HLA-B27 misfolding in transgenic rats is associated with activation of the unfolded protein response.

The mechanism by which the MHC class I allele, HLA-B27, contributes to spondyloarthritis pathogenesis is unknown. In contrast to other alleles that have been examined, HLA-B27 has a tendency to form high m.w. disulfide-linked H chain complexes in the endoplasmic reticulum (ER), bind the ER chaperone BiP/Grp78, and undergo ER-associated degradation. These aberrant characteristics have provided biochemical evidence that HLA-B27 is prone to misfold. Recently, similar biochemical characteristics of HLA-B27 were reported in cells from HLA-B27/human beta2-microglobulin transgenic (HLA-B27 transgenic) rats, an animal model of spondyloarthritis, and correlated with disease susceptibility. In this study, we demonstrate that the unfolded protein response (UPR) is activated in macrophages derived from the bone marrow of HLA-B27 transgenic rats with inflammatory disease. Microarray analysis of these cells also reveals an IFN response signature. In contrast, macrophages derived from premorbid rats do not exhibit a strong UPR or evidence of IFN exposure. Activation of macrophages from premorbid HLA-B27 transgenic rats with IFN-gamma increases HLA-B27 expression and leads to UPR induction, while no UPR is seen in cells from nondisease-prone HLA-B7 transgenic or wild-type (nontransgenic) animals. This is the first demonstration, to our knowledge, that HLA-B27 misfolding is associated with ER stress that results in activation of the UPR. These observations link HLA-B27 expression with biological effects that are independent of immunological recognition, but nevertheless may play an important role in the pathogenesis of inflammatory diseases associated with this MHC class I allele.

Enhanced intracellular replication of Salmonella enteritidis in HLA-B27-expressing human monocytic cells: dependency on glutamic acid at position 45 in the B pocket of HLA-B27.

OBJECTIVE: To reveal the cause of the impaired elimination of Salmonella enteritidis in HLA-B27-transfected human monocytic cells and to study whether the B pocket of HLA-B27 contributes to these modulatory effects. METHODS: Stable U937 cell transfectants expressing HLA-A2, B27, or different forms of B27 with amino acid substitutions in the B pocket were prepared. Mock-transfected cells were prepared using the antibiotic resistance vector (pSV2neo) alone. Cells were differentiated, infected with S enteritidis, and the number of live intracellular S enteritidis organisms was determined using the colony-forming unit method. To visualize intracellular S enteritidis, the bacteria were transformed with green fluorescent protein (GFP), and studied by confocal microscopy. RESULTS: Cells expressing wild-type HLA-B27 were more permissive of intracellular replication of S enteritidis compared with mock-transfected or A2-transfected controls. Cells expressing B27 with an altered B pocket composition having either 6 amino acid substitutions (B27.A2B; substitutions H9F, T24A, E45M, I66K, C67V, and K70H) or a single substitution (B27.E45M) were no longer permissive of S enteritidis replication. In contrast, cells expressing B27 with the single substitution of F for H at position 9 (B27.H9F) retained their permissiveness. Studies using GFP-transformed S enteritidis confirmed that the increase in the amount of intracellular bacteria in B27-expressing cells was due to replication of the bacteria. CONCLUSION: Our data indicate that HLA-B27 expression modulates the host-microbe interaction that results in an impaired capacity of monocytes to resist intracellular replication of S enteritidis. The phenotype is dependent on glutamic acid at position 45 in the B pocket and, thus, may be due to properties of the B27 heavy chain that are related to this residue. The ability of HLA-B27 to confer susceptibility to Salmonella-triggered reactive arthritis may occur, at least in part, through these modulatory effects.