Use of HLA-B27 tetramers to identify low-frequency antigen-specific T cells in Chlamydia-triggered reactive arthritis

Chlamydia-triggered reactive arthritis (Ct-ReA) is strongly associated with HLA-B27 like other spondylarthropathies, and especially ankylosing spondylitis [1,2]. ReA occurs 1 to 4 weeks after urogenital infection with Chlamydia trachomatis or gastroenteral infection with enterobacteria such as Yersinia enterocolitica [3]. After acute onset, most patients have a self-limiting course, but up to 20% suffer from a disease duration of more than 1 year [4]. Of HLA-B27+-reactive arthritis patients, 20–40% move on to ankylosing spondylitis after 10–20 years, suggesting that the ReA-associated bacteria can cause ankylosing spondylitis [5] and that immune mechanisms triggering the disease are induced by T cell responses to microbial antigens. The main hypothesis advanced for the association between HLA-B27 and spondylarthropathies is the arthritogenic peptide theory. It states that some HLA-B27 subtype alleles, owing to their unique amino acid residues, bind a specific arthritogenic peptide that is recognized by CD8+ T cells [6-9]. Recently we and several other groups have reported on Chlamydia-specific CD8+ T cells capable of lysing target cells primed with Chlamydia antigens [10-12]. CD8+ T cell responses in spondylarthropathies other than Ct-ReA have also been described [13-15].Recently a new method for antigen-specific T cell recognition has been established by using multimerized MHC/peptide molecules [16]. These molecules are called tetramers because they contain four soluble and biotinylated MHC molecules linked to labelled streptavidin that specifically bind with high avidity to T cell receptors. In comparison with intracellular cytokine staining, the major advantage of tetramer technology is the identification of antigen-specific T cells independently of their cytokine secretion profile, the possibility of sorting unstimulated T cells and of having a tool for the antigen-specific detection of T cells in experiments in situ [17].In humans, MHC class I tetramers are wid