A Translocated Bacterial Protein Protects Vascular Endothelial Cells from Apoptosis

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Abstract
The modulation of host cell apoptosis by bacterial pathogens is of critical importance for the outcome of the infection process. The capacity of Bartonella henselae and B. quintana to cause vascular tumor formation in immunocompromised patients is linked to the inhibition of vascular endothelial cell (EC) apoptosis. Here, we show that translocation of BepA, a type IV secretion (T4S) substrate, is necessary and sufficient to inhibit EC apoptosis. Ectopic expression in ECs allowed mapping of the anti-apoptotic activity of BepA to the Bep intracellular delivery domain, which, as part of the signal for T4S, is conserved in other T4S substrates. The anti-apoptotic activity appeared to be limited to BepA orthologs of B. henselae and B. quintana and correlated with (i) protein localization to the host cell plasma membrane, (ii) elevated levels of intracellular cyclic adenosine monophosphate (cAMP), and (iii) increased expression of cAMP-responsive genes. The pharmacological elevation of cAMP levels protected ECs from apoptosis, indicating that BepA mediates anti-apoptosis by heightening cAMP levels by a plasma membrane–associated mechanism. Finally, we demonstrate that BepA mediates protection of ECs against apoptosis triggered by cytotoxic T lymphocytes, suggesting a physiological context in which the anti-apoptotic activity of BepA contributes to tumor formation in the chronically infected vascular endothelium. The capacity of infected host cells to die by apoptosis (programmed cell death) is critical for controlling pathogen replication and survival. Bacterial pathogens have thus developed strategies to inhibit host cell apoptosis, allowing them to preserve their cellular habitat during chronic infection. For instance, the capacity of the facultative intracellular pathogen Bartonella henselae to trigger tumor formation as a consequence of chronic infection of the human vasculature is linked to the inhibition of endothelial cell apoptosis. This study describes the identification and functional characterization of the anti-apoptotic bacterial effector protein BepA of B. henselae, which is shown to be sufficient to inhibit endothelial cell apoptosis, i.e., as triggered by activated cytotoxic T lymphocytes. Upon translocation into endothelial cells via a bacterial type IV secretion system, BepA localizes to the plasma membrane, where it triggers the production of second messenger cyclic adenosine monophosphate in quantities effective for blocking apoptosis. Strikingly, the capacity of BepA to mediate membrane localization, cyclic adenosine monophosphate production, and the resulting inhibition of apoptosis is confined to a conserved domain that originally evolved in bacteria as a signal for type IV secretion. This study thus highlights the convergent evolution of an anti-apoptotic effector protein of purely bacterial origin.