The human parvovirus B19 is now divided into three genotypes: type 1 (prototype), type 2 (A6-and LaLi-like), and type 3 (V9-like). In overall DNA sequence, the three virus types differ by ϳ10%. The most striking DNA dissimilarity, of >20%, is observed within the p6 promoter region. Because of the scarcity of data on the biological activities and pathogenetic potentials of virus types 2 and 3, we examined the functional characteristics of these virus types. We found the activities of the three p6 promoters to be of equal strength and to be most active in B19-permissive cells. Virus type 2 capsid protein VP2, alone or together with VP1, was expressed with the baculovirus system and was shown to assemble into icosahedral parvovirus-like particles, which were reactive in the hemagglutination assay. Furthermore, sera containing DNA of any of the three B19 types were shown to hemagglutinate. The infectivities of these sera were examined in two B19-permissive cell lines. Reverse transcription-PCR revealed synthesis of spliced B19 mRNAs, and immunofluorescence verified the production of NS and VP proteins in the infected cells. All three genotypes showed similar functional characteristics in all experiments performed, showing that the three virus types indeed belong to the same species, i.e., human parvovirus B19. Additionally, the antibody activity in sera from B19 type 1-or type 2-infected subjects (long-term immunity) was examined with homo-and heterologous virus-like particles. Cross-reactivity of 100% was observed, indicating that the two B19 genotypes comprise a single serotype.Human parvovirus B19, a member of the genus Erythrovirus within the subfamily Parvovirinae, has long been considered the only human pathogen of its family, in which the adeno-associated viruses of the genus Dependovirus conceivably are apathogenic. However, new parvoviruses distinct from the genus Erythrovirus were recently detected in plasma (PARV4 and PARV5) (20, 28) and in nasopharyngeal aspirates (human bocavirus) (1), the last of which is supposedly associated with severe respiratory illness in small children.Although infection with parvovirus B19 typically results in erythema infectiosum or fifth disease (4), more severe or even lethal manifestations can occur among predisposed individuals. The virus replicates in erythroid progenitor cells of bone marrow (49, 64), causing aplastic crisis in patients with hemolytic anemia of various etiologies (2, 53, 56). During pregnancy, B19 can be transmitted from the infected mother to the fetus and cause fetal hydrops and death (9). In the immunocompromised, B19 infection may remain persistently productive, leading to chronic anemia (31).The B19 virus is small and nonenveloped and encapsidates a linear single-stranded DNA genome of ϳ5.6 kb. The two genomic ends contain identical inverted terminal repeats of ϳ380 nucleotides that are imperfect palindromes and form hairpin loops (13). The genome contains only one functional promoter, p6, located in the 3Ј palindrome (15). The p6 promoter regulates...

Torque teno virus (TTV) is a non-enveloped human virus with a circular ( approximately 3800 nt) ssDNA genome. TTV transcription results in three viral mRNAs and six proteins, the function or antigenicity of which are unknown. The six open reading frames of TTV genotype 6 were expressed in bacteria and insect cells. Expression of the ORF1/1-encoded protein was inefficient, while expression of the others was successful, with ORF1 and ORF1/2 as arginine-rich region depleted. All six recombinant TTV proteins were antigenic. Of healthy adults, 11/25 (44%) showed strong IgG reactivity with one or more proteins. Four subjects, two of whom were genotype-6-DNA positive, were followed. One of the latter showed concurrently a strong IgG response against the ORF1 protein. The other showed appearance of IgG against the ORF2 protein concomitantly with resolution of the genotype-6 viremia. The genotype-6 sequences remained unaltered for years, suggesting that some mechanisms other than amino acid substitutions play a role in TTV immune evasion.

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