Identification of an Immunodominant ABC Transporter in Methicillin-Resistant Staphylococcus aureus Infections

Immunoblotting sera from 26 patients with septicemia due to an epidemic strain of methicillin-resistant Staphylococcus aureus (EMRSA-15), 6 of whom died, revealed an immunodominant EMRSA-15 antigen at 61 kDa. There was a statistically significant correlate (P < 0.001) between survival and immunoglobulin G to the 61-kDa band. The antigen was identified by sequencing positive clones obtained by screening a genomic expression library of EMRSA-15 with pooled sera from patients taken after the septicemic episode. Eluted antibody reacted with the 61-kDa antigen on immunoblots. The amino terminus was obtained by searching the S. aureus NCTC 8325 and MRSA strain COL databases, and the whole protein was expressed in Escherichia coli TOP 10F′. The derived amino acid sequence showed homology with ABC transporters, with paired Walker A and Walker B motifs and 73% homology to YkpA from Bacillus subtilis. Epitope mapping of the derived amino acid sequence with sera from patients who had recovered from EMRSA-15 septicemia delineated seven epitopes. Three of these epitopes, represented by peptides 1 (KIKVYVGNYDFWYQS), 2 (TVIVVSHDRHFLYNNV), and 3 (TETFLRGFLGRMLFS), were synthesized and used to isolate human recombinant antibodies from a phage antibody display library. Recombinant antibodies against peptides 1 and 2 gave logarithmic reductions in organ colony counts, compared with control groups, in a mouse model of the infection. This study suggests the potential role of an ABC transporter as a target for immunotherapy.