Detection of HTLV‐1 by means of <i>HBZ</i> gene <i>in situ</i> hybridization in formalin‐fixed and paraffin‐embedded tissues

<jats:p>Adult T‐cell leukemia/lymphoma (ATLL) is a T‐cell malignancy associated with HTLV‐1. The HTLV‐1 provirus genome has the pX region that encodes <jats:italic>tax</jats:italic> and HTLV‐1 basic leucine zipper factor (<jats:italic>HBZ</jats:italic>). Previous studies have reported that the <jats:italic>tax</jats:italic> gene is expressed in few ATLL cases, but the <jats:italic>HBZ</jats:italic> gene in all ATLL cases. In this study, we used <jats:italic>HBZ</jats:italic> gene <jats:italic>in situ</jats:italic> hybridization (HBZ‐ISH) for detection of the <jats:italic>HBZ</jats:italic> gene in formalin‐fixed paraffin‐embedded tissues. This method showed that all cases (<jats:italic>n</jats:italic> = 19) were positive for the ATLL cell line (MT‐1, MT‐2, and MT‐4) and ATLL mouse model (HBZ‐Tg mice and NOD/SCID/β2‐microglobulin<jats:sup>null</jats:sup> mice with ATLL transplanted), and the <jats:italic>HBZ</jats:italic> gene was also detected in all human ATLL cases (<jats:italic>n</jats:italic> = 16). The percentage of positive cells in HBZ‐ISH was 5–70%. Immunohistochemical staining for Tax protein showed positivity in seven of 11 cases in NOD/SCID/β2‐microglobulin<jats:sup>null</jats:sup> mice with ATLL transplanted and in six of eight human ATLL cases, but the percentage of positive cells was very low (range, 1–5%). Although HBZ‐ISH is unsuitable to detect HTLV‐1 clonality, this method is convenient and can be useful for the histological diagnosis of ATLL in HTLV‐1 sero‐indeterminate patients. (<jats:italic>Cancer Sci</jats:italic> 2011; 102: 1432–1436)</jats:p>