Analysis of Staphylococcus aureus proteins secreted inside infected human epithelial cells.

Staphylococcus aureus, an opportunistic pathogen is able to invade into and persist inside non-professional phagocytic cells. To do so, this bacterium possesses a wide range of secreted virulence factors which enable attachment to the host as well as intracellular survival. Hence, a monitoring of virulence factors specifically produced upon internalization might reveal targets for prevention or therapy of S. aureus infections. However, previous proteome approaches enriching S. aureus from lysed host cells after infection did not cover secreted virulence factors. Therefore, we used density gradient centrifugation and mass spectrometry to identify S. aureus HG001 proteins which were secreted into compartments of infected human bronchial epithelial S9 cells. Because shotgun mass spectrometry revealed only few bacterial proteins amongst 1905 host proteins, we used highly sensitive and selective single reaction monitoring mass spectrometry as an alternative approach and quantified 37 bacterial proteins within the S. aureus containing host cell compartment 2.5 h and 6.5 h post infection. Among them were secreted bacterial virulence factors like lipases, pore forming toxins, and secreted adhesins which are usually hard to detect from infected sample material by proteomics approaches due to their low abundance. S. aureus adapted its proteome to improve its response to oxidative and cell wall stress occurring inside the host, but also, increased the amounts of some adhesins and pore-forming toxins, required for attachment and host cell lysis.

Global spread of mouse-adapted Staphylococcus aureus lineages CC1, CC15, and CC88 among mouse breeding facilities.

We previously reported that laboratory mice from all global vendors are frequently colonized with Staphylococcus aureus (S. aureus). Genotyping of a snap sample of murine S. aureus isolates from Charles River, US, showed that mice were predominantly colonized with methicillin-sensitive CC88 strains. Here, we expanded our view and investigated whether laboratory mice from other global animal facilities are colonized with similar strains or novel S. aureus lineages, and whether the murine S. aureus isolates show features of host adaptation. In total, we genotyped 230 S. aureus isolates from various vendor facilities of laboratory mice around the globe (Charles River facilities in the USA, Canada, France, and Germany; another US facility) and university- or company-associated breeding facilities in Germany, China and New Zealand. Spa typing was performed to analyse the clonal relationship of the isolates. Moreover, multiplex PCRs were performed for human-specific virulence factors, the immune-evasion cluster (IEC) and superantigen genes (SAg). We found a total of 58 different spa types that clustered into 15 clonal complexes (CCs). Three of these S. aureus lineages had spread globally among laboratory mice and accounted for three quarters of the isolates: CC1 (13.5%), CC15 (14.3%), and CC88 (47.0%). Compared to human colonizing isolates of the same lineages, the murine isolates frequently lacked IEC genes and SAg genes on mobile genetic elements, implying long-term adaptation to the murine host. In conclusion, laboratory mice from various vendors are colonized with host-adapted S. aureus-strains of a few lineages, predominantly the CC88 lineage. S. aureus researchers must be cautioned that S. aureus colonization might be a relevant confounder in infection and vaccination studies and are therefore advised to screen their mice before experimentation.

Laboratory Mice Are Frequently Colonized with Staphylococcus aureus and Mount a Systemic Immune Response-Note of Caution for In vivo Infection Experiments.

Whether mice are an appropriate model for S. aureus infection and vaccination studies is a matter of debate, because they are not considered as natural hosts of S. aureus. We previously identified a mouse-adapted S. aureus strain, which caused infections in laboratory mice. This raised the question whether laboratory mice are commonly colonized with S. aureus and whether this might impact on infection experiments. Publicly available health reports from commercial vendors revealed that S. aureus colonization is rather frequent, with rates as high as 21% among specific-pathogen-free mice. In animal facilities, S. aureus was readily transmitted from parents to offspring, which became persistently colonized. Among 99 murine S. aureus isolates from Charles River Laboratories half belonged to the lineage CC88 (54.5%), followed by CC15, CC5, CC188, and CC8. A comparison of human and murine S. aureus isolates revealed features of host adaptation. In detail, murine strains lacked hlb-converting phages and superantigen-encoding mobile genetic elements, and were frequently ampicillin-sensitive. Moreover, murine CC88 isolates coagulated mouse plasma faster than human CC88 isolates. Importantly, S. aureus colonization clearly primed the murine immune system, inducing a systemic IgG response specific for numerous S. aureus proteins, including several vaccine candidates. Phospholipase C emerged as a promising test antigen for monitoring S. aureus colonization in laboratory mice. In conclusion, laboratory mice are natural hosts of S. aureus and therefore, could provide better infection models than previously assumed. Pre-exposure to the bacteria is a possible confounder in S. aureus infection and vaccination studies and should be monitored.

De novo phosphorylation and conformational opening of the tyrosine kinase Lck act in concert to initiate T cell receptor signaling.

The enzymatic activity of the Src family tyrosine kinase p56Lck (Lck) is tightly controlled by differential phosphorylation of two tyrosine residues, Tyr394 and Tyr505 Phosphorylation of Tyr394 and the conformational opening of Lck are believed to activate the kinase, whereas Tyr505 phosphorylation is thought to generate a closed, inactive conformation of Lck. We investigated whether the conformation of Lck and its phosphorylation state act in concert to regulate the initiation of T cell receptor (TCR) signaling. With a sensitive biosensor, we used fluorescence lifetime imaging microscopy (FLIM) to investigate the conformations of wild-type Lck and its phosphorylation-deficient mutants Y394F and Y505F and the double mutant Y394F/Y505F in unstimulated T cells and after TCR stimulation. With this approach, we separated the conformational changes of Lck from the phosphorylation state of its regulatory tyrosines. We showed that the conformational opening of Lck alone was insufficient to initiate signaling events in T cells. Rather, Lck additionally required phosphorylation of Tyr394 to induce T cell activation. Consistent with the FLIM measurements, an optimized immunofluorescence microscopy protocol revealed that the TCR-stimulated phosphorylation of Lck at Tyr394 occurred preferentially at the plasma membrane of Jurkat cells and primary human T cells. Our study supports the hypothesis that T cell activation through the TCR complex is accompanied by the de novo activation of Lck and that phosphorylation of Tyr394 plays a role in Lck function that goes beyond inducing an open conformation of the kinase.

Exosomes derived from Burkitt's lymphoma cell lines induce proliferation, differentiation, and class-switch recombination in B cells.

Exosomes, nano-sized membrane vesicles, are released by various cells and are found in many human body fluids. They are active players in intercellular communication and have immune-suppressive, immune-regulatory, and immune-stimulatory functions. EBV is a ubiquitous human herpesvirus that is associated with various lymphoid and epithelial malignancies. EBV infection of B cells in vitro induces the release of exosomes that harbor the viral latent membrane protein 1 (LMP1). LMP1 per se mimics CD40 signaling and induces proliferation of B lymphocytes and T cell-independent class-switch recombination. Constitutive LMP1 signaling within B cells is blunted through the shedding of LMP1 via exosomes. In this study, we investigated the functional effect of exosomes derived from the DG75 Burkitt's lymphoma cell line and its sublines (LMP1 transfected and EBV infected), with the hypothesis that they might mimic exosomes released during EBV-associated diseases. We show that exosomes released during primary EBV infection of B cells harbored LMP1, and similar levels were detected in exosomes from LMP1-transfected DG75 cells. DG75 exosomes efficiently bound to human B cells within PBMCs and were internalized by isolated B cells. In turn, this led to proliferation, induction of activation-induced cytidine deaminase, and the production of circle and germline transcripts for IgG1 in B cells. Finally, exosomes harboring LMP1 enhanced proliferation and drove B cell differentiation toward a plasmablast-like phenotype. In conclusion, our results suggest that exosomes released from EBV-infected B cells have a stimulatory capacity and interfere with the fate of human B cells.