Panton-Valentine leukocidin facilitates the escape of Staphylococcus aureus from human keratinocyte endosomes and induces apoptosis.

Skin and soft-tissue infections (SSTIs) caused by community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) have emerged as major health problems throughout the world. Most SSTI CA-MRSA strains produce Panton-Valentine leukocidin (PVL), but its contribution to CA-MRSA pathogenesis is poorly defined. Here, we used an endemic PVL-positive SSTI-causing CA-MRSA strain from Taiwan, together with an isogenic PVL-knockout mutant (Δpvl) and complemented PVL-positive derivative, to evaluate the role of PVL in the pathogenesis of CA-MRSA in the RHEK-1 human keratinocyte cell line and a rabbit skin infection model. We found that both PVL-positive CA-MRSA and isogenic Δpvl strains attached and were engulfed into endosomes of RHEK-1 cells within 1 hour following infection. However, by 2 hours after infection PVL-positive CA-MRSA more effectively disrupted endosomes, escaped into the cytoplasm, and replicated intracellularly. By 6 hours after infection, the PVL-positive strain caused significantly more caspase-dependent keratinocyte apoptosis than the isogenic Δpvl mutant. In the rabbit infection model, 1 week following infection the wild-type strain produced significantly more widespread lesions and cell apoptosis than the isogenic Δpvl mutant. These findings indicate that PVL is an important virulence factor that enables CA-MRSA to produce necrotizing skin infections by allowing the bacteria to escape from endosomes, replicate intracellularly, and induce apoptosis.

Conditioning vaccination site with irradiated MIP-3alpha-transfected tumor cells enhances efficacy of dendritic cell-based cancer vaccine.

Macrophage inflammation protein-3alpha (MIP-3alpha) is a chemokine expressed in inflamed tissue and capable of inducing migration of immature dendritic cells (DCs) or Langerhans cells. We postulated that conditioning vaccination sites with MIP-3alpha might enhance the efficacy of subsequently administered DC-based cancer vaccines. Our results demonstrate that subcutaneously injection of irradiated tumor cells expressing MIP-3alpha induces substantial cell infiltration to the injection site. Vaccination of irradiated tumor cells expressing MIP-3alpha followed by DCs pulsed with irradiated tumor cells can effectively suppress tumor growth in animals, which is significantly better than vaccination with irradiated MIP-3alpha-producing tumor cells or DCs pulsed with tumor cells alone. The protective effect was most evident when the MIP-3alpha-producing tumor cells and DC-based vaccines were injected at the same site. These results support the notion that this combination vaccination strategy might generate a more effective immune response to suppress the growth of tumor cells in animals.

Retrograde axonal transport: a major transmission route of enterovirus 71 in mice.

Inoculation of enterovirus 71 (EV71) by the oral (p.o.), intramuscular (i.m.), or intracranial route resulted in brain infection, flaccid paralysis, pulmonary dysfunction, and death of 7-day-old mice. The lag time of disease progression indicated that neuroinvasion from the inoculation sites was a prerequisite for the development of the clinical signs. Although EV71 p.o. inoculation led to a persistent viremia and a transient increase in blood-brain barrier permeability at the early stage of the infection, only low levels of virus, which led to neither severe infection nor clinical illness, could be detected in the brain, suggesting that hematogenous transport might not represent a major transmission route. In the spinal cord, following both p.o. and hind limb i.m. inoculation, the virus first appeared and increased rapidly in the lower segments, especially at the anterior horn areas, and then spread to the upper segments and brain in the presence of viremia. A reverse pattern, with the virus being first detected in the upper segment, was observed when the virus was i.m. inoculated in the forelimb. Colchicine, a fast axonal transport inhibitor, but not sciatic nerve transection reduced EV71 neuroinvasion in a dose-dependent manner, indicating a neuronal transmission of the virus.