Disruption of the annexin A1/S100A11 complex increases the migration and clonogenic growth by dysregulating epithelial growth factor (EGF) signaling.

Endocytosis of activated growth factor receptors regulates spatio-temporal cellular signaling. In the case of the EGF receptor, sorting into multivesicular bodies (MVBs) controls signal termination and subsequently leads to receptor degradation in lysosomes. Annexin A1, a Ca(2+)-regulated membrane binding protein often deregulated in human cancers, interacts with the EGF receptor and is phosphorylated by internalized EGF receptor on endosomes. Most relevant for EGF receptor signal termination, annexin A1 is required for the formation of internal vesicles in MVBs that sequester ligand-bound EGF receptor away from the limiting membrane. To elucidate the mechanism underlying annexin A1-dependent EGF receptor trafficking we employed an N-terminally truncated annexin A1 mutant that lacks the EGF receptor phosphorylation site and the site for interaction with its protein ligand S100A11. Overexpression of this dominant-negative mutant induces a delay in EGF-induced EGF receptor transport to the LAMP1-positive late endosomal/lysosomal compartment and impairs ligand-induced EGF receptor degradation. Consistent with these findings, EGF-stimulated EGF receptor and MAP kinase pathway signaling is prolonged. Importantly, depletion of S100A11 also results in a delayed EGF receptor transport and prolonged MAP kinase signaling comparable to the trafficking defect observed in cells expressing the N-terminally truncated annexin A1 mutant. These results strongly suggest that the function of annexin A1 as a regulator of EGF receptor trafficking, degradation and signaling is critically mediated through an N-terminal interaction with S100A11 in the endosomal compartment. This interaction appears to be essential for lysosomal targeting of the EGF receptor, possibly by providing a physical scaffold supporting inward vesiculation in MVBs. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Comprehensive analysis of CD4+ T cells in the decision between tolerance and immunity in vivo reveals a pivotal role for ICOS.

We have established a comprehensive in vivo mouse model for the CD4(+) T cell response to an "innocuous" versus "dangerous" exogenous Ag and developed an in vivo test for tolerance. In this model, specific gene-expression signatures, distinctive upregulation of early T cell-communication molecules, and differential expansion of effector T cells (Teff) and regulatory T cells (Treg) were identified as central correlates of T cell tolerance and T cell immunity. Different from essentially all other T cell-activation molecules, ICOS was found to be induced in the immunity response and not by T cells activated under tolerogenic conditions. If expressed, ICOS did not act as a general T cell costimulator but selectively caused a massive expansion of effector CD4(+) T cells, leaving the regulatory CD4(+) T cell compartment largely undisturbed. Thus, ICOS strongly contributed to the dramatic change in the balance between Ag-specific Teff and Treg from ∼1:1 at steady state to 21:1 at the height of the immune response. This newly defined role for the balance of Teff to Treg, together with its known key function in T cell help for B cells, establishes ICOS as a central mediator of immunity. Given its exceptionally selective induction on CD4(+) T cells under inflammatory, but not tolerogenic, conditions, ICOS emerges as a pivotal effector molecule in the early decision between tolerance and immunity to exogenous Ag.

Expression of XCR1 Characterizes the Batf3-Dependent Lineage of Dendritic Cells Capable of Antigen Cross-Presentation.

Cross-presentation of antigen by dendritic cells (DCs) to CD8(+) T cells is a fundamentally important mechanism in the defense against pathogens and tumors. Due to the lack of an appropriate lineage marker, cross-presenting DCs in the mouse are provisionally classified as "Batf3-IRF-8-Id2-dependent DCs" or as "CD8(+) DCs" in the spleen, and as "CD103(+)CD11b(-) DCs" in the periphery. We have now generated a mAb to XCR1, a chemokine receptor which is specifically expressed on CD8(+) DCs and a subpopulation of double negative DCs in the spleen. Using this antibody, we have determined that only XCR1(+)CD8(+) (around 80% of CD8(+) DCs) and their probable precursors, XCR1(+)CD8(-) DCs, efficiently take up cellular material and excel in antigen cross-presentation. In lymph nodes (LNs) and peripheral tissues, XCR1(+) DCs largely, but not fully, correspond to CD103(+)CD11b(-) DCs. Most importantly, we demonstrate that XCR1(+) DCs in the spleen, LNs, and peripheral tissues are dependent on the growth factor Flt3 ligand and are selectively absent in Batf3-deficient animals. These results provide evidence that expression of XCR1 throughout the body defines the Batf3-dependent lineage of DCs with a special capacity to cross-present antigen. XCR1 thus emerges as the first surface marker characterizing a DC lineage in the mouse and potentially also in the human.