The eicosanoid Leukotriene B4 (LTB4) relays chemotactic signals to direct neutrophil interstitial migration through its receptor, BLT1. However, whether the LTB4-BLT1 axis relays signals during intravascular neutrophil response has not been addressed. Here, we report that LTB4 produced by neutrophils acts as an autocrine/paracrine signal to drive neutrophil recruitment, arrest, and extravasation during infection in living mice. Using Intravital Subcellular Microscopy (ISMic), we reveal that LTB4 elicits sustained cell polarization and adhesion response during neutrophil arrest in vivo. Specifically, LTB4 signaling coordinates the dynamic redistribution of non-muscle Myosin IIA (NMIIA) and b2-integrin (Itgb2), whose retention at the cell surface facilitates neutrophil arrest. Notably, we also found that inhibition of the machinery regulating exosomes release from the cell blocks the autocrine/paracrine LTB4-dependent extravasation response. Overall, our study reveals a critical function for LTB4 in promoting neutrophil communication in the vasculature during early inflammation response.
RESULTS
The LTB4-BLT1 axis is required for the persistent recruitment, rolling, arrest and extravasation of neutrophils in response to infectionTo assess the role of the LTB4-BLT1 axis during neutrophil extravasation, we developed an in vivo inflammation model based on the injection of heat-killed E. coli into the hind footpad of anesthetized mice (Figs. 1A and 1B). First, we set up the optimal conditions for imaging neutrophil extravasation in a mouse strain expressing GFP in myeloid cells (LyzM-GFP) 13 . Upon infection, neutrophils accumulated in the blood vessels and extravasated ( Fig. 1A right panel, pink arrows, and Movie S1), whereas under control conditions, we observed no neutrophil retention in the vasculature (Fig. 1A left panel and Movie S1). Similar results were obtained using an adoptive transfer model, where neutrophils were purified from the bone marrow of a WT mouse, labeled with a cellpermeant fluorescent dye and injected into a recipient WT mouse (Fig. 1B, pink arrows and Movie S2). Using the latter approach, we infected the footpad of Alox5 -/mice, which are incapable of producing LTB4, and introduced neutrophils purified from either WT or Alox5 -/mice (Fig. 1C). We observed that up to 3 hours post-infection, the recruitment of Alox5 -/neutrophils to the infected footpad was significantly reduced in comparison to WT neutrophils (Figs. 1C & 1D). To gain further insights into this process, we visualized the intravascular dynamics of WT and Alox5 -/neutrophils introduced into the infected Alox5 -