ATL is characterized by pleomorphic lobular leukemic cells with mature CD4+ T cell markers. HTLV-1 infection, usually subclinical, is diagnosed by demonstrating antibodies in an enzyme immunoassay (ElA) or gelatin particle agglutination. Today, genetically cloned HTLV antigen is preferred to lysates of virus-transformed cell lines as capture antigen, in order to reduce the excessive number of false positives. Because of extensive sharing of epitopes between HTLV-1 and HTLV-2, appropriate synthetic peptides are required to distinguish the respective antibodies. Western blotting is used to confirm provisionally positive results from EIA.
The polymerase chain reaction (PCR) followed by Southern blotting is used to demonstrate HTLV DNA from lymphocytes and has the additional advantage of discriminating between HTLV-2 and HTLV-1, providing suitable pairs of primers are chosen. However, as always, the exquisite sensitivity of PCR is a two-edged sword; HTLV infection can be recognized even in seronegative individuals, but false positives may occur in any but the most experienced hands.
Virus can be propagated only with difficulty, classically by in vitro culture of the patient's peripheral blood leukocytes, either with a mitogen or with an HTLV-transformed T-cell line. Because virus is almost exclusively cell-associated, infection occurs by fusion of infected and uninfected CD4^ lymphocytes, leading to transformation and immortalization of the latter Cells other than T lymphocytes can also be infected, albeit with difficulty, by co-cultivation with infected CD4+ T cells.
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