Although the number of human T-cell leukemia virus type-I (HTLV-I)-infected individuals in the world has been estimated at over 10 million, no prophylaxis vaccines against HTLV-I infection are available. In this study, we took a new approach for establishing the basis of protective vaccines against HTLV-I. We show here the potential of a passively administered HTLV-I neutralizing monoclonal antibody of rat origin (LAT-27) that recognizes epitopes consisting of the HTLV-I gp46 amino acids 191–196. LAT-27 completely blocked HTLV-I infection in vitro at a minimum concentration of 5 μg/mL. Neonatal rats born to mother rats pre-infused with LAT-27 were shown to have acquired a large quantity of LAT-27, and these newborns showed complete resistance against intraperitoneal infection with HTLV-I. On the other hand, when humanized immunodeficient mice were pre-infused intravenously with humanized LAT-27 (hu-LAT-27), all the mice completely resisted HTLV-I infection. These results indicate that hu-LAT-27 may have a potential for passive immunization against both horizontal and mother-to-child vertical infection with HTLV-I.

Transgenic silkworms are promising host organisms for the production of therapeutic recombinant proteins due to their superior protein synthesis ability and human-like posttranslational modifications. In this study, we generated transgenic silkworms that secrete a recombinant human monoclonal antibody (mAb) against gp120 of human immunodeficiency virus (HIV) into their cocoons. Variations in the rearing temperature and humidity conditions had little effect on mAb properties, such as N-glycosylation. Next, we performed pilot-scale production of the mAb using three batches of transgenic silkworms. Rearing 22,000–45,000 silkworm larvae yielded 4–8 kg of cocoon shells per batch. Larval growth and development, as well as cocoon quality, were highly consistent across production batches. We extracted and purified the mAb from cocoon shells, yielding 6.1–7.6 g of purified mAb per kg of cocoons in each batch. Characterization of the purified mAb showed that the contents of oligomeric antibodies and host cocoon-derived proteins were less than 0.2% and 10 ppm, respectively, with high consistency among batches. From these results, we conclude that the transgenic silkworm system is sufficiently robust and reproducible for high-quality therapeutic mAb production.