Acyloxyacyl Hydrolase: Studies on Its Regulation and Function in Mus Musculus
Acyloxyacyl hydrolase (AOAH) is an enzyme that detoxifies Gram-negative bacterial lipopolysaccharides (LPS) by selectively removing secondary acyl chains from the lipid A moiety. Originally found in neutrophils, it is also produced by monocyte-macrophages and renal proximal tubule cells. In the studies described here, I found that both immature dendritic cells (DCs) of the XS52 cell line and bone marrow-derived DCs produce AOAH. AOAH expression decreased when DCs were incubated with IL-4, IL-1ᬠTNFa and an agonistic CD40 antibody (maturation cocktail), and increased following treatment with microbial agonists that engage 3 distinct Toll-like receptors (LPS, TLR4; CpG oligodeoxynucleotides, TLR9; and a Gram-positive bacterium (Micrococcus luteus), TLR2). Maturation cocktail treatment also diminished, while LPS treatment enhanced or maintained, the cells' ability to kill E. coli, deacylate LPS, and degrade bacterial proteins. Enzymatic deacylation of LPS is thus an intrinsic, regulated mechanism by which DCs may modulate host responses to this potent bacterial agonist. To study the biological functions of AOAH, AOAH-deficient mice were generated by targeted gene disruption. AOAH did not protect mice from lethal doses of LPS or Gram-negative bacterial challenge. In response to subcutaneous injections of LPS, however, AOAH-deficient mice produced significantly higher levels of non-specific (polyclonal) IgM and IgG3 than did wild type mice. Anti-double-stranded DNA and anti-nucleosome IgM and IgG antibody levels were also higher in LPS-immunized AOAH-deficient mice than in wild type control mice. In addition, the partially-deacylated LPS product (dLPS) induced lower polyclonal antibody responses in vivo than did mock-treated LPS, yet the anti-LPS specific responses to dLPS and LPS were equivalent. These results suggest that AOAH may diminish potentially harmful polyclonal antibody responses to Gram-negative infection but maintain the protective anti-LPS specific response. Since B cells do not produce the enzyme, my results also point to an important role for macrophages and DCs in modulating B-cell responses to LPS antigens. In addition, the absence of AOAH did not alter the ability of LPS to function as an adjuvant, indicating that this activity is mechanistically distinct from stimulation of polyclonal antibody production. Finally, the ability of a bacterial lipopeptide to stimulate polyclonal antibody production only in AOAH -/- mice suggests that the enzyme may also regulate immune responses to non-LPS bacterial agonists.