Staphylococcus aureus is a species of Gram-positive staphylococcus. It can cause sinusitis, respiratory infections, skin infections, and food poisoning. Recently, it was discovered that S. aureus infects epithelial cells, but the interaction between S. aureus and the host is not well known. In this study, we confirmed S. aureus to be internalized by HaCaT cells using the ESAT-6-like protein EsxB and amplified within the host over time by escaping host immunity. S. aureus increases the expression of decay-accelerating factor (CD55) on the surfaces of host cells, which inhibits the activation of the complement system. This mechanism makes it possible for S. aureus to survive in host cells. S. aureus, sufficiently amplified within the host, is released through the initiation of cell death. On the other hand, the infected host cells increase their surface expression of UL16 binding protein 1 to inform immune cells that they are infected and try to be eliminated. These host defense systems seem to involve the alteration of tight junctions and the induction of ligand expression to activate immune cells. Taken together, our study elucidates a novel aspect of the mechanisms of infection and immune system evasion for S. aureus.

Cell cycles, ordered series of events modulating cell growth and division, are tightly regulated by complexes containing cyclin-dependent kinases (CDKs) and cyclins. Cyclin O is a novel cyclin family protein which interacts with CDK2. However, the molecular effects of cyclin O on the activity of CDK2 have not been fully evaluated. In this study, an interaction between cyclin O and CDK2 was identified by co-immunoprecipitation and the effect of cyclin O on the kinase activity of CDK2 was investigated using cyclin O point mutants. Co-immunoprecipitation was achieved using using HEK293 human embryonic kidney cells which were transiently transfected with vectors expressing cyclin O and CDK2, which revealed that cyclin O interacted with CDK2, particularly with the active form of endogenous CDK2. Cyclin O was expressed as several different bands with molecular weights between 45 and 50 kDa, possibly due to different post-translational modifications. When co-expressed with CDK2, cyclin O appeared as a band with a molecular weight of 50 kDa. Treatment with calf intestinal phosphatase reduced the intensity of the uppermost band. Mass spectroscopic analysis of cyclin O co-expressed with CDK2 revealed that the 81st serine residue of cyclin O was phosphorylated. The in vitro kinase activity of CDK2 phosphorylating histone H1 was markedly increased in the cells overexpressing cyclin O. This activity was reduced in cells overexpressing cyclin O, in which the 81st serine had been replaced with alanine (S81A). These results suggest that cyclin O is a novel cyclin family protein that regulates CDK2 kinase activity, which is mediated by the phosphorylation of the 81st serine residue of cyclin O.

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We investigated the production of influenza A/ Korea/01-2-9/2009 (H1N1) virus-like particles (VLPs) containing four structural proteins, matrix protein 1 (M1), matrix protein 2 (M2), neuraminidase (NA) and hemagglutinin (HA), using stably transfected Trichoplusia ni BT1 TN-5B1-4 (TN-5B1-4) cells. Recombinant M1, M2, NA and HA were expressed as bands with molecular weights of 28, 17, 60, and 70 kDa, respectively, in stably transfected TN-5B1-4 (TN-5B1-4/M1-M2-NA-HA) cells. VLPs were purified from the culture medium of the TN-5B1-4/M1-M2-NA-HA cells by pelleting on a sucrose cushion, followed by ultracentrifugation in a sucrose density gradient. The four structural proteins were released together from the TN-5B1-4/M1-M2-NA-HA cells, and were co-purified from the same fractions of the sucrose density gradient, indicating that recombinant M1, M2, NA, and HA self-assembled into VLPs in the TN-5B1-4/M1-M2-NA-HA cells. Recombinant baculoviruses (rBac/NA and rBac/HA) expressing recombinant NA and HA were generated and used to co-infect stably transfected TN-5B1-4 (TN-5B1-4/M1-M2) cells expressing recombinant M1 and M2, resulting in the production of high-molecularweight VLPs and the release of self-assembled VLPs with diameters of 80 ~ 120 nm. The production of VLPs was greatly enhanced in the TN-5B1-4/M1-M2 cells co-infected with rBac/NA and rBac/HA, compared with the TN-5B1-4/M1-M2-NA-HA cells. Our results suggest that a stably transfected-insect cell expression system might be useful for producing VLPs for the development of recombinant vaccines against influenza.

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Staphylococcus aureus inhibits complement activity by secreting a variety of toxins. However, the underlying mechanism of complement component regulation by lipoteichoic acid (LTA), a cell wall component of S. aureus, has not been elucidated. In this study, we observed that aLTA (LTA of S. aureus) increased C3 expression in THP-1 cells. The mechanism of aLTA-mediated C3 induction includes an aLTA-toll-like receptor (TLR) 2 interaction, interleukin 1 receptor associated kinase (IRAK) 2 recruitment, and nuclear factor kappa B (NF-kB) activation. In HepG2 cells, C3 protein production begins to increase from 3 h and increases steadily until 48 h. On the other hand, CD55 levels increased up to 6 h after aLTA treatment and started to decrease after 24 h and levels were decreased at 48 h by more than 50% compared to untreated cells. The expression of CD55 in HepG2 cells was shown to be regulated by IRAK-M induced by aLTA. Serum C3 levels increased in mice injected with aLTA, which resulted in an increase in the amount and activity of the membrane attack complex (MAC). We also observed that CD55 mRNA was increased in the liver 24 h after aLTA injection, but was decreased 48 h after injection. These results suggest that aLTA increases complement levels via induction of C3 and inhibition of CD55, which may cause associated MAC-mediated liver damage.