Characterization and Sequencing of a Full-Length Form of HTLV-I from an HTLV-I/II Seroindeterminate Individual

Abstract

SP-26 Screening for infection with HTLV-I is initiated by ELISA and confirmed by Western blot. There are a significant number of cases in which an ELISA positive specimen shows incomplete reactivity in the Western blot, a condition described as HTLV-I/II seroindeterminate. The majority of these cases are negative by PCR. It remains unclear what the cause of seroindeterminate status is and what implications this status may have for infected individuals and the medical field in general. We have generated an EBV transformed B-cell line (TG-B) from an HTLV-I/II seroindeterminate individual; during routine screening this cell line was found to be positive by primary PCR for HTLV-I tax. HLA matching of the TG-B line with the patient has ruled out potential contamination. Southern blotting of genomic DNA from the TG-B cell line demonstrated the expected banding pattern when digested with PstI or HindII and probed with full-length genomic HTLV-I, and numerous truncated forms are detected when a digest is performed with EcoRI. We have demonstrated that the cell line is positive by PCR for overlapping regions spanning the entire genome of HTLV-I. In addition, the TG-B line expresses tax by RT-PCR; however, we cannot detect any expression of gag by an antigen-capture assay. Sequencing of the overlapping clones, generated by PCR and spanning the entire HTLV-I genome, has resulted in the representation of a full-length virus from the TG-B line with greater than 96% homology to the Cosmopolitan form of HTLV-I. We therefore propose that at least a subset of individuals that are seroindeterminate for HTLV-I by Western blot may be infected with full-length HTLV-I at a viral load which may not be detectable by PCR.