FTY720 exerts a survival advantage through the prevention of end-stage glomerular inflammation in lupus-prone BXSB mice
Introduction
The FTY720 molecule is a synthetic analog of a natural product isolated from the filamentous fungus Isaria sinclairii, and causes immunosuppression by inducing lymphocyte retention in secondary lymphoid organs [1], [2], [3]. FTY720 has structural similarities to the lysophospholipid, and becomes phosphorylated in vivo by sphingosine kinases. Phosphorylated FTY720 acts as a high-affinity agonist for the sphingosine 1-phosphate (S1P) receptor on lymphocytes, inducing aberrant internalization of the receptor [4]. This renders the cells unresponsive to serum S1P, depriving them of a signal necessary for egress from lymphoid organs. Therefore it induces the sequestration of circulating mature lymphocytes into secondary lymphoid organs and thereby decreases the number of lymphocytes in peripheral blood [1]. FTY720 also modulates chemotactic responses and lymphocyte trafficking to divert lymphocytes from inflammatory lesions and graft sites [2]. So far, great potentials of FTY720 has been reported for the treatment of relapsing multiple sclerosis [5] and the prevention of kidney transplant rejection [6] in human clinical trials.
Systemic lupus erythematosus (SLE) is refractory autoimmune disease characterized by the production of a wide variety of autoantibodies and immune complex (IC)-mediated tissue inflammation. IC-type lupus nephritis is a common complication that significantly worsens mortality. Treatment with corticosteroids and various immunosuppressive agents may be useful in many cases; however, these drugs have an immunosuppressive effect on immune cell function and toxicity-related complications limit their long-term usage. Therefore, it is necessary to develop new treatment strategies. In this context, FTY720 is a possible alternative, since FTY720 exerts its immunosuppressive effect without impairing lymphocyte function. Trials for FTY720 treatment for SLE were carried out using SLE-prone MRL/lpr[7] and (NZB × NZW) F1 mice [8]. While each study demonstrated a therapeutic effect of FTY720 treatment, the underling mechanism may differ from each other. In MRL/lpr mice, FTY720 suppressed autoantibody production in association with the marked decrease in splenic lymphocytes [7]. In contrast, FTY720 had no effect on autoantibody production and glomerular mesangial expansion, while proteinuria was greatly improved in FTY720-treated (NZB × NZW) F1 mice [8]. Thus, further studies are needed to thoroughly understand the FTY720-mediated suppressive effect on lupus nephritis.
The SLE-prone BXSB mice develop severe lupus nephritis in association with the production of various autoantibodies [9]. The disease is more severe in males than in females due to involvement of the Y chromosome-linked autoimmune acceleration (Yaa) mutation, which is a consequence of translocation of the telomeric segment of the X chromosome including the gene encoding Toll-like receptor 7 onto the Y chromosome [10], [11]. Unique features in BXSB male mice are the disease-associated increase in monocytes in periphery [12], and the defect in the development of marginal zone B (MZ B) cells [13]. Intriguingly, it has been shown that the localization of B cells in the splenic white pulp is highly dependent on S1P receptor signals, and that FTY720 rapidly induces MZ B cell migration into follicles [14]. In the present study, we examined the effect of FTY720 on the lymphocyte localization and on the disease features in BXSB male mice.
Section snippets
Mice and treatment
BXSB mice were purchased from the Shizuoka Laboratory Animal Center, Shizuoka, Japan. Three-month-old BXSB male mice were given per os FTY720 (1 mg/kg in distilled water) or distilled water alone three times a week for 5 months. FTY720 was generously provided by Mitsubishi Tanabe Pharma Corporation (Osaka, Japan). All mice were housed under identical conditions, and all experiments were performed in accordance with our institutional guidelines. Only male mice were analyzed.
Cell count and flow cytometric analysis
Peripheral blood
Effects of FTY720 on the clinical course of BXSB lupus
To confirm the effect of FTY720, numbers of PBL were examined one month after the treatment (Fig. 1A). In control vehicle-treated mice, major population was composed of B220+ B cells and CD11b+ monocytes, and numbers of CD4+ and CD8+ T cells were small. In FTY720-treated mice, CD4+ and CD8+ T cells almost disappeared and B220+ B cells dramatically decreased, thus indicating the significant effect on lymphocyte depletion in peripheral blood. In contrast, there was no difference in number of CD11b
Discussion
The present study provided evidence that long-term treatment with FTY720 suppress end-stage lupus nephritis, but was not able to suppress autoantibody production and IC deposition in renal glomeruli. It has been shown that FTY720 may induce apoptosis in splenic lymphocyte, thus suppress spleen weight and autoantibody production in MRL/lpr mice [7]. However, this is not the case in the present study, since FTY720 showed no effect on not only spleen weight but also autoantibody production. The
Acknowledgments
We thank Naoki Ishihara, Naomi Ohtsuji, and Keiko Nishikawa for excellent technical help.
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