Inhibition of Toxic Shock by Human Monoclonal Antibodies against Staphylococcal Enterotoxin B

Abstract

Background Staphylococcus aureus is implicated in many opportunistic bacterial infections around the world. Rising antibiotic resistance and few alternative methods of treatment are just two looming problems associated with clinical management of S. aureus. Among numerous virulence factors produced by S. aureus, staphylococcal enterotoxin (SE) B is a secreted protein that binds T-cell receptor and major histocompatibility complex class II, potentially causing toxic shock mediated by pathological activation of T cells. Recombinant monoclonal antibodies that target SEB and block receptor interactions can be of therapeutic value.Methodology/Principal FindingsThe inhibitory and biophysical properties of ten human monoclonal antibodies, isolated from a recombinant library by panning against SEB vaccine (STEBVax), were examined as bivalent Fabs and native full-length IgG (Mab). The best performing Fabs had binding affinities equal to polyclonal IgG, low nanomolar IC50s against SEB in cell culture assays, and protected mice from SEB-induced toxic shock. The orthologous staphylococcal proteins, SEC1 and SEC2, as well as streptococcal pyrogenic exotoxin C were recognized by several Fabs. Four Fabs against SEB, with the lowest IC50s, were converted into native full-length Mabs. Although SEB-binding kinetics were identical between each Fab and respective Mab, a 250-fold greater inhibition of SEB-induced T-cell activation was observed with two Mabs.Conclusions/SignificanceResults suggest that these human monoclonal antibodies possess high affinity, target specificity, and toxin neutralization qualities essential for any therapeutic agent.

Mechanisms Mediating Enhanced Neutralization Efficacy of Staphylococcal Enterotoxin B by Combinations of Monoclonal Antibodies

“…SEB fractions were pooled and further purified using a size exclusion column preequilibrated with NMR buffer (20 mM Tris, pH 7.5). NMR labeled samples were grown in M9 medium using either 15 Nlabeled ammonium chloride and/or 13 C-labeled glucose as sole source for 15 N and 13 C isotopic labeling (Cambridge Isotope Laboratory). Purity of the protein was verified by SDS-PAGE.…”

“…In some cases, a combination of mAbs was required to achieve optimal protection (8 -13). However, the administration of potent neutraliz-ing SEB-specific mAbs, either individually or as cocktails (14,15), constitutes a challenge, because the onset of life-threatening symptoms after aerosol exposure occurs within 24 h (16). Given the short window for therapeutic intervention after exposure, lead clinical mAb candidates need to be optimized for postexposure treatment against SEB intoxication.…”

Staphylococcal Infection, Antibiotic Resistance and Therapeutics

“…Virulence factor-specific antibodies derived from vaccination or employed as therapeutics represent a potential defense against bacterial diseases (Larkin et al, 2010). Staphylococcal enterotoxins are considered potential biowarfare agents that can be spread through ingestion or inhalation (Drozdowski et al, 2010).…”

“…The vaccine is being tested for prophylactic and therapeutic use (http://clinicaltrials.gov/ct2/show/NCT00974935) . Larkin et al, (2010) selected human monoclonal antibodies from a phage display library, using a recombinant SEB vaccine (STEBVax) incorporating site-specific mutations that prevent MHC II interactions. This group discovered that some antibody clones cross-react with SEC1, SEC2, and streptococcal pyrogenic exotoxin C (SpeC), while others were highly specific for SEB.…”

“…These antibodies prevented intoxication by interfering with toxin binding to MHC II and/or T-cell antigen receptors. The author suggested the potential applications for treating toxic shock syndrome, human monoclonal antibodies recognizing SEB or other bacterial superantigens may be useful if employed as an adjunct therapy along with antibiotics in treating difficult S. aureus infections (Larkin et al, 2010).…”