Levamisole suppresses activation and proliferation of human T cells by the induction of a p53-dependent DNA damage response

Levamisole (LMS) is a small molecule used in the treatment of idiopathic nephrotic syndrome (INS). The pathogenesis of INS remains unknown, but most evidence points towards an immunological basis of the disease. Recently, LMS has been shown to increase the relapse-free survival in INS patients treated in combination with corticosteroids with relatively few side effects. While LMS has been hypothesized to exert an immunomodulatory effect, its mechanism of action remains unknown. To provide insight into the working mechanism of LMS, we studied its immunomodulatory activity on in vitro activated human T cells. We show here that treatment with LMS decreased activation and proliferation of human CD4+ and CD8+ T cells. In addition, production of T cell activation-associated cytokines such as IL-2, TNF-α and IFN-γ were reduced upon LMS treatment, whereas IL-4 and IL-13 production was increased. Gene expression profiling confirmed the suppressive effects of LMS on proliferation as numerous genes involved in cell cycle progression were downregulated. Furthermore, genes associated with p53 activation and cell cycle arrest were upregulated by LMS. In agreement, LMS treatment resulted in p53 phosphorylation and increased expression of the p53 target gene FAS. Accordingly, LMS sensitized activated T cells for Fas-mediated apoptosis. Cell cycle analysis showed that LMS induced a mid-S phase arrest indicating the activation of a replication stress-associated checkpoint. In support, LMS treatment resulted in γH2AX-foci formation and phosphorylation of CHK1. Our findings indicate that LMS acts as an immunosuppressive drug that directly affects the activation and proliferation of human T cells by induction of DNA damage and the activation of a p53-dependent DNA damage response.