Estimating parameters from T cell proliferation data. The best fit of a heterogeneous discrete-time branching process model to a CFSE timecourse obtained by in vitro stimulation of 2.5 × 104 human CD8+ T cells with anti-CD3 and anti-CD28 in saturating quantities of IL-2. γ refers to division probability, α to death. Cells from independent cultures were recovered and analysed for CFSE content at 24 h intervals after stimulation. The histograms show total cell numbers in each generation (grey bars, observed counts obtained by the EM algorithm; black bars, predicted counts with best-fit branching process model). The best fit model gave a timestep of 12 h and indicated that division (γ) and death (α) probabilities changed with generation number (Table 2).