Antibody-mediated enhancement of community-acquired methicillin-resistant Staphylococcus aureus infection

Abstract

Community-acquired infections caused by methicillin-resistant Staphylococcus aureus (MRSA) expressing the Panton-Valentine leukocidin (PVL) are rampant, but the contribution of PVL to bacterial virulence remains controversial. While PVL is usually viewed as a cytotoxin, at sublytic amounts it activates protective innate immune responses. A leukotoxic effect might predominate in high inoculum studies, whereas protective proinflammatory properties might predominate in settings with lower bacterial inocula that more closely mimic what initially occurs in humans. However, these protective effects might possibly be neutralized by antibodies to PVL, which are found in normal human sera and at increased levels following PVL(+) S. aureus infections. In a low-inoculum murine skin abscess model including a foreign body at the infection site, strains deleted for the pvl genes replicated more efficiently within abscesses than isogenic PVL(+) strains. Coinfection of mice at separate sites with isogenic PVL(+) and PVL(-) MRSA abrogated the differences in bacterial burdens, indicating a systemic effect on host innate immunity from production of PVL. Mice given antibody to PVL and then infected with seven different PVL(+) strains also had significantly higher bacterial counts in abscesses compared with mice given nonimmune serum. Antibody to PVL had no effect on MRSA strains that did not produce PVL. In vitro, antibody to PVL incapacitated PVL-mediated activation of PMNs, indicating that virulence of PVL(+) MRSA is enhanced by the interference of PVL-activated innate immune responses. Given the high rates of primary and recurring MRSA infections in humans, it appears that antibodies to PVL might contribute to host susceptibility to infection.

Fig. 1.

Bacterial counts from mouse abscesses induced with PVL⁺ and isogenic ∆pvl S. aureus strains. (A) Comparison of the bacterial counts in 72-h-old abscesses in the skin of mice induced by four wild-type MRSA strains (NRS193, NRS194, LAC, and MW2) with their respective isogenic ∆pvl counterparts. (B) Comparison of bacterial counts from mouse abscesses induced with two wild-type S. aureus strains (NRS193 and LAC) and their isogenic ∆pvl strains at different skin sites within the same mouse. P values are from unpaired t tests. Bars represent means; error bars represent SEM.

Fig. 2.

Bacterial counts from mice injected with nonimmune rabbit sera (NIS) or PVL-immune rabbit antisera followed by induction of abscesses with different S. aureus strains. (A) CFU/abscess for eight different PVL⁺ S. aureus strains determined in mice given NIS or antibody to PVL. (B) CFU/abscess for three strains deleted for the pvl genes following injection with NIS or antibody to PVL. P values are from t tests. Bars represent means; error bars represent SEM.

Fig. 3.

Modulation of PMN activation and antibacterial effects on S. aureus by antibody to PVL. (A) Influx of calcium into PMNs exposed to sublytic amounts of purified PVL in the presence of NIS or antibody to PVL. (B) Susceptibility of PVL⁺ and isogenic ∆pvl S. aureus strains to the antimicrobial activity of PMNs. Bars represent means; error bars represent SEM. the percent change in S. aureus CFU in the presence of antibody to PVL was significantly higher in strains LAC, NRS193, and NRS194 producing PVL compared with cultures incubated with NIS (P <.05, t test). No significant differences were seen with the ∆pvl strains or with strain MW2.

Fig. 4.

Effect of antibody to PVL on production of an extracellular antibacterial factor by human PMNs exposed to PVL-positive S. aureus. S. aureus strains LAC, NRS193, NRS194, and MW2 were grown to midlogarithmic stage, diluted, and incubated with supernatants from PMNs exposed to S. aureus NRS194 in the presence of NIS or antibody to PVL. Samples were obtained after 30 min and 1 h for enumeration of viable S. aureus. Four separate experiments were conducted and analyzed. P values are from unpaired t tests. Bars represent means; error bars represent SEM.