Third, the diminished protection of the antibody 2B11 when it was administered after GAS inoculation might be due to the reduced access of 2B11 to the infection site after the early hours of the infection. of SsE is critical for its function. An anti-SsE IgG1 monoclonal antibody (MAb), 2B11, neutralized the PAF acetylhydrolase activity of SsE. Passive immunization with 2B11 increased neutrophil infiltration, reduced skin invasion, and protected mice against MGAS5005 infection. However, 2B11 did not protect mice when it was administered after MGAS5005 inoculation. SCH 23390 HCl MGAS5005 induced vascular effusion at infection sites at early hours after GAS inoculation, suggesting that 2B11 did not always have access to infection sites. Thus, the enzymatic activity of SsE mediates its function, and SsE has the potential to be included in a vaccine but is not a therapeutic target. An SCH 23390 HCl effective MAb-based immunotherapy for severe invasive GAS infections may need to target virulence factors that SCH 23390 HCl are critical for systemic survival of GAS. INTRODUCTION Group A (GAS) is a major human pathogen that commonly causes pharyngitis and superficial skin infections (1). This pathogen can also cause severe invasive infections, such as necrotizing fasciitis, streptococcal toxic shock syndrome, pneumonia, and bacteremia. Streptococcal necrotizing fasciitis results from GAS infection of the subcutaneous tissue, which progresses rapidly, causes necrosis of the fascia and subcutaneous tissue, and leads to systemic infection (2). Annually in the United States, there are more than 10 million cases of streptococcal pharyngitis and about 10,000 cases of invasive GAS infections (3.5 cases per 100,000 persons), with a fatality rate of 13.7%, and invasive infections are most frequently caused by serotype M1, M3, and M12 GAS strains (3). While antibiotic treatment is effective to treat pharyngitis patients, it is not effective for treating severe invasive GAS infections (4). Prompt surgical debridement, fluid and electrolyte management, and analgesia are mainstays of therapy for necrotizing fasciitis (5,C7). Clindamycin, hyperbaric oxygen therapy, and intravenous immunoglobulin are Rabbit Polyclonal to SLC33A1 used as adjunctive treatments of severe invasive GAS infections (4,C7). New strategies to treat severe GAS infections are desirable. Severe invasive GAS isolates are usually more virulent than pharyngitis isolates (8, 9). Hypervirulence of some invasive GAS isolates is attributable to natural mutations in the two-component regulatory system CovRS (also known as CsrRS) or to their capacity to acquire CovRS mutations during infection (8,C11). CovRS negatively regulates many virulence factors (12,C15), including the capsule synthase HasA (12), streptolysin S (15), DNase Sda1 (16), interleukin-8/CXC chemokine peptidase SpyCEP (17), and platelet-activating factor (PAF) acetylhydrolase SsE (18, 19). Many CovRS-regulated virulence factors evade neutrophil responses. Natural CovRS mutations are selected by neutrophils to SCH 23390 HCl enhance the expression of multiple virulence factors and downregulate the production of the nonspecific protease SpeB, maximizing the potential of GAS to evade neutrophil responses and resulting in hypervirulence (11, 16, 20,C22). CovRS-regulated virulence factors that critically contribute to the hypervirulence of invasive GAS are potential targets for the development of therapeutics for the treatment SCH 23390 HCl of severe invasive GAS infections. We target SsE to test this potential. SsE is a protective antigen and is required for the skin invasion and dissemination of hypervirulent M1T1 GAS in a mouse model of necrotizing fasciitis (23). SsE has potent PAF acetylhydrolase activity and critically contributes to the inhibition of neutrophil recruitment by GAS (19, 24). If the enzymatic activity of SsE is critical for its functions, an inhibitory antibody of SsE is likely protective and can have the potential to be developed as a therapeutic agent for treatment of invasive GAS infection. In this study, we tested the essentiality of the SsE activity for GAS function, generated a neutralizing monoclonal antibody (MAb), and evaluated the protection and therapeutic value of the inhibitory MAb. We found that a derivative of M1T1 strain MGAS5005 that produced an inactive SsE mutant displayed the phenotype of an deletion mutant of MGAS5005 and that the inhibitory anti-SsE MAb protected mice against MGAS5005 infection when it was administered prior to, but not after, GAS inoculation. MATERIALS AND METHODS Declaration of ethical approvals. All animal procedures were carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health (25). The protocols for the experiments were approved by the Institutional Animal Care and Use Committee at Montana State University ([MSU] permit numbers 2011-57 and 2014-45). Blood was collected from healthy donors in.