PdpC, a secreted effector protein of the type six secretion system, is required for erythrocyte invasion by Francisella tularensis LVS

Abstract

Francisella tularensisis a gram negative, intracellular pathogen that is the causative agent of the potentially fatal disease, tularemia. During infection,F. tularensisis engulfed by and replicates within host macrophages. Additionally, this bacterium has also been shown to invade human erythrocytes and, in both cases, the Type Six Secretion System (T6SS) is required for these host-pathogen interaction. One T6SS effector protein, PdpC, is important for macrophage infection, playing a role in phagolysosomal escape and intracellular replication. To determine if PdpC also plays a role in erythrocyte invasion, we constructed apdpC-null mutant in the live vaccine strain,F. tularensisLVS. We show that PdpC is required for invasion of human and sheep erythrocytes duringin vitroassays and that reintroduction of a copy ofpdpC,in trans, rescues this phenotype. The interaction with human erythrocytes was further characterized using double-immunofluorescence microscopy to show that PdpC is required for attachment ofF. tularensisLVS to erythrocytes as well as invasion. To learn more about the role of PdpC in erythrocyte invasion we generated a strain ofF. tularensisLVS expressingpdpC-emgfp. PdpC-EmGFP localizes as discrete foci in a subset ofF. tularensisLVS cells grown in broth culture and accumulates in erythrocytes during invasion assays. Our results are the first example of a secreted effector protein of the T6SS shown to be involved in erythrocyte invasion and indicate that PdpC is secreted into erythrocytes during invasion.