Stimulation of the respiratory burst in peripheral blood monocytes by lipoteichoic acid. The involvement of calcium ions and phospholipase A₂

Lipoteichoic acid (LTA) from Streptococcus faecalis stimulates the respiratory burst in peripheral blood monocytes (mon), as measured by cytochrome C reduction. The effect of LTA was time and dose dependent. LTA stimulated the respiratory burst in a biphasic manner with a range of 1 to 1000 ng/ml·10⁶ mon, with maximal activity at 50 ng/ml. At this concentration LTA increased the activity from 0.97 ± 0.2 to 4.88 ± 0.2 nmol·10⁶ mon/20 min. The role of calcium ions in the effect of LTA in stimulating respiratory burst was studied by changing the availability of calcium ions in the medium, and by measuring the effect of LTA on ⁴⁵Ca²⁺ uptake and on intracellular Ca²⁺ levels. Removal of extracellular calcium ions in the presence of the calcium chelator EGTA, abolished the LTA-stimulated respiratory burst. LTA (50 ng/ml) was found to increase ⁴⁵Ca²⁺ uptake into monocytes within seconds (from 2200 ± 242 in the untreated cells to 4642 ± 365 cpm/min in the LTA-treated mon). At this concentration, LTA stimulated an immediate rise in the intracellular free Ca²⁺ concentration to 155 ± 15 nM as compared with 120 ± 14 nM in the unstimulated monocytes. LTA caused a specific release of arachidonic acid indicating the involvement of phospholipase A₂ in the transduction signal stimulating the respiratory burst by LTA.