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* Department of Medicine, Oklahoma University Health Sciences Center, and Veterans Affairs Medical Center, Oklahoma City, OK 73014;
Department of Molecular Cell Biology, Free University Medical Center, Amsterdam, The Netherlands;
Institute of Experimental Dermatology and Department of Dermatology, University of Münster, Münster, Germany;
Department of Dermatology and Allergy, University of Ulm, Ulm, Germany; and
¶ Department of Immunology, Erasmus MC, Rotterdam, The Netherlands
Mononuclear phagocytes can be used by intracellular pathogens to disseminate throughout the host. In the bloodstream these cells are generically referred to as monocytes. However, blood monocytes are a heterogeneous population, and the exact identity of the leukocyte(s) relevant for microbial spreading is not known. Experiments reported in this study used Listeria monocytogenes-infected mice to establish the phenotype of parasitized blood leukocytes and to test their role in systemic dissemination of intracellular bacteria. More than 90% of the blood leukocytes that were associated with bacteria were CD11b+ mononuclear cells. Analysis of newly described monocyte subsets showed that most infected cells belonged to the Ly-6Chigh monocyte subset and that Ly-6Chigh and Ly-6Cneg-low monocytes harbored similar numbers of bacteria per cell. Interestingly, systemic infection with wild-type or 
actA mutants of L. monocytogenes, both of which escape from phagosomes and replicate intracellularly, caused expansion of the Ly-6Chigh subset. In contrast, this was not evident after infection with hly mutants, which neither escape phagosomes nor replicate intracellularly. Importantly, when CD11b+ leukocytes were isolated from the brains of lethally infected mice, 88% of these cells were identified as Ly-6Chigh monocytes. Kinetic analysis showed a significant influx of Ly-6Chigh monocytes into the brain 2 days after systemic infection. This coincided with both bacterial invasion and up-regulation of brain macrophage chemoattractant protein-1 gene expression. These data indicate that the Ly-6Chigh monocyte subset transports L. monocytogenes into the brain and establish their role as Trojan horses in vivo.
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