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In the United States, melioidosis is an emerging infectious disease caused by Burkholderia pseudomallei (1,2). In melioidosis-endemic areas, the bacterium causes pneumonia and fatal bacteremia with diverse mortality rates that [...]

Ebolavirus is a hemorrhagic fever virus associated with high mortality. Although much has been learned about the viral lifecycle and pathogenesis, many questions remain about virus entry. We recently showed that binding of the receptor binding region (RBR) of the ebolavirus glycoprotein (GP) and infection by GP pseudovirions increase on cell adhesion independently of mRNA or protein synthesis. One model to explain these observations is that, on cell adhesion, an RBR binding partner translocates from an intracellular vesicle to the cell surface. Here, we provide evidence for this model by showing that suspension 293F cells contain an RBR binding site within a membrane-bound compartment associated with the trans-Golgi network and microtubule-organizing center. Consistently, trafficking of the RBR binding partner to the cell surface depends on microtubules, and the RBR binding partner is internalized when adherent cells are placed in suspension. Based on these observations, we reexamined the claim that lymphocytes, which are critical for ebolavirus pathogenesis, are refractory to infection because they lack an RBR binding partner. We found that both cultured and primary human lymphocytes (in suspension) contain an intracellular pool of an RBR binding partner. Moreover, we identified two adherent primate lymphocytic cell lines that bind RBR at their surface and strikingly, support GP-mediated entry and infection. In summary, our results reveal a mode of determining viral entry by a membrane-trafficking event that translocates an RBR binding partner to the cell surface, and they suggest that this process may be operative in cells important for ebolavirus pathogenesis (e.g., lymphocytes and macrophages). filovirus | negative strand RNA virus | virus receptor | lymphocytes | macrophages doi/ 10.1073/pnas.1008509107

Integrins are involved in the binding and internalization of both enveloped and nonenveloped viruses. By using 3 distinct cell systems--CHO cells lacking expression of [[alpha].sub.5][[beta].su.1]-integrin, HeLa cells treated with siRNA to [[alpha].sub.5]-integrin, and mouse [[beta].sub.1]-integrin knockout fibroblasts, we show that [[alpha].sub.5][[beta].sub.1]-integrin is required for efficient infection by pseudovirions bearing the ebolavirus glycoprotein (GP). These integrins are necessary for viral entry but not for binding or internalization. Given the need for endosomal cathepsins B and L (CatB and CatL) to prime GPs for fusion, we investigated the status of CatB and CatL in integrin-positive and integrin-negative cell lines. [[alpha].sub.5][[beta].sub.1]-Integrin-deficient cells lacked the double-chain (DC) forms of CatB and CatL, and this correlated with decreased CatL activity in integrin-negative CHO cells. These data indicate that [[alpha].sub.5][[beta].sub.1]-integrin-negative cells may be refractory to infection by GP pseudovirions because they lack the necessary priming machinery (the double-chain forms of CatB and CatL). In support of this model, we show that GP pseudovirions that have been preprimed in vitro to generate the 19-kDa form of GP overcome the requirement for [[alpha].sub.5][[beta].sub.1]-integrin for infection. These results provide further support for the requirement for endosomal cathepsins for ebolavirus infection, identify the DC forms of these cathepsins as previously unrecognized factors that contribute to cell tropism of this virus, and reveal a previously undescribed role for integrins during viral entry as regulators of endosomal cathepsins, which are required to prime the entry proteins of ebolavirus and other pathogenic viruses. Ebola | filovirus | virus entry | proteolytic priming