Ankylosing spondylitis (AS) is chronic inflammatory rheumatic disorder particularly affecting the axial skeleton. A link between AS and the major histocompatibility antigen HLA-B27 has been well established : over 95% of patients with AS are HLA-B27 positive [2,3]. There are seven subtypes of HLA-B27 ranging from B*2701 to B*2707. It has been suggested that B*2705 is the most common HLA-B27 subtype in Caucasian populations, whilst it is proposed that B*2703 is the most common B27 antigen in black populations, where AS is rare. This observation has led to the suggestion that B*2703 is not associated with AS . Subtypes of HLA-B27, other than B*2705, containing different amino acid sequences are also involved in AS. In this paper the HLA-B27 subtype under examination is B*2705. The cause of AS remains controversial, although an environmental agent in the form of the gram negative bacterium, Klebsiella pneumoniae, present in the gut, has been suggested as a possible aetiological agent  to which antibodies have been shown in several centres . Any aetiological agent implicated in AS must provide an explanation for the link with HLA-B27. Schwimmbeck et al.  identified an amino acid homology, QTDRED found in residues 72-77 of B*2705 and
*Corresponding author. Infection and Immunity Group, Division of Life Sciences, King"s College, London W8 7AH, UK.
residues 188-193 of K. pneumoniae nitrogenase enzyme. The validity of this homology, as a cross reactive antigen, has however, been criticised, in both biochemical and immunological terms. For nitrogenase production, an extremely low level of fixed nitrogen is required  and this condition is not achieved in the human gut. Additionally, extremely anaerobic conditions are required for nitrogenase production in Klebsiella , and whilst the human gut has a low level of oxygen, debate persists as to whether this degree of anaerobiosis is sufficient to allow nitrogenase production. Furthermore, HLA-B27 is a self protein and therefore should not trigger an immune response. The part of K. pneumoniae nitrogenase that contains the sequence QTDRED should been seen as self by the immune system, and therefore should not evoke an immune response. These criticisms have led to a re-evaluation of the relevance of K. pneumoniae nitrogenase in AS and initiated the search for different sequence homologies between K. pneumoniae proteins and B*2705. In this study a computer search has revealed a novel homology between B*2705 and another component of K. pneumoniae in the form of the secretion protein (pulD) of the induc-ible, starch debranching enzyme pullulanase. Additionally, amino acid homology has been described between the extracellular starch induced enzyme pullulanase (pulA) and types I, III and IV collagen . In the present study the effect of the induction of the pullulanase system on serum antibody in AS patients has been studied using bacteria grown in the presence or absence of the starch substrate pullulan. Furthermore, the levels of cross reactive antibodies between B*2705 and pulD synthetic peptides, pulA and two types of collagen (I and IV), have been measured in order to demonstrate their relevance in the development of the disease.