Small Heterodimer Partner Regulates Dichotomous T Cell Expansion by Macrophages

The involvement of small heterodimer partner (SHP) in the inhibition of hepatic bile acid synthesis from cholesterol has been established. However, extrahepatic expression of SHP implies that SHP may have regulatory functions other than those in the liver. Here, we find that SHP mRNA expression is high in murine bone marrow cells, suggesting a physiological role within macrophages. Indeed, expression of SHP in macrophages decreases the transcriptional activity and nuclear localization of nuclear factor κB, whereas downregulation of SHP has the opposite effects. Expression of genes associated with macrophage-T cell crosstalk were altered by overexpression or downregulation of SHP. Intriguingly, increasing SHP expression in macrophages resulted in decreased T cell expansion, a hallmark of T cell activation, whereas knockdown of SHP resulted in increased expansion. Analyses of the expanded T cells revealed a dichotomous skewing between effector T cells and regulatory T cells (T_regs), with SHP overexpression reducing T_regs and downregulation of SHP increasing their expansion. The expanded T_regs were confirmed to be suppressive via adoptive transfers. IL-2 and TGF-β, known inducers of T_reg differentiation, were found to be regulated by SHP. Furthermore, SHP occupancy at the promoter region of IL-2 was increased after macrophages were challenged with lipopolysaccharide. Neutralizing antibodies to IL-2 and TGF-β inhibited the expansion of T_regs mediated by downregulation of SHP. This study demonstrates that expression and activity of SHP within macrophages can alter T cell fate and identifies SHP as a potential therapeutic target for autoimmune diseases or solid cancers.