Inhibition of Macrophage Nitric‐Oxide Production and Ia‐Expression by Docosahexaenoic Acid, a Constituent of Fetal and Neonatal Serum

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Abstract
PROBLEM: We previously demonstrated profound inhibition of macrophage activation in the murine placenta in vivo. Given the importance of macrophages both in initiating cellular immunity by presenting antigen in the context of Ia to CD4+ T cells, and in killing cellular targets by producing nitric oxide (NO), inhibition of macrophage functions in the placenta may account for the increased susceptibility of the placenta to infection. We have also showed that docosahexaenoic acid (DHA), at concentrations present in the fetal circulation, has a major role in inhibiting macrophage Ia‐expression and NO production in the placenta. The concentration of DHA in fetal serum perfusing the placenta is 50x higher than in the adult. DHA has previously been reported to profoundly affect prostanoid production, to be metabolized by lipoxygenases, and to affect lipoxygenases. We now determine if these activities of DHA account for its inhibition of macrophage NO production and Ia‐expression. METHODS: Murine macrophages were cultured in vitro, exposed to IFNγ, endotoxin, DHA, and various eicosanoids, and their ability to produce NO or express Ia determined.RESULTS: Although the cyclooxygenase inhibitor, indomethacin, did inhibit NO production, DHA inhibited by a different mechanism. DHA further inhibited NO production by macrophages exposed to doses of indomethacin known to maximally inhibit prostanoid production. Stable, biologically active prostanoids did not reverse the inhibitory effect of DHA. Although DHA is metabolized by lipoxygenases, the lipoxygenase inhibitor NDGA did not reverse the inhibition of either NO production nor Ia expression. This indicates that lipoxygenase products of DHA did not mediate inhibition. NDGA itself inhibited NO production and Ia expression. However, DHA did not inhibit by inhibiting lipoxygenase activity because DHA further inhibited macrophages exposed to doses of DHA known to maximally inhibit lipoxygenases. Furthermore, stable biologically active analogs of lipoxygenase products did not reverse DHA inhibition. DHA also did not inhibit by preventing PAF production because PAF did not reverse inhibition of NO production.CONCLUSION: DHA did not inhibit Ia‐expression or NO production via its known effects on eicosanoid or PAF metabolism, nor by being metabolized by lipoxygenases.