Role of Conformational Epitopes Expressed by Human Papillomavirus Major Capsid Proteins in the Serologic Detection of Infection and Prophylactic Vaccination

Human papillomaviruses (HPVs) cause a variety of cutaneous warts, mucosal condylomata, and dysplasias and are etiologic in cervical cancer. Papillomavirus (PV) conformational epitopes on the surface of virions are type-specific and are the target of neutralizing antibodies. In this study, we describe two methods of in vitro expression of HPV major capsid (L1) proteins which mimicked conformational epitopes and demonstrate their type specificity and ability to react with neutralizing and/or conformation-dependent antibodies. The L1 open reading frames (ORFs) for HPV-1, 6, 11, and 16 were molecularly cloned into a SV 40 expression vector and the encoded gene products were expressed in mammalian (cos) cells. Similarly, the L1 ORFs for HPV-6, 11, 16, and 18 were molecularly cloned into recombinant baculovirus and the encoded gene products were expressed in insect (SF9) cells. The expressed L1 proteins reacted by immunofluorescence and immunoprecipitation with polyclonal and monoclonal antibodies generated against their corresponding native virions and by Western blotting with antibodies that recognized nonconformational epitopes of denatured virions. The recombinant L1 proteins expressed conformational epitopes in both cos and Sf9 cells that were type-specific and displayed neutralizing epitopes. The ability to express, purify, and qualitate the reactivity of recombinant L1 proteins will now permit the serologic analysis of host response to HPV infection and the development of prophylactic PV subunit vaccines.