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- Material and methods
Behçet's disease (BD) is a multisystemic disorder of unknown etiology characterized by chronic relapsing oral aphthous lesions, genital ulcers, and uveitis. During the ensuing 65 years, multiple systemic associations of the disease, including articular, vascular, gastrointestinal, cardiopulmonary, and neurological involvement, have become increasingly apparent. Although the etiology and pathogenesis is not clearly defined, genetic predisposition, infections, and immunological dysfunctions have been implicated. Microchimerism indicates the presence of cells from one individual in another individual. Recently, it has been suggested that microchimerism plays a role in the pathogenesis of autoimmune diseases, including lupus erythematosus, scleroderma, and Sjögren's syndrome,[5-7] and it may lead to autoimmune disease in some individuals with a permissive genetic make-up. Autoimmune diseases in recipients of transplanted hematopoietic cells are well known. These include graft-versus-host disease (GVHD), which can be lupus-like or scleroderma-like, Sjögren's syndrome, primary biliary cirrhosis, and myositis. Disease in transplant recipients resembles that in individuals with the corresponding primary autoimmune diseases. The dysregulation of tolerance in the transplant recipient is thought to allow development of autoreactive T-cell clones, leading to autoimmune disease. Whether this results from microchimerism or simply from immunocompromise is not known. The best-studied example of immune cell microchimerism as a possible factor in primary autoimmune disease is scleroderma, which has a peak incidence in women after the childbearing years. Women with scleroderma are more likely than matched control subjects to have had an HLA-compatible fetus, and Y-chromosome-positive cells can be found in fibrotic maternal tissue of women with scleroderma many years after delivery of a male baby. Fetal microchimerism has been proposed as an explanation for scleroderma in males and children. Young individuals, 20–40 years of age, are most commonly affected in BD. In different series, high prevalence of ocular, nervous system and pulmonary system involvement, large vessel thrombosis, thrombophlebitis, and pathergy positivity has been found in male patients with BD, and in view of these data, a more severe course in male patients can be expected. Microchimerism might clarify this condition. BD is an immune-mediated disease that affects several organs and has a variety of clinical symptoms. This disease is characterized by the presence of autoantibodies, particularly autoantibodies against endothelial cell, oral mucosal antigens, retinal-S antigen, and heat shock proteins. Despite extensive research, the etiology of BD is still unknown but is probably multifactorial. Chimerism is a candidate factor that may be responsible for the development of BD. The presence of chimeric cells in tissues affected by BD may indicate the pathogenic potential of chimeric cells. For example, their presence could be interpreted as GVH or host-versus-graft (HVG) reaction. In these scenarios, chimeric cells are involved in the initiation of disease. These observations have led us to the hypothesis that microchimerism may be involved in the pathogenesis of BD. This study was undertaken to evaluate microchimerism in the circulation of patients with BD.
- Top of page
- Material and methods
The term chimerism is used to indicate the presence of cells from one individual in another. Individuals can become chimeric in several ways. Pregnancy-related causes of chimerism can lead to chimerism of either the pregnant woman or the fetus. The former comprise completed pregnancy, miscarriage, and induced abortion.[4-7] Chimeric fetal cells may present as hematopoietic progenitor cells (CD34+ and CD34+ CD38+ cells), trophoblast cells, nucleated erythrocytes, T lymphocytes, and other leukocytes.[10-12]
To determine what types of experiments will most efficiently elucidate the role of chimeric cells in BD and other autoimmune diseases, it is necessary to determine the various pathogenic mechanisms that theoretically could be involved. We can discuss three hypotheses on the role of chimeric cells in immune-mediated diseases, with emphasis on the role of chimerism in BD.
The first hypothesis is that the chimeric cell is a T cell that induces a GVH reaction. Because of the numerous similarities between autoimmune diseases and GVHD,[4, 13] it seems likely that a GVH-like reaction is involved in the pathogenesis of these diseases. Autoreactive T cells, the hallmark of true-to-form autoimmune diseases, seem to be important in BD, and the once-popular proposed role for increased T- and B-cell responses to heat shock proteins seems to be of central importance. As in many forms of uveitis, BD is characterized by organ- and/or tissue-specific inflammation resulting from an abnormal immune response or an otherwise normal response but directed against self antigens. At least three conditions are required for a chimeric cell to induce a GVH reaction. First, the host must accept the presence of chimeric cells. Secondly, chimeric cells must be immunologically competent T cells. Thirdly, the chimeric cells must recognize the cells of the host as foreign. Microchimerism may be associated with the autoreactive T-cell response in BD.
The second hypothesis is that the chimeric cell is the target of an HVG-like reaction. Antigens from chimeric cells induce an immune response leading to an autoimmune-like reaction, either by direct response to chimeric cells or by cross-reactivity due to molecular mimicry. Retinal-S antigen is an immunologically privileged retinal antigen that can produce experimental autoimmune uveitis and is thought to be a target for ocular inflammatory disease. Although the exact trigger for the onset of inflammatory BD is unknown, higher chimeric immune cells can be recruited to the site of inflammation once sensitization to retinal-S antigen has developed.
There is evidence in humans for a direct HVG-like reaction leading to an autoimmune-like response. In this scenario, the host has to recognize the chimeric cell as foreign. Fetus-derived chimeric cells may be considered foreign cells because they contain inherited paternal antigens. During and after pregnancy, antipaternal human leukocyte antigen (HLA) antibodies have been found in up to 30% of mothers. A number of studies investigated whether HLA class alleles of fetus and mother were related to the occurrence of autoimmune disease. Stevens et al. investigated maternal HLA class II compatibility in men with lupus erythematosus and observed that men with this disorder significantly more often showed bidirectional HLA class II compatibility with their mothers compared with healthy controls. Also, in women with scleroderma, HLA class II compatibility with their children was found more often than in healthy controls. It has been confirmed in different ethnic groups that the primary gene involved in the pathogenesis of BD is HLA-B. HLA-B51, one of the split antigens of HLA-B5, has been found the most accurate genetic marker for BD to date.
The third hypothesis is that the chimeric cell is not directly involved in the pathogenesis of autoimmune disease but that its presence in host tissues represents the result of a repair mechanism. This hypothesis suggests that chimeric cells develop from progenitor cells into parenchymal cells and replace damaged host cells after tissue injury. Immunohistopathologic studies of specimens taken from active inflammatory sites of patients with BD support the findings of those found in peripheral blood and indicate immune-complex-mediated disease. Necrotizing, neutrophilic, obliterative perivasculitis and venous thrombosis with lymphocytic and monocytic cellular infiltration of the veins, capillaries, and arteries of all sizes, with or without fibrin deposition in the vessel wall, is the hallmark of BD. In BD, chimeric cells might develop from progenitor cells into parenchymal cells and replace damaged host cells after vasculitic injury.
In addition to scleroderma, other examples of microchimerism and skin disease due to the two-way traffic between mother and fetus have been proposed. For instance, it has been suggested that fetal cells in maternal circulation produce the eruptions associated with pregnancy, such as pruritic urticarial papules and plaques. Pemphigoid gestationis is thought to be an immune reaction to the father's histocompatibility antigens on fetal cells in maternal tissue. Erythema toxicum neonatorum, which occurs in the first few days postpartum, may be a mild GVHD-like reaction to maternal cells in the newborn. In BD, we might see mucocutaneous lesions as a mild GVHD-like reaction to maternal cells.
To date, the connection between microchimerism and disease has not yet been definitively made. In fact, it has also been suggested that fetal cells may provide a renewable source of stem cells that can help with repair of maternal tissues, which is a positive rather than negative outcome of microchimerism. One hypothesis is that stem cells may migrate to sites of inflammation and differentiate into cells that participate in repair.[24, 25] Thus, finding microchimeric cells in diseased tissue does not necessarily mean they were responsible for the initial injury.
A new paradigm – the presence of cells other than leukocytes in microchimerism as a possible factor in skin disease – is explored in the article by Khosrotehrani et al. The authors identified maternally derived cytokeratin-positive cells in archival paraffin-embedded sections of the skin of 11 of 12 male children with pityriasis lichenoides by fluorescent in situ hybridization with X and Y chromosome-specific probes. These maternally derived cells were also present in biopsies of skin from male control subjects without skin disease but with a lower frequency (four of seven) and density (approximately 20-fold lower in patients with pityriasis lichenoides than in normal subjects). In summary, microchimerism, not only of leukocytes but also of other cell types, may be a common event in both normal and diseased tissues, a “true-true unrelated” situation.
To the best of our knowledge, this study presents for the first time the presence of microchimerism in BD. One of the factors held responsible for the etiopathogenesis of BD is increased neutrophil chemotaxis and autoreactive T-cell clones. The histopathological and immunofluorescence study of the lesional specimens revealed an immune-complex-mediated leukocytoclastic vasculitis as compared with non-lesional skin in patients with BD. Retinal-S antigen, a protein, is the most potent uveitic autoantigen, which is localized to the photoreceptor area of the retina. This autoantigen may contribute or help to perpetuate the inflammatory response in ocular BD. Enhanced cell-mediated cytotoxicity with demonstrated circulating immune complex response against oral mucosal antigens, especially during an exacerbation period, supports the presence of both T-helper 1 and 2 types of the immune reaction in BD. Immune complexes in cerebrospinal fluid and intrathecal production of IgM, IgG, and IgA during active encephalitis have been reported in BD along with elevated cerebrospinal fluid concentrations of C3, C4, and CD8 subset T lymphocytes. Anti-endothelial cell antibodies are commonly present in both vascular and non-vascular BD, and they increase the expression of ICAM-1 on endothelial cells, implicating endothelial activation in such patients. Microbial HSP-65-derived peptides and their mammalian counterpart HSP-60, both of which cause autoimmunity with 60% of sequence homology, have been demonstrated to stimulate the lymphoproliferative response in patients with BD. All these findings suggest that at least some clinical aspects of BD have autoimmune components because elevated levels of circulating immune complexes exist and are associated with both immunoglobulins and antibodies that react with oral mucosa in such patients. Our data may suggest that in some patients with BD microchimeric cells represent an immunologically active peripheral blood mononuclear cell in contrast to healthy controls, what may be the effect of differentiation of microchimeric progenitor CD34+ stem cells into functional leukocytes, as postulated before.