The spatial and temporal control of polarity is fundamental to the survival of all organisms. Cells define their polarity using highly conserved mechanisms that frequently rely upon the action of small GTPases, such as Ras and Cdc42. Schizosaccharomyces pombe is an ideal system with which to study the control of cell polarity since it grows from defined tips using Cdc42-mediated actin remodeling. Here we have investigated the importance of Ras1-GTPase activity for the coordination of polarized cell growth during fission yeast mating. Following pheromone stimulation, Ras1 regulates both a MAPK cascade and the activity of Cdc42 to enable uni-directional cell growth towards a potential mating partner. Like all GTPases, when bound to GTP, Ras1 adopts an active conformation returning to an inactive state upon GTP-hydrolysis, a process accelerated through interaction with negative regulators such as GAPs. Here we show that, at low levels of pheromone stimulation, loss of negative regulation of Ras1 increases signal transduction via the MAPK cascade. However, at the higher concentrations observed during mating, hyperactive Ras1 mutations promote cell death. We demonstrate that these cells die due to their failure to coordinate active Cdc42 into a single growth zone resulting in disorganized actin deposition and unsustainable elongation from multiple tips. These results provide a striking demonstration that the deactivation stage of Ras signaling is fundamentally important in modulating cell polarity.

Natural killer (NK) cells rapidly reconstitute following allogeneic stem cell transplantation (allo‐SCT), at the time when alloreactive T cell immunity is being established. We investigated very early NK cell reconstitution in 82 patients following T cell‐depleted allo‐SCT. NK cell number rapidly increased, exceeding T cell reconstitution such that the NK:T cell ratio was over 40 by day 14. NK cells at day 14 (NK‐14) were donor‐derived, intensely proliferating and expressed chemokine receptors targeted to lymphoid and peripheral tissue. Spontaneous production of the immunoregulatory cytokine IL‐10 was observed in over 70% of cells and transcription of cytokines and growth factors was augmented. NK‐14 cell number was inversely correlated with the incidence of grade II‐IV acute graft versus host disease (GVHD). These findings reveal that robust reconstitution of immunoregulatory NK cells by day 14 after allo‐SCT is an important determinant of the clinical outcome, suggesting that NK cells may suppress the development of the T cell‐mediated alloreactive immune response through production of IL‐10.

Background: G protein-coupled receptors (GPCRs) interact with regulator of G protein signaling (RGS) proteins, but the mechanistic/physiological importance is unclear. Results: A GPCR-RGS interaction is mapped that localizes RGS to the plasma membrane, a requirement for physiological signaling. Conclusion:The interaction spatially regulates RGS activity to the activated GPCR. Significance: Compartmentalized RGS activity could be a novel mechanism for modulating numerous GPCR signaling pathways.

CD117 (cKit) is the receptor for stem cell factor (SCF) and plays an important role in early haemopoiesis. We show that CD117 is also expressed following priming of mature human CD8+ T cells in vitro and is detectable following primary infection in vivo. CD117 expression is mediated through an intrinsic pathway and is suppressed by IL-12. Importantly, the extent of CD117 expression is inversely related to the strength of the activating stimulus and subsequent engagement with cell-bound SCF markedly increases susceptibility to apoptosis. CD117 is therefore likely to shape the pattern of CD8+ T cell immunodominance during a primary immune response by rendering cells with low avidity for antigen more prone to apoptosis. Furthermore, CD117+ T cells are highly sensitive to apoptosis mediated by galectin-1, a molecule commonly expressed within the tumor microenvironment, and CD117 expression may therefore represent a novel and potentially targetable mechanism of tumor immune evasion.

HIGHLIGHTS g-chain cytokines revert newly differentiated CD8 + T cells to a naive-like phenotype These ''naive-revertant'' are primed for secondary challenge Their chromatin landscape is reminiscent of memory cells Specific signaling pathways and transcription factors are involved