Detection And Characterization Techniques

The diagnosis of HTLV infection is usually performed by detecting antibodies to HTLV-1 and/or 2 antigens using a screening enzyme immunosorbent assay (EIA) and a confirmatory Western blot. According to U.S. Public Health Service criteria, a confirmed positive finding involves reactivity to the GAG p24 and ENV gp46 proteins.17 Antigen preparations are usually made of purified virions supplemented with recombinant ENV peptides. Some, but not all, serologic assays allow for type-specific discrimination between HTLV-1 and HTLV-2 infection.18 However, unlike HIV infection, a significant minority of seronegative or seroindeterminate HTLV-1- and HTLV-2-infected individuals exist, partly because of the predominant use of HTLV-1 antigens to screen for both HTLV-1 and HTLV-2 antibodies.19,20 Because the two viruses are 40 percent divergent, complete sero-cross-reactivity is not found. Also, as described below, because of the low replication rate of HTLV, seroconversion can often take up to 2 years compared with 3 to 6 months that is almost absolutely the case for HIV.20 Finally, persistent HTLV-1 seronegative T-cell lymphoma/leukemia patients have been described whose malignant cells contain defective HTLV-1 nucleic acid (vide infra).21,22

Currently, the most sensitive and specific assay to detect the HTLV viruses is the DNA-dependent polymerase chain reaction (PCR), which is available in a variety of non-FDA licensed research formats by which samples can be amplified and detected in under 6 hours.21,23 The use of primers and probes in the HTLV pol, Tax, and LTR (vide infra) region of the HTLV genome allows for 100 percent detection of all members of PTLV. The assay can be used on fresh, frozen, or formalin (but not B5) fixed cells or tissue samples. The amplified DNA can be subsequently sequenced for phylogenetic comparisons. It should be noted that, unlike the case for HIV, because of its low replication rate, RNA-directed PCR is a relatively insensitive means of detecting HTLV infection.24

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