Pro-inflammatory differentiation by GM-CSF reduces prostanoid release and phagocytic activity in murine bone marrow-derived macrophages

Murine bone marrow-derived macrophages (BMDMs) are widely used to study macrophage functions in vitro. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) are routinely used to differentiate monocytes into M1- and M2-like macrophages, respectively. Although macrophage-derived eicosanoids regulate both inflammation and its resolution, the impact of these differentiation factors on eicosanoid production remains poorly understood. Additionally, eicosanoid secretion and transportation has never been characterised in these macrophage populations. In the present study, we show that BMDMs differentiated in the presence of GM-CSF (hereafter referred to as GM-BMDMs) produce markedly lower levels of arachidonic acid (AA)-derived prostanoids following lipopolysaccharide (LPS) activation than macrophages differentiated with M-CSF (hereafter referred to as M-BMDMs). Moreover, we found that GM-BMDMs failed to rapidly release LPS-induced prostanoids. Mechanistically, this delayed release of prostanoids likely arises from reduced expression of the prostaglandin efflux transporter multidrug resistance protein-4 (MRP4) alongside a concomitant upregulation of the influx prostaglandin transporter (PGT). Our results also highlight that analyses of both cell pellets and supernatants are essential when comparing oxylipin profiles between M1- and M2-like macrophages. We next studied the phagocytic capacity of GM-BMDMs and found that GM-BMDMs display a blunted increase in phagocytosis of fluorescent E. coli bioparticles after LPS stimulation compared to M-BMDMs. Pharmacological inhibition of microsomal prostaglandin E synthase-1 (mPGES-1), but not cyclooxygenase-2 (COX-2), promotes phagocytic capacity, suggesting that mPGES-1 inhibitors may be superior to COX-2 inhibitors for suppressing inflammation. Collectively, our findings reveal that GM-CSF not only modulates the production and trafficking of prostanoids but also constrains phagocytic activity in response to LPS, which can be enhanced by mPGES-1 inhibition.