Specificity of CD4 T-cell responses to aggrecan in BALB/c and DR1 transgenic mice

It has been suggested that rheumatoid arthritis (RA) is initiated or perpetuated by CD4 T cells activated by presentation of joint antigens by susceptible MHC class II molecules including HLA-DR4 and DR1. A number of candidate antigens have been investigated including cartilage-derived type II collagen and aggrecan. Immunisation of susceptible strains of mice with aggrecan, including BALB/c (H-2^d), induces autoimmune arthritis, providing an animal model of human RA [1]. Immunisation with peptides predicted to bind to HLA-DR4 also induces T-cell responses in HLA-DR4 transgenic mice [2]. The auto-immune T-cell response focuses on only a few epitopes within restricted regions of aggrecan such as the G₁ globular domain, for reasons that are poorly understood. One explanation is a failure of self-tolerance due to poor presentation of these aggrecan epitopes in the thymus, and presentation of the same epitopes in joints following release and processing of the normally sequestered aggrecan induced by early inflammatory events. T-cell responses have also been observed in RA patients mapping epitopes within the G₁ domain.

To characterise aggrecan-specific T-cell hybridomas from BALB/c (H-2^d) and DR1 transgenic mice to investigate the mechanisms of antigen presentation of arthritogenic CD4 T-cell epitopes of aggrecan.

Aggrecan-specific T-cell hybridomas were generated from spleen cells of BALB/c and DRB1*0101-transgenic mouse-MHC class II knockout (DR1-tg) mice immunised with deglycosylated bovine aggrecan. Synthetic peptides of three known arthritogenic H-2^d-restricted epitopes (amino acids 68–82, 84–103 and 169–189) and three DR1-restricted epitopes known to be recognised by RA patients (aminoacids 148–165, 201–213 and 292–311) from the G₁ domain of bovine aggrecan were used to define the specificity of T-cell hybridomas. Cultured bone marrow macrophages were treated with deglycosylated bovine aggrecan or synthetic peptides and used as antigen-presenting cells. T-cell hybridoma responses were measured by proliferation of CTLL-2 indicator cells to T-cell hybridoma supernatants in the presence of tritiated thymidine and expressed as counts per minute.

The specificity of most aggrecan-specific T-cell hybridomas mapped to the aggrecan G₁ domain using synthetic peptides representing known H-2^d-restricted or DR1-restricted CD4 T-cell epitopes. However, most cloned (and recloned) T-cell hybridomas recognised two of the H-2^d-restricted epitopes (84–103 and 169–189) as well as two of the DR1-restricted epitopes (148–165 and 292–311). The same synthetic peptides were not recognised by H-2^d-restricted and DR1-restricted T-cell hybridomas of unrelated specificities and were not mitogenic for spleen cells from BALB/c or DR1-tg mice. None of the T-cell hybridomas recognised aggrecan epitopes 68–82 or 201–213.

We are investigating this unexpected pattern of cross-reactivity between a number of known arthritogenic epitopes of aggrecan using lymph node T-cell responses of peptide-immunised BALB/c and DR1-tg mice. In addition we are investigating the mechanisms of antigen presentation of these epitopes, which may shed light on why this particular region of aggrecan is antigenic for autoreactive T cells.