Age-Related Loss of Function of Alloactivated Interleukin 2-Propagated Human Primed Lymphocyte Typing Clones

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Abstract
Human lymphocytes alloactivated in vitro were cloned by limiting dilution in the presence of filler cells and interleukin 2 (IL 2)‐containing supernatants of phytohaemagglutinin‐stimulated lymphocytes. Clones with allospecific proliferative reactivity (PLT clones), measured by tritiated thymidine (3H‐TdR) incorporation, were selected for extensive IL 2‐dependent expansion. The cloned lines had finite lifespans, ranging from an estimated minimum of 28 to > 65 doublings. Function as PLT reagents, however, was retained in all cases for only an estimated 30 cell doublings. This apparent cessation of function was not caused by loss of the ability to metabolize thymidine, since lines continuing to grow for > 30 doublings still incorporated 3H‐TdR in the presence of IL 2. An altered requirement for stimulating antigen (number of stimulating cells), or altered response kinetics, did not contribute to loss of PLT function. Exogenous IL 2 added during restimulation to responders previously ‘rested’ overnight without IL 2 did not restore the response. Thus, under present experimental conditions, functional lifespans of cloned PLT reagents appear fixed at ˜ 30 cell doublings.