Phagocytosis of Staphylococcus aureus by Macrophages Exerts Cytoprotective Effects Manifested by the Upregulation of Antiapoptotic Factors

Abstract

It is becoming increasingly apparent that Staphylococcus aureus are able to survive engulfment by macrophages, and that the intracellular environment of these host cells, which is essential to innate host defenses against invading microorganisms, may in fact provide a refuge for staphylococcal survival and dissemination. Based on this, we postulated that S. aureus might induce cytoprotective mechanisms by changing gene expression profiles inside macrophages similar to obligate intracellular pathogens, such as Mycobacterium tuberculosis. To validate our hypothesis we first ascertained whether S. aureus infection could affect programmed cell death in human (hMDMs) and mouse (RAW 264.7) macrophages and, specifically, protect these cells against apoptosis. Our findings indicate that S. aureus-infected macrophages are more resistant to staurosporine-induced cell death than control cells, an effect partly mediated via the inhibition of cytochrome c release from mitochondria. Furthermore, transcriptome analysis of human monocyte-derived macrophages during S. aureus infection revealed a significant increase in the expression of antiapoptotic genes. This was confirmed by quantitative RT-PCR analysis of selected genes involved in mitochondria-dependent cell death, clearly showing overexpression of BCL2 and MCL1. Cumulatively, the results of our experiments argue that S. aureus is able to induce a cytoprotective effect in macrophages derived from different mammal species, which can prevent host cell elimination, and thus allow intracellular bacterial survival. Ultimately, it is our contention that this process may contribute to the systemic dissemination of S. aureus infection.

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

“…rsp expression is not required for dissemination of S. aureus from the blood or for deep abscess formation, and we have identified and characterized rsp loss-of-function mutants found in human bacteremia. Thus, our data suggest that S. aureus can adopt an attenuated cytotoxic phenotype, with prolonged intracellular residence (51)(52)(53), which permits effective dissemination of the organism with few initial symptoms, followed by deep abscess establishment. The attenuated toxicity phenotype was not noted in isolates from skin and soft tissue infection (17), suggesting that such attenuated toxicity and intracellular survival may be particularly important in bloodstream infection, as opposed to other forms of infection, such as skin and soft tissue infection.…”

Animal and human antibodies to distinct Staphylococcus aureus antigens mutually neutralize opsonic killing and protection in mice

“…The OPK assays followed published protocols (17,20,23), with one modification: in this study, S. aureus strains were grown overnight and in Columbia broth plus 2% NaCl (15,18,21).…”

“…It has been reported that S. aureus internalized by mouse polymorphonuclear cells (PMNs) and human macrophages may survive intracellularly (16), and in the case of infected macrophages there is an increased resistance to apoptosis (17). Using a gentamicin protection assay to measure viable intracellular S. aureus cells, we found that when dilutions of antibody to CP8 were added to a constant amount of antibody to dPNAG, and vice versa, there was enhanced intracellular survival of S. aureus over that achieved with the opsonically active mixtures of single antigenic specificities (Supplemental Figure 1).…”

Autosomal dominant and sporadic monocytopenia with susceptibility to mycobacteria, fungi, papillomaviruses, and myelodysplasia

“…Microarray analysis was performed using HU133 ϩ 2 human Affymetrix GeneChips as described previously. 16 A separate GeneChip was used for each donor. Genes were defined as differentially expressed when changes in transcript levels were statistically significant by t test and/or analysis of variance as indicated, were at least 1.5-fold increased or decreased compared with cells from control subjects, and transcripts passed all quality filters.…”