Although macrophages (Mphi) and monocyte-derived dendritic cells (MDDC) come from a common precursor, they are distinct cell types. This report compares the two cell types with respect to the metabolism of
platelet-activating factor (PAF), a biologically active
lipid mediator. These experiments were prompted by our studies of localized
juvenile periodontitis, a disease associated with high
IgG2 production and a propensity of monocytes to differentiate into MDDC. As the
IgG2 Ab response is dependent on PAF, and MDDC selectively induce
IgG2 production, we predicted that PAF levels would be higher in MDDC than in Mphi. To test this hypothesis, human MDDC were prepared by treating adherent monocytes with
IL-4 and
GM-CSF, and Mphi were produced by culture in
M-CSF. Both Mphi and MDDC synthesized PAF; however, MDDC accumulated significantly more of this
lipid. We considered the possibility that PAF accumulation in MDDC might result from reduced turnover due to lower levels of
PAF acetylhydrolase (PAFAH), the
enzyme that catabolizes PAF. Although PAFAH increased when monocytes differentiated into either cell type, MDDC contained significantly less PAFAH than did Mphi and secreted almost no PAFAH activity. The reduced levels of PAFAH in MDDC could be attributed to lower levels of expression of the
enzyme in MDDC and allowed these cells to produce
PGE(2) in response to exogenous PAF. In contrast, Mphi did not respond in this manner. Together, these data indicate that PAF metabolism may impinge on regulation of the immune response by regulating the accessory activity of MDDC.