Biochemical transformation of bacterial lipopolysaccharides by acyloxyacyl hydrolase reduces host injury and promotes recovery

Abstract

Animals can sense the presence of microbes in their tissues and mobilize their own defenses by recognizing and responding to conserved microbial structures (often called microbe-associated molecular patterns (MAMPs)). Successful host defenses may kill the invaders, yet the host animal may fail to restore homeostasis if the stimulatory microbial structures are not silenced. Although mice have many mechanisms for limiting their responses to lipopolysaccharide (LPS), a major Gram-negative bacterial MAMP, a highly conserved host lipase is required to extinguish LPS sensing in tissues and restore homeostasis. We review recent progress in understanding how this enzyme, acyloxyacyl hydrolase (AOAH), transforms LPS from stimulus to inhibitor, reduces tissue injury and death from infection, prevents prolonged post-infection immunosuppression, and keeps stimulatory LPS from entering the bloodstream. We also discuss how AOAH may increase sensitivity to pulmonary allergens. Better appreciation of how host enzymes modify LPS and other MAMPs may help prevent tissue injury and hasten recovery from infection.

Keywords: AOAH; CD14; Toll-like receptor 4 (TLR4); acyloxyacyl hydrolase; colitis; inflammation; lipid A; lipopolysaccharide (LPS); macrophage; natural killer cells (NK cells); tolerance.

Figure 1.

AOAH removes secondary acyl chains from LPS. E. coli LPS and lipid A are shown. The diglucosamine backbone is phosphorylated at 1 and 4′, and four primary 3-hydroxy fatty acyl chains are attached to the backbone in ester or amide linkage. Secondary acyl chains (red), usually myristate or laurate, are attached via acyloxyacyl linkage to two of the primary chains. Six acyl chains and both phosphates are required for optimal recognition by the MD-2–TLR4 receptor on animal cells. AOAH cleaves the acyloxyacyl linkages (arrows), converting stimulatory hexaacyl LPS into antagonistic tetraacyl LPS.

Figure 2.

A, soluble cofactors enable LPS signaling. A single Gram-negative bacterium or outer membrane vesicle can contain >10⁶ LPS molecules, all present in the outer leaflet of the outer membrane. 1, LBP binds to membrane particles containing hexaacyl (stimulatory) LPS and initiates the extraction and transfer of LPS monomers to sCD14 or mCD14. 2, LPS can be exchanged between sCD14 and mCD14. Either sCD14 or mCD14 can transfer the LPS monomer to MD2-TLR4 (3), which initiates TLR4 signaling following formation and dimerization of LPS-MD2-TLR4 ternary complexes (see text for more details) (4). B, AOAH deacylates LPS. 5, sCD14 can transfer LPS to mCD14 or to extracellular AOAH, which removes the two secondary acyl chains (red) to produce tetraacyl (≪stimulatory, antagonistic) dLPS. 6, dLPS is transferred back to sCD14 (no protein-free form of either hexaacyl or tetraacyl LPS is detected). sCD14 can transfer the tetraacyl dLPS to MD2-TLR4, but this ternary complex, unlike that of hexaacyl LPS-MD-2–TLR4 (Fig. 2A), does not form a stable dimerized ternary complex that can initiate signaling. 7, dLPS may also compete with hexaacyl LPS for binding MD2-TLR4. 8, mCD14 can promote uptake of LPS into an endosomal compartment, where it can be deacylated by AOAH.

Figure 3.

AOAH promotes recovery from tolerance. 1, LPS is injected into the peritoneal cavity of a naive Aoah^+/+ or Aoah^−/− mouse. 2, the resident and recruited peritoneal macrophages are stimulated by LPS and may internalize it. 3, after the initial inflammatory responses subside, the macrophages become tolerant; bioactive LPS (orange stars) remains. 4, in the Aoah^+/+ mouse, AOAH transforms LPS to dLPS (green stars), and the macrophages recover from tolerance. 5, if AOAH is absent, LPS cannot be inactivated (orange stars). Bioactive LPS in extracellular fluid keeps stimulating macrophages locally, preventing them from recovering from tolerance. These mice are more likely to respond slowly and die quickly after they are challenged with live E. coli on day 10.

Figure 4.

Intestinal AOAH modifies pulmonary immune responses. Colonic AOAH reduces translocation of bioactive LPS to the lungs, decreasing induction of tolerance in pulmonary epithelial cells to Th2 allergens, such as HDM.