Canine Adenovirus Type 2 Attachment and Internalization: Coxsackievirus-Adenovirus Receptor, Alternative Receptors, and an RGD-Independent Pathway

The best-characterized receptors for adenoviruses (Ads) are the coxsackievirus-Ad receptor (CAR) and integrins α(v)β(5) and α(v)β(3), which facilitate entry. The α(v) integrins recognize an Arg-Gly-Asp (RGD) motif found in some extracellular matrix proteins and in the penton base in most human Ads. Using a canine adenovirus type 2 (CAV-2) vector, we found that CHO cells that express CAR but not wild-type CHO cells are susceptible to CAV-2 transduction. Cells expressing α(M)β(2) integrins or major histocompatibility complex class I (MHC-I) molecules but which do not express CAR were not transduced. Binding assays showed that CAV-2 attaches to a recombinant soluble form of CAR and that Ad type 5 (Ad5) fiber, penton base, and an anti-CAR antibody partially blocked attachment. Using fluorescently labeled CAV-2 particles, we found that in some cells nonpermissive for transduction, inhibition was at the point of internalization and not attachment. The transduction efficiency of CAV-2, which lacks an RGD motif, surprisingly mimicked that of Ad5 when tested in cells selectively expressing α(v)β(5) and α(v)β(3) integrins. Our results demonstrate that CAV-2 transduction is augmented by CAR and possibly by α(v)β(5), though transduction can be CAR and α(v)β(3/5) independent but is α(M)β(2), MHC-I, and RGD independent, demonstrating a transduction mechanism which is distinct from that of Ad2/5.