Proteomic Identification of saeRS -Dependent Targets Critical for Protective Humoral Immunity against Staphylococcus aureus Skin Infection

Abstract

Recurrent Staphylococcus aureus skin and soft tissue infections (SSTIs) are common despite detectable antibody responses, leading to the belief that the immune response elicited by these infections is not protective. We recently reported that S. aureus USA300 SSTI elicits antibodies that protect against recurrent SSTI in BALB/c but not C57BL/6 mice, and in this study, we aimed to uncover the specificity of the protective antibodies. Using a proteomic approach, we found that S. aureus SSTI elicited broad polyclonal antibody responses in both BALB/c and C57BL/6 mice and identified 10 S. aureus antigens against which antibody levels were significantly higher in immune BALB/c serum. Four of the 10 antigens identified are regulated by the saeRS operon, suggesting a dominant role for saeRS in protection. Indeed, infection with USA300⌬sae failed to protect against secondary SSTI with USA300, despite eliciting a strong polyclonal antibody response against antigens whose expression is not regulated by saeRS. Moreover, the antibody repertoire after infection with USA300⌬sae lacked antibodies specific for 10 saeRS-regulated antigens, suggesting that all or a subset of these antigens are necessary to elicit protective immunity. Infection with USA300⌬hla elicited modest protection against secondary SSTI, and complementation of USA300⌬sae with hla restored protection but incompletely. Together, these findings support a role for both Hla and other saeRS-regulated antigens in eliciting protection and suggest that host differences in immune responses to saeRS-regulated antigens may determine whether S. aureus infection elicits protective or nonprotective immunity against recurrent infection. Staphylococcus aureus is the most common cause of skin and soft tissue infections (SSTIs) in the United States (1, 2). Recurrent SSTIs are common, leading to the belief that they do not elicit immune responses that protect against subsequent infection. Because of the substantial morbidity and mortality associated with S. aureus infections, as well as increasing resistance of S. aureus isolates to antimicrobials, developing a vaccine to prevent these infections is a public health priority (3). Unfortunately, several vaccines comprising single S. aureus antigens have failed in phase III trials, most recently Merck's V710 (4).S. aureus has a wide array of factors that contribute to its virulence and survival in host tissues (5). The redundancy in the function of many of the virulence factors, as well as the myriad ways in which S. aureus evades protective immune responses, complicates the selection of antigens to incorporate into prospective vaccines. For example, most S. aureus isolates have multiple factors that bind IgG (6), have superantigen activity (7), inhibit complement activity (8), or are toxic to leukocytes or other immune cells (9, 10). Although the cellular mechanisms by which many of these molecules interact with the host immune system have been defined, how they function in concert during S. aureus infection is less well understo...

Natural mutations in a Staphylococcus aureus virulence regulator attenuate cytotoxicity but permit bacteremia and abscess formation

“…Our findings that rsp contributed to lethality in pulmonary disease is also supported by a recent study of cutaneous S. aureus disease (54). In both models, toxins have been shown to be key mediators of disease (13,55). Fig.…”

Inhibition of protective immunity against Staphylococcus aureus infection by MHC-restricted immunodominance is overcome by vaccination

“…First, we have not yet identified the immunodominant epitopes in C57BL/6 mice that outcompete protective responses. We hypothesize that these will come from one or more antigens that elicited strong antibody or T cell responses in C57BL/6 mice, but not BALB/c mice (17). Second, future studies should compare the protection afforded by full-length LukE and LukS-PV with LkT/C or LukE 153-168 -and LukE 185-200 -specific T cell responses.…”

“…These findings suggested that the failure of SSTI to elicit protective anti-Hla antibody responses in C57BL/6 mice was due to the specificity of the MHC haplotype. We previously demonstrated that protective immune responses against S. aureus SSTI in BALB/c mice were directed against sae-regulated antigens, including Hla, LukE, and LukS-PV (11,17). However, while Hla-specific antibody responses correlated with protection, the LukS-PV-and LukE-specific antibody levels did not (Fig.…”

“…In addition to Hla, LukE, and LukS-PV, we previously reported that responses against two other sae-regulated antigens, SplB and SspB, were associated with protection in BALB/c mice (17). Therefore, we compared effector T cell responses against each of these antigens following infection of BALB/c and C57BL/6 mice by ELISpot for IL-17A and IFN.…”

“…We previously showed that T H 17-polarized immune responses, independent of antibody, also contributed to protection in BALB/c mice (11,17). Therefore, we quantified antigen-specific effector T cell responses by interleukin-17A (IL-17A) and interferon  (IFN) ELISpots in the BALB/c, C57BL/6, C.B10-H2 b , and B6.C-H2 d mice.…”