Provided herein are methods that relate, in some aspects, to the incubation or culturing, such as to induce stimulation of expansion (proliferation), activation, costimulation and/or survival, of a composition of cells, such as a population of lymphocytes. In some aspects, provided are methods and reagents for the stimulation, e.g., of expansion (proliferation), survival or persistence, activation, costimulation, or other effect, of cell populations that involve binding of agents to a molecule on the surface of the cells, thereby providing one or more signals to the cells. In some cases, the reagents are reagents containing a plurality of binding sites for agents, such as multimerization reagents, and thus the one or more agents are multimerized by reversibly binding to the reagent, e.g., thereby creating a stimulatory reagent (multimerized agent), having stimulatory agents multimerized thereon. In some aspects, the multimerized agent can provide for expansion or proliferation or other stimulation of a population of cells, and then such stimulatory agents can be removed by disruption of the reversible bond. Also provided are compositions, apparatus and methods of use thereof.