The mechanisms underlying the appearance of asymmetry between cells in the early embryo and consequently the specification of distinct cell lineages during mammalian development remain elusive. Recent experimental advances have revealed unexpected dynamics of and new complexity in this process. These findings can be integrated in a new unified framework that regards the early mammalian embryo as a self-organizing system.

Natural killer (NK) cells are effector cells of the immune system whose activation is carefully regulated by the interplay of signals from activating and inhibitory receptors. Signals from activating receptors induce phosphorylation of the guanine nucleotide exchange factor Vav1, whereas those from inhibitory receptors lead to the dephosphorylation of Vav1 by the Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1). Here, we used mathematical modeling and experiments with NK cells to gain insight into this integration of positive and negative signals at a molecular level. Our data showed a switch-like regulation of Vav1 phosphorylation, the extent of which correlated with the cytotoxic activity of NK cells. Comparison of our experimental results with the predictions that we derived from an ensemble of 72 mathematical models showed that a physical association between Src family kinases and activating receptors on NK cells was essential to generate the cytotoxic response. Our data support a central role for Vav1 in determining the cytotoxic activity of NK cells and provide insight into the molecular mechanism of the integration of positive and negative signals during lymphocyte activation.

A balance of positive and negative signals that are transmitted by different surface receptors controls the effector functions of NK cells. To date our understanding about the integration of positive and negative signals and the decision-making process inside NK cells remains poor.With the help of bioinformatic modelling we try to understand how NK cells first integrate antagonising signals and then compute a reliable killing decision. Gradual signal input through activating and inhibitory receptors is integrated to come to a "yes or no" decision by the NK cell to kill an attached target cell. Triggering of activating receptors leads to Src kinase activation and Vav-1 phosphorylation, whereas inhibitory receptors dephosphorylate Vav-1 via the phosphatase SHP-1. Therefore, we proposed in a first hypothesis, that Vav-1 is the decision making point in the signal transduction network. With this hypothesis we created a family of simplified models describing NK cell activation upon various stimuli. The predictions derived from these models were compared with experimental data. Our experiments showed that increased clustering of activating receptors lead to a rapid switch-like increase in Vav-1 phosphorylation. Similarly, titrating the engagement of inhibitory receptors resulted in switch-like dephosphorylation of Vav-1. Testing NK cell activity after various amounts of activating and inhibitory receptor engagement revealed a functional dominance of inhibitory receptors. Our current model is consistent with a central role of Vav-1 in the decision making process of NK cells and enables a novel insight into the integration of positive and negative signals during lymphocyte activation.