Taming C-terminal peptides of Staphylococcus aureus leukotoxin M for B-cell response: Implication in improved subclinical bovine mastitis diagnosis and protective efficacy in vitro.

Leukotoxin M/F'-PV (LukM/F'-PV) produced by bovine mastitis causing Staphylococcus aureus structurally comprises three domains, the β-sandwich, rim and stem domain. The rim and stem domains interacting with target cell membrane lipid rafts contributes to the virulent trait of the toxin. In the present study, two facts were hypothesized that neutralization of these domains will ebb LukM/F'-PV leukotoxicity. Secondly, the neutralizing antibodies can improve the leukotoxin detection sensitivity in bovine mastitis milk samples. The in silico mapping of S. aureus LukM C-termini comprising these domains predicted seven linear B-cell antigenic epitopes. The immune response of C-terminal truncated recombinant peptides rCtM19 (19 kDa; near carboxy-terminal) having four epitopes and rCtM15 (15 kDa; C-terminal) with three epitopes were evaluated for their diagnostic and neutralization potential. Anti-rCtM19 and anti-rCtM15 antibodies with enhanced immunogenicity had the most striking outcome in IgG-ELISA for detecting native determinants of leukotoxin. For the obtained ELISA values, ROC curve inferred a cut-off score of >0.102 OD405. The assay sensitivity in the range of 90-96% along with 100% specificity and AUC of 0.93-0.98 categorized subclinical and clinical from healthy bovine milk samples. As observed through in vitro neutralization and LDH assays, C-terminus specific antibodies (1:42 titer) deactivating leukotoxicity abolished LukM from interacting with lipid bilayer and LukF for forming pores on bovine neutrophil membrane. As a proof of concept, it was proved that peptide antibodies can be a more specific serodiagnostic and passive therapeutic molecules.