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.

Production and Characterization of F(Ab')2 Fragments Obtained by Enzymatic Digestion from Murine Anti-MRSA PBP2a Monoclonal Antibodies.

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a worldwide health problem. In a previous study, a murine monoclonal antibody (mMAB), capable of binding to PBP2a within MRSA strains, was generated. F(ab')2 antibody fragments are widely described in the literature as immunochemical tools and reagents for diagnostics and therapeutics, particularly because of their low immunogenicity and rapid pharmacokinetics. In this study, F(ab')2 fragments from mMAB were generated by enzymatic digestion, using pepsin. They were purified by affinity chromatography using protein A and concentrated by a MWCO 50 kDa filtration unit. The results indicate that it is possible to obtain F(ab')2 fragments by pepsin digestion. ELISA, western blotting, and fluorescence microscopy data demonstrated that F(ab')2 affinity for PBP2a is not lost even after the enzymatic digestion process. As expected, in the pharmacokinetics tests, F(ab')2 presented a faster elimination (between 12 and 18 h) compared to IgG. These F(ab')2 fragments could be used in future immunodiagnostic applications, including in vitro or in situ radiolabeling and in the treatment of infections caused by this important pathogen.