Lipopolysaccharide modulates receptors for leukotriene B₄, C5a, and formyl-methionyl-leucyl-phenylalanine on rabbit polymorphonuclear leukocytes

Peripheral blood polymorphonuclear leukocytes (PMNL) isolated from rabbits after an i.v. injection of endotoxin exhibited decreased chemotactic migration in response to leukotriene B₄ (LTB₄) and C5a, but not N-formyl-methionyl-leucyl-phenylalanine (fMLP), after endotoxin treatment. The binding of radiolabeled LTB₄, fMLP, and C5a to isolated PMNL was assessed in order to determine whether altered receptor expression could account for the observed functional changes. Control PMNL expressed binding sites for fMLP, LTB₄, and C5a similar to those previously characterized from human PMNL. Control PMNL expressed a single class of 14,600 ± 2700 receptors for fMLP with a mean dissociation constant (Kd) of 2.0 ± 0.6 nM at 0°C, whereas two subclasses of binding sites were expressed for LTB₄: 10,300 ± 6800 high-affinity and 85,600 ± 53,000 low-affinity binding sites per PMNL with mean Kd for LTB₄ of 0.75 ± 0.43 nM and 70 ± 58 nM (mean ± SD, n = 5), respectively. Control PMNL bound [¹²⁵I]-C5a in a dose-dependent and saturable manner at 24°C. At saturating concentrations of C5a, PMNL obtained from control rabbits bound 270,000 ± 50,000 molecules of [¹²⁵I]-C5a with half-maximal binding occurring at [¹²⁵I]-C5a concentrations of 5.5 ± 1.9 nM. The binding of LTB₄ and C5a to PMNL obtained 24 hr after an i.v. injection of endotoxin was markedly decreased compared with control PMNL. PMNL from endotoxin-treated rabbits exhibited 68% fewer high-affinity binding sites per PMNL for LTB₄ and a 51% decrease in the amount of [¹²⁵I]-C5a bound at saturating concentrations compared with control PMNL. There was no significant change in the Kd of the high-affinity binding sites for LTB₄, no change in the Kd and number of the low-affinity binding sites for LTB₄, and a small decrease in the apparent Kd for C5a to 3.3 ± 1.1 nM. Even though the pretreatment with i.v. endotoxin did not alter chemotactic or degranulation responses elicited by fMLP, the endotoxin pretreatment induced an eightfold increase in the receptor density without altering the Kd for fMLP. Decreased receptor expression could account in large part for the decreased chemotactic responsiveness towards C5a and LTB₄ induced by LPS. The finding that a substantial increase in receptors for fMLP need not be accompanied by a comparable functional change suggests that decreased efficiency in receptor coupling to intracellular biochemical events may also result from i.v. endotoxin.