Increased cyclooxygenase-2 and 5-lipoxygenase activating protein expression in peritoneal macrophages during ovalbumin immunization of mice and cytosolic phospholipase A(2) activation after antigen challenge.

The present study investigates phenotypic and functional differentiation of peritoneal macrophages during ovalbumin-induced subcutaneous immunization of mice. For the first time we show that, in mouse peritoneal macrophages, ovalbumin immunization induces an increase in cyclooxygenase-2 (COX-2) and 5-lipoxygenase activating protein (FLAP) expression whereas it inhibits cytosolic phospholipase A(2) (cPLA2) expression. The study of arachidonic acid (AA) metabolism in peritoneal macrophages from control (cPM) and ovalbumin-immunized (iPM) mice shows that the reduced cPLA2 expression is correlated to a reduced basal AA metabolism, but is not a limiting factor for the opsonized zymosan-, PMA-, or A23187-triggered AA metabolism. We also show that in vitro ovalbumin challenge induces, only in iPM, cPLA2 activation through phosphorylation of serine residues, via a mechanism involving MAP kinases, and through increased intracellular calcium concentrations, leading to eicosanoid production. In parallel, we report that, in peritoneal macrophages, ovalbumin immunization induces the expression of CD23, the low affinity receptor for IgEs known for its involvement in allergic diseases. Thus, the modified expression of the enzymes involved in AA metabolism and the difference of response of cPM and iPM toward the antigen are important elements to understand the underlying mechanisms of ovalbumin-induced allergic responses.

IL-13 induces serine phosphorylation of cPLA2 in mouse peritoneal macrophages leading to arachidonic acid and PGE2 production and blocks the zymosan-induced serine phosphorylation of cPLA2 and eicosanoid production.

In a recent investigation, we demonstrated that long-term treatment of macrophages with IL-13 enhances cPLA2 expression and modulates zymosan-stimulated AA mobilization. In the present study, we examine the ability of IL-13 to modify the cPLA2 activity and the AA mobilization of macrophages after a short-period of treatment. We demonstrate that in resting macrophages, IL-13 induces, through a MAP kinase-dependent process, (1) an increase of free AA release within 15 min, followed by increased PGE2 production and (2) a time-dependent serine phosphorylation of cPLA2. Conversely, in macrophages stimulated by zymosan, IL-13 added 30 min before zymosan inhibited the AA release and the serine phosphorylation of cPLA2 induced by the phagocytic agonist. In conclusion, these findings show for the first time that a Th2-type cytokine can upregulate cPLA2 activity and downregulate zymosan-induced AA metabolism. Thus, establishment of the connection between these two events may help to understand the complex regulatory role of IL-13 on the macrophage AA metabolism.

Effect of interleukin-4 on allergen-induced arachidonic acid metabolism of rat peritoneal macrophages during immediate hypersensitivity reactions.

The aim of this study was to investigate the [3H]arachidonic acid metabolism of rat peritoneal macrophages, induced by allergen (ovalbumin) and the impact of interleukin-4 on this process. We established that ovalbumin induces an increase of [3H]arachidonic acid mobilisation from membrane lipids and of [3H]arachidonic acid catabolism, principally by the 5-lipoxygenase pathway, when the macrophages are sensitized and when serum is present. The allergen effect is not modified by the presence of interleukin-4 in the culture medium of macrophages 15 h before the allergen challenge. We also showed that, whereas the basal [3H]arachidonic acid metabolism of macrophages from control and actively sensitized rats is not different, interleukin-4 increases the [3H]arachidonic acid mobilisation and catabolism by cyclooxygenase and 5-lipoxygenase pathways in macrophages from control rats although it does not in macrophages from actively sensitized rats. In macrophages from control rats, the interleukin-4 effect is diminished by the addition of IgEs to their culture medium. In summary, interleukin-4 has an enhancer effect on the macrophage arachidonic acid catabolism that depends on the sensitization condition of the cell but that has no consequences on the further increased arachidonic acid metabolism induced by the allergen.