Abstract
Monocytes (MO) influenced phosphoinositide metabolism when human T lymphocytes, isolated from peripheral blood, were activated by polyclonal mitogens. In the 3 hr immediately following mitogenic challenge, the synthesis of phosphatidylinositol (PI) was augmented and the synthesis of PI‐4‐phosphate (PIP) and PI‐4,5‐bisphosphate (PIP~2~) was induced in cultures of T lymphocytes and MO. In addition, MO induced a rapid and transient degradation of PIP and PIP~2~ in T cells prelabeled with [^32^P]PL and subsequently activated by mitogen. Induction of a PIP/PIP~2~ response correlated well with induction of DNA replication by MO when T cells were activated by phytohemagglutinin or by neuraminidase plus galactose oxidase. MO did not influence polyphosphoinositide metabolism when T cells were stimulated by the nonmitogenic lectin wheat germ agglutinin. Interleukin 1 could not substitute for monocytes in inducing a polyphosphoinositide response. By causing a rapid and transient release of the second messengers diacylglycerol and inositol phosphates and by subsequently increasing their cellular precursors, MO may induce the interleukin 2 responsive state in T lymphocytes.