Low pH Environmental Stress Inhibits LPS and LTA-Stimulated Proinflammatory Cytokine Production in Rat Alveolar Macrophages

Abstract

Gastric aspiration increases the risks for developing secondary bacterial pneumonia. Cytokine elaboration through pathogen recognition receptors (PRRs) is an important mechanism in initiating innate immune host response. Effects of low pH stress, a critical component of aspiration pathogenesis, on the PRR pathways were examined, specifically toll-like receptor-2 (TLR2) and TLR4, using isolated rat alveolar macrophages (aMØs). We assessed the ability of aMØs after brief exposure to acidified saline to elaborate proinflammatory cytokines in response to lipopolysaccharide (LPS) and lipoteichoic acid (LTA) stimulation, known ligands of TLR4 and TLR2, respectively. Low pH stress reduced LPS- and LTA-mediated cytokine release (CINC-1, MIP-2, TNF-α, MCP-1, and IFN-β). LPS and LTA increased intracellular Ca2+ concentrations while Ca2+ chelation by BAPTA decreased LPS- and LTA-mediated cytokine responses. BAPTA blocked the effects of low pH stress on most of LPS-stimulated cytokines but not of LTA-stimulated responses. In vivo mouse model demonstrates suppressed E. coli and S. pneumoniae clearance following acid aspiration. In conclusion, low pH stress inhibits antibacterial cytokine response of aMØs due to impaired TLR2 (MyD88 pathway) and TLR4 signaling (MyD88 and TRIF pathways). The role of Ca2+ in low pH stress-induced signaling is complex but appears to be distinct between LPS- and LTA-mediated responses.

Lactic Acid Inhibits Lipopolysaccharide-Induced Mast Cell Function by Limiting Glycolysis and ATP Availability

“…We also observed that both acidification with formic acid and elevated concentrations of sodium lactate could mimic the suppressive effects of lactic acid. These results support many studies that have reported immunosuppressive effects of acidification (21,72,73), high concentrations of lactate (10-100 mM) (20)(21)(22), and slightly lower concentrations of lactic acid (5-20 mM) (23,42,43). These results also support our data, showing that lactic acid suppresses glycolysis because H + inhibits phosphofructokinase, the rate-limiting enzyme in the glycolytic pathway, and lactate can inhibit different isoforms of lactate dehydrogenase, an example of product inhibition that can further reduce glycolysis (74,75).…”

Synergy between acid and endotoxin in an experimental model of aspiration-related lung injury progression

“…Our results rather suggest that the synergy observed in our model is due to an initial low pH-mediated release (possibly from direct injury of the epithelium) of some soluble mediator into the alveolar lining fluid that can amplify LPS signaling at the point of injury. In fact, a low pH environment, if anything, appears to have a downregulatory effect on alveolar macrophage function and cytokine release (12), lending greater support to our theory that the synergy imparted by acid in our in vivo model is due to a direct caustic injury to the epithelium and barrier function.…”

“…To evaluate the time course of inflammatory change and barrier disruption independent of any potential effect from mechanical ventilation, additional mice (n ϭ 6 per group and time point) were again anesthetized with isoflurane for aspiration of 4 l/gm of normal saline or HCl in saline (pH 1.3) and assigned to receive aerosolized LPS or nothing 1 h later. The mice were euthanized at 2, 5, 13, 25, or 49 h following aspiration (1,4,12,24, or 48 h following LPS), at which time BALF was collected in the same manner described above. The spun BALF cell pellets were resuspended and stained, and the total cell counts were determined manually using a hemacytometer.…”

Influence of Mushroom Polysaccharide, Nano-Copper, Copper Loaded Chitosan, and Lysozyme on Intestinal Barrier and Immunity of LPS-mediated Yellow-Feathered Chickens

“…Similar changes in gene expression of TNF-α in the spleen were observed in LPS treatment ( Table 5). These results are consistent with previous reports, which indicated that LPS induce the production and expression of proinflammatory cytokines [62][63][64]. Except for antibiotic, nano-copper, and copper loaded chitosan treatments, the tested dietary additives here decreased the content of IFN-γ in plasma, and IL-1β content in jejunal mucosa.…”