Characteristics of HTLV-1-infected human T cells recovered from hu-PBMC-NOG-spl mice. Tax protein expression in human lymphocytes recovered from the spleens of hu-PBMC-NOG-spl mice by flow cytometry. A. Human lymphocytes recovered from mouse spleens express very low levels of Tax protein (upper panel). After a short-term (16 h) cultivation ex vivo, Tax expression was rapidly induced (lower panel). The numbers represent the percentage of the Tax-positive cell population within the HLA-class I-positive gate. B. Tax-expressing cells are more frequently positive for CCR4 than Tax-negative cells.