  A promising direction in drug development is to exploit the ability of
natural killer cells to kill antibody-labeled target cells. Monoclonal
antibodies and drugs designed to elicit this effect typically bind cell-surface
epitopes that are overexpressed on target cells but also present on other
cells. Thus it is important to understand adhesion of cells by antibodies and
similar molecules. We present an equilibrium model of such adhesion,
incorporating heterogeneity in target cell epitope density and epitope
immobility. We compare with experiments on the adhesion of Jurkat T cells to
bilayers containing the relevant natural killer cell receptor, with adhesion
mediated by the drug alefacept. We show that a model in which all target cell
epitopes are mobile and available is inconsistent with the data, suggesting
that more complex mechanisms are at work. We hypothesize that the immobile
epitope fraction may change with cell adhesion, and we find that such a model
is more consistent with the data. We also quantitatively describe the parameter
space in which binding occurs. Our results point toward mechanisms relating
epitope immobility to cell adhesion and offer insight into the activity of an
important class of drugs.
