The Innate Immune Response Against Staphylococcus aureus

Antimicrobial Peptide MPX with Broad-Spectrum Bactericidal Activity Promotes Proper Abscess Formation and Relieves Skin Inflammation

Bacteria have developed antibiotic resistance during the large-scale use of antibiotics, and multidrug-resistant strains are common. The development of new antibiotics or antibiotic substitutes has become an important challenge for humankind. MPX is a 14 amino acid peptide belonging to the MP antimicrobial peptide family. In this study, the antibacterial spectrum of the antimicrobial peptide MPX was first tested. The antimicrobial peptide MPX was tested for antimicrobial activity against the gram-positive bacterium S. aureus ATCC 25923, the gram-negative bacteria E. coli ATCC 25922 and Salmonella enterica serovar Typhimurium CVCC541, and the fungus Candida albicans ATCC 90029. The results showed that MPX had good antibacterial activity against the above four strains, especially against E. coli, for which the MIC was as low as 15.625 μg/mL. The study on the bactericidal mechanism of the antimicrobial peptide revealed that MPX can destroy the integrity of the cell membrane, increase membrane permeability, and change the electromotive force of the membrane, thereby allowing the contents to leak out and mediating bacterial death. A mouse acute infection model was used to evaluate the therapeutic effect of MPX after acute infection of subcutaneous tissue by S. aureus. The study showed that MPX could promote tissue repair in S. aureus infection and alleviate lung damage caused by S. aureus. In addition, skin H&E staining showed that MPX treatment facilitated the formation of appropriate abscesses at the subcutaneous infection site and facilitated the clearance of bacteria by the skin immune system. The above results show that MPX has good antibacterial activity and broad-spectrum antibacterial potential and can effectively prevent the invasion of subcutaneous tissue by S. aureus, providing new ideas and directions for the immunotherapy of bacterial infections.

Combinatory effects of Dipterocarpus alatus twig emulgel: Wound-restoring, antibacterial, and anti-inflammatory activities against methicillin-resistant Staphylococcus aureus-infected mouse superficial wounds

Dipterocarpus alatus has been used for the treatment of infectious skin diseases and ulcerative wounds in Thai traditional medicine. A major pathogen in human superficial skin infections is methicillin-resistant Staphylococcus aureus (MRSA). This study determined the wound healing, antibacterial, and anti-inflammatory activities of D. alatus twig emulgel against MRSA-infected mouse superficial skin wounds. Ethyl acetate-methanol crude extract of D. alatus twig was incorporated into emulgel at concentrations of 20 and 40 mg/g (D20 and D40) and its activity was compared to tetracycline emulgel (160 μg/g, Tetra). MRSA-infected superficial wounds demonstrated decreased skin barrier strength, increased transepidermal water loss (TEWL), and mast cell accumulation. Expression of toll-like receptor 2 (TLR-2), NF-κβ, TNFα, IL-1β, IL-6 and IL-10 genes were induced after MRSA infection. Daily application of 100 μL of D20 or D40 for 9 days restored skin barrier strength and TEWL while reducing mast cell and MRSA numbers compared to the non-treated group (MRSA-NT). The wounds treated with D20 and D40 were entirely healed on day 9. Expression of TLR-2 and cytokine-related genes NF-κβ, TNFα, IL-1β, IL-6 and IL-10 were normalized by treatment with either D20 or D40. Therefore, emulgel containing 20 to 40 mg/g ethyl acetate-methanol crude D. alatus twig extract is a good candidate for development as a topical formulation for MRSA-infected ulcerated wounds.

The Influence of Antibiotic Resistance on Innate Immune Responses to Staphylococcus aureus Infection

Staphylococcus aureus (S. aureus) causes a broad range of infections and is associated with significant morbidity and mortality. S. aureus produces a diverse range of cellular and extracellular factors responsible for its invasiveness and ability to resist immune attack. In recent years, increasing resistance to last-line anti-staphylococcal antibiotics daptomycin and vancomycin has been observed. Resistant strains of S. aureus are highly efficient in invading a variety of professional and nonprofessional phagocytes and are able to survive inside host cells. Eliciting immune protection against antibiotic-resistant S. aureus infection is a global challenge, requiring both innate and adaptive immune effector mechanisms. Dendritic cells (DC), which sit at the interface between innate and adaptive immune responses, are central to the induction of immune protection against S. aureus. However, it has been observed that S. aureus has the capacity to develop further antibiotic resistance and acquire increased resistance to immunological recognition by the innate immune system. In this article, we review the strategies utilised by S. aureus to circumvent antibiotic and innate immune responses, especially the interaction between S. aureus and DC, focusing on how this relationship is perturbed with the development of antibiotic resistance.

ATF3 Positively Regulates Antibacterial Immunity by Modulating Macrophage Killing and Migration Functions

The clinical severity of Staphylococcus aureus (S. aureus) respiratory infection correlates with antibacterial gene signature. S. aureus infection induces the expression of an antibacterial gene, as well as a central stress response gene, thus activating transcription factor 3 (ATF3). ATF3-deficient mice have attenuated protection against lethal S. aureus pneumonia and have a higher bacterial load. We tested the hypothesis that ATF3-related protection is based on the increased function of macrophages. Primary marrow-derived macrophages (BMDM) were used in vitro to determine the mechanism through which ATF3 alters the bacterial-killing ability. The expression of ATF3 correlated with the expression of antibacterial genes. Mechanistic studies showed that ATF3 upregulated antibacterial genes, while ATF3-deficient cells and lung tissues had a reduced level of antibacterial genes, which was accompanied by changes in the antibacterial process. We identified multiple ATF3 regulatory elements in the antibacterial gene promoters by chromatin immunoprecipitation analysis. In addition, Wild type (WT) mice had higher F4/80 macrophage migration in the lungs compared to ATF3-null mice, which may correlate with actin filament severing through ATF3-targeted actin-modifying protein gelsolin (GSN) for the macrophage cellular motility. Furthermore, ATF3 positively regulated inflammatory cytokines IL-6 and IL-12p40 might be able to contribute to the infection resolution. These data demonstrate a mechanism utilized by S. aureus to induce ATF3 to regulate antibacterial genes for antimicrobial processes within the cell, and to specifically regulate the actin cytoskeleton of F4/80 macrophages for their migration.

Staphylococcus aureus Protein A Induces Human Regulatory T Cells Through Interaction With Antigen-Presenting Cells

Despite continuous exposure and development of specific immunity, Staphylococcus aureus (Sa) remains one of the leading causes of severe infections worldwide. Although innate immune defense mechanisms are well understood, the role of the T cell response has not been fully elucidated. Here, we demonstrate that Sa and one of its major virulence factors protein A (SpA) induce human regulatory T cells (Tregs), key players in immune tolerance. In human PBMC and MoDC/T cell cocultures CD4⁺CD25⁺CD127^dim Tregs were induced upon stimulation with Sa and to a lower extent with SpA alone. Treg induction was strongly, but not exclusively, dependent on SpA, and independent of antigen presentation or T cell epitope recognition. Lastly, soluble factors in the supernatant of SpA-stimulated MoDC were sufficient to trigger Treg formation, while supernatants of MoDC/T cell cocultures containing Sa-triggered Tregs displayed T cell suppressive activity. In summary, our findings identify a new immunosuppressory function of SpA, which leads to release of soluble, Treg-inducing factors and might be relevant to establish colonization.

Skin microbiota in health and disease: From sequencing to biology

Microbiota live in a closely regulated interaction with their environment, and vice versa. The presence and absence of microbial entities is greatly influenced by features of the niche in which they thrive. Characteristic of this phenomenon is that different human skin sites harbor niche‐specific communities of microbes. Microbial diversity is considerable, and the current challenge lies in determining which microbes and (corresponding) functionality are of importance to a given ecological niche. Furthermore, as there is increasing evidence of microbial involvement in health and disease, the need arises to fundamentally understand microbiome processes for application in health care, nutrition and personal care products (e.g. diet, cosmetics, probiotics). This review provides a current overview of state‐of‐the‐art sequencing‐based techniques and corresponding data analysis methodology for profiling of complex microbial communities. Furthermore, we also summarize the existing knowledge regarding cutaneous microbiota and their human host for a wide range of skin diseases.

A Supramolecular Platform Technology for Bacterial Cell Surface Modification

In an era of antimicrobial resistance, a better understanding of the interaction between bacteria and the sentinel immune system is needed to discover new therapeutic targets for combating bacterial infectious disease. Sentinel immune cells such as macrophages phagocytose intact bacteria and thereby initiate ensuing immune responses. The bacterial surface composition is a key element that determines the macrophage signaling. To study the role of the bacterial cell surface composition in immune recognition, we developed a platform technology for altering bacterial surfaces in a controlled manner with versatile chemical scaffolds. We show that these scaffolds are efficiently loaded onto both Gram-positive and -negative bacteria and that their presence does not impair the capacity of monocyte-derived macrophages to phagocytose bacteria and subsequently signal to other components of the immune system. We believe this technology thus presents a useful tool to study the role of bacterial cell surface composition in disease etiology and potentially in novel interventions utilizing intact bacteria for vaccination.

Research Techniques Made Simple: Mouse Bacterial Skin Infection Models for Immunity Research

Bacterial skin infections are a major societal health burden and are increasingly difficult to treat owing to the emergence of antibiotic-resistant strains such as community-acquired methicillin-resistant Staphylococcus aureus. Understanding the immunologic mechanisms that provide durable protection against skin infections has the potential to guide the development of immunotherapies and vaccines to engage the host immune response to combat these antibiotic-resistant strains. To this end, mouse skin infection models allow researchers to examine host immunity by investigating the timing, inoculum, route of infection and the causative bacterial species in different wild-type mouse backgrounds as well as in knockout, transgenic, and other types of genetically engineered mouse strains. To recapitulate the various types of human skin infections, many different mouse models have been developed. For example, four models frequently used in dermatological research are based on the route of infection, including (i) subcutaneous infection models, (ii) intradermal infection models, (iii) wound infection models, and (iv) epicutaneous infection models. In this article, we will describe these skin infection models in detail along with their advantages and limitations. In addition, we will discuss how humanized mouse models such as the human skin xenograft on immunocompromised mice might be used in bacterial skin infection research.

Differential Responses of Human Dendritic Cells to Live or Inactivated Staphylococcus aureus: Impact on Cytokine Production and T Helper Expansion

Understanding Staphylococcus aureus (S. aureus)-host immune system interaction is crucial to meet the tremendous medical need associated with this life-threatening bacterial infection. Given the crucial role of dendritic cells (DC) in dictating immune responses upon microbial challenge, we investigated how the bacterial viability and the conservation of structural integrity influence the response of human DC to S. aureus. To this end, human primary DC were stimulated with the methicillin-resistant S. aureus USA300 live strain, USA300 inactivated by heat (HI), ultraviolet irradiation (UVI), or paraformaldehyde treatment (PFAI) and subsequently analyzed for cell phenotype and immune-modulatory properties. Although no differences in terms of DC viability and maturation were observed when DC were stimulated with live or inactivated bacteria, the production of IL-12, IL-23, and other cytokines differed significantly. The Th1 and Th17 expansion was also more pronounced in response to live vs. inactivated S. aureus. Interestingly, cytokine production in DC treated with live and inactivated USA300 required phagocytosis, whereas blocking endosomal Toll-like receptor signaling mainly reduced the cytokine release by live and HI USA300. A further analysis of IFN-β signaling revealed the induction of a cyclic GMP-AMP synthase stimulator of interferon genes (cGAS-STING)-independent and IRF3-dependent signaling pathway(s) in UVI-stimulated DC. This study underscores the capacity of human DC to discriminate between live and inactivated S. aureus and, further, indicates that DC may represent a valuable experimental setting to test different inactivation methods with regard to the retention of S. aureus immunoregulatory properties. These and further insights may be useful for the development of novel therapeutic and prophylactic anti-S. aureus vaccine strategies.

Neutralization of MMP-2 and TNFR1 Regulates the Severity of S. aureus-Induced Septic Arthritis by Differential Alteration of Local and Systemic Proinflammatory Cytokines in Mice

Despite advancement in the field of antibiotics septic arthritis remains a serious concern till date. Staphylococcus aureus is the most common bacterium that causes septic arthritis. Severity of this disease is directly correlated with chronic inflammation induced by proinflammatory cytokines like TNF-α, interleukin (IL)-1β, IL-6, and induction of matrix metalloproteinases (MMPs) including MMP-2. The objective of our study was to evaluate the role of MMP-2 and tumor necrosis factor receptor 1 (TNFR1) in the pathogenesis of S. aureus infection-induced septic arthritis. Mice were infected with live S. aureus (5 × 10⁶ cells/ml) followed by administration of MMP-2 inhibitor and TNFR1 antibody. Arthritis index showed highest reduction in severity of arthritis in mice treated with both MMP-2 inhibitor and TNFR1 antibody after infection. Combined neutralization of MMP-2 and TNFR1 led to marked diminution in bacterial count in the combined group. Lowest levels of pro inflammatory cytokines like TNF-α, IL-1β, IL-6, and IFN-γ were observed in both serum and synovial tissues indicating maximum protection in S. aureus arthritis during combination treatment. Increment in the level of IL-10 in the combination group could be positively correlated with the recovery of arthritis. Similarly, expressions of COX-2 and iNOS, markers of acute inflammation were also significantly reduced in the combination group due to resolution of inflammation. Levels of O2^.- and NO also showed a significant fall in case of the group treated with MMP-2 inhibitor and TNFR1 antibody both. Neutralization of both MMP-2 and TNFR1 caused rapid decline in recruitment of neutrophil and macrophages in the synovial tissues as evident from reduced MPO and MCP-1 levels, respectively, compared to other groups. Overall, it can be suggested that administration of MMP-2 inhibitor and TNFR1 antibody in combination is protective against the severity of inflammation and cartilage destruction associated with S. aureus infection-induced septic arthritis by altering the levels of cytokines.

Neutrophils: Beneficial and Harmful Cells in Septic Arthritis

Septic arthritis is an inflammatory joint disease that is induced by pathogens such as Staphylococcus aureus. Infection of the joint triggers an acute inflammatory response directed by inflammatory mediators including microbial danger signals and cytokines and is accompanied by an influx of leukocytes. The recruitment of these inflammatory cells depends on gradients of chemoattractants including formylated peptides from the infectious agent or dying cells, host-derived leukotrienes, complement proteins and chemokines. Neutrophils are of major importance and play a dual role in the pathogenesis of septic arthritis. On the one hand, these leukocytes are indispensable in the first-line defense to kill invading pathogens in the early stage of disease. However, on the other hand, neutrophils act as mediators of tissue destruction. Since the elimination of inflammatory neutrophils from the site of inflammation is a prerequisite for resolution of the acute inflammatory response, the prolonged stay of these leukocytes at the inflammatory site can lead to irreversible damage to the infected joint, which is known as an important complication in septic arthritis patients. Thus, timely reduction of the recruitment of inflammatory neutrophils to infected joints may be an efficient therapy to reduce tissue damage in septic arthritis.

Identification and Characterization of Staphylococcus aureus Strains with an Incomplete Hemolytic Phenotype

Staphylococcus aureus is a common pathogen causing both hospital and community-acquired infections. Hemolysin is one of the important virulence factors for S. aureus and causes the typical β-hemolytic phenotype which is called complete hemolytic phenotype as well. Recently, S. aureus with an incomplete hemolytic phenotype (SIHP) was isolated from clinical samples. To study the microbiologic characteristics of SIHP, the special hemolytic phenotype of SIHP was verified on the sheep blood agar plates supplied by different manufacturers. Expression of hemolysin genes hla, hlb, hlgC, and hld of SIHP was detected by qRT-PCR and it was showed that expression of hlb in SIHP was obviously increased compared to the control S. aureus strains with complete hemolytic phenotype (SCHP), while the expression of hla, hlgC, and hld in SIHP was significantly decreased. In addition, the α-hemolysin encoded by gene hla was decreased obviously in SIHP compared to SCHP by western blot. All 60 SIHP strains were identified to be the methicillin resistant S. aureus (MRSA), and moreover these SIHP strains all contains mecA gene. The virulence gene tst were all present in SIHP, and the intracellular survival ability of SIHP was much greater than that of the gene tst negative S. aureus. We also found that IL-2, IL-6, and IL-17A secreted in the supernatant of SIHP infected macrophages increased significantly compared to tst negative control strains infected ones. MLST analysis showed that all of SIHP strains were classified into ST5 clone. To our knowledge, this study firstly showed that SIHP strains are a kind of methicillin resistant strains which express β-hemolysin highly and possess a potential high virulence, and it was suggested that SIHP should be paid more attention in hospital.

The many faces of interleukin-17 in inflammatory skin diseases

Interleukin (IL)-17 is an emerging target for inflammatory skin disorders. Given the remarkable success of its therapeutic inhibition in psoriasis, the pathogenic role of this cytokine is being explored in other immune-mediated diseases. Interestingly, IL-17 is linked to particular skin conditions where its activation coincides with disease flares. The leading hypothesis for its contribution to proinflammatory signalling cascades is driving inflammasome activation. However, IL-17 stimulation also releases a range of noninflammasome-related cytokines from human skin. Furthermore, a role in cytotoxic responses and an important interplay with the microbiome is hypothesized. While treatment failure would be surprising in neutrophilic dermatoses, the picture might be more complex in lymphocyte-mediated conditions. Nonetheless, increasing insights into the pathogenesis suggest that beneficial responses are also probable in the latter conditions. Study of this pathway in the skin reveals some intriguing aspects of the IL-17-related immunological network.