Accessory cell requirement for activation antigen expression and cell cycle progression by human T lymphocytes

Stringent accessory cell (AC) depletion by a three-step procedure--plastic adherence, nylon wool adherence, followed by simultaneous treatment with two anti-AC monoclonal antibodies + complement--has allowed the demonstration of several AC-dependent stages in the T cell activation pathway. Simultaneous analysis of DNA content and cell surface immunofluorescence (correlation of activation antigen expression with cell cycle position) or DNA and RNA content (cell cycle position) of cultured cells was accomplished by dual parameter flow cytometry. AC-depleted, PHA-stimulated human peripheral blood T lymphocytes (PBTL) failed to exhibit "early" indicators of activation, including increased RNA content, expression of three activation-associated cell surface proteins (IL 2 receptor, transferrin receptor, and 4F2 protein), and the production of IL 2. The AC-depleted PBTL that failed to express these "early" markers of activation also failed to progress into the "late" phase of activation, DNA synthesis. All indicators of PHA responsiveness were fully replenished upon addition of AC but were only reconstituted to intermediate levels by addition of excess quantities of either highly purified IL 1 or crude AC-conditioned medium with lymphocyte-activating factor activity. These data suggest that the AC membrane plays a key and as yet undefined role in the stimulation of T cells by PHA.