The characteristics of 4 T-cell clones, each capable of producing phagocytosis-inducing factor (PIF), were compared before and after transformation with human T-lymphotropic virus Type 1 (HTLV-I). Before transformation, the four clones produced PIF transiently after stimulation with antigen or mitogen and expressed the phenotype T3 (CD3)+, T4 (CD4)+, T8 (CD8)-, 4B4+, and 2H4-; the three clones that could be studied also expressed the OKT17 marker. After transformation, the cells expressed the same phenotypic markers, except for two clones that lost the CD3 antigen. The clones that were available for study before and after transformation also expressed the antigen detected by the monoclonal antibody 5 9. In addition, all clones secreted PIF constitutively after transformation. These characteristics of the four transformed T-cell clones closely resembled those of three long-term HTLV-I-transformed T-cell lines, HUT-102, C5/MJ, and MT-2, which also produced PIF constitutively and expressed the CD4 and 4B4, but not 2H4, markers. In addition, two other HTLV-I-transformed lines generated in the present study produced PIF constitutively. Since all nine HTLV-I transformed cell lines and all four untransformed clones secreted PIF, and since our previous studies have shown that only approximately 20% of CD4+ peripheral blood lymphocytes secrete PIF, these results suggest that HTLV-I may preferentially transform PIF-secreting CD4+ lymphocytes. The predominant 4B4+, 5 9+, 2H4- phenotype (characteristic of antigen-responsive T cells) of the untransformed and transformed clones as well as the long-term HTLV-I-transformed lines also suggests that the subset of CD4+ lymphocytes that proliferates in response to soluble antigen may be especially susceptible to transformation with this virus.
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