Passive immunization against methicillin resistant Staphylococcus aureus recombinant PBP2a in sepsis model of mice: Comparable results with antibiotic therapy

Staphylococcus aureus vaccine strategy: Promise and challenges

Staphylococcus aureus (S. aureus) is a leading and crucial infectious threat to global public health due to the widespread emergence of antibiotic-resistant strains such as Methicillin-Resistant S. aureus (MRSA). MRSA infects immunocompromised patients and healthy individuals and has rapidly spread from the healthcare setting to the outside community. The development of flawless vaccines become a medical need worldwide against multi-drug resistant S. aureus. Therefore, protection by an immune-based strategy may provide valuable measures to contain the spread of invasive S. aureus infections. Several vaccine candidates have been prepared which are either in the preclinical phase or in the early clinical phase, whereas several candidates have failed to show a protective efficacy in human subjects. Currently, research is focusing on identifying novel vaccine formulations able to elicit potent humoral and cellular immune responses. Several approaches have also been made to the development of monoclonal or polyclonal antibodies for passive immunization to protect against S. aureus infections. In recent years, a multi-epitope vaccine has emerged as a novel platform for subunit vaccine design by using computational approaches. Therefore, in this review, we have summarized and discussed the mechanistic overview of different strategies used to develop potential vaccine candidates and passive interventions which are in different stages of clinical trials to fight multi-drug resistant S. aureus infections.

In Silico Method for the Screening of Phytochemicals against Methicillin-Resistant Staphylococcus Aureus

Methicillin-resistant Staphylococcus aureus (MRSA) has evolved resistance even against the last resort β-lactam antibiotics. This is because of the acquisition of an additional penicillin-binding protein 2a (PBP2a) which is a resistance determinant in MRSA. Currently, available PBP2a inhibitors are ineffective against life-threatening and fatal infections caused by microorganisms. Therefore, there is an urgent need to screen natural compounds that could overpass the resistance issue alone or in combination with antibacterial drugs. We studied the interactions of different phytochemicals with PBP2a so that crosslinking of peptidoglycans could be inhibited. In structure-based drug designing, in silico approach plays a key role in determining phytochemical interactions with PBP2a. In this study, a total of 284 antimicrobial phytochemicals were screened using the molecular docking approach. The binding affinity of methicillin, -11.241 kcal/mol, was used as the threshold value. The phytochemicals having binding affinities with PBP2a stronger than methicillin were identified, and the drug-likeness properties and toxicities of the screened phytochemicals were calculated. Out of the multiple phytochemicals screened, nine were found as good inhibitors to be PBP2a, among which cyanidin, tetrandrine, cyclomorusin, lipomycin, and morusin showed strong binding potential with the receptor protein. These best-selected phytochemicals were also docked to the allosteric site of PBP2a, and most of the compounds revealed strong interactions with the allosteric site. These compounds were safe to be used as drugs because they did not show any toxicity and had good bioactivity scores. Cyanidin had the highest binding affinity (S-score of -16.061 kcal/mol) with PBP2a and with high gastrointestinal (GI) absorption. Our findings suggest that cyanidin can be used as a drug against MRSA infection either in purified form or that its structure can lead to the development of more potent anti-MRSA medicines. However, experimental studies are required to evaluate the inhibitory potential of these phytochemicals against MRSA.

Glucomannan as a polysaccharide adjuvant improved immune responses against Staphylococcus aureus: Potency and efficacy studies

Staphylococcus aureus is a gram-positive bacterium, representing one of the most important nosocomial pathogens. The treatment of infections, caused by S. aureus, has become increasingly intricate due to the emergence of highly resistant strains. Therefore, it is obvious that an effective prevention strategy against this bacterium could significantly decrease such infections. In the present study, the protective efficacy and immunological properties of recombinant autolysin, formulated in Montanide ISA266 and Alum adjuvants with Glucomannan as a polysaccharide, were assessed in the systemic mouse model of infection. Mice were immunized with the purified recombinant protein in various formulations in different groups and, subsequently, mice were challenged with 5 × 10⁸ CFU of bacteria for the evaluation of their survival and bacterial clearances in the internal organs. ELISA was performed to determine the type of induced immunity, cytokine secretion (IFN-γ, IL-4, IL-2, and IL-17), and isotyping (IgG1 and IgG2a). In addition, we measured the opsonophagocytic activities of the antibodies. Results showed that immunization with r-autolysin + Alum + Glucomannan and r-autolysin + MontanideISA266+Glucomannan formulations significantly increased total IgG and isotypes (IgG1 and IgG2a), as compared with other vaccinated and control groups. Furthermore, the formulation of r-autolysin in Alum and MontanideISA266 adjuvants with Glucomannan enhanced IFN-γ, IL-4, and IL-17 cytokine secretion as well as protectivity, following experimental challenge. We concluded that Glucomannan has the potential to induce immune responses and would be used as an adjuvant factor in vaccine formulation.

Evaluation of the PBP2 transglycosylase region of Staphylococcus aureus as a target for immunotherapeutic approaches

Infections caused by Staphylococcus aureus are increasingly prevalent, and treatment has become more difficult due to the emergence of strains that are resistant to multiple drugs, such as methicillin-resistant Staphylococcus aureus (MRSA). Penicillin-binding proteins (PBPs) are essential enzymes in peptidoglycan biosynthesis. Only found in bacteria, they are an excellent target for the development of bacterial control strategies. S. aureus has 4 PBPs, and only PBP2 has transglycosylation activity, making it a good model to evaluate whether the inactivation of the transglycosylase domain (PBP2t) could lead to bacterial death. (His6)-tagged PBP2t was purified from the E. coli cell lysate using Ni-charged resin, and ELISA and immunoblotting assays demonstrated that PBP2t is immunogenic. Flow cytometry analysis was performed to verify the binding of polyclonal antibodies to the bacterial cell surface. In order to verify the ability to provide protection, immunized mice were challenged with a sublethal dose of MRSA, and the bacterial loads in kidneys and spleen were evaluated. A reduction of 2-2.5 logs was seen in organs from immunized mice compared with the negative controls in two independent assays (p < 0.01). Our results demonstrate that the PBP2t is a promising target for the development of novel antimicrobial strategies, but further testing should be performed to validate the protection conferred by immunization with this protein.

Monoclonal antibodies against LipL32 confer prophylactic protection against lethal leptospirosis challenge in animal model

Leptospirosis is a widespread zoonotic disease caused by pathogenic spirochetes of the genus Leptospira. The commercially available vaccines are bacterins that offer limited protection, short-term effect, and serovar-specific immunity. The development of novel immunization strategies is crucial to control the infection and decrease the chances of new outbreaks. In this study, purified monoclonal antibodies (mAbs) anti-LipL32 (1D9 and mAb3) were evaluated by their capacity to bind and neutralize the pathogen improving host survival. For that, an in vitro growth inhibition assay, and in vivo passive immunization were performed in animal model. Syrian hamsters were passively immunized by three different strategies. Hamsters immunized with mAb3 6 h prior to the lethal challenge showed a significantly higher survival rate of 61.1%, and a significant reduction in tissue damage in the lungs. Cumulatively, our results showed that anti-LipL32 mAbs inhibited the growth of L. interrogans in vitro, and that passive immunization offered significant protection in animal model when administered prior to infection.

Recombinant PBP2a of methicillin-resistant S. aureus formulation in Alum and Montanide ISA266 adjuvants induced cellular and humoral immune responses with protection in Balb/C mice

Staphylococcus aureus is an important cause of both hospital and community acquired infections worldwide. S.aureus can develop multidrug resistance; thus, immunotherapy can be a rational alternative. High level β-lactam resistance of S. aureus has been attributed to the penicillin binding protein 2a (PBP2a). In this study, we assessed the immunogenicity and protectivity of PBP2a formulated in Montanide ISA266 and Alum adjuvants. Recombinant PBP2a with a molecular weight of approximately 13 kDa was expressed and purified by nickel-nitrilotriacetic acid (NI-NTA) affinity chromatography and characterized by SDS-PAGE and Western blot. To investigate the immunogenicity and protective effects of recombinant protein, 20 μg of r-PBP2a in various formulations were subcutaneously injected in different groups. Two booster vaccinations were carried out in two-week intervals and blood samples were collected two weeks after each injection. To determine the type of induced immune response, sera and splenocytes were analyzed by ELISA for total IgG and isotypes (IgG1 and IgG2a) and cytokine secretion (IFN-γ, IL-4, IL-17 and TNF-α), respectively. Three weeks following the last immunization, experimental mice were challenged with 5 × 10⁸ CFU of bacteria intraperitoneally and mortality rate and bacterial load were assessed. Interestingly, analysis of humoral immune responses revealed that administration of r-PBP2a with Montanide ISA266 significantly increased specific IgG responses and also IgG1 isotype compared to alum-adjuvanted vaccine group. Also, r-PBP2a formulation with alum and MontanideISA266 adjuvants raised IFN-γ, IL-4, IL-17 cytokines secretion, and protectivity following experimental challenge. The results of the present study provide evidences for immunogenicity and protectivity of PBP2a protein as a vaccine candidate.

Monoclonal antibody anti-PBP2a protects mice against MRSA (methicillin-resistant Staphylococcus aureus) infections

Methicillin-resistant Staphylococcus aureus (MRSA) is a multidrug-resistant bacterium responsible for serious nosocomial and community-acquired infections worldwide. Since few antibiotics are effective for treating MRSA infections, the development of new therapies is of great importance. Previous studies demonstrated that PBP2a is a target that generates protective antibodies against MRSA. A murine monoclonal antibody (MAb) that recognizes PBP2a from MRSA strains was previously isolated and characterized. In this report, we evaluated the biodistribution of this MAb in blood and tissues, as well as the extent of protection conferred using prophylactic and therapeutic assays compared to vancomycin treatment. Biodistribution was evaluated 12–96 h after MAb administration. It predominantly remained in the serum, but it was also detectable in the kidneys, lungs, and spleen at low concentrations (about 4.5% in the kidneys, 1.9% in the lungs, and 0.7% the spleen) at all observed timepoints. Prophylactic studies in a murine model demonstrated a significant bacterial load reduction in the kidneys of the groups treated with either with IgG (greater than 3 logs) or F(ab’)₂ (98%) when compared to that of the control groups (untreated). Mice were challenged with a lethal dose, and the survival rate was higher in the treated mice. Treatment with the MAb resulted in a bacterial load reduction in the kidneys similar to that of mice treated with vancomycin, and a MAb/vancomycin combination therapy was also effective. These results demonstrate that an anti-PBP2a MAb may be a promising therapeutic for treating MRSA infections.

Origin of Nontreponemal Antibodies During Treponema pallidum Infection: Evidence From a Rabbit Model

The origin of nontreponemal antibodies during syphilis infection is hotly debated. Here, we analyzed the immune response in rabbits immunized with various antigens. Inactivated treponemes elicited the production of low-titer nontreponemal antibodies in some rabbits. Cardiolipin combined with bovine serum albumin also induced anticardiolipin antibody production. These findings indicate that Treponema pallidum contained a cardiolipin antigen with weak immunogenicity. However, active T. pallidum induced higher nontreponemal antibody production with strong immunogenicity at an earlier time point, and the antibody titer was consecutive, suggesting the high nontreponemal antibody titer resulted from the combined effects of both the T. pallidum cardiolipin antigen and the damaged host-cell cardiolipin antigen during syphilis infection, the latter of which plays a major role in the induction of nontreponemal antibody production. Our study provides direct animal evidence of the origin of nontreponemal antibodies during T. pallidum infection.

Recent advances in Staphylococcus aureus infection: focus on vaccine development

Staphylococcus aureus normally colonizes the nasal cavity and pharynx. After breaching the normal habitat, the organism is able to cause a number of infections at any site of the body. The development of antibiotic resistance has created a global challenge for treating infections. Therefore, protection by vaccines may provide valuable measures. Currently, several vaccine candidates have been prepared which are either in preclinical phase or in early clinical phase, whereas several candidates have failed to show a protective efficacy in human subjects. Approaches have also been made in the development of monoclonal or polyclonal antibodies for passive immunization to protect from S. aureus infections. Therefore, in this review we have summarized the findings of recently published scientific literature to make a concise report.

Purification and functional characterization of serum transferrin from Nile tilapia (Oreochromis niloticus)

Transferrin (TF), an iron-binding multifunctional protein, could participate in the iron-withholding strategy, an effective antimicrobial defense mechanism in innate immunity, and is involved in host defense against pathogenic infection. In this study, a TF homologue (OnTF) was purified from serum of Nile tilapia (Oreochromis niloticus) through a two-step affinity chromatography, and characterized its antibacterial function and the role in inflammatory response. The identification by mass spectrometry showed that peptide sequence of the purified OnTF was highly consistent with its amino acids sequence, containing two conserved iron binding lobes: N-lobe and C-lobe. The native OnTF was able to bond iron ions, and possessed capability to inhibit the growth of both bacterial pathogens (Streptococcus agalactiae and Aeromonas hydrophila) in vitro. Upon infections of S. agalactiae and A. hydrophila, the expression of OnTF protein was significantly up-regulated in vivo and in vitro. In addition, the OnTF participated in the regulation of inflammation, migration, and enhancement of phagocytosis and respiratory burst activity in head kidney macrophages/monocytes. Taken together, the results of this study indicated that OnTF is likely to involve in innate immunity to play a role in host defense against bacterial infection in Nile tilapia.

Passive immunotherapy with specific IgG fraction against autolysin: Analogous protectivity in the MRSA infection with antibiotic therapy

Staphylococcus aureus is a leading infectious cause of life-threatening diseases in human beings, with no effective vaccine available to date against this bacterium. Treatment of methicillin-resistant S. aureus (MRSA) infections has become increasingly difficult because of the emergence of multidrug-resistant isolates. Immunotherapy represents a potential approach to prevent S. aureus-related infections. Autolysin is one of the virulence factors, which controls the growth, cell lysis, daughter-cell separation, and biofilm formation. Our study focused on passive immunization against MRSA infection. Herein, rabbit polyclonal IgG was produced following the preparation of r-autolysin. Specificity of IgG against r-autolysin was investigated by ELISA and western blotting assays. IgG fraction was prepared using sulfate ammonium precipitation, and the ability of antiserum to promote phagocytosis of bacteria was assessed by opsonophagocytosis assay. Then, passive immunization of mice was carried out with polyclonal IgG fraction and, mice were sacrificed three days after challenge and their kidneys, liver, and spleen were collected. Results exhibited that the passive immunization with rabbit polyclonal anti-IgG fraction tremendously improved survival rates of mice challenged by S. aureus as well as vancomycin treatment compared with the negative control groups. In addition, a remarkable decrease in bacterial numbers was observed in mice treated with rabbit polyclonal anti-IgG. Importantly, our findings demonstrated that passive immunotherapy and antibiotic therapy lead to decreased histopathological damage in mice infected by S. aureus as compared with control groups. Our results suggested that the passive immunization may result in the introduction of excellent strategies to control infections caused by MRSA, like antibiotic therapy.

Antibody-Based Agents in the Management of Antibiotic-Resistant Staphylococcus aureus Diseases

Staphylococcus aureus is a human pathogen that can cause a wide spectrum of diseases, including sepsis, pneumonia, arthritis, and endocarditis. Ineffective treatment of a number of staphylococcal infections with antibiotics is due to the development and spread of antibiotic-resistant strains following decades of antibiotic usage. This has generated renewed interest within the scientific community in alternative therapeutic agents, such as anti-S. aureus antibodies. Although the role of antibodies in the management of S. aureus diseases is controversial, the success of this pathogen in neutralizing humoral immunity clearly indicates that antibodies offer the host extensive protection. In this review, we report an update on efforts to develop antibody-based agents, particularly monoclonal antibodies, and their therapeutic potential in the passive immunization approach to the treatment and prevention of S. aureus infections.