Effects of acute phase high density lipoprotein on gram-negative and gram-positive bacterial growth

Abstract

High-density lipoprotein (HDL) plays and important role not only in protecting against atherosclerosis but also in innate immunity. Several lines of evidence have shown that HDL could inhibit the growth of gram-positive bacteria and ameliorate the toxic effects of endotoxin or lipopolysaccharide (LPS). In this study, we examined whether normal HDL or acute-phase HDL (AP-HDL) could directly inhibit the growth of gram-negative Escherichiacoli (E. coli) and gram-positive Staphylococus epidermidis (S. edpidermidis) in vitro and whether it could attenuate LPS-induced leukocyte adhesion on endothelial cells in vivo. Normal HDL and acute-phase HDL (AP-HDL) were purified form plasma of hamsters injected with NSS and LPS, respectively. In an in vitro study, cultures of E. coli and S. epidermidis were incubated with normal HDL or AP-HDL and amount of bacteria were determined using a spread plate method. In an in vivo study, LPS preincubated with NSS, normal HDL, AP-HDL, apoHDL, lipids of HDL or apo A-I were given intravenously into Wistar rats and the number of leukocytes adhered on endothelial cells of the mesenteric post-capillary venules were determined using intravital fluorescence microscopy. The results showed that both normal HDL and SP-HDL had no effect on suppressing the growth of E. Coli and S. Epidermidis after incubation for 0.5, 1, 2, 4, 6, or 24 hours. Varying concentration of HDL, 50, 100, 200, 400, 800 and 1,670 μg protein of HDL/100 g BW, did not have significant effects on the bacterial growth. Intravenous injection of LPS enhanced leukocyte adhesion on endothelial cells in response to LPS was significantly attenuated in a dose-dependent manner. AP-HDL was also able to significantly decrease LPS-induced leukocyte adhesion on endothelial cells and appeared to be more effective than normal HDL (5 μg protein of AP-HDL/100 g BW) since lower concentrations were required. This inhibitory effect of HDL was not due to HDL itself because it required preincubation of HDL with LPS. When HDL was separated into protein and lipid fractions, it was found that lipid-free apoHDL protein was able to significantly inhibit LPS-induced leukocyte adhesion, whereas the lipid component of HDL had no effect. Apo A-I, the major protein of HDL, could significantly decrease LPS-induced leukocyte adhesion. This effect of apo A-I aoso required preincubation of apo A-I with LPS. In conclusion, our studies suggested that HDL, both normal and acute phase, up to physiological concentration, could not suppress the living bacteria in vitro but could inhibit and inflammatory effect of LPS on endothelial cells in vivo. AP-HDL was more potent than normal HDL in inhibiting LPS-induced leukocyte adhesion, and this effect was attributed to the protein component of HDL. Apo A-I is one of the proteins of HDL responsible for this effect.