A 42‐kDa recombinant protein, PX141, consisting of the trans‐activator protein encoded by human T‐cell leukemia virus (HTLV‐I) (tax, antigen) and the amino‐terminal fusion peptide of 12 amino acid residues of the a‐peptide encoded by the plasmid pUC19 was produced. In order to investigate the immunogenicity of the tax, antigen, mice were immunized with the purified PX141 and 4 anti‐tax, monoclonal antibodies (MAbs) designated TAXY‐I, TAXY‐6, TAXY‐7 and TAXY‐8 were generated, and their reactivity was characterized along with another anti‐tax, MAb, Lt‐4. Immunoblot assays showed that all the MAbs reacted with the PX141, the native tax, antigen expressed in various HTLV‐I‐infected cell lines and the gp68 of MT‐2 cells expressing the tax, amino acids 94–353. Immunoblot assays using recombinant, truncated tax, antigens, XD59 (expressing amino acids 180–338) and XD128 (expressing amino acids 1–47 and 286–353) showed that: (I) TAXY‐I and Lt‐4 did not react with either antigen; (2) TAXY‐6 and TAXY‐8 reacted with only XD128; and (3) TAXY‐7 reacted with both. In addition, TAXY‐I, but not the other MAbs, reacted with a putative tax antigen of an STLV‐I‐infected cell line, designated RfM26‐I. Competitive binding assays showed that TAXY‐6 and TAXY‐8 did not compete against each other. Sera from HTLV‐I‐infected humans interfered with the binding of all of these anti‐tax, MAbs. These results indicate that the tax1 antigen and the PX141 express at least 5 distinct epitopes recognized by human and mouse antibodies.
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