Smif (Smad4 Interacting Factor) Is a Negative Regulator of T Cell Activation and Autoimmunity In Vivo.

Abstract 712

Transforming growth factor β (TGFβ) plays a critical role in regulating cellular processes like proliferation, extracellular matrix production, vasculogenesis and angiogenesis as well as immunomodulation. TGFβ is a pluripotent cytokine with a pronounced immunosuppressive effect by controlling proliferation, differentiation and activation of immune cells. TGFβ binding to its receptor leads to the phosphorylation of R-Smads. R-Smads again form a heteromeric complex with the cytosolic common Smad4. This Smad complex, together with additional cofactors, translocate into the nucleus, where they control the transcription of TGFβ target genes. Smif was originally identified in our lab as an interaction partner of Smad4. Functional analysis revealed a stimulatory effect in regulating TGFβ-dependent genes like the early target gene JunB. After TGFβ stimulation, Smif tranlocates, together with Smad4, into the nucleus, where Smif acts as a coactivator. To investigate the role of Smif in mammals, we generated a Smif knockout mouse. To this end exon 2 of Smif was replaced by GFP and an inverted neomycin selection cassette. Smif-deficient mice were viable but exhibit a shortened life span. On the average, these mice die at 12 month of age due to multifocal inflammatory disease. Overall pathological analysis of diseased mice revealed extensive lymphocytic infiltrates in multiple organs. Moreover, Smif-deficiency caused immune complex induced glomerulonephritis associated with proteinuria. In line with these findings, autoantibodies could be detected in the serum of Smif knockout mice. Interestingly, we identified T cells and not B-cells as the important target in Smif-deficient mice. T cells lacking Smif were spontaneously activated. In addition, TGFβ was not able to block T cell proliferation of CD4+ cells in vitro, whereas B cells isolated from Smif knockout spleens behave as wildtype. Transcription of TGFβ responsive reporter constructs was greatly reduced in Smif knockout Mefs and could be rescued by the reexpression of functional Smif. Taken all together, the observed autoimmune phenotype found in Smif-deficient mice is at least partially caused by overactivated T cells due to downregulation of the inhibitory TGFβ pathway.