Effect of immunization on HTLV-I infection in rabbits.

Two groups of 3 rabbits, each immunized with heat-inactivated HTLV-I or a synthetic env peptide (env175-196), developed antibodies to viral proteins including gp68 and gp46. These immunized rabbits were then challenged with a transfusion of blood from HTLV-I-infected rabbits of the opposite sex. After transfusion challenge, antibody titers further rose in both groups and antibodies to HTLV-I proteins p24 and p19 newly appeared in the env 175-196 group. In addition, 3 more rabbits were infused with hyperimmune rabbit anti-HTLV-I IgG and similarly challenged with virus-infected blood. Pre-challenge sera from these rabbits showed high anti-HTLV-I titers with antibodies to envelope and core proteins. Despite transfusion challenge, the antibody titers gradually declined to undetectable levels in all 3 rabbits over a period of 16 weeks. Virus isolation was attempted from peripheral lymphocytes harvested 1 to 6 months after challenge infection and cultured in the presence of interleukin-2 (IL-2). HTLV-I-carrying lymphoid cell lines of recipient origin were established from all 6 rabbits given active immunization, whereas HTLV-I could not be isolated from any of the 3 rabbits given passive immunization. Absence of virus infection in the latter group was confirmed by negative blood transfusion assay to normal rabbits. These results indicate that hyperimmune IgG, but neither heat-inactivated HTLV-I nor env 175-196, were protective against HTLV-I infection induced by blood transfusion.