The CD36 and SR-A/CD204 scavenger receptors fine-tune Staphylococcus aureus-stimulated cytokine production in mouse macrophages.

The occurring in SR-A/CD204- or CD36-deficient mice increased susceptibility to infections with Staphylococcus aureus (Sa) had traditionally been ascribed to the impairment of macrophage-mediated phagocytosis, which is, however, inconsistent with low effectiveness of unopsonized Sa killing within macrophages and redundant roles of both receptors in this process. We have found that Sa-stimulated cytokine production in mouse macrophages seems to be exclusively mediated by TLR2, mainly from within endosomes in response to Sa-derived lipoteichoic acid. By driving endocytic trafficking of TLR2 and its ligands through the clathrin-dependent pathway, CD36 and SR-A sensitize macrophages to activation by Sa as well as regulate the type and amount of cytokines produced. Additionally, upon direct Sa binding, both receptors autonomously generate anti-inflammatory signaling. Consequently, the delayed induction of acute inflammation in knockout mice may allow for the initial, uncontrolled multiplication of bacteria, stimulating excessive, septic shock-inducing production of inflammatory cytokines in later stages of infection.

Combined Biological Effects of N-Bromotaurine Analogs and Ibuprofen. Part I: Influence on Inflammatory Properties of Macrophages.

Taurine haloamines (N-chlorotaurine, N-bromotaurine) due to their strong antiseptic and anti-inflammatory properties are good candidates for topical application in treatment of skin inflammatory/infectious disorders. Recently, we have demonstrated that more stable N-bromotaurine analogs (N-dibromo-dimethyl taurine, N-monobromo-dimethyl taurine) and bromamine T show strong microbicidal and anti-inflammatory properties at concentrations well tolerated by human cells and tissue. Non-steroidal anti-inflammatory drugs (NSAIDs) with cyclooxygenase (COX) inhibitory activity are commonly used in various inflammatory diseases. However, systemic administration of NSAIDs may result in adverse side effects. For example, the use of ibuprofen in children with varicella is associated with enhanced serum levels of TNF-α and with increased risk of necrotizing soft tissue infections and secondary skin infections caused by invasive streptococci. The aim of this study was to examine combined immunomodulatory effects of bromamines and ibuprofen on J774.A1 macrophages. We have shown that the primary activity of ibuprofen, the inhibition of PGE2 production by activated macrophages was intensified in the presence of bromamines. Most importantly, the stimulatory effect of ibuprofen on production of inflammatory cytokines (TNF-α, IL-6) was inhibited by all tested bromamines. These observations indicate that bromamines may neutralize massive production of TNF-α at sites of inflammation, a side effect of ibuprofen. Therefore, we suggest that systemic administration of ibuprofen (NSAIDs) in treatment of inflammatory/infectious skin diseases should be supported by topical application of bromamines as an adjunctive therapy.

Combined Biological Effects of N-Bromotaurine Analogs and Ibuprofen. Part II: Influence on a Local Defense System.

The stable N-bromotaurine analogs (N-dibromo-dimethyl taurine, N-monobromo-dimethyl taurine), and bromamine T (BAT) show anti-inflammatory and microbicidal properties. These bromamines are good candidates for a treatment of skin infectious/inflammatory diseases as local antiseptics. Ibuprofen, a non-steroidal anti-inflammatory drug (NSAID), is commonly used in various infectious/inflammatory diseases due to its analgesic and antipyretic therapeutic effects. However, systemic administration of ibuprofen may also result in adverse side effects. It has been reported that ibuprofen enhances serum levels of TNF-α and worsens secondary skin infections caused by invasive streptococci (S. pyogenes). Recently we have demonstrated that bromamines inhibit the stimulatory effect of ibuprofen on the production of inflammatory cytokines (TNF-α, IL-6). The aim of this study was to examine the combined antibacterial actions of ibuprofen and bromamines against S. pyogenes and their joint effect on the generation of reactive oxygen species (ROS) by activated neutrophils and macrophages. We have shown that the microbicidal activity of bromamines against S. pyogenes was not altered by ibuprofen. On the other hand, co-administration of ibuprofen and bromamines markedly decreased the generation of ROS by activated neutrophils and macrophages. Finally, we discuss how the antioxidant combined effect of bromamines and ibuprofen may affect a local defense system.

Phagocytosis of live versus killed or fluorescently labeled bacteria by macrophages differ in both magnitude and receptor specificity.

Scavenger receptor (SR)-mediated opsonin-independent phagocytosis of bacteria by macrophages has been suggested to represent an important, early mechanism of anti-bacterial host defense. However, although the ability to bind bacteria has been demonstrated to be a shared feature of all types of SRs, in many cases the evidence is limited to the demonstration of increased binding of killed, fluorescently labeled bacteria to non-phagocytic cells transfected with these receptors. We sought to verify the ability of SRs to mediate non-opsonic phagocytosis of live Escherichia coli (Ec) and Staphylococcus aureus (Sa), model species of Gram-negative and -positive bacteria, respectively, and to assess the relative contributions of different SRs expressed on murine macrophages in this process. We found that the class A SR SR-A/CD204 was the major receptor mediating phagocytosis of fluorescently labeled Sa, whereas different SRs had highly redundant roles in the phagocytosis of live Sa. Conversely, different SRs contributed to the phagocytosis of fluorescently labeled Ec. In comparison, phagocytosis of live Ec was of much lower magnitude and was selectively mediated by SR-A. These results question the use of fluorescently labeled bacteria as valid replacements for live bacteria. The low magnitude of opsonin-independent phagocytosis of Ec and unimpaired phagocytosis of Sa in SR-A- or CD36-deficient macrophages indicate that the defect in this process might not be responsible for the reported impaired bacteria clearance in mice deficient in these receptors. We postulate that this impairment might result to a larger extent from inhibition of intracellular bacteria killing caused by pro-inflammatory cytokines, produced in excessive amounts by SR-deficient cells in response to bacterial products.

CD36 Differently Regulates Macrophage Responses to Smooth and Rough Lipopolysaccharide.

Lipopolysaccharide (LPS) is the major pathogen-associated molecular pattern of Gram-negative bacterial infections, and includes smooth (S-LPS) and rough (R-LPS) chemotypes. Upon activation by LPS through CD14, TLR4/MD-2 heterodimers sequentially induce two waves of intracellular signaling for macrophage activation: the MyD88-dependent pathway from the plasma membrane and, following internalization, the TRIF-dependent pathway from endosomes. We sought to better define the role of scavenger receptors CD36 and CD204/SR-A as accessory LPS receptors that can contribute to pro-inflammatory and microbicidal activation of macrophages. We have found that CD36 differently regulates activation of mouse macrophages by S-LPS versus R-LPS. The ability of CD36 to substitute for CD14 in loading R-LPS, but not S-LPS onto TLR4/MD-2 allows CD14-independent macrophage responses to R-LPS. Conversely, S-LPS, but not R-LPS effectively stimulates CD14 binding to CD36, which favors S-LPS transfer from CD14 onto TLR4/MD-2 under conditions of low CD14 occupancy with S-LPS in serum-free medium. In contrast, in the presence of serum, CD36 reduces S-LPS binding to TLR4/MD-2 and the subsequent MyD88-dependent signaling, by mediating internalization of S-LPS/CD14 complexes. Additionally, CD36 positively regulates activation of TRIF-dependent signaling by both S-LPS and R-LPS, by promoting TLR4/MD-2 endocytosis. In contrast, we have found that SR-A does not function as a S-LPS receptor. Thus, by co-operating with CD14 in both R- and S-LPS loading onto TLR4/MD-2, CD36 can enhance the sensitivity of tissue-resident macrophages in detecting infections by Gram-negative bacteria. However, in later phases, following influx of serum to the infection site, the CD36-mediated negative regulation of MyD88-dependent branch of S-LPS-induced TLR4 signaling might constitute a mechanism to prevent an excessive inflammatory response, while preserving the adjuvant effect of S-LPS for adaptive immunity.