Staphylococcus aureus-stimulated mononuclear leucocyte-conditioned medium increases the neutrophil bactericidal activity, and augments oxygen radical production and degranulation in response to the bacteria.

Conditioned medium produced by human mononuclear leucocytes (MNL) stimulated with formalin-fixed, heat-killed Staphylococcus aureus enhanced the killing of opsonized S. aureus by human neutrophils. This enhancement was not seen when conditioned medium from non-stimulated MNL or medium cultured in the absence of both bacteria and MNL was used. There was a five-fold decrease in survival of bacteria when neutrophils were treated with conditioned medium. Conditioned medium-treated neutrophils showed increased initial rate of killing but after 20-30 min the rate of killing was similar to that of non-conditioned medium-treated leucocytes. The neutrophils treated with conditioned medium showed increased production of chemiluminescence, superoxide and lysosomal enzyme release (from both azurophilic and specific granules) in response to opsonized S. aureus. The results demonstrate that bacterial interaction with MNL leads to the release of mediators (cytokines) which potentiate the anti-bacterial function of neutrophils.