Reduced bacterial dissemination and liver injury in CD14-deficient mice following a chronic abscess-forming peritonitis induced by Bacteroides fragilis

Differential expression of CD14-dependent and independent pathways for chemokine induction regulates neutrophil trafficking in infection

Previous studies have shown that CD14(-/-) mice are resistant to peritoneal infection with some clinical isolates of Escherichia coli and that this resistance is accompanied by an enhanced ability to clear the bacteria; in contrast, normal mice expressing CD14 fail to clear the bacteria, causing severe sepsis and death. The enhanced clearance in CD14(-/-) mice is dependent on early neutrophil recruitment to the local foci of infection in the PC. The studies described show that neutrophil recruitment in CD14(-/-) mice occurs as a result of the local induction of the CXCL1 and CXCL2 chemokines, KC and MIP-2. Although local induction of these chemokines also occurs in normal mice, their effects on neutrophil recruitment to the PC appear to be counterbalanced by very high levels of these chemokines in the blood of normal, but not CD14(-/-), mice. Neutrophil recruitment to the PC is also inhibited in normal mice in response to LPS, which also induces high chemokine levels in the blood of normal, but not CD14(-/-), mice. However, MPLA, a monophosphorylated derivative of LPS, is able to induce early neutrophil recruitment in normal mice; this is because MPLA, unlike LPS or E. coli, induces MIP-2 and KC in the PC but not in the blood of normal mice. The pretreatment of normal mice with MPLA is able to protect them from a lethal E. coli infection. Thus, stimulation of a local CD14-independent chemokine induction pathway without triggering a systemic CD14-dependent chemokine pathway can protect against severe E. coli infections.

CD14 impairs host defense against gram-negative sepsis caused by Burkholderia pseudomallei in mice

BACKGROUND

CD14 is a pattern-recognition receptor that can facilitate the presentation of bacterial components to either Toll-like receptor 2 (TLR2) or TLR4. We have recently shown that during melioidosis, a severe infection caused by the gram-negative bacterium Burkholderia pseudomallei, TLR2 but not TLR4 impacts the immune response of the intact host in vivo.

METHODS

The function of CD14 in melioidosis was analyzed by means of in vitro and in vivo approaches, using wild-type (WT) and CD14 knockout (KO) mice.

RESULTS

CD14-deficient macrophages and whole blood leukocytes released less tumor necrosis factor (TNF)-alpha on stimulation with B. pseudomallei or B. pseudomallei lipopolysaccharide in vitro, compared with WT cells. Strikingly, CD14 KO mice intranasally inoculated with B. pseudomallei demonstrated reduced lethality and significantly decreased bacterial outgrowth, compared with WT mice. Administration of recombinant soluble CD14 to CD14 KO mice partially reversed their phenotype to that of WT mice. Lastly, CD14 deficiency did not alter the capacity of macrophages or neutrophils to phagocytose or kill B. pseudomallei.

CONCLUSION

CD14 is crucially involved in the recognition of B. pseudomallei by innate immune cells but plays a remarkable detrimental role in the host response against B. pseudomallei. Inhibition of CD14 may be a novel treatment strategy in melioidosis.

Recombinant soluble CD14 reduces severity of intramammary infection by Escherichia coli

The interaction among gram-negative bacteria, the innate immune system, and soluble CD14 (sCD14) has not been well documented. The effect of recombinant bovine sCD14 (rbosCD14) on milk somatic cell count (SCC), bacterial clearance, and cytokine production was investigated by using a bovine intramammary Escherichia coli infection model. We first determined whether rbosCD14 would increase the SCC during a lipopolysaccharide (LPS) challenge. Three quarters of each of six healthy lactating cows were injected with either 0.3 microg of LPS, 0.3 microg of LPS plus 100 micro g of rbosCD14, or saline. In comparison with quarters injected with LPS alone, the SCC was twofold higher (P < 0.05) in quarters injected with LPS plus rbosCD14 after the challenge. We therefore hypothesized that when E. coli bacteria invade the mammary gland, sCD14 in milk would interact with LPS and rapidly recruit neutrophils from the blood to eliminate the bacteria before establishment of infection. To test this hypothesis, two quarters of each of nine healthy cows were challenged with either 50 CFU of E. coli plus saline or 50 CFU of E. coli plus 100 microg of rbosCD14. Quarters challenged with E. coli plus rbosCD14 had a more rapid recruitment of neutrophils, which was accompanied by a faster clearance of bacteria, lower concentrations of tumor necrosis factor alpha and interleukin-8 in milk, and milder clinical symptoms, than challenged quarters injected with saline. Results indicate that increasing the concentration of sCD14 in milk may be a potential strategy with which to prevent or reduce the severity of infection by coliform bacteria.

Mycobacterium avium infection in CD14-deficient mice fails to substantiate a significant role for CD14 in antimycobacterial protection or granulomatous inflammation

CD14 is a pattern-recognition receptor implicated in the inflammatory response to microbial components such as lipopolysaccharide, peptidoglycan and lipoarabinomannan. In this work, we made use of CD14-deficient (CD14-/-) mice to evaluate the relative importance of CD14 in response to infection with viable, intact cells of Mycobacterium avium in vitro and in vivo. Following co-incubation of either bone marrow-derived macrophages (Mphi) or thioglycollate-elicited peritoneal Mphi from CD14-/- mice with viable M. avium, tumour necrosis factor (TNF) production was significantly reduced and delayed compared to TNF secretion by infected CD14+/+ Mphi. However, following intravenous infection with a M. avium strain of either high virulence (TMC724) or intermediate virulence (SE01), there was no difference in the bacterial loads of lungs, livers or spleens at 3, 5 and 8 weeks postinfection in CD14-/- mice when compared with syngeneic CD14+/+ mice. At these time-points, TNF and interferon-gamma (IFN-gamma) mRNA expression in the liver was similar in infected CD14+/+ and CD14-/- mice, and granuloma formation and expression of inducible nitric oxide synthase within granuloma Mphi was the same in both mouse groups. In conclusion, although the absence of CD14 results in significantly reduced and delayed TNF production in response to stimulation with M. avium in vitro, there is no evidence that CD14 plays a significant role in either the antibacterial defence or the chronic granulomatous reaction to M. avium infection in vivo.

Critical role of CD14 for production of proinflammatory cytokines and cytokine inhibitors during sepsis with failure to alter morbidity or mortality

We investigated the immunopathophysiologic responses during sepsis induced by cecal ligation and puncture (CLP) in CD4-deficient (CD14 knockout [CD14KO]) mice. Our studies were designed to specifically test the role of CD14 in the inflammatory response to sepsis and to ascertain if alterations would improve morbidity or mortality. Sepsis was induced using the CLP model with appropriate antibiotic treatment. The severity of sepsis increased in the CD14KO mice with increasing puncture size (18 gauge [18G], 21G, and 25G). Following CLP, body temperature (at 12 h) and gross motor activity levels of the sham and 25G CLP groups recovered to normal, while the 21G and 18G CLP groups exhibited severe hypothermia coupled with decreased gross motor activity and body weight. There were no significant differences in survival, temperature, body weight, or activity levels between CD14KO and control mice after 21G CLP. However, CD14KO mice expressed two- to fourfold less pro-inflammatory (interleukin-1beta [IL-1beta], tumor necrosis factor [TNF], and IL-6) and anti-inflammatory (IL-10, IL-1 receptor antagonist, and TNF receptors I and II) cytokines in the blood after 21G CLP. Plasma levels of the chemokines macrophage inflammatory protein 2alpha and KC were similarly reduced in CD14KO mice. A similar trend of decreased cytokine and cytokine inhibitor levels was observed in the peritoneal cavity of CD14KO mice. Our results indicate that the CD14 pathway of activation plays a critical role in the production of both pro-inflammatory cytokines and cytokine inhibitors but has minimal impact on the morbidity or mortality induced by the CLP model of sepsis.

Induction of a novel mechanism of accelerated bacterial clearance by lipopolysaccharide in CD14-deficient and Toll-like receptor 4-deficient mice

Despite the lack of a proinflammatory response to LPS, CD14-deficient mice clear Gram-negative bacteria (Escherichia coli 0111) at least 10 times more efficiently than normal mice. In this study, we show that this is due to an early and intense recruitment of neutrophils following the injection of Gram-negative bacteria or LPS in CD14-deficient mice; in contrast, neutrophil infiltration is delayed by 24 h in normal mice. Similar results of early LPS-induced PMN infiltration and enhanced clearance of E. coli were seen in Toll-like receptor (TLR) 4-deficient mice. Furthermore, the lipid A moiety of LPS induced early neutrophil infiltration not only in CD14-deficient and TLR-4-deficient mice, but also in normal mice. In conclusion, the lipid A component of LPS stimulates a unique and critical pathway of innate immune responses that is independent of CD14 and TLR4 and results in early neutrophil infiltration and enhanced bacterial clearance.