Macrophage migration inhibitory factor (MIF) was reported to be the first lymphokine produced by activated T cells in guinea pigs to prevent the migration of macrophages out of capillary tubes. 1 The biological role of MIF has been unclear for a long time. Recent studies have established that MIF is a critical cytokine in delayed-type hypersensitivity responses, an important mediator of endotoxin shock, and a counterregulator of glucocorticoid action. 2–4 Moreover, inhibition of T-cell proliferation and interleukin-2 (IL-2) production in vitro by anti-MIF antibodies suggested that MIF had an important regulatory role in the activation of T cells induced by mitogenic or antigenic stimuli. 5
In this chapter, we first discuss serum MIF levels in uveitis patients. We have quantified serum MIF levels in patients with various forms of uveitis, including Behçet's disease, Vogt-Koyanagi-Harada (VKH) disease, sarcoidosis, and idiopathic iridocyclitis, and the influence of corticosteroid therapy on MIF. Second, we discuss MIF in experimental autoimmune uveoretinitis (EAU). We have studied the kinetics of MIF in EAU and have treated EAU with anti-MIF monoclonal antibody.
Serum MIF Levels in Uveitis Patients
In Japan, Behçet's disease, VKH disease, and sarcoidosis are the major causes of uveitis. 6 These diseases affect not only the eyes but also other organs. Figure 1 shows the serum MIF levels in patients with these diseases. No patients had been administered systemic corticosteroids. Serum MIF levels of healthy controls were generally low, the average level being 5.4 ng/ml. In contrast, the uveitis patients had high levels of MIF. The mean serum MIF levels were 60.4 ng/ml in Behçet's disease, 16.5 ng/ml in VKH disease, and 27.1 ng/ml in sarcoidosis. These figures were significantly higher than those in healthy control subjects. In particular, the levels of MIF were extremely high in patients with Behçet's disease.
We divided the patients into two groups according to the status of uveitis. In Figure 1, open circles indicate the patients with active uveitis, and closed circles indicate the patients with inactive uveitis. All patients with VKH disease were in the active stage. Behçet's disease and sarcoidosis patients showing active uveitis had high levels of serum MIF. The sarcoidosis patients with severe uveitis showed higher levels of MIF (63.3 ± 11.9 ng/ml) than did those with mild uveitis (10.3 ± 4.4 ng/ml). Behçet's patients in the exacerbation stage showed higher levels of MIF (80.9 ± 15.4 ng/ml) than did those in the convalescent stage (39.9 ± 4.1 ng/ml).
Patients with Behçet's disease have recurrent attacks. Figure 2 shows the serial observations of 4 individual patients at various stages. The serum MIF levels in these patients in the exacerbation stage are significantly higher than those in the convalescent stage.
The question remains, “What is the source of MIF?” In a previous study, we observed high levels of MIF in the uvea. 7 This is thought to be one of the major sites of origin. However, as Behçet's, VKH, and sarcoidosis are systemic inflammatory diseases, we considered that MIF production in the local lesion was relatively low and that serum MIF might be secreted by vascular endothelial cells 8 in organs other than eyes.
Next, we determined whether MIF was increased in the sera of patients with iridocyclitis. Systemic symptoms, such as are seen in ankylosing spondylitis and other collagen diseases, are not implicated in these patients. The mean MIF concentrations were 28.8 ng/ml in acute iridocyclitis, 18.5 in chronic iridocyclitis, and 4.4 in healthy control subjects. The average levels of MIF in the sera of patients with iridocyclitis were significantly higher than those in healthy control subjects. We divided the acute cases into two groups on the basis of the presence of HLA-B27. No significant difference was detected in MIF levels between HLA-B27-positive and -negative patients.
From these results, we can speculate three possible origins of MIF. The first possibility is that uveal tissues are the main source of MIF. The second possibility is that the origin of MIF is peripheral blood mononuclear cells in uveitis patients. The third possibility is the pituitary gland, which, when stimulated by ocular inflammation, releases MIF into the serum. In the murine model, MIF circulated normally as a hormone and was released in part by the pituitary gland in response to systemic inflammatory stimuli. 3,9 MIF production in the ocular tissue of patients with iridocyclitis may trigger the systemic production of MIF.
Serum MIF Levels Under Corticosteroid Therapy
We next explored the relationship between corticosteroids and MIF. It was reported that corticosteroids increased the serum MIF levels in rats. Although the production of most inflammatory cytokines is suppressed by corticosteroids both in vivo and in vitro, it is unclear whether corticosteroids influence MIF production in humans.
This study was performed on the patients with VKH disease. Immediately after the disease was diagnosed, patients were given systemic corticosteroids. Figure 3 shows the serial observations of serum MIF levels in 9 patients with VKH disease. The serum MIF levels in most patients were markedly increased on day 7 or day 14. This is the first report that shows the influence of high-dose corticosteroid administration on MIF in humans. MIF may be a unique cytokine that acts together with corticosteroids to regulate inflammation and immune responses.
MIF Kinetics in Experimental Autoimmune Uveoretinitis
EAU is an intraocular inflammatory disease that can be induced in a variety of animals by immunization with ocular-specific antigens. EAU has been considered to be an animal model for human endogenous uveitis. In our experiment, male Lewis rats were immunized with interphotoreceptor retinoid-binding protein–derived peptide, R16. The MIF concentration in aqueous humor was extremely high during the EAU development period (between days 9 and 13), whereas the serum MIF level was low.
To elucidate the role of MIF in EAU induction, we administered anti-MIF monoclonal antibody to rats immunized with a uveitogenic peptide, R16. The table summarizes the histological disease severity on day 18 and the mean disease onset. The histological disease severity was decreased and onset of EAU was delayed in rats treated with anti-MIF monoclonal antibody every second day from days 0 to 6. However, no inhibitory effect was noted after similar treatment from days 8 to 14. The T-cell proliferative responses against the immunized peptide from rats treated with anti-MIF monoclonal antibody were markedly suppressed.
To summarize, we found that high levels of MIF were detected in the serum of uveitis patients with Behçet's disease, VKH disease, and sarcoidosis, as well as idiopathic iridocyclitis. In Behçet's disease and sarcoidosis, the MIF levels were higher in the active uveitis stage than in the inactive stage. Serum MIF increased after corticosteroid therapy in VKH disease. A high aqueous humor MIF concentration was detected at the onset of EAU. Finally, anti-MIF antibody suppressed EAU when administered in the early phase. MIF is thought to be an important cytokine in the pathogenesis of uveitis.
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