Abstract
A tissue antigen, HLA-B27, is strongly associated with a group of rheumatic diseases called spondyloarthritides. Despite the intensive research, the exact role of HLA-B27 in the pathogenesis of these diseases is still unclear. Here we studied whether HLA-B27 modulates the phosphorylation of signal transducer and activator of transcription 1 (STAT-1) serine 727 residue and the localization of STAT-1 in Salmonella-infected human monocytic cells. In addition, we studied the role of signaling molecule double-stranded RNA activated protein kinase (PKR) in these modulatory effects. U937 human monocytic cell transfectants stably expressing wild type HLA-B27 or mutated HLA-B27 heavy chains with amino acid substitutions in the B pocket were prepared. The PMA-differentiated cells were infected with S. enteritidis. Western blotting was used to detect the phosphorylation of STAT-1, and to visualize the localization of STAT-1 in the cells confocal microscopy was used. Specific inhibitors were employed to study the role of PKR in STAT-1 phosphorylation. We discovered that the phosphorylation of STAT-1 serine 727 is prolonged in cells expressing misfolding forms of HLA-B27 after S. enteritidis infection, whereas in mock cells and in cells expressing mutated, non-misfolding HLA-B27 the phosphorylation of serine 727 is transient. Interestingly, STAT-1 serine 727 phosphorylation is partly dependent on PKR. In addition, more STAT-1 is localized in the nucleus of HLA-B27-expressing cells, even before an external trigger, when compared to mock cells. In conclusion, our results show that the phosphorylation of STAT-1 serine 727 residue is prolonged in HLA-B27-expressing monocyte-macrophage U937 cells after bacterial infection. This is of interest since the phosphorylation of serine 727 on STAT-1 is suggested to contribute to macrophage activation and promote inflammatory responses. Therefore, our results provide a mechanism which explains how the expression of an HLA-B27 molecule can impact the course of Salmonella infection and reactive arthritis.
Highlights
An MHC class I tissue antigen, HLA-B27, is strongly associated with a group of rheumatic diseases called spondyloarthritides (SpA), including an acute inflammatory joint disease reactive arthritis (ReA) [1,2]
We aimed to examine whether the phosphorylation of serine 727 residue of signal transducer and activator of transcription 1 (STAT-1) is altered in U937 cells expressing HLA-B27 upon Salmonella enteritidis infection and whether the phosphorylation is dependent on PKR activity
We studied the localization of STAT-1 in U937 cells
Summary
An MHC class I tissue antigen, HLA-B27, is strongly associated with a group of rheumatic diseases called spondyloarthritides (SpA), including an acute inflammatory joint disease reactive arthritis (ReA) [1,2]. There is evidence that triggering bacteria or parts of them can persist for an unusually long time in patients suffering from ReA [4,5,6,7]. Since most ReA patients are HLA-B27 positive, it is proposed that interaction between host cells and ReA-triggering bacteria might be altered [8]. Recent studies suggest that both antigen presenting and non-antigen presenting functions (e.g. dimer formation and the misfolding of HLA-B27 heavy chains [HCs] in the endoplasmic reticulum [ER]) might be involved in the pathogenesis of SpA. Recent studies suggest that both antigen presenting and non-antigen presenting functions (e.g. dimer formation and the misfolding of HLA-B27 heavy chains [HCs] in the endoplasmic reticulum [ER]) might be involved in the pathogenesis of SpA. [3,9] Results show that HLA-B27 can misfold in the ER and cause abnormal HC/b2-microglobulin complexes. [10]
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