Epitope mapping of type-specific antibodies. A) Elimination of HPV33-specific epitopes by loop exchanges in capsid protein L1. Recombinant HPV L1 capsid proteins expressed in HUTK- cells were tested by immunofluorescence analysis for the presence of epitopes for antibodies H16.56E, H33.E12, H33.J3 and H33.B6. Loss of reactivity is marked by (-), gain of antibody reactivity by (+). B) Functional transfer of HPV33-specific epitopes to HPV 16 by loop swapping, leading to reactivity (+) with the respective HPV33-specific antibody. Note that the correct presentation of corresponding epitopes is also influenced by neighboring loops.