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Abstract

A remarkable feature of cancer-associated Human papillomavirus (HPV) types (e.g. HPV16/18) is that the open reading frame (ORF) of the major capsid protein L1 comprises alternative translation initiation codons, thereby potentially encoding different isoforms. Notably, the same can be found within the genome of Mastomys natalensis papillomavirus (MnPV), an etiological agent for non-melanoma skin cancer in the rodent Mastomys coucha.

Capsid formation of these L1 variants was tested using both baculovirus and pseudovirus production systems. L1 translated from the first and second ATG (referred as L1LONG and L1MIDDLE) inefficiently induced capsid formation. In contrast, virions could be efficiently formed with capsomeres derived from L1SHORT protein starting from the third ATG. Even after adding L1SHORT and L2 proteins, L1LONG and L1MIDDLE were still unable to form the spherically correct virus particles.

Analysis of MnPV-infected animals during early infection revealed strong serological responses against MnPV L1LONG and L1MIDDLE protein, but these antibodies were not protective. Contrary, neutralizing antibodies against conformational epitopes of L1SHORT only appeared during the late phase of infection, apparently enabling the virus to accumulate. Therefore, the additional N-terminal part of L1LONG might play a decisive role in capsid formation and form a loop which prevents the assembly of correct capsomeres in vivo. Such a transitory conformational epitope is apparently only recognized during early infection, allowing the virus to escape from humoral immune surveillance. Moreover, using competition assays during pseudoviruses formation revealed that L1LONG protein could interfere with the capsid forming process in the presence of L1SHORT. This could be also shown by changes in intracellular localization when L1LONG or L1MIDDLE is co-expressed with L1SHORT.

In summary, these data provide evidence of an adaptive immune escape initial after infection by avoiding the production of protective antibodies. Since other “high-risk” mucosal and cutaneous HPVs can also encode such L1 isoforms, these results may have important implications in the establishment of a persistent viral infection and the outcome of the disease.