Vertebrates vary in resistance and resilience to infectious diseases, and the mechanisms regulating the trade-off between these two often opposing protective processes are not well understood. Variability in the sensitivity of species to induction of damaging inflammation in response to equivalent pathogen loads (resilience) complicates the use of animal models that reflect human disease. We found that induction of pro-inflammatory cytokines from macrophages in response to inflammatory stimuli in vitro is regulated by proteins in the sera of species in inverse proportion to their in vivo resilience to lethal doses of bacterial lipopolysaccharide over a range of 10,000-fold. This finding suggests that proteins in serum rather than intrinsic cellular differences may play a role in regulating variations in resilience to microbe-associated molecular patterns between species. Involvement of circulating proteins as key molecules raises hope that the process might be manipulated to create better animal models and potentially new drug targets.

Detection of LPS in tissues is an integral component of innate immunity that acts to protect against invasion by Gram-negative bacteria. Plasma down-regulates LPS-induced cytokine production from macrophages, thereby limiting systemic inflammation in blood and distant tissues. To identify the protein(s) involved in this process, we used classical biochemical chromatographic techniques to identify fractions of mouse sera that suppress LPS-induced TNF from bone marrow-derived macrophages (BMDMs). Fractionation yielded microgram quantities of a protein that was identified by MS to be hemopexin (Hx). Mouse Hx purified on hemin-agarose beads and rhHx decreased the production of cytokines from BMDMs and peritoneal macrophages induced by LPS. Preincubation of LPS with Hx did not affect the activity of LPS on LAL, whereas preincubation of Hx with macrophages followed by washing resulted in decreased activity of these cells in response to LPS, suggesting that Hx acts on macrophages rather than LPS. Heme-free Hx did not stimulate HO-1 in the macrophages. Purified Hx also decreased TNF and IL-6 from macrophages induced by the synthetic TLR2 agonist Pam3Cys. Our data suggest that Hx, which is an acute-phase protein that increases during inflammation, limits TLR4 and TLR2 agonist-induced macrophage cytokine production directly through a mechanism distinct from HO-1.

Sepsis is initiated by interactions between microbial products and host inflammatory cells. Toll-like receptors (TLRs) are central innate immune mediators of sepsis that recognize different components of microorganisms. Peptidoglycan-associated lipoprotein (PAL) is a ubiquitous gram-negative bacterial outer-membrane protein that is shed by bacteria into the circulation of septic animals. We explored the inflammatory effects of purified PAL and of a naturally occurring form of PAL that is shed into serum. PAL is released into human serum by Escherichia coli bacteria in a form that induces cytokine production by macrophages and is tightly associated with lipopolysaccharide (LPS). PAL activates inflammation through TLR2. PAL and LPS synergistically activate macrophages. These data suggest that PAL may play an important role in the pathogenesis of sepsis and imply that physiologically relevant PAL and LPS are shed into serum and act in concert to initiate inflammation in sepsis.

Background and Aims-Intestinal alkaline phosphatase (IAP) is a gut mucosal defense factor known to dephosphorylate lipopolysaccharide (LPS); however, the role of IAP in the gut response to luminal bacteria remains poorly defined. We investigated immune responses of wild-type (WT) and IAP-knockout (IAP-KO) mice to LPS and Salmonella typhimurium challenges. Methods-Cryostat sectioning and standard indirect immunohistochemical staining for major histocompatibility complex (MHC) class II molecules were performed on liver tissue from WT and IAP-KO mice. WT and IAP-KO mice were orally gavaged with S. typhimurium; bacterial translocation to mesenteric nodes, liver, and spleen was determined by tissue homogenization and plating. In other experiments, WT and IAP-KO mice received intraperitoneal injections of LPS, with subsequent quantification of complete blood counts and serum interleukin (IL)-6 by enzymelinked immunosorbent assay (ELISA). WT and IAP-KO whole blood were plated and stimulated with LPS and Pam-3-Cys, followed by cytokine assays. Results-Immunohistologic liver examinations showed increased expression of MHC class II molecules in IAP-KO mice. Following S. typhimurium challenge, WT mice appeared moribund compared with IAP-KO mice, with increased bacterial translocation. WT mice had [50% decrease (P \ .005) in platelets and 1.8-fold (P \ .05) increased serum IL-6 compared with IAP-KO mice in response to LPS injections. IAP-KO whole-blood stimulation with LPS and Pam-3-Cys resulted in increased IL-6 and tumor necrosis factor (TNF)-alpha secretion compared with WT. Conclusions-IAP-KO mice exhibit characteristics consistent with local LPS tolerance. Wholeblood response of IAP-KO mice did not reflect systemic tolerance. These data suggest that IAP is a local immunomodulating factor, perhaps regulating LPS-toll-like receptor 4 (TLR4) interaction between commensal microflora and intestinal epithelium.

Multiple older studies report that immunoglobulin directed to rough mutant bacteria, such as E. coli J5, provides broad protection against challenge with heterologous strains of Gram-negative bacteria. This protection was initially believed to occur through binding of immunoglobulin to bacterial lipopolysaccharide (LPS). However, hundreds of millions of dollars have been invested in attempting to develop clinically-effective anti-LPS monoclonal antibodies without success, and no study has shown that IgG from this antiserum binds LPS. Identification of the protective mechanism would facilitate development of broadly protective human monoclonal antibodies for treating sepsis. IgG from this antiserum binds 2 bacterial outer membrane proteins: murein lipoprotein (MLP) and peptidoglycan-associated lipoprotein (PAL). Both of these outer membrane proteins are highly conserved, have lipid domains that are anchored in the bacterial membrane, are shed from bacteria in blebs together with LPS, and activate cells through Toll-like receptor 2. Our goal in the current work was to determine if passive immunization directed to MLP and PAL protects mice from Gram-negative sepsis. Neither monoclonal nor polyclonal IgG directed to MLP or PAL conferred survival protection in 3 different models of sepsis: cecal ligation and puncture, an infected burn model, and an infected fibrin clot model mimicking peritonitis. Our results are not supportive of the hypothesis that either anti-MLP or anti-PAL IgG are the protective antibodies in the previously described anti-rough mutant bacterial antisera. These studies suggest that a different mechanism of protection is involved.