Early Steps of Clathrin-Mediated Endocytosis Involved in Phagosomal Escape of Fcγ Receptor-Targeted Adenovirus

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Abstract
Adenovirus type 2 (Ad2) and Ad5 enter epithelial cells via the coxsackievirus B Ad receptor (CAR) and αvintegrin coreceptors. In the absence of CAR, they can be directed to the Fcγ receptor 1 of hematopoietic cells by an adaptor comprising the extracellular CAR domain and the Fc portion of a human immunoglobulin G (CARex-Fc). This gives rise to Ad aggregates and single particles which together enhance gene delivery up to 250-fold compared to adaptor-less viruses. A small interfering RNA knockdown of the clathrin heavy chain and quantitative electron microscopy of hematopoietic leukemia cells showed that the majority of Ads were phagocytosed as clusters of 1 to 3 μm in diameter and that about 10% of the particles entered cells by clathrin-mediated endocytosis. The clathrin knockdown did not affect phagocytosis but, surprisingly, inhibited viral escape from phagosomes. Similarly, blocking an early stage of clathrin-coated pit assembly inhibited phagosomal escape and infection but not aggregate uptake, unlike blocking of a late stage of clathrin-coated pit formation. We propose a cooperative interaction of clathrin-mediated endocytosis and phagocytosis triggering phagosomal lysis and infection.