity). For the quantitative analysis Results: Comparison of amino acidic sequences of sclGVHD patients and dSSc patients with severe skin fibrosis (fibrotic phenotype), with data of lSSc patients (non-fibrotic phenotype) showed the lower prevalence of selected amino acidic motifs in patients with fibrotic phenotype compared with those with non-fibrotic phenotype. Thus these motifs displayed protective effect against skin fibrosis. Simultaneous presence of shared amino acidic motifs at different HLA molecules conferred higher degree of protection against skin fibrosis, compared to single epitopes (Table 2). Qualitative amino acidic analysis showed the major prevalence of hydrophobic and non-polar side chains at positions 11 and 26 of HLA – DP (p = 0.026) and HLA - DQ (p = 0.047) β chains respectively. Quantitative analysis showed that patients with non-fibrotic phenotype who did not possess the longest common amino acidic sequence of HLA-DP and HLA-DQ molecules had fewer differences from the common motifs than patients with fibrotic phenotype (p=0.0007 and p= for HLA – DP and HLA – DQ respectively). Conclusion: This is the first study which describes common sequences (shared epitopes), codified by three of the most important genes associated with SSc that seem to display a protective effect against severe fibrotic involvement in SSc and sclGVHD. We identified common amino acidic motifs within the hypervariable regions of β chains of HLA – DP, HLA – DQ and HLA – DR molecules present in patients with minimal skin fibrosis but not in patients who developed severe skin fibrosis. Additionally, we proposed to quantify functional differences within the HLA motifs to compare apparently different sequences for their functional ability of binding specific peptide residues. Our findings indicate this approach can add value to simple genotyping of HLA. Further studies on large populations are necessary to confirm our preliminary findings.