Another cell surface molecule that may act as a co-stimulator for T-cells is VCAM-1 (CD106) that binds very-late-antigen-4 (VLA-4) on T-cells. [0099] One of skill in the art will appreciate that cells other than T-cells may be stimulated by binding of an agent that ligates a cell surface moiety and induces aggregation of the moiety, which in turn results in activation of a signaling pathway.