Linezolid Is Superior to Vancomycin in Experimental Pneumonia Caused by Superantigen-Producing Staphylococcus aureus in HLA Class II Transgenic Mice

Abstract

Superantigens (SAg), the potent activators of the immune system, are important determinants of Staphylococcus aureus virulence and pathogenicity. Superior response to SAg in human leukocyte antigen (HLA)-DR3 transgenic mice rendered them more susceptible than C57BL/6 mice to pneumonia caused by SAg-producing strains of S. aureus. Linezolid, a bacterial protein synthesis inhibitor, was superior to vancomycin in inhibiting SAg production by S. aureus in vitro and conferred greater protection from pneumonia caused by SAg-producing staphylococci.T he pathogenicity and virulence of Staphylococcus aureus are determined by several exotoxins, and the superantigens (SAg) are one such family of exotoxins. SAg are the most powerful biological activators of T lymphocytes and other cells of the immune system (9). Through this property, SAg divert the immune response against the bacterium, thereby helping in bacterial immune evasion (5, 15). At the same time, massive immune activation caused by SAg is by itself pathogenic. A higher prevalence of many exotoxins, including the SAg, in community-associated methicillin-resistant S. aureus (CA-MRSA) strains may facilitate infection of healthy individuals (4,8,(10)(11)(12)19).Considering these factors, antibacterials such as linezolid that inhibit staphylococcal exotoxin (including the SAg) synthesis may be advantageous over bactericidal agents in treating infections caused by toxigenic S. aureus. While some murine studies have supported this hypothesis, others do not (1, 2, 14). The lack of considerable benefit with linezolid over vancomycin in mouse models of S. aureus infection has raised uncertainties about the potential benefits of linezolid in humans (6). Considering the enormous differences in the sensitivities of humans and conventional laboratory mice to SAg (conventional mice are believed to be 10 11 times more resistant to SAg than humans [10]), we hypothesized that the benefits of linezolid or similar antibacterial agents do not become apparent in conventional mice. On the other hand, these agents might in fact be useful in humans. Since transgenic mice expressing HLA class II molecules respond robustly to SAg similarly to humans (barring certain species-level differences such as absence of emetic response in mice), they are more susceptible to S. aureus and Streptococcus pyogenes (which also produces SAg) infections than conventional mice (13,16,17). Therefore, we evaluated the activities of linezolid and vancomycin, particularly their abilities to inhibit SAg production, and compared their effectiveness in pneumonia induced by toxigenic S. aureus strains, using human leukocyte antigen (HLA) class II transgenic mice.In support of the divergent response between conventional and HLA class II transgenic mice to SAg, splenocytes from HLA-DR3 transgenic mice responded more robustly to a purified staphylococcal SAg, staphylococcal enterotoxin B (SEB), than splenocytes from B6 mice (Fig. 1A). In addition, culture supernatants from a clinical S. aureus isolate capable of produ...

Passive therapy with humanized anti-staphylococcal enterotoxin B antibodies attenuates systemic inflammatory response and protects from lethal pneumonia caused by staphylococcal enterotoxin B-producingStaphylococcus aureus

“…None of the mice (4-5 mice in each group), either treated with antibodies or left untreated succumbed to pneumonia. This study reaffirmed our earlier report that SAg (SEB in our model) plays a lethal role in pneumonia 54 Moreover, this result also demonstrated that these antibodies are safe and do not potentiate pneumonia caused by S. aureus that do not produce any SAg. In the final set of experiments, we wished to establish the specificity of anti-SEB monoclonal antibodies and to verify that these chimeric antibodies do not have any non-specific antistaphylococcal activity.…”

“…We had previously reported that the S. aureus strain IDRL-7419 which produces SEB, but not its isogenic counterpart IDRL-7420 that does not produce SEB, causes lethal pneumonia in HLA-DR3 transgenic mice. 54 We had also demonstrated that this is due to higher susceptibility of HLA-DR3 transgenic mice to the immune-stimulating effects of SEB compared with C57Bl/6 mice. 54 To specifically show that SEB produced by the S. aureus strain IDRL-7419 causes early and robust activation of T cells during pneumonia, we generated HLA-DR3.Nur77-eGFP transgenic mice.…”

“…54 We had also demonstrated that this is due to higher susceptibility of HLA-DR3 transgenic mice to the immune-stimulating effects of SEB compared with C57Bl/6 mice. 54 To specifically show that SEB produced by the S. aureus strain IDRL-7419 causes early and robust activation of T cells during pneumonia, we generated HLA-DR3.Nur77-eGFP transgenic mice. Experimental pneumonia was induced in HLA-DR3.Nur77-eGFP transgenic mice with SEBproducing S. aureus strain IDRL-7419 (SEBCSA) and the non-SEB-producing isogenic strain, IDRL-7420 (SEB-SA).…”

Superantigens produced by catheter-associated Staphylococcus aureus elicit systemic inflammatory disease in the absence of bacteremia

“…IDRL-7971 is a methicillin-susceptible S. aureus clinical isolate producing SEA and SEB [16,22]. The S. aureus strain RN6734 contains the intact cloned seb pRN5543::seb (pRN7114) and produces SEB (referred to as SEB + SA).…”

“…After 48 hours, 1 mg tritiated [ 3 H]thymidine was added, and the cells were incubated for an additional 18 hours. The extent of proliferation was determined by measuring incorporated radioactivity [16,22].…”