Priming induces functional coupling of N-formyl-methionyl-leucyl-phenylalanine receptors in equine neutrophils.

The synthetic formylpeptide fMLP is widely used as a model chemoattractant and secretagogue for mammalian neutrophils. Despite possessing fMLP receptors, equine neutrophils do not produce superoxide anions in response to fMLP and there is no inflammatory reaction in the horse when fMLP is injected intradermally. The functional capability of these receptors was investigated after pretreatment with recognized priming agents. Purified neutrophils were pretreated with lipopolysaccharide (LPS), platelet-activating factor (PAF), or tumor necrosis factor alpha (TNF-alpha) and superoxide anion generation and shape change quantified by lucigenin-dependent chemiluminescence (LDCL) and flow cytometry, respectively. LPS, TNF-alpha, and PAF pretreatment induced significant LDCL in response to fMLP; similarly LPS pretreatment was a prerequisite for fMLP-stimulated neutrophil polarization in response to fMLP. However, LPS failed to induce fMLP-mediated chemotaxis of equine neutrophils. These data indicate that equine neutrophil fMLP receptors are not vestigial as previously thought but can trigger both respiratory burst activity and cell polarization responses after priming.