Mechanisms of action of ‘lymphocyte activating factor’ (LAF)—II.

The effect of cyclic AMP on LAF-induced potentiation of DNA synthesis in PHA-stimulated rat lymphocytes (LAF effect) was investigated. LAF preparations were dialyzed 24-hr supernatants of partially purified rat macrophages and lacked mitogenic properties. Agents that increase cellular cAMP by different mechanisms (l-isoproterenol, prostaglandin E₁, theophylline, and dibutyryl cAMP) were added before during, or after the period of optimal cell sensitivity to LAF. This period began approximately 16 hr after initiation of the cultures and lasted for about 8 hr. Addition of any of these reagents to the cultures during the first 8–12 hr of culture, i.e. before the period of cell sensitivity to LAF, resulted incomplete inhibition of both the PHA response and the LAF effect. Addition at 16 hr or later, though still inhibiting the PHA response markedly, gave insignificant inhibition of the LAF effect itself. Thus occurrence of a LAF effect requires effective lymphocyte triggering in order to reach the period of optimal cell sensitivity to LAF, a process which is sensitive to elevation of cellular cAMP, but the LAF effect on triggered cells is insensitive to cAMP. We conclude that the lymphocytes tested are heterogenous in their sensitivity both to LAF and to cAMP.