Figure 2

Activation curves of one cell and of two interacting cells. (A) A single cell. The stable fixed points for the level of bound IL-2 receptor, B, were analytically calculated as a function of the TCR signal strength, m (blue circles). The system has two branches of stable solutions in an intermediate range of TCR signal strengths. The actual state of the system as calculated numerically (black line) exhibits a sharp transition between low and high levels of B when the TCR signal strength exceeds a threshold m value. Also shown is the response in the case of no feedback, where the induced synthesis rate of IL-2 receptors is constant (red line). In this case, the number of bound receptors increases linearly with m for almost all m values, and no threshold exists. (B) Two interacting cells. Red line: fixed points of B were calculated for a cell that is interacting with a strongly activated cell (m₁ = 1). The black line is the same as in A (for one cell alone), shown for comparison. Due to the intercellular interaction, the activation curve of Cell2 shifts to the right and the threshold m level increases. The dashed red line shows the number of bound receptors of Cell1 which increases since it can utilize the IL-2 secreted by Cell2. Inset: The normalized interaction index, C (see text and Eq. 5) of the two cells. In the range where C₂ = 0, Cell2 (black line) is indifferent to the presence of Cell1. Where C₂< 0, Cell2 suffers from the interaction (competition and exclusion). In the range where C₁> 0, Cell1 (dashed red line) benefits from the interaction (cooperation).