Differential enhancement of HLA-B27 by interferon

The microbiome in HLA-B27-associated disease: implications for acute anterior uveitis and recommendations for future studies

The pathogenesis of human leukocyte antigen (HLA)-B27-associated diseases such as acute anterior uveitis (AAU) and ankylosing spondylitis (AS) remains poorly understood, though Gram-negative bacteria and subclinical bowel inflammation are strongly implicated. Accumulating evidence from animal models and clinical studies supports several hypotheses, including HLA-B27-dependent dysbiosis, altered intestinal permeability, and molecular mimicry. However, the existing literature is hampered by inadequate studies designed to establish causation or uncover the role of viruses and fungi. Moreover, the unique disease model afforded by AAU to study the gut microbiota has been neglected. This review critically evaluates the current literature and prevailing hypotheses on the link between the gut microbiota and HLA-B27-associated disease. We propose a new potential role for HLA-B27-driven altered antibody responses to gut microbiota in disease pathogenesis and outline recommendations for future well-controlled human studies, focusing on AAU.

Measurement of HLA class I expression in ankylosing spondylitis

The importance of HLA-B27 in the pathogenesis of ankylosing spondylitis is uncertain: current evidence favours a role for the B27 molecule itself. The possibility that quantitative differences in HLA-B27 expression may exist between patients with ankylosing spondylitis, family members, and control subjects positive for B27 was examined using appropriate monoclonal antibodies, flow cytometry, and a 'model lymphocyte' coated with a known number of mouse immunoglobulin binding sites. No differences were found between the groups. HLA-A2, examined for comparison, was expressed in greater amounts than HLA-B27, but each contributed only 10-20% of the total class I antigens. Homozygotes expressed twice the amount of antigen expressed by heterozygotes. Synovial lymphocytes expressed more class I antigens than peripheral lymphocytes.

In vivo induction of HLA molecules in patients with myeloproliferative syndrome during IFN alpha treatment

Eighteen patients with myeloproliferative syndrome (14 with chronic myeloid leukemia, four with essential thrombocytosis) were investigated for modulation of HLA antigens on peripheral blood lymphocytes, monocytes, and hematopoietic precursors during IFN alpha therapy as a sign of potentially increased immune recognition of malignant cells. After 1 month of IFN alpha therapy, an increased number of monocytes and hematopoietic precursor cells, but not of lymphocytes, expressed HLA-DQ antigens. In addition, a strong induction of HLA class-I antigens was found on both hematopoietic progenitors and normal peripheral blood mononuclear cells. With daily injections of IFN in the first month of therapy stimulation continuously increased, suggested a major effect of IFN alpha on hematopoietic progenitors with sustained enhanced expression of HLA class-I antigens during differentiation of myelomonocytic cells. HLA class-I antigen expression was consistently augmented by IFN alpha in all patients, irrespective of their hematological response.

Acute anterior uveitis and HLA-B27

Acute anterior uveitis is a common ocular disease characterized by inflammation of the iris and ciliary body. In the majority of patients presenting with an acute attack of anterior uveitis, the only clues to the pathogenesis of this disease are its close association with the genetic marker HLA-B27 and the likely triggering role of a variety of gram negative bacteria. HLA-B27 acute anterior uveitis appears to be a distinct clinical entity frequently associated with the seronegative arthropathies, such as ankylosing spondylitis and Reiter's syndrome. Recent advances in our understanding of the structure and function of class I HLA molecules have revealed their fundamental function in antigen presentation and this has led to a reevaluation of their role in disease predisposition.

Differential transcription inducibility by interferon of the HLA-A3 and HLA-B7 class-I genes

HLA-A3 and HLA-B7 class-I genes are differentially regulated in human T lymphoma Jurkat cells, at the transcriptional level, the expression of the HLA-B7 gene being selectively increased following alpha, beta or gamma interferon (IFN) treatment. Using a series of hybrid CAT constructs, associating HLA-A3 and HLA-B7 complete or fragmented promoters, the differential regulation was shown to be associated with 2 nucleotide differences at positions -176 and -175 in the interferon regulatory sequence (IRS) of the HLA-A3 and the HLA-B7 genes. Replacement, using site-directed mutagenesis, of the 2 thymidine in the HLA-A3-IRS by adenine and cytidine found at the same positions in the HLA-B7-IRS was sufficient to restore IFN inducibility of the HLA-A3 promoter and efficient interaction with HeLa nuclear factors. Since the same nucleotide differences are shared by all sequenced HLA-A and HLA-B class-I genes, the differential induction by IFN of the transcription of the HLA-A3 and B7 genes might be a general locus-related property.

Dysfunction of HLA-B27

Every HLA antigen, as defined by the WHO-HLA Nomenclature, is unique. The major function of these molecules is to present antigen-peptides to the T-cell receptor, thereby contributing to the immunological defence mechanism. This function is regulated for each MHC antigen by its unique structure, with the peptide-binding pockets of the three-dimensional groove of the corresponding molecules playing the critical role. However, HLA-B27 is special by virtue of its disease association(s). Various aspects which might provide an explanation for--or at least a clue to an understanding of the specific role of--B27 in its disease associations are reviewed. Since it appears that there are no published experimental data which would support either of the alternative hypothetical possibilities, the bulk of current theories must therefore be purely speculative. The only lead to a better understanding of the function of B27 in disease associations is the postinfectious reactive arthritis. If it is the B27 molecule itself which is involved, further in vivo work on B27 transgenic animals might help solve this problem with its numerous unknown factors.