Abstract
Blood transfusion services around the world screen donations for antibodies to hepatitis C virus (HCV). One potential problem of this is that the antibody response only becomes detectable several weeks after infection and potentially infectious donations in the "window period" go undetected. The screening ELISA tests are based upon genotype 1 sequence. Six distinct HCV genotypes exist, which show approximately 30 % amino acid sequence divergence in their encoded proteins. Sequence variability is not uniform across the genome, being greatest in the regions encoding envelope proteins and most restricted in the core gene. Sequence variation in the antigenic determinants in core, NS3, NS4A/B and NS5A/B proteins, may produce genotype-specific antibodies which do not cross-react with the antigens used in cunent screening assays for detection of antibody to HCV. Reduction in test sensitivity caused by genotype-specific reactivity may particularly affect detection of antibody around the time of seroconversion, as antibody reactivity is likely to be weak and restricted to a limited number of epitopes.
The extent of type-specific and type-common components of the humoral immune response were investigated by measuring the antibody levels in samples of each genotype with type-homologous and type-heterologous antigens. Reactivity directed against the individual genotype 1 antigens used in a commercially available anti-HCV screening assay (VK48; Murex Biotech) was compared with corresponding core, NS3 and NS4A/B antigens of genotype 3, and NS3, NS4A/B and NS5A/B antigens from genotype 4 using a panel of samples from individuals infected with different genotypes (genotype 1: n=33, 3: n=34 and 4: n=43). A combined ELISA was subsequently assembled using the core, NS3 and NS5A/B recombinant proteins and NS4A/B synthetic peptide of genotype 3a sequence, and serological reactivity in this ELISA compared quantitatively with reactivity in the (type 1-based) Murex VK48 assay, using a similar panel of samples from individuals infected with different HCV genotypes. The overall proportion of type-specific reactivity in the combined ELISAs was 46% (with the type-common component making up the other 54%). Type-specificity of reactivity to each region depended on the extent of amino acid divergence between genotypes, with the more conserved core region eliciting 26% type-specific reactivity, compared with 60% and 62-77% to the NS3 and NS5A/B regions.
To investigate whether antigenic variability influenced the effectiveness oftype-1 based serological assays for screening, blood donor samples collected in regions where non-genotype 1 HCV predominated, such as genotype 3 infection in Pakistan and genotype 4a infection in Egypt and the Middle East, were screened by ELISAs based on genotype 3a or 4a antigens. Additionally, samples from seroconversion panels, individuals identified with a serum positive PCR result, but a commercial assay negative result and individuals with cryptogenic hepatitis, were tested for reactivity to antigens of genotype 2, 3 or 4. Although several samples were identified with type-specific reactivity confined to non-type 1 antigens, none were PCR positive. These results therefore indicate that while some (past, resolved) HCV infections may not be detectable using commercially available type 1-based screening assays, the Jack of detectable viraemia in the samples provides no evidence at present that these serological "misses" risk the safety of blood transfusion. However, the results do indicate that a proportion of past HCV infections may remain undetected by current diagnostic methods.
