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Antigen-Specific IgG Responses from Naive Human Splenocytes: In Vitro Priming Followed by Antigen Boost in the SCID Mouse¹

Peter Brams^2,*, Mai-Lan Nguyen^*, Soulaima Chamat^*, Ivor Royston and Phillip R. Morrow

^* IDEC Pharmaceuticals Corporation, and The Sidney Kimmel Cancer Center, San Diego, CA 92121; and LIDAK Pharmaceuticals, La Jolla, CA 92037

High titers of Ag-specific human IgG were consistently achieved in SCID mice reconstituted with human splenocytes that had been primed with Ag in vitro and then boosted with Ag after engraftment into SCID mice. Specific human IgG titers in the hu-SPL-SCID mice reached approximately 1:4 x 10⁵ when the mice were immunized with a neo-antigen, whereas titers reached 1:2 x 10⁶ when recall responses were induced. Booster immunizations with Ag 21 days after the initial in vivo boost further enhanced this response, and specific human IgG titers of 1:17 x 10⁶ were achieved. This represented an essentially monospecific IgG population. These responses were CD4⁺ T cell dependent. In addition, affinity maturation of the human Ab responses was observed. Spleens of hu-SPL-SCID mice with Ag-specific titers 1:1 x 10⁶ were often significantly enlarged and often displayed visible tumors. Fourteen of sixteen B cell tumors removed from spleens of five such hu-SPL-SCID mice, produced Abs that were specific for the immunizing Ags. From such tumor, cloned cell lines were established. One such mAb, MLN-7 (1,), was raised to tetanus toxoid and had no identified cross-reactivity.