Efficiency of HPV 16 L1/E7 DNA immunization: influence of cellular localization and capsid assembly

Infections by human papillomaviruses (HPV) are the major cause of uterine cancer in women worldwide. Aiming to develop a combined prophylactic and therapeutic vaccine we have previously demonstrated immunogenicity of chimeric virus-like particles consisting of a C-terminally truncated HPV 16 L1 capsid protein fused to an E7 portion. Here we show that genetic vaccination with a corresponding DNA was inefficient in the induction of a L1-specific prophylactic immune response. DNA immunization with C-terminally truncated HPV 16 L1 genes of different lengths revealed that only short deletions (L1(1-498)) were tolerated for eliciting a humoral immune response against viral capsids. This correlates with the observation that the C-terminal sequences are critical for nuclear localization, capsomere and capsid assembly. However, only the ability of L1 protein to form capsomeres or capsids showed a direct influence on the outcome of the immune response. C-terminal insertion of 60 amino acids of E7 was tolerated in fusion constructs, whereas insertion of full-length E7(1-98) or shuffled E7 (149 aa) completely abolished the humoral immune response. The L1(1-498)/E7(1-60) fusion construct not only induced L1-specific antibodies but also L1- and E7-specific CTL responses after DNA vaccination.