Substantial confusion exists regarding the origins and corneal tropism of HAdV-D19. The prototype virus (HAdV-D19p) was characterized first in 1955,
21 but has not been associated with ocular infection, while a virus identified approximately 20 years later with similar serum neutralization characteristics, but a unique restriction endonuclease digest pattern, is an important etiologic agent of EKC.
22–24 Historically, adenoviral serotypes were distinguished in the laboratory by testing for reactivity of virus-infected cells with virus-specific immune sera.
25 The critical epitope for seroreactivity of the hexon protein is coded for by two adjacent regions on the adenovirus hexon gene, loops 1 and 2, corresponding to hypervariable regions 1–6 and 7, respectively, and representing approximately 2.6% of the genome.
26–29 This general region of the adenovirus hexon gene has been shown to be highly susceptible to homologous recombination between HAdV-Ds and, therefore, is an important source for adenovirus evolution and diversity.
5,9,30–32 Our laboratory previously sequenced and annotated the whole genome of a clinical EKC isolate of HAdV-19, strain C (HAdV-D19 [C]),
33 as well as the whole genome of HAdV-D37,
34 an even more virulent EKC agent. In the present study, we investigated the apparent paradox of HAdV-D19 corneal tropism through whole genome sequencing and bioinformatics analysis of HAdV-D19p and three newly described clinical EKC isolates of HAdV-D19. While all the clinical EKC isolates we sequenced essentially had identical genomes, the HAdV-D19p genome was dissimilar, with only the serologic determinant of the adenovirus hexon gene conserved. Application of in vitro and in vivo models of infection demonstrated that HAdV-D19p uniquely fails to enter and replicate in human corneal epithelial cells, thus explaining its lack of corneal tropism. EKC-associated viruses characterized previously as HAdV-D19, now retyped as HAdV-D64, represent the product of adenoviral recombination and evolution, and are unique and distinct from HAdV-D19p. HAdV-D64 is yet another example of adenovirus mistyping due to reliance solely on serum neutralization and/or hexon genotyping.
28,35