Human melanoma therapy in the SCID mouse:In vivo targeting and reactivation of melanoma-specific cytotoxic T cells by bi-specific antibody fragments

The adoptive transfer of tumor-specific cytotoxic T cells (CTL) offers a promising perspective in cancer immunotherapy. However, the ex vivo-generated T lymphocytes are mostly IL-2-dependent. Here we explored the possibility of circumventing the requirement for IL-2, known for severe side effects in the patient, and of simultaneously targeting the CTL towards the tumor by the use of 2 bi-specific antibody fragments. As a model system, we used SCID mice bearing an s.c.-implanted human melanoma line (BLM-gp100) and in vitro-generated CTL specific for the gp100-derived immunogenic peptide YLEPGPVTA, which were injected i.v. with delay. To maintain the cytotoxic potential of the transferred CTL, 2 bi-specific antibody (biAb) fragments were generated which bound with one arm either CD3 or CD28, a combination known to support the activation of CTL. For targeting the CTL, both biAbs contained the F(ab') part of HD-Me13, an antibody recognizing p97, a non-immunogenic melanoma-associated surface molecule. In vitro and in vivo, the addition of the 2 biAbs increased the cytotoxic potential of the gp100-specific CTL and supported their clonal expansion in the absence of IL-2. Correspondingly, significantly higher numbers of CTL were recovered from melanoma-bearing SCID-mice that received the 2 biAb than from mice treated with the CTL only. In animals treated with CTL plus both biAbs, the primary tumor did not grow, and none of the mice developed metastases. Thus, this set of bi-specific antibody fragments was proved to target effector cells in the tumor-bearing host and to efficiently support in vivo clonal expansion and cytolytic activity of in vitro-generated CTL.