Yersinia enterocolitica serotype O:3 alters the expression of serologic HLA-B27 epitopes on human monocytes

Enteropathic spondyloarthritis: from diagnosis to treatment

Enteropathic arthritis (EA) is a spondyloarthritis (SpA) which occurs in patients with inflammatory bowel diseases (IBDs) and other gastrointestinal diseases. Diagnosis is generally established on the medical history and physical examination. It was, generally, made according to the European Spondyloarthropathy Study Group (ESSG) criteria. Rheumatic manifestations are the most frequent extraintestinal findings of IBD with a prevalence between 17% and 39%, and IBD is associated, less frequently, with other rheumatic disease such as rheumatoid arthritis, Sjogren syndrome, Takayasu arteritis, and fibromyalgia. Although the pathogenesis of EA has not been plainly clarified, the most popular theory supposes that joint inflammation occurs in genetically predisposed subjects with bacterial gut infections, provided an important evidence for a possible relationship between inflammation of the gut mucosa and arthritis. The management of patients with EA requires an active cooperation between the gastroenterologist and rheumatologist.

Endosomal processing for antigen presentation mediated by CD1 and Class I major histocompatibility complex: roads to display or destruction

The presentation of antigen in a form that can be recognized by T lymphocytes of the immune system requires antigen processing and association of antigen-derived fragments with molecules encoded by the major histocompatibility complex (MHC) locus or by the CD1 locus. Much emphasis on antigen processing and presentation in the last decades has focused on what we consider 'conventional routes' of antigen processing and presentation, whereby extracellular antigens are processed for presentation via Class II MHC complexes and cytosolic antigens are presented as peptide-Class I MHC complexes. We here highlight two other pathways in myeloid dendritic cells, those of lipid antigen presentation in association with CD1 and of peptide cross-presentation via Class I MHC complexes. Some pathogens evade immune recognition through inhibition of antigen presentation of phagosomal origin. Deviations in endosomal antigen processing and presentation are also seen in individuals suffering from glycosphingolipid lysosomal lipid storage diseases. We summarize recent developments in the endosomal antigen processing and presentation pathway, for display as lipid-CD1 complexes to natural killer T cells and as peptide-Class I MHC complexes to CD8 T cells.

Yersinia EnterocoliticaO:3 Isolated from Patients with or without Reactive Arthritis Induces Polyclonal Activation of B Cells and Autoantibody ProductionIn Vivo

The mechanisms by which arthritis-provoking pathogens such as Yersinia enterocolitica interact with the human immune system to produce inflammatory synovitis are not well known. One of the immunomodulating mechanisms used against these pathogens is the polyclonal activation of lymphocytes. In this study, we investigated the extent of the B-lymphocyte activation induced in mice by a strain of Y. enterocolitica O:3 (FCF 526) isolated from a patient with arthritis, and compared it with two other strains, a virulent one (FCF 397[+]) isolated from a patient without arthritis and its plasmidless isogenic pair (FCF397[-]). Also we investigated the production of autoantibodies in mice infected with these different strains. SPF Swiss mice were infected intravenously with a suspension of Y. enterocolitica. Spleen cells were taken on days 7, 14, 21 and 28 after infection and the number of cells secreting nonspecific and specific antibodies of IgG1, IgG2a, IgG2b, IgG3, IgM and IgA isotypes were determined by the ELISPOT technique. The presence of autoantibodies in mouse serum was investigated by the dot-blot assay. The pattern of infection of the three bacterial strains were almost the same. We observed a general increase in the number of nonspecific Ig-secreting cells with all three strains, and the greatest increases observed were in the IgG2a and IgG3 isotypes. Only a small fraction of the immunoglobulins detected were antibacterial, suggesting that the rest resulted from polyclonal B cell activation. The strain isolated from the patient with arthritis (FCF526) induced the greatest production of autoantibodies, coinciding with the period in which the greatest activation of nonspecific B lymphocytes was seen. There were no signs of arthritis or inflammation in the joints of the infected animals. Based on our results, we were unable to determine whether there is an association between the arthritogenic capability of Y. enterocolitica and polyclonal activation of B cells.

Y. enterocolitica translocated Yops impair stimulation of T-cells by antigen presenting cells

As T helper cells play a crucial role in the defense of the mouse immune system against Yersinia enterocolitica, an effective subversion strategy for the pathogen would be the inhibition of T-cell activation. In this study, we investigated whether Y. enterocolitica impairs this process on the level of antigen presentation. For this purpose, we used T-cells to measure the antigen presentation capacity of dendritic cells after they had been incubated with different types of Yersinia mutants. We could show that Y. enterocolitica impairs the processing of antigens by dendritic cells, that this effect is dependent on factors translocated by the pathogenicity-plasmid-encoded type III secretion system and that the most important factor appears to be YopP. The YopP effect is partly mediated by the killing of APCs, but in addition to this there appears to be an alternative way of action that results in the inhibition of antigen processing. The YopP effect is not mediated by soluble factors. In contrast to antigen processing, antigen presentation was only weakly affected by pathogenicity plasmid encoded factors in dendritic cells, but obviously in A20.J B-cells.

HLA-B27-associated reactive arthritis: pathogenetic and clinical considerations

Current evidence supports the concept that reactive arthritis (ReA) is an immune-mediated synovitis resulting from slow bacterial infections and showing intra-articular persistence of viable, non-culturable bacteria and/or immunogenetic bacterial antigens synthesized by metabolically active bacteria residing in the joint and/or elsewhere in the body. The mechanisms that lead to the development of ReA are complex and basically involve an interaction between an arthritogenic agent and a predisposed host. The way in which a host accommodates to invasive facultative intracellular bacteria is the key to the development of ReA. The details of the molecular pathways that explain the articular and extra-articular manifestations of the disease are still under investigation. Several studies have been done to gain a better understanding of the pathogenesis of ReA; these constitute the basis for a more rational therapeutic approach to this disease.

Identification by mass spectrometry of CD8(+)-T-cell Mycobacterium tuberculosis epitopes within the Rv0341 gene product

Identification of Mycobacterium tuberculosis proteins that can provide immunological protection against tuberculosis is essential for the development of a more effective vaccine. To identify new vaccine targets, we have used immunoaffinity chromatography to isolate class I HLA-A*0201-peptide complexes from M. tuberculosis-infected cells and sequenced the isolated peptides by mass spectrometry. From this material, we have identified three peptides derived from a single M. tuberculosis protein that is encoded by the M. tuberculosis Rv0341 gene. Although no known protein encoded by the Rv0341 gene has been described, it is predicted to give rise to a 479-amino-acid protein with a molecular mass of 43.9 kDa. The three peptides identified are all nested and were found to be antigenic, in that they were capable of inducing peptide-specific, CD8(+) T cells from healthy blood donors in vitro and capable of recognizing and lysing M. tuberculosis-infected dendritic cells. This methodology provides a powerful tool for the identification of M. tuberculosis proteins that can be evaluated as potential vaccine candidates.

New pathogenic hypotheses for spondyloarthropathies

Pathogenic hypotheses for spondyloarthropathies are evolving. Several candidates have been added to the list of inciting microorganisms, and genes other than HLA-B27 are under scrutiny. Above all, the chiefly 'immunological' theory of spondyloarthropathies incriminating a cross-reaction between self-proteins and bacterial peptides is giving way to a more 'microbiological' concept in which latent bacteria residing within macrophagic or dendritic cells undergo reactivation through a process facilitated by HLA-B27. This molecule is prone to misfolding, which decreases the presentation of bacterial peptides to the immune system and stimulates the Nf-KB inflammation pathway within infected macrophages and/or dendritic cells. Migration of these infected cells from the mucous membranes to the tissues targeted by spondyloarthropathies, particularly the bone marrow located near entheses, may facilitate transient reactivation of dormant intracellular bacteria by creating a favorable cytokine environment. This environment may include high levels of TGFbeta and IL-10, noted also at other sites that enjoy immune privilege, such as the eye. The reactivation may be stopped by a local response of CD4+ and/or CD8+ T cells at the expense of local inflammation responsible for clinical manifestations. This scenario seems consistent with results from studies of murine models transgenic for the HLA-B27 antigen: exposure to bacteria is necessary to the development of spondyloarthropathy, but the disease occurs even when only the heavy chain of HLA-B27 is present (i.e., beta2-microgloblin is not indispensable). Improved understanding of the mechanisms that confer to some bacterial strains a strong potential for persisting within cells, including macrophagic cells, may open the way toward new treatment approaches capable of complementing antagonists of TNF-alpha and other monokines, which merely suspend the disease process, and antibiotic therapy, which fails to kill dormant bacteria located within cells. Pathogenic hypotheses for spondyloarthropathies are evolving. This review presents the most recent concepts. These concepts have not all received confirmation from experimental data. However, the high degree of consistency among them prompted us to consolidate them into a single picture. Although this approach may yield a motley composite of fact and speculation, it may open up new avenues of thought for rheumatologists interested in the links between chronic intracellular infections and inflammatory joint disease.

HLA-B27 expression does not modulate intracellular Chlamydia trachomatis infection of cell lines

Chlamydia trachomatis is an obligate intracellular pathogen. Infection of susceptible individuals with this bacterium can trigger the development of reactive arthritis, an acute inflammation that is associated with the expression of the class I major histocompatibility antigen, HLA-B27. Other facultative intracellular pathogens, such as Yersinia and Salmonella spp., are also known triggers of reactive arthritis. Previous studies report conflicting results concerning whether the presence of HLA-B27 modulates the infection of cells with these enteric pathogens. In the present study, we have examined whether the expression of HLA-B27 can influence the infection of cell lines with C. trachomatis and also whether the replication of these bacteria is altered in HLA-B27-expressing cell lines. To do this, we have used a sensitive flow cytometric approach. We fixed and permeabilized cells and used fluorescein isothiocyanate-conjugated monoclonal antibody specific for chlamydia lipopolysaccharide to detect intracellular bacteria. The staining pattern obtained closely resembled the intracellular life cycle of chlamydia, with the appearance of brightly staining cells correlating to the microscopic detection of mature inclusion bodies. Moreover, since the percentage of cells that stained with the antibody was proportional to the infectious inoculum used, we were able to use the technique to quantitate the number of infectious organisms recoverable from infected cell lines. An important component of our study was the use of heparin to prevent reinfection of cells and thus enable the infection to be followed from a discrete time point. Our results suggest that HLA-B27 influences neither the infection nor replication of C. trachomatis serovar L2 within cell lines. Consequently, the role of HLA-B27 in the pathogenesis of reactive arthritis may lie downstream of the invasion and replication stages of the triggering pathogenic infection.

HLA-B27 and immunogenetics of spondyloarthropathies

The arthritogenic peptide hypothesis has inspired research aimed at defining the peptide-presenting properties of HLA-B27 subtypes and their relation to ankylosing spondylitis. Various studies have shed new light on the influence of HLA-B27 polymorphism in modulating peptide binding and T-cell antigen presentation. Moreover, multiple factors along the antigen processing-loading pathway, including tapasin, contribute to shaping the HLA-B27 repertoire. Other studies have revealed significant peptide-binding similarities between HLA-B27 and subtypes of HLA-B39, supporting a role of this antigen in spondyloarthropathy. A putative pathogenetic role of the HLA-B27 heavy chain, initially suggested from studies in transgenic mice, is claimed on the basis of novel, yet circumstantial, evidence concerning an apparently unusual capacity of the heavy chain to form stable homodimers or misfold after biosynthesis. Finally, it appears that arthritogenic infections might downregulate HLA-B27 expression, favoring bacterial survival. The specificity and mechanism of this phenomenon are yet to be defined.

Bacterial modulation of antigen processing and presentation

This review examines the mechanisms by which bacteria influence the antigenic processing of endogenous and exogenous antigens presented by class I, class II, and nonclassical MHC molecules. Consequent effects on presentation of bacterial antigens, the ability of bacteria to evade host defences, and the potential induction of autoimmunity are discussed.

A molecular insight on the association of HLA-B27 with spondyloarthropathies

This article focuses on molecular studies concerning HLA-B27 and their relevance for its pathogenetic role in spondyloarthropathy. The peptide binding and T-cell antigen presenting properties of HLA-B27 are discussed, mainly in connection with differential subtype association to ankylosing spondylitis. Molecular studies in transgenic rodents are also considered, with an emphasis on their relevance to the various pathogenetic mechanisms. Recent studies on the putative role of HLA-B27 in bacteria-host interactions are also discussed, as they suggest another level of implication of HLA-B27 in disease whose molecular basis is obscure.

Enterobacterial infection modulates major histocompatibility complex class I expression on mononuclear cells

Major histocompatibility complex (MHC) class I expression is reduced in several viral infections, but it is not known whether the same happens during infections caused by intracellular enterobacteria. In this study, the expression of MHC class I antigens on peripheral blood mononuclear cells (PBMC) from 16 patients with Salmonella, Yersinia, or Klebsiella infection was investigated. During or after the acute infection, the expression of MHC class I antigens was markedly decreased in eight patients, all with genotype HLA-B27, and six out of eight with reactive arthritis (ReA). A significant decrease of monomorphic MHC class I was found in three patients, of HLA-B27 in eight (P<0.05) and of HLA-A2 in two. However, patients negative for the HLA-B27 genotype, or healthy HLA-B27-positive individuals, did not have a significant decrease of MHC class I antigens. During the decreased expression on the cell surface, intracellular retention of MHC class I antigens was observed, whereas HLA-B27 mRNA levels did not vary significantly. This is the first evidence that enterobacterial infection may down-regulate expression of MHC class I molecules in vivo and that down-regulation is predominant in patients with the HLA-B27 genotype.

Ankylosing spondylitis in monozygotic twins: studies on immunological parameters

OBJECTIVE

To examine immunological parameters that might explain disease discordance in monozygotic twin pairs with ankylosing spondylitis (AS).

METHODS

11 monozygotic twin pairs (nine with AS, two with undifferentiated spondyloarthropathy) were investigated. The peripheral T cell receptor Vbeta repertoire was investigated using FACS analysis and 14 different Vbeta antibodies. In addition serum samples were tested for antibodies to Klebsiella pneumoniae, Streptococcus pyogenes, Candida albicans, Proteus mirabilis, and Escherichia coli. Peripheral blood lymphocyte reactivity against a number of bacteria was investigated by interferon gamma ELISPOT assays.

RESULTS

Twins suffering from AS showed cellular hyporeactivity against K pneumoniae, S pyogenes, C albicans in the ELISPOT assays compared with healthy twins. In contrast with the antibody data, where no significant differences were observed between the two groups, AS concordant twins showed the most pronounced differences in their Vbeta repertoire on CD4+ and CD8+ lymphocytes.

CONCLUSIONS

Cellular hyporeactivity of peripheral blood cells to bacterial antigens might reflect defective T cell responses allowing bacterial antigens to persist in diseased patients. There are probably other environmental factors that influence disease concordance.

Immunogenetics, HLA-B27 and spondyloarthropathies

HLA-B27 is the strongest HLA molecule associated with a disease. However, the reason only a small fraction of HLA-B27 positive individuals develop spondyloarthropathies is still unknown. Recent advances in genetics support the fact that additional genetic factors influence the disease and that the environmental factors may be ubiquitous. The mechanism of association of HLA-B27 and disease remains unknown, but recent studies reveal some peculiar properties of accessory molecules in antigen presentation of B27. Furthermore, research has focused on the analysis of HLA-B27-restricted processing and presentation of a bacteria-derived peptide as playing a key role in the development of spondyloarthropathy. Other studies support a more complex interaction between bacteria and HLA-B27 and suggest that other roles unrelated to antigen presentation might contribute to the development of SpA.

Cutting Edge: HLA-B27 Can Form a Novel β2-Microglobulin-Free Heavy Chain Homodimer Structure

HLA-B27 has a striking association with inflammatory arthritis. We show that free HLA-B27 heavy chains can form a disulfide-bonded homodimer, dependent on residue Cys67 in their extracellular α1 domain. Despite the absence of β2-microglobulin, HLA-B27 heavy chain homodimers (termed HC-B27) were stabilized by a known peptide epitope. HC-B27 complexes were recognized by the conformation-specific Ab W6/32, but not the ME1 Ab. Surface labeling and immunoprecipitation demonstrated the presence of similar W6/32-reactive free heavy chains at the surface of HLA-B27-transfected T2 cells. HC-B27 homodimer formation might explain the ability of HLA-B27 to induce spondyloarthropathy in β2-microglobulin-deficient mice.

Yersinia enterocolitica: overview and epidemiologic correlates

Yersinia enterocolitica comprises both pathogenic and nonpathogenic members. Distinguished by biogrouping, serogrouping, and ecological distribution, commonly occurring pathogenic serobiogroups, e.g., O:3/4; O:5,27/2; O:8/1b; O:9/2, possess both chromosomal and plasmid-mediated virulence traits. Studies have revealed several (oral, blood transfusion) modes of acquisition, elucidated the putative role of chromosomal and plasmid-encoded virulence factors in the pathogenesis of human infection, and have identified major animal reservoirs, e.g., the pig. Diagnosis has been refined though use of selective media, monoclonal antibodies directed against outer membrane proteins, and of purified yersiniae outer membrane proteins for antibody detection. Epidemiological investigations of foodborne outbreaks have been advanced through the use of molecular biology techniques such as ribotyping and pulsed-field gel electrophoresis.

The role of HLA-B27 polymorphism and molecular mimicry in spondylarthropathy

Ankylosing spondylitis (AS), reactive arthritis (ReA) and other related spondyloarthropathies (SpAs) are characterized by a strong association with the major histocompatibility complex allele HLA-B27. Experimental evidence from humans and transgenic rodents suggests that HLA-B27 is itself involved in the pathogenesis of SpA. Population and peptide-specificity analysis of HLA-B27 suggest it has a pathogenic function related to antigen presentation. Putative roles for infectious agents have been proposed in ReA and suggested in AS. However, the mechanism by which HLA-B27 and bacteria interact to induce arthritis is not clear. Molecular mimicry between bacterial epitopes that cross-react with self-B27 peptides is the most persuasive explanation for the pathogenesis of SpA. The experimental studies reviewed here have greatly increased our knowledge of the structure, function and disease association of HLA-B27.

Pathogenesis of human leukocyte antigen B27-positive arthritis. Information from clinical materials

In the spondyloarthropathies human leukocyte antigen (HLA) B27 confers a strong genetic predisposition to the development and to the chronicity of disease after extra-articular infection with certain gram-negative bacteria. The close relationships between infection, HLA-B27, other genetic factors, and the host immune system, however, still are unexplained. HLA-B27-positive arthritis continues to be an area of intensive investigation in basic and clinical research. New animal models with HLA-B27 transgenic mice and rats, as well as recent developments in understanding the processes involved in signal transduction, cytokine production, and human T-lymphocyte activation, contribute to the development of new pathogenic models of the spondyloarthropathies. This article summarizes the current concepts of the cause and pathogenesis of the spondyloarthropathies resulting from studies of clinical materials. The host-microbial interplay in human disease, namely in bacteria-induced reactive arthritis, may eludicate principle disease mechanisms in acute disease and in the development of chronic autoimmune arthritis or ankylosing spondylitis.

Aetiological agents: their molecular biology and phagocyte-host interaction

Inflammatory joint disease can develop following an extra-articular infection. The term reactive arthritis was coined in order to differentiate this arthritis, which is often characterized by lack of culturable organisms in the joint, from septic arthritides. Bacteria known to trigger reactive arthritis include Campylobacter, Chlamydia, Salmonella, Shigella and Yersinia. Demonstration of bacteria or bacterial macromolecules in the joint has elicited the idea that reactive arthritis is a sterile process induced and maintained by antigenic material in the synovium. Continued synthesis of antigens to maintain synovial inflammation probably requires establishment of persistent bacterial infection in the joint, or at the primary site of infection. In the case of Chlamydia trachomatis, viable, metabolically-active organisms have been demonstrated to exist for extended periods in the joints of patients with reactive arthritis. In this chapter, we review the aetiological agents, and their molecular biology and phagocyte-host interactions, that are involved in reactive arthritis and spondylarthropathy.

Invasion by Salmonella typhimurium induces increased expression of the LMP, MECL, and PA28 proteasome genes and changes in the peptide repertoire of HLA-B27

We have analyzed proteasomal adaptation and associated changes in the B27-bound peptide repertoire in response to cellular invasion with Salmonella. The peptide repertoire of HLA-B27 complexes was analyzed by two different methods: (i) high-pressure liquid chromatography (HPLC) profiles of newly synthesized peptides eluted from B27 following metabolic labeling with arginine and (ii) reactivities with two B27 monoclonal antibodies, Ye-2 and B27.M2, sensitive to peptide-induced conformational changes. LMP, MECL, and PA28 expression was analyzed by reverse transcription-PCR (RT-PCR) of mRNA and by Western blot analysis for LMP2. Invasion of HLA-B27-transfected HeLa cells by Salmonella typhimurium induced significant changes in the reactivities of HLA-B27 with these two antibodies, which was accompanied by significant quantitative and qualitative changes in the HPLC profile of peptides eluted from HLA-B27. We also observed increases in the RT-PCR values for the LMP2, LMP7, and MECL proteasome subunit genes, as well as the proteasomal activator PA28alpha and -beta genes, and increased expression of the LMP2 protein by Western blotting. Upregulation of LMP2, but not LMP7, gene expression showed a close correlation with the changes in antibody reactivities observed upon bacterial invasion. We observed similar changes in reactivity with the Ye-2 or the B27.M2 antibody of lymphoblastoid cells upon gamma interferon treatment, which significantly correlated with the increased RT-PCR values for the LMP2 gene. This was accompanied by consistent HPLC profile changes for eluted peptides. Thus, Salmonella invasion leads to serologically recognizable changes in the B27-bound peptide repertoire, which may include peptides of host origin potentially through modulation of proteasome LMP2 subunit expression and, as a consequence, proteasomal activities.

Infectious agents as triggers of reactive arthritis

Reactive arthritis was originally defined as a sterile joint inflammation after infection elsewhere in the body, but this view has been challenged in the past decade since different antigens and DNA and RNA of various triggering microbes have been shown to exist at the sites of inflammation in the joints. It has been suggested that microbial antigens, or intact pathogens, are important for the pathogenesis of reactive arthritis, at least in the early phase of the disease, but the exact mechanism of how the pathogens contribute to the development of this usually self-limiting polyarthritis has not been discovered. This article reviews the theories on the role of infectious agents as triggers of reactive arthritis.

Alteration of HLA-B27 peptide presentation after infection of transfected murine L cells by Shigella flexneri

Shigella flexneri is a triggering agent for reactive arthritis in HLA-B27-susceptible individuals. Considering the intracellular multiplication of bacteria, it seems likely that bacterial peptides may be presented by the major histocompatibility complex (MHC) class I pathway. To examine this hypothesis, we infected HLA-B*2705- and/or human beta2-microglobulin-transfected murine L-cell lines with M90T, an invasive strain of S. flexneri. Bacterial infection induced no detectable modifications in the biosynthesis and expression level of HLA-B27, as assessed by immunoprecipitation, Northern blot analysis, and flow cytometry. Using confocal microscopy, we observed that bacterial infection induced a clustering of HLA-B27 molecules during macropinocytosis and before bacterial dissemination from cell to cell. Peptides naturally bound to HLA-B27 molecules were acid eluted from infected cells and separated by high-performance liquid chromatography. Major differences were observed in high-performance liquid chromatography profiles and in the nature of peptides presented following bacterial infection. Although most of the antigens presented were not accessed by Edman degradation, we obtained two sequences partially homologous to bacterial proteins. These peptides lacked the major HLA-B27 peptide anchor (Arg) at position 2, and one had an unusual length of 14 amino acids. These data suggest that alterations in the peptide presentation by HLA-B27 occur during infection, which could be relevant to the pathogenesis of HLA-B27-related arthritis.

The pathogenetic role of HLA-B27 in chronic arthritis

Population and peptide specificity analyses and studies in transgenic rodents support a role of HLA-B27 as an antigen-presenting molecule in spondyloarthropathy. The interplay between HLA-B27 and arthritogenic bacteria on infected cells suggests that HLA-B27 might also influence disease by other mechanisms. Recent genetic advances promise the identification of additional susceptibility genes.

HLA-B27 modulates the survival of Salmonella enteritidis in transfected L cells, possibly by impaired nitric oxide production

Reactive arthritis is triggered by certain microbes that cause primary infections mainly on the gastrointestinal or urogenital mucosa. The disease is strongly associated with HLA-B27. Long persistence of causative microbes or their structures in the body has been thought to have an important role in the pathogenesis of reactive arthritis. This suggests that the elimination of the microbes causing reactive arthritis is ineffective or disturbed in HLA-B27-positive individuals developing this complication. We examined the role of the HLA-B27 antigen in microbe-host interaction in vitro by monitoring the invasion and intracellular survival of Salmonella enteritidis in mouse fibroblasts transfected with HLA-B27, HLA-B7, or beta2-microglobulin only. S. enteritidis invaded into all the three transfectants with the same efficiency. However, at 6 and 10 days after incubation, there were more living intracellular Salmonella organisms in HLA-B27 transfectants than in the other transfected cell lines (P < 0.05), suggesting that the bactericidal effect is impaired in these cells. Impaired NO production in HLA-B27-transfected cells was indicated as a possible mechanism, since the amount of nitrite in the supernatants of the Salmonella-infected HLA-B27-transfected cells was smaller than that in the supernatants of the Salmonella-infected HLA-B7- or beta2-microglobulin-transfected cells (P < 0.001). The inhibition of NO synthesis by N-monomethyl-L-arginine resulted in impaired elimination of Salmonella also in HLA-B7and beta2-microglobulin-transfected cells. The inverse correlation between intracellular survival of Salmonella and the amount of nitrite detected in culture supernatants supports the hypothesis that the L-arginine-dependent NO pathway plays an important role in the murine fibroblast response against Salmonella. We suggest that a major histocompatibility complex class I antigen, HLA-B27, may contribute to the intracellular persistence of Salmonella by a mechanism which involves the NO pathway.