HLA-DR3 restricted T cell epitope mimicry in induction of autoimmune response to lupus-associated antigen SmD

Abstract

Although systemic lupus erythematosus (SLE) is a multigenic autoimmune disorder, HLA-D is the most dominant genetic susceptibility locus. This study was undertaken to investigate the hypothesis that microbial peptides bind HLA-DR3 and activate T cells reactive with lupus autoantigens. Using HLA-DR3 transgenic mice and lupus-associated autoantigen SmD protein, SmD_79–93 was identified to contain a dominant HLA-DR3 restricted T cell epitope. This T cell epitope was characterized by using a T–T hybridoma, C1P2, generated from SmD immunized HLA-DR3 transgenic mouse. By pattern search analysis, 20 putative mimicry peptides (P2–P21) of SmD_79–93, from microbial and human origin were identified. C1P2 cells responded to SmD, SmD_79–93 and a peptide (P20) from Vibro cholerae. Immunization of HLA-DR3 mice with P20 induced T cell responses and IgG antibodies to SmD that were not cross-reactive with the immunogen. A T–T hybridoma, P20P1, generated from P20 immunized mice, not only responded to P20 and SmD_79–93, but also to peptides from Streptococcus agalactiae (P17) and human-La related protein (P11). These three T cell mimics (P20, P11 and P17) induced diverse and different autoantibody response profiles. Our data demonstrates for the first time molecular mimicry at T cell epitope level between lupus-associated autoantigen SmD and microbial peptides. Considering that distinct autoreactive T cell clones were activated by different microbial peptides, molecular mimicry at T cell epitope level can be an important pathway for the activation of autoreactive T cells resulting in the production of autoantibodies. In addition, the novel findings reported herein may have significant implications in the pathogenesis of SLE.

Introduction

Systemic lupus erythematosus (SLE) is a multigenic autoimmune disease with diverse clinical manifestations at the initial diagnosis and subsequent relapses [1], [2]. SLE is characterized by the presence of autoantibodies to a number of cellular antigens, including dsDNA, phospholipids and several ribonucleoproteins [3], [4]. Susceptibility to SLE is dependent on both genetic and environmental factors. Family studies show there is a significant genetic influence in SLE [5]. The genetic element that exerts the most influence is the HLA-D region [6], [7], [8], [9]. Genetic predisposition to SLE manifests into several cellular defects such as: hyperactivity at T and B cell level [10]; defective clearance of apoptotic cells [11]; regulatory T cell defects [12] and Toll-like receptor driven autoantibody production [13]. However, these defects do not explain the strongest association of SLE with HLA-D.

The present study was designed to investigate the mechanisms responsible for the dominant role of HLA-DR in the pathogenesis of SLE. It was hypothesized that HLA-DR influences the selection and enrichment of autoreactive T cells that provide cognate help to B cells recognizing the same autoantigen. One plausible pathway for the activation and enrichment of these autoreactive T cells is through the presentation of molecular mimics. This pathway is eminently feasible in view of the well established facts that positive selection of CD4+ T cell repertoire is based on self MHC Class II molecules and those TCRs are polyreactive [14]. While the role of molecular mimicry at T cell epitope level in initiating autoimmune responses has been explored in various autoimmune disease such as multiple sclerosis, evidence for it in SLE is still lacking. Thus, in this investigation, HLA-DR3 transgenic mice were used as an experimental model system, as HLA-DR3 is strongly associated with SLE [7], [15]. SmD protein, which is a part of the small nuclear ribonucleoprotein (snRNP) complex and is involved with the splicing and processing of pre-mRNA was selected as the model autoantigen [16]. Antibodies against SmD are seen in lupus patients in North America [17] and the presence of anti-Sm antibodies is one of the classification criteria for SLE as established by the American College of Rheumatology [18]. In addition, mouse SmD is completely homologous to human SmD, thereby rendering it to be an ideal autoantigen.

In this study, we identified a dominant DR3 restricted T cell epitope on SmD_79–93 and mimicry peptides capable of activating T cells reactive with this epitope. SmD_79–93 and its molecular mimics induced autoantibodies against SmD and other lupus-related autoantigens. These data provide a mechanism for association of specific HLA-D haplotypes with SLE and demonstrate the potential of T cell epitope mimicry, for initiating autoimmune responses in SLE. They suggest that autoimmune responses in SLE may be initiated through multiple exposures to environmental antigens over a long period.