Host immune responses to Toxoplasma gondii

Macroautophagy is mediated by a unique organelle, the autophagosome, which encloses a portion of cytoplasm for delivery to the lysosome. Autophagosome formation is dynamically regulated by starvation and other stresses and involves complicated membrane reorganization. Since the discovery of yeast Atg-related proteins, autophagosome formation has been dissected at the molecular level. In this review we describe the molecular mechanism of autophagosome formation with particular focus on the function of Atg proteins and the long-standing discussion regarding the origin of the autophagosome membrane.

Several subsets of Foxp3+ regulatory T (Treg) cells work in concert to maintain immune homeostasis. However, the molecular bases underlying the phenotypic and functional diversity of Treg cells remain obscure. We show that in response to interferon-γ, Foxp3+ Treg cells upregulated the T helper 1 (TH1)-specifying transcription factor T-bet. T-bet promoted expression of the chemokine receptor CXCR3 on Treg cells, and T-bet+ Treg cells accumulated at sites of TH1-mediated inflammation. Furthermore, T-bet expression was required for the homeostasis and function of Treg cells during type-1 inflammation. Thus, within a subset of CD4+ T cells, the activities of Foxp3 and T-bet are overlaid, resulting in Treg cells with unique homeostatic and migratory properties optimized for suppression of TH1 responses in vivo.

Autophagy is a bulk degradation process in eukaryotic cells; autophagosomes enclose cytoplasmic components for degradation in the lysosome/vacuole. Autophagosome formation requires two ubiquitin-like conjugation systems, the Atg12 and Atg8 systems, which are tightly associated with expansion of autophagosomal membrane. Previous studies have suggested that there is a hierarchy between these systems; the Atg12 system is located upstream of the Atg8 system in the context of Atg protein organization. However, the concrete molecular relationship is unclear. Here, we show using an in vitro Atg8 conjugation system that the Atg12-Atg5 conjugate, but not unconjugated Atg12 or Atg5, strongly enhances the formation of the other conjugate, Atg8-PE. The Atg12-Atg5 conjugate promotes the transfer of Atg8 from Atg3 to the substrate, phosphatidylethanolamine (PE), by stimulating the activity of Atg3. We also show that the Atg12-Atg5 conjugate interacts with both Atg3 and PE-containing liposomes. These results indicate that the Atg12-Atg5 conjugate is a ubiquitin-protein ligase (E3)-like enzyme for Atg8-PE conjugation reaction, distinctively promoting protein-lipid conjugation.