[Monocyte locomotion inhibiting factor (MLIF) produced by E. histolytica induces an increase of cAMP in human...

The supernatant fluid of axenically grown E. histolytica contains a factor (MLIF) which inhibits the locomotion of human monocytes (including chemotaxis) without affecting that of human polymorphonuclear leucocytes. Locomotion, like other cellular functions, is modulated by changes in intracellular cAMP and cGMP. The consensus--with some exceptions--is that while rises in cGMP accompany locomotion, an increase in cAMP (without a concomitant fall in -cGMP) occurs with inhibition of cellular movement. We measured by radioimmunoassay the cAMP concentration of human monocytes exposed to inhibitory concentrations of MLIF. A significant (p less than 0.005) rise in monocyte cAMP was found, comparable to that observed with the use of forskolin, a well known cAMP stimulator. The control studies using plain axenic medium, not only failed to reveal any rise in cAMP but disclosed a small, yet not significant drop in intracellular cAMP. These results suggest that MLIF (like other locomotion inhibitors, i.e. prostaglandins E1, A1 and isoproterenol) produces a significant increase in intracellular monocyte cAMP. This modification in intracellular signals may contribute to the inhibition in monocyte locomotion, an event during which an increase in pericentriolar microtubules has also been observed.