Human neutrophils require activation by mononuclear leucocyte conditioned medium to kill the pathogenic free-living amoeba, Naegleria fowleri.

Naegleria fowleri is a free-living amoeba which causes a fulminant and rapidly fatal meningoencephalitis in man. Human neutrophils fail to kill the amoeba in vitro, but can do so if they are exposed to conditioned medium (CM) from PHA stimulated mononuclear leucocytes (MNLs). Specific antibody or complement was required to effect amoeba killing by CM modified neutrophils. Only short time exposure of the leucocytes to CM was required to endow them with amoebicidal properties. The CM was also shown to contain neutrophil migration inhibition activity and an activity(ies) which induced a respiratory burst in neutrophils. The results highlight the importance of MNL products other than specific antibody in neutrophil anti-microbial activity.