Supporting Text Model Equations and Parameter Estimation IFN-γ Receptor-Ligand Binding. IFN-γ binds to receptors on the macrophage surface, initiating a cascade of events leading to an increase in MHC class II

IFN-γ Receptor-Ligand Binding. IFN-γ binds to receptors on the macrophage surface, initiating a cascade of events leading to an increase in MHC class II expression (1). We assume that this increase is the primary mechanism by which IFN-γ facilitates antigen presentation. To represent IFN-γ receptor-ligand binding, we use the general reaction scheme ligand + receptor ↔ complex. Other processes are likely to affect the number of IFN-γ receptor-ligand complexes on the time scales of IFN-γ treatment used experimentally, 20-36 h (2, 3). Celada et al. (4) observed that IFN-γ levels in solution decrease 13% and 83% after 4 h in the absence and presence of macrophages, respectively, indicating that appreciable levels of IFN-γ both degrade in solution and are taken up by macrophages. Therefore, in addition to representing IFN-γ receptor-ligand binding, we also represent degradation of IFN-γ in solution and within the macrophage following uptake (Eqs. 1-3).