Regulatory T (Treg) cells play an important role in suppressing unwanted immune responses, including preventing the development of autoimmune disease. However, recent studies also show an increased frequency of Treg cells in cancer patients, suggesting that cancer cells can escape from the immune responses by using the immunosuppressive activity of Treg cells. Development and function of Treg cells are regulated by a transcription factor, Foxp3. We are therefore studying regulation of Foxp3 gene expression to control Foxp3-mediated transcription. Interestingly, Ox40 signaling inhibits Treg development in periphery by blocking TGF-β/TCR signal-mediated induction of Foxp3, and Gitr signaling has been shown to inhibit the suppressive activity of Foxp3+ Treg cells. Gitr (Glucocorticoid-induced tumor necrosis factor receptor, TNFRSF18) and Ox40 (CD134, TNFRSF4) are members of TNF receptor superfamily, and are expressed on activated T cells. In particular, the expression of both receptors is kept at high levels on CD4+CD25+Foxp3+ Treg cells. To investigate Foxp3-mediated transcriptional upregulation, we are also studying Gitr and Ox40 gene expression. To understand regulation of protein expression on lymphoid cells, we are studying posttranscriptional regulation as well.
Regulation of Foxp3-mediated transcription
Foxp3 is a forkhead transcription factor, and in many cases, this transcription factor functions as a repressor by binding to other transcription factors such as NFAT, NF-κB p65 and Runx. However, some gene expression (e.g., CTLA4, IL-35, CD25, Ox40 and Gitr) is positively regulated by Foxp3. Importantly, these upregulated molecules play critical roles in regulating the Treg cell functions. To understand the Foxp3-mediated Treg cell development and transcriptional regulation, we are studying gene expression of Foxp3, Ox40 and Gitr. Foxp3 gene expression is regulated by signaling through T cell receptor (TCR) and TGF-β receptor in periphery. We demonstrated that Foxp3 gene expression is regulated by a promoter and two enhancers, and the Enhancer 1 activity is regulated by transcription factors NFAT and Smad3, which are activated by TCR and TGF-β signaling, respectively.
Roles of Gitr and Ox40 in regulating Teff and Treg cell activities
Signalings through Gitr and Ox40 have been shown to inhibit the suppressive activity of Treg cells. However, these receptors are not only regulators on Treg cells, but are also known as co-stimulatory molecules on Teff cells. Previously, we cloned the mouse Gitr ligand, and subsequently discovered that this signaling can enhance the proliferation of antigen-stimulated T cells from monospecific TCR-transgenic mice. Ox40 signaling can promote survival signals in Teff cells. To understand regulation of T cell-mediated immune responses, we are studying Gitr and Ox40 gene expression in T cells and Gitr ligand expression in APCs.
Controlling alternative splicing for cancer therapy
Alternative splicing is a key molecular mechanism for increasing the function diversity of the eukaryotic proteomes. Interestingly, some alternative splicing events are differentially regulated. Previously, we have demonstrated that CD40 protein expression is differentially regulated by the alternative splicing mechanism. CD40 signaling seems to be upregulated in early-activated cells, but downmodulated in late-activated cells through alternative splicing. We are developing new cancer therapeutic strategies by controlling alternative splicing.