Toxoplasma gondii excreted-secreted antigens suppress Foxp3 via PI3K-AKT-mTOR signaling pathway

Toxoplasma gondii excreted-secreted antigens (ESA) cause spontaneous abortion or fetal teratogenesis during the pregnancy in mice, especially in the early stage. Those adverse pregnancy outcomes are due to the deficit in regulatory T cells (Tregs). Forkhead box P3 (Foxp3), a critical transcription factor, modulates Tregs differentiation and its function. Besides, phosphatidylinositol 3-kinase-protein kinase B-mammalian target of rapamycin (PI3K-AKT-mTOR) signaling network is implicated in interfering with Foxp3 induction. We previously demonstrated that ESA diminished the number of Tregs and inhibited its function. And ESA suppressed Foxp3 expression via the attenuation of transforming growth factor β RII/Smad2/Smad3/Smad4 pathway. The current study aimed to investigate whether the PI3K-AKT-mTOR signaling network is involved in Foxp3 downregulation induced by ESA. We found that ESA upregulated PI3K, P-AKT, mTOR, and P-mTOR. Knockdown of PI3K cooperated with ESA to restore Foxp3 expression mediated by ESA. This suppressive role of ESA on Foxp3 expression was abrogated by AKT inhibitor. In addition, neutralization of Toll-like receptor 4 could restore the expression of Foxp3, PI3K, and its downstream effectors induced by ESA. Collectively, the findings indicated that ESA inhibited Foxp3 expression via the upregulation of PI3K-AKT-mTOR signaling pathway.

Regulatory T cell subsets are differentially dependent on CD28 for their proliferation

It is thought that CD28 plays a crucial role in the maintenance of regulatory T cell (Treg) pool size through promoting the development and proliferation of these cells. However, recently we found that the dependency on CD28 co-stimulation for their development is different between Treg subsets, thymus-derived Tregs (tTregs, CD28-dependent) and peripherally-derived Tregs (pTregs, CD28-independent), suggesting that CD28 may also have differential influences on the homeostasis of each Treg subset. Here, we demonstrated that both Treg subsets were reduced in secondary lymphoid organs of CD28 deficient mice, and that this reduction was due to impaired proliferation in both Treg subsets by the intrinsic CD28 defect. However, we found that the massive proliferation of both Treg subsets under lymphopenic condition was regulated by CD28, whereas the proliferative activity of tTregs but not pTregs in the steady state was dependent on CD28. Also, experiments using mutant CD28 knock-in mice revealed that proliferation of pTregs under lymphopenic condition required only the Lck-NFκB pathway of CD28, whereas tTregs required an additional unknown pathway. These findings indicate that the dependency on CD28 for proliferation in each Treg subset differs depending on the environment.

Mechanisms for T cell tolerance induced with granulocyte colony-stimulating factor

Granulocyte colony-stimulating factor (G-CSF) has been widely accepted as a mediator of T cell tolerance. The immune modulatory effect of G-CSF on T cells is believed to be mediated exclusively through other effector cells, such as monocytes, tolerogenic dendritic cells (DC), and myeloid-derived suppressor cells. Recent advances confirmed the direct effects of G-CSF in inducing immune tolerance of T cells through the G-CSF-G-CSF receptor pathway and related molecular mechanisms. This review aims to summarize the findings associated with the direct and indirect mechanisms for T cell tolerance induced with G-CSF. The role of G-CSF in preventing graft-versus-host disease (GVHD) and in treating autoimmune diseases (ADs) is also discussed. It is conceivable that G-CSF and immune cell compositions, such as tolerogenic DC and CD4(+)CD25(+)Foxp3(+) T cells, modulated by G-CSF could become an integral part of the immunomodulatory therapies against GVHD and ADs in the future.