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.