Neutralizing activity of human antibodies against the structural protein of human T-cell lymphotropic virus type I.

We have identified and mapped the regions responsible for neutralization in the human T-cell leukemia virus type I (HTLV-I) structural proteins by using region-specific human antibodies derived from seropositive blood donors. We have obtained 18 kinds of region-specific antibody (2 in the p19 gag, 10 in the gp46 env and 6 in the gp21 env proteins) from seropositive human plasma by means of an affinity column coupled with the synthetic peptides corresponding to the antigenic regions of the HTLV-I structural proteins. These antibodies were highly specific in ELISA using synthetic peptides as an antigen. Subsequently, we examined the neutralizing activity expressed by the inhibition of virion-induced syncytium formation by region specific antibodies. Twelve of 16 antibodies derived from the env protein were able to inhibit syncytium formation induced by co-cultivation of 8C cells with HTLV-I antigen-positive T cells. The antibodies derived from the p19 gag protein and the seronegative plasma used as the control showed no significant activity. The sequences recognized by the 10 neutralizing antibodies were sites corresponding to amino acids 20 to 49, 89 to 115, 136 to 160, 175 to 199, 213 to 236, 235 to 254, 277 to 292, 332 to 352, 350 to 386, 382 to 403, 426 to 448 and 458 to 488 from the amino terminal of the env protein. These observations suggest that the neutralizing epitopes were widely distributed in the env proteins of HTLV-I.