Evidence for pH Sensitivity of Tumor Necrosis Factor-α Release by Alveolar Macrophages

Abstract.

Alveolar macrophages (mφ) participate in inflammatory and immune responses in acidic microenvironments such as the interstitial fluids of tumors and abscesses. Two plasmalemmal H⁺ extruders interact to control the acid-base status of alveolar mφ, namely a V-type H⁺ pump (V-ATPase) and a Na⁺/H⁺ exchanger. The present study examined the effects of extracellular pH (pH_o) and H⁺ transport inhibitors on tumor necrosis factor-α (TNF-α) release induced by endotoxin (lipopolysaccharide) in rabbit alveolar mφ. The amount and activity of TNF-α in mφ-conditioned media were determined by enzyme-linked immunosorbent assay and L929 fibroblast bioassay, respectively. TNF-α release was suppressed progressively at lower pH_o values (≤7.0). Also, bafilomycin A₁ (a specific inhibitor of V-ATPases) significantly reduced the amount and activity of TNF-α in mφ-conditioned media (pH_o 7.4). However, bafilomycin caused a significant increase in the nonspecific cytotoxicity (i.e. bioactivity insensitive to TNF-α antibody) of mφ-conditioned media. The effects of bafilomycin specifically on TNF-α release followed a time course similar to that of acidic pH_o, suggesting that both treatments acted on similar events in the lipopolysaccharide signal transduction pathway. Amiloride (an inhibitor of Na⁺ transporters including the Na⁺/H⁺ exchanger) also suppressed TNF-α release but displayed a time course of action different from the acidic pH_o or bafilomycin.