Nontoxigenic protein A vaccine for methicillin-resistant Staphylococcus aureus infections in mice

Identifying protective antigens of Staphylococcus aureus, a pathogen that suppresses host immune responses

Staphylococcus aureus infections result in abscesses as well as septicemia. Even with therapy, abscesses can persist or even reoccur, as staphylococcal infections fail to induce protective immune responses. Here, we show that prior infection with certain attenuated strains may elicit protective immunity. A closer examination reveals that protection correlates with antibody responses elicited on exposure to particular attenuated variants. Linear regression analysis was used to compare reduction in staphylococcal disease and antibody responses to infection with wild-type and attenuated variants. This analysis identified protective antigens that, when tested as vaccines in mice, elicited disease protection. Protection afforded by attenuated strains correlates in part with the ability of Staphylococcus aureus to modulate B cell responses via protein A (spa encoded). We designate this approach "genetic vaccinology," since it exploits genetic variants to draw a correlation between disease protection and humoral immune responses for the deduction of vaccine antigens. Genetic vaccinology is particularly useful for microbes that do not elicit natural protective immunity during infection.

Immunotherapeutic approaches against Staphylococcus aureus

Staphylococcus aureus is a major cause of life-threatening infections such as bacteremia and endocarditis. Unfortunately, many strains of this bacterial species have become resistant to certain antibiotics, including methicillin and amoxicillin. These strains are known as methicillin-resistant S. aureus (MRSA). Therefore, the prophylactic and therapeutic potential of antistaphylococcal vaccines is currently being explored with priority. In animal models, (passive) immunization with (antibodies directed against) certain S. aureus surface components, staphylococcal toxins and capsular polysaccharides protects against S. aureus colonization or infection. However, immunization studies performed in humans show less promising results. So far, not a single antistaphylococcal vaccine successfully passed clinical trials. This article focuses on the results that were obtained with immunotherapeutic approaches directed against S. aureus in animal and human studies. In addition, it is discussed whether effective immunization approaches against S. aureus are feasible in humans.

Staphylococcus aureus biofilms: properties, regulation, and roles in human disease

Increasing attention has been focused on understanding bacterial biofilms and this growth modality's relation to human disease. In this review we explore the genetic regulation and molecular components involved in biofilm formation and maturation in the context of the Gram-positive cocci, Staphylococcus aureus. In addition, we discuss diseases and host immune responses, along with current therapies associated with S. aureus biofilm infections and prevention strategies.

Immunity against Staphylococcus aureus cutaneous infections

Complications arising from cutaneous and soft tissue infections with Staphylococcus aureus are a major clinical problem owing to the high incidence of these infections and the widespread emergence of antibiotic-resistant bacterial strains. If prophylactic vaccines or immunotherapy for certain patient populations are to be developed as an alternative to antibiotics, it will be essential to better understand the immune mechanisms that provide protection against S. aureus skin infections. Recent discoveries have identified a key role for interleukin-1 (IL-1)- and IL-17-mediated immune responses in promoting neutrophil recruitment to the site of infection in the skin, a process that is required for host defence and bacterial clearance. This Review describes these new insights and discusses their potential impact on immune-based therapies and vaccination strategies.

A play in four acts: Staphylococcus aureus abscess formation

Staphylococcus aureus is an important human pathogen that causes skin and soft tissue abscesses. Abscess formation is not unique to staphylococcal infection and purulent discharge has been widely considered a physiological feature of healing and tissue repair. Here we present a different view, whereby S. aureus deploys specific virulence factors to promote abscess lesions that are distinctive for this pathogen. In support of this model, only live S. aureus is able to form abscesses, requiring genes that act at one or more of four discrete stages during the development of these infectious lesions. Protein A and coagulases are distinctive virulence attributes for S. aureus, and humoral immune responses specific for these polypeptides provide protection against abscess formation in animal models of staphylococcal disease.

EsaD, a secretion factor for the Ess pathway in Staphylococcus aureus

Staphylococcus aureus encodes the Sec-independent Ess secretion pathway, an ortholog of mycobacterial T7 secretion systems which is required for the virulence of this Gram-positive microbe. The Ess (ESX secretion) pathway was previously defined as a genomic cluster of eight genes, esxA, esaA, essA, essB, esaB, essC, esaC, and esxB. essABC encode membrane proteins involved in the stable expression of esxA, esxB, and esaC, genes specifying three secreted polypeptide substrates. esaB, which encodes a small cytoplasmic protein, represses the synthesis of EsaC but not that of EsxA and EsxB. Here we investigated a hitherto uncharacterized gene, esaD, located downstream of esxB. Expression of esaD is activated by mutations in esaB and essB. EsaD, the 617-amino-acid product of esaD, is positioned in the membrane and is also accessible to EsaD-specific antibodies on the bacterial surface. S. aureus mutants lacking esaD are defective in the secretion of EsxA. Following intravenous inoculation of mice, S. aureus esaD mutants generate fewer abscesses with a reduced bacterial load compared to wild-type parent strain Newman. The chromosomes of Listeria and Bacillus species with Ess pathways also harbor esaD homologues downstream of esxB, suggesting that the contributory role of EsaD in Ess secretion may be shared among Gram-positive pathogens.

Host chemokines bind to Staphylococcus aureus and stimulate protein A release

There are few examples of host signals that are beneficial to bacteria during infection. Here we found that 31 out of 42 host immunoregulatory chemokines were able to induce release of the virulence factor protein A (SPA) from a strain of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA). Detailed study of chemokine CXCL9 revealed that SPA release occurred through a post-translational mechanism and was inversely proportional to bacterial density. CXCL9 bound specifically to the cell membrane of CA-MRSA, and the related SPA-releasing chemokine CXCL10 bound to both cell wall and cell membrane. Clinical samples from patients infected with S. aureus and samples from a mouse model of CA-MRSA skin abscess all contained extracellular SPA. Further, SPA-releasing chemokines were present in mouse skin lesions infected with CA-MRSA. Our data identify a potential new mode of immune evasion, in which the pathogen exploits a host defense factor to release a virulence factor; moreover, chemokine binding may serve a scavenging function in immune evasion by S. aureus.

Contribution of coagulases towards Staphylococcus aureus disease and protective immunity

The bacterial pathogen Staphylococcus aureus seeds abscesses in host tissues to replicate at the center of these lesions, protected from host immune cells via a pseudocapsule. Using histochemical staining, we identified prothrombin and fibrin within abscesses and pseudocapsules. S. aureus secretes two clotting factors, coagulase (Coa) and von Willebrand factor binding protein (vWbp). We report here that Coa and vWbp together are required for the formation of abscesses. Coa and vWbp promote the non-proteolytic activation of prothrombin and cleavage of fibrinogen, reactions that are inhibited with specific antibody against each of these molecules. Coa and vWbp specific antibodies confer protection against abscess formation and S. aureus lethal bacteremia, suggesting that coagulases function as protective antigens for a staphylococcal vaccine.