Behçet's disease, the Silk Road and HLA-B51: historical and geographical perspectives

Behçet's disease (BD), also known as the Silk Road disease, is a blinding inflammatory disorder of young adults found predominantly between the Mediterranean basin and the Orient, and is strongly associated with the major histocompatibility complex (MHC) antigen HLA-B51. In this article we review the history of Behçet's disease since its first description by Hippocrates, the development of the trading routes collectively known as the Silk Road and the effect of population movement on the distribution of HLA-B51. The global distribution of this antigen among healthy control populations bears a striking similarity both to the ancient trading routes and the distribution of Behçet's disease, suggesting a genetic risk that migrated in parallel with population movement between the Mediterranean and Asia. However, certain indigenous Amerindian peoples have a high prevalence of HLA-B51 but no reported cases of BD. Furthermore, a clear genealogical relationship exists between eastern, but not central, Siberian populations with the Amerindians. Since a high level of recombination within the MHC is known to have occurred in these eastern populations before their migration into Beringia, we suggest that disruption of genetic loci in linkage disequilibria with HLA-B51 may be one reason for the absence of disease in these high HLA-B51-bearing populations. However, a contributory influence of environmental factors is not excluded by this data, and the wide variation that exists in relative risk of HLA-B51 even within Europe would support other non-genetic risk factors on the Silk Road which may be absent, or non-contributory to disease, in the Americas.

Serum prolactin levels in Behçet's Syndrome

Since prolactin (PRL) has been implicated as playing a role in the pathogenesis of certain autoimmune diseases and since Behcet's Syndrome (BS) is a unique systemic vasculitis, we investigated serum PRL levels in patients with BS. We found that mean PRL levels in patients with clinically active BS, were not significantly higher than patients with clinically inactive BS and healthy controls. This finding may be regarded as evidence that a contribution of hyperprolactinemia to the aetiopathogenesis of BS seems unlikely.

[Behçet disease. 162 cases]

We report 162 cases of Behçet's disease, seen at the Internal Medicine Unit of Ibn Sina Hospital at Rabat, between January 1983 and June 1996. This series concerned 124 men et 38 women, Moroccans, whose mean age at first hospitalization was 32 years, and mean age at disease onset was 26 years. Diagnosis of Behçet's disease was established on Mason and Barnes and/or International Study Group for Behçet's Disease criteria. There was a muco-cutaneous involvement in 100%, eye involvement in 50%, joint involvement in 45%, neurological symptoms in 43.2%, vascular involvement in 62.34%, thoracic involvement symptoms in 13%, digestive involvement in 8 cases, cardiac involvement in 5 patients, long term fever in 5 patients, and one case of amyloidosis. We compared our results to the literature and we noticed that our series had an elevated frequency of neurological involvement, mostly benign intra-cranial hypertension and deep vein thrombosis. We also found that gut involvement was particularly low.

[Behçet's disease]

Clinicopathologically, Behçet's disease is characterized by neutrophil and platlet hyperfunction. Lesions are formed presumably because neutrophils infiltrating the affected tissue release active oxygen and lysosomal enzymes in large amounts. The mechanisms of neutrophil hyperfunction was obscure, but in recent years attention has focused on the effects of cytokines released by mononuclear cells. As more has been learned about the etiopathogenesis of Behçet's disease, novel anti-inflammatory drugs, immunosuppressants, anti-thrombotic agents, and anticoagulants have helped to achieve remarkable progress of treatment. Particularly notable is the effect on severe uveitis of cyclosporine, which is more effective than colchicine and conventional immunosuppressants. Low dose weekly methotrexate therapy might have a beneficial effect in the treatment of patients with neuro-Behçet's disease.

Behçet's Disease

OBJECTIVES

To review the new data on the epidemiology, etiopathogenesis, clinicolaboratory spectrum, prognosis, and treatments of Behçet's disease (BD).

METHODS

The information concerning the etiopathogenesis of the disease is divided into infection, immune, and genetic factors. The clinical features of the disease are discussed according to the organ or system involved. Treatment is described as general, local, and systemic.

RESULTS

BD is a multisystem vasculitis with recurrent symptoms. It affects mainly people living around the Mediterranean basin and in Japan. The mean age at onset is the third decade. Children are rarely affected, and few neonatal cases have been reported. In large series of patients, men predominate over women. Infectious agents, immune mechanisms, and genetic factors are implicated in the etiopathogenesis of the disease, which remains to be elucidated. The pathology of the lesions consists of widespread vasculitis. Eyes, skin, joints, the oral cavity, blood vessels, and central nervous system are usually involved, although less frequently the heart, lung, kidney, genital system, and gastrointestinal tract may be affected. The prognosis of the disease has been improved because of early diagnosis and suitable treatment. Local remedies and systemic administration of colchicine, corticosteroids, immunosuppressives, and other agents have been applied.

CONCLUSION

BD is a widespread vasculitis affecting young people and involving concurrently or consecutively nearly all organs and systems. Treatment results in better prognosis even when vital organs are involved.

[Aphthae and aphthous lesions of the mouth mucosa]

Recurrent aphthae (or aphthoses) are the most frequent inflammatory lesions of the oral mucosa. Aphthae can clearly be defined, but may also be barely discernible from aphthoid erosions and ulcers of heterogeneous origin. Although a subject of molecular research, the etiopathogenesis of oral aphthoses is still unknown. However, the clinical differentiation of a "benign" type of aphthosis from a "pernicious" one, such as Behçet's disease (BD), is a diagnostic matter of great significance. BD has been found clinically in increasing numbers in central Europe and represents an (auto-)hyperimmune multisystem life-threatening vasculitis. In this disorder possible damage can occur to many visceral organs and/or the cerebrospinal system. The prognosis of BD may depend on the changing involvement of very different organs, as well as the early recognition of the disease per se. The most prominent feature is a multilesional aphthous stomatitis that is almost never absent during acute episodes of BD. Hence, the occurrence of this peculiar type of aphthosis strongly indicates an active BD. The plethora of other aphthoid lesions can be challenging to the diagnostic competence of all physicians and may confuse the correct recognition of the severity of the disease present. This review particularly details the wide range of diagnostic, therapeutic and prognostic aspects of the various oral aphthoses.

Etiopathology of Behçet's disease: immunological aspects

Behçet's disease is recognized as a systemic inflammatory disease of unknown etiology. The disease has a chronic course with periodic exacerbations and progressive deterioration. Previous reports have shown at least three major pathophysiologic changes in Behçet's disease; excessive functions of neutrophils, vasculitis with endothelial injuries, and autoimmune responses. Many reports suggested that immunological abnormalities and neutrophil hyperfunction may be involved in the etiology and the pathophysiology of this disease. HLA-B51 molecules by themselves may be responsible, in part, for neutrophil hyperfunction in Behçet's disease. T cells in this disease proliferated vigorously in response to a specific peptide of human heat shock protein (hsp) 60 in an antigen-specific fashion. T cells reactive with self-peptides produced Th1-like proinflammatory and/or inflammatory cytokines. This leads to tissue injury, possibly via delayed-type hypersensitivity reaction, macrophage activation, and activation and/or recruitment of neutrophils. These data shed new light on the autoimmune nature of Behçet's disease; molecular mimicry mechanisms may induce and/or exacerbate Behçet's disease by bacterial antigens that have activated T cells which are reactive with self-peptide(s) of hsp. This would lead to positive selection of autoreactive T cells in this disease.

Streptococcal infection in the pathogenesis of Behçet's disease and clinical effects of minocycline on the disease symptoms

Although the precise pathoetiology of Behçet's disease (BD) remains obscure, patients with BD have a high incidence of chronic infectious foci, indicating an enhanced susceptibility to chronic tonsillitis, and dental caries. Sometimes, clinical symptoms appear after treatment of these foci in BD patients. It is believed that BD might be related to an allergic reaction to a bacterial infection in view of the many clinical symptoms, especially the presence of aphthous and genital ulcerations. An attempt to obtain cutaneous responses to bacterial antigens has been carried out using various vaccines developed from bacteria isolated from the ulcerative lesions and oral cavities of BD patients. BD patients often show intense hypersensitivity to various strains of streptococci, not only by their cutaneous reactions but also by in vitro testing. In this report, we describe our previous studies on the correlation between streptococcal antigens and the pathogenesis of BD and also discuss the recent reports of other authors. The intense hypersensitivity to streptococcal antigens acquired after streptococcal infection is thought to play an important role in the appearance of symptoms in BD patients since the production of pro-inflammatory cytokines by peripheral blood mononuclear cells (PBMC) was enhanced when stimulated with streptococcal antigen in a culture system. Minocycline, an antibiotic to which certain strains of streptococci are sensitive, reduced the frequency of clinical symptoms in BD patients as well as the production of pro-inflammatory cytokines by BD-PBMC stimulated with streptococcal antigen.

Etiopathology of Behçet's disease: herpes simplex virus infection and animal model

The etiology of Behçet's disease has been proposed as being a viral, bacterial, genetic and immunological disorder. After Hulusi Behçet's viral etiological hypothesis, many investigators have tried to confirm this. Scrapings and fluids from patients were applied to subculture in chorioallantoic membranes of fertilized eggs as well as in mice and rabbits by direct intracerebral injection. Since the 1980s, in situ hybridization, blotting, and polymerase chain reaction has also been applied to identify the herpes simplex virus DNA and RNA in patients. Animal models were developed based on environmental pollutants, bacterial and human heat shock protein derived peptides and virus injection. Using these animal models separately and/or concurrently, allows for a more effective investigation into Behçet's disease.

Peripheral blood T cell expansions in patients with Behcet's disease

Behçet's disease (BD) is a chronic multisystemic inflammatory disorder characterized mainly by recurrent oral and genital aphthous ulcerations and uveitis. Etiology and pathogenesis of BD remain unknown. T cell receptor (TCR) V alpha/V beta gene product expression as well as Jbeta gene segment expression in peripheral blood of BD patients were analysed to investigate the possible role of T lymphocytes in the etiopathogenesis of BD. Flow cytometry with 12 TCR V-specific MoAbs was used for TCRV analyses. Jbeta gene segment usage by T cell populations expressing certain V betas was determined by polymerase chain reaction (PCR) technique with V beta- and C beta-specific primers, Southern blotting of PCR products, and subsequent hybridization with radiolabelled Jbeta gene segment-specific probes. Although 13 of the 23 BD patients exhibited increases in expression of one or more TCR V-gene products, only expansions among the CD4+ T cell subset were significantly more frequent in BD patients (7/23) compared with healthy controls (0/15) (P = 0.019). Six out of eight cases followed for up to 20 months had at least one expansion correlated with disease activity. A strict preference for particular Jbeta gene segments implicating clonality was apparent in all analysed T cell expansions and correlated well with disease activity. These results suggest a possible involvement of antigen-specific T lymphocytes in the pathogenesis of BD.

Triplet repeat polymorphism in the transmembrane region of the MICA gene: a strong association of six GCT repetitions with Behçet disease

A member of a novel family of the human major histocompatibility complex (MHC) class I genes termed MIC (MHC class I chain-related genes), MICA, has been recently identified near the HLA-B gene on the short arm of human chromosome 6. The predicted amino acid sequence of the MICA chain suggests that it folds similarly to typical class I chains and may have the capacity to bind peptides or other short ligands. Therefore, MICA is predicted to have a specialized function in antigen presentation or T cell recognition. During nucleotide sequence analyses of the MICA genomic clone, we found a triplet repeat microsatellite polymorphism of (GCT/AGC)n in the transmembrane (TM) region of the MICA gene. In 68 HLA homozygous B cell lines, 5 distinct alleles of this microsatellite sequence were detected. One of them contained an additional one base insertion that created a frameshift mutation resulting in a premature termination codon in the TM region. This particular allele may encode a soluble, secreted form of the MICA molecule. In addition, we have investigated this microsatellite polymorphism in 77 Japanese patients with Behcet disease, which is known to be associated with HLA-B51. The microsatellite allele consisting of 6 repetitions of GCT/AGC was present at significantly higher frequency in the patient group (Pc = 0.00055) than in a control population. Furthermore, the (GCT/AGC)6 allele was present in all B51 positive patients and in an additional 13 B51 negative patients. These results suggest the possibility of a primary association of Behcet disease with MICA rather than HLA-B.

Cerebrospinal fluid interleukin-6 in progressive Neuro-Behçet's syndrome

Central nervous system (CNS) involvement in Behçet's disease, usually called neuro-Behçet's syndrome (NB), is one of the most serious complications of the disease. The present study examined cerebrospinal fluid (CSF) interleukin-6 (IL-6) activity in patients with NB to explore its relevance to the progressive CNS disease. Paired CSF and serum specimens were obtained from 11 patients who were diagnosed as having progressive NB based on careful clinical observation and from 11 patients with active Behçet's disease but lacking progressive CNS disease. IL-6 levels in the CSF and sera were determined using IL-6-dependent murine hybridoma MH60.BSF2 cells. All 11 patients with progressive NB showed marked elevation of CSF IL-6 activity [0.18-3.90 U/ml, 1.19 +/- 1.18 (mean +/- SD), normal range, <0.010 U/ml]. In contrast, only 5 of the 11 control patients showed very modest CSF IL-6 activity below 0.10 U/ml, and CSF IL-6 was not detected in the other 6 patients. There was no difference in the serum IL-6 activities of patients with progressive NB and control patients. There was no significant correlation of CSF IL-6 activity with serum IL-6 activity, CSF cell counts, CSF total protein levels, or the CSF/serum albumin quotient. These results indicate that persistent chronic CNS inflammation, as evidenced by the enhanced production of IL-6 within the CNS, plays an important role in the pathogenesis of progressive neuropsychiatric manifestations in Behcet's disease.

Hyperreactivity of neutrophils and abnormal T cell homeostasis: a new insight for pathogenesis of Behçet's disease

Neutrophil hyperfunction of Behçet's disease is in part regulated by genetical factors, especially related to HLA-B51 genes and by immunological abnormalities. Regarding the latter points, Behçet's disease worsens with abnormal regulation by gamma delta T cells and alpha beta T cells. Indeed, our own studies and those of other laboratories suggest that human heat shock protein may be one of the triggering factors for activation of both the gamma delta T cells and alpha beta T cells. These activated T cells may produce proinflammatory and/or inflammatory cytokines, and lead to tissue injury possibly via delayed-type hypersensitivity reaction, macrophage activation, and activation and/or recruitment of neutrophils. Here we discuss the crosstalk between the immune system and neutrophils in this disease.

The role of heat shock protein, microbial and autoimmune agents in the aetiology of Behçet's disease

Investigation of the aetiology of Behçet's disease (BD) has focused predominantly on herpes simplex virus immunopathology, autoimmunity to oral mucosa or cross-reactive microbial antigens, and streptococcal infection. These aetiological factors might have a common denominator in microbial heat shock protein (HSP) which shows significant homology with the human mitochondrial HSP. Indeed, the uncommon serotypes of Streptococcus sanguis found in BD cross-react with the 65 kD HSP which also shares antigenicity with an oral mucosal antigen. T cell epitope mapping has identified 4 peptides derived from the sequence of the 65 kD HSP which stimulate specifically TCR gamma delta + lymphocytes from patients with BD. These peptides (111-125, 154-172, 219-233 and 311-325) show significant homology with the corresponding peptides derived from the human 60 kD HSP. The specific proliferative response of TCR gamma delta + lymphocytes elicited by the 4 peptides can be used as a laboratory test for the diagnosis of BD. The pathogenic significance of these peptides has been established by inducing uveitis in rats.

Arthropathy in Behçet's disease

We analyzed the incidence, clinical features, and the long-term sequelae of arthropathy in 84 patients with Behçet's disease. After a mean disease duration of 44.7 months, 59 patients (70%) had at least 1 episode of arthropathy. Of these, arthropathy was present at the time of diagnosis in the majority of patients (75%). In the remainder, arthropathy developed 4 to 80 months (mean 29) after the diagnosis of Behçet's disease. Female patients were significantly more likely to have arthropathy than male patients (89 versus 56%, p = 0.006). The pattern of involvement was oligoarticular in 75%, monoarticular in 23%, and polyarticular in 2% of these patients. The most commonly involved site was the knee, followed by the ankle and the wrist. Radiologic examination of the involved site was normal in 80% of the patients. After a mean duration of arthropathy of 43.2 months, by the American College of Rheumatology (ACR) criteria, the functional class of the arthropathy was I-II in 88%, and III-IV in 12%. In conclusion arthropathy is a fairly common manifestation in patients with Behçet's disease, but crippling arthritis is relatively rare.

Pathogenic gene responsible for the predisposition of Behçet's disease

HLA-B51 is well known to be associated with Behçet's disease (BD) in many different ethnic groups. The hypothesis may be presented that B51 molecules are primarily involved in BD development through specific antigen presentation. Furthermore, HLA-C genotyping by the polymerase chain reaction-sequence specific primers method suggests that the BD pathogenic gene is not the HLA-C gene itself but some other gene located near the HLA-B gene. Polymorphic analysis of the Tau-a microsatellite between the HLA-B and TNF genes indicates that the pathogenic gene of BD is not the HLA-B51 gene itself but other gene located around the HLA-B gene. Recent studies suggest that many novel genes exist in the region between the TNF and HLA-B or HLA-C genes such as MIC and PERB, etc. and furthermore, many unidentified new genes have been suggested to exist in this region. In this paper, the present situation of the investigations on the genetic predisposition responsible for BD was reviewed.

Pathogenesis and therapy of Behçet's disease

Behçet's disease is a relapsing-remitting systemic vasculitis characterized by oral and genital ulcers, uveitis and thrombophlebitis which can involve many organs. Although its pathogenesis is not fully understood, a possible pathogenetic model can be proposed on the basis of recent studies. Genetic factors, in particular, have been investigated and the role of the genes encoding tumor necrosis factor, transporter in antigen processing proteins and MIC (MHC class I chain related) has been emphasized. In addition, a possible polarization of T lymphocytes towards the Th1 phenotype in Behçet's disease has been suggested by recent observations in experimental uveoretinitis and by preliminary data in humans. Neutrophils may also play a role in the pathogenesis of this disease, as they are attracted by macrophage-released cytokines at the site of the lesions, and thus contribute to tissue damage and self-maintenance of inflammation. New strategies for the treatment of Behçet's disease are being devised. In particular, immunosuppressive drugs used in association or in sequence may be administered to patients with particular clinical features or very severe disease.

New perspective on Behçet's disease

There are many distinct differences between Behçet's disease of Silk Route and that of outside Silk Route; genetic factors, role of neutrophils, and severity of this disease. We have thus emphasized that we prefer the term "Behçet's syndrome" rather than "Behçet's disease". In this chapter, Behçet's disease seen along the Silk Route will be mainly discussed. HLA-B51 molecules themselves may be responsible, at least in part, for the neutrophil hyperfunction in Behçet's disease; a significant correlation was observed between the neutrophil hyperfunction and the possession of HLA-B51 phenotype, regardless of the presence of the disease, in both humans and HLA-B transgenic mice. T cells in this disease, proliferated vigorously in response to a specific peptide of human heat shock protein (HSP)-60; however, T cells from normal subjects or patients with rheumatoid arthritis, did not. This peptide has the amino acid sequence of 336-351 of human HSP-60, which is similar, but not identical to specific peptide of mycobacterial HSP-65. We have further analyzed T cell receptor (TCR) usage of HSP-responsive T cells by means of TCR V beta subfamily specific monoclonal antibodies and polymerase chain reaction and single strand conformation polymorphism-based technique. We found that T cells with specific TCR V beta subfamilies proliferated and increased in number in response to the peptide by an antigen-specific fashion. The result of recurrent exposure to the HSP may break the tolerance to self-HSP, and provoke T cell responses to self- and microbial-HSP. Such T cells produced Th1-like proinflammatory and/or inflammatory cytokines. This leads to tissue injury, possibly via delayed-type hypersensitivity reaction, macrophage activation, and activation and/or recruitment of neutrophils. Our data shed a new light on the autoimmune nature of Behçet's disease; a novel multistep molecular mimicry mechanisms may induce and/or exacerbate Behçet's disease by bacterial antigens that activate T cells previously educated by self-peptides of HSP. This would lead to positive selection of autoreactive T cells in this disease.

The place of Behçet's syndrome among the autoimmune diseases

The most popular research interest in Behçet's Syndrome (BS) is directed to immunological mechanisms. However there are many ways in which BS differs from a classic autoimmune disease. The most important differences lie in the male dominance in severe disease, lack of association with other autoimmune diseases, lack of association with HLA alleles usually seen in autoimmune diseases, lack or paucity of autoantibodies and B cell hyperfunction-especially Sjögren's syndrome- and no definite T cell hypofunction in BS. Perhaps less important points are the peculiar geographic and ethnic distribution, the peculiar clinical features and the lack of response of BS to steroids. The value of immunological data on BS will much increase of we include in each experiment patients with classic autoimmune diseases along with "pure" inflammatory conditions like gout and infectious diseases.