The initial steps leading to papillomavirus infection occur on the basement membrane prior to cell surface binding.

Using a murine challenge model, we previously determined that human papillomavirus (HPV) pseudovirions initially bind preferentially to the cervicovaginal basement membrane (BM) at sites of trauma. We now report that the capsids undergo a conformational change while bound to the BM that results in L2 cleavage by a proprotein convertase (PC), furin, and/or PC5/6, followed by the exposure of an N-terminal cross-neutralization L2 epitope and transfer of the capsids to the epithelial cell surface. Prevention of this exposure by PC inhibition results in detachment of the pseudovirions from the BM and their eventual loss from the tissue, thereby preventing infection. Pseudovirions whose L2 had been precleaved by furin can bypass the PC inhibition of binding and infectivity. Cleavage of heparan sulfate proteoglycans (HSPG) with heparinase III prevented infection and BM binding by the precleaved pseudovirions, but did not prevent them from binding robustly to cell surfaces. These results indicate that the infectious process has evolved so that the initial steps take place on the BM, in contrast to the typical viral infection that is initiated by binding to the cell surface. The data are consistent with a dynamic model of in vivo HPV infection in which a conformational change and PC cleavage on the BM allows transfer of virions from HSPG attachment factors to an L1-specific receptor on basal keratinocytes migrating into the site of trauma.