Antitumor × Anti-CD3 Bifunctional Antibodies Redirect T-Cells Activated in Vivo with Staphylococcal Enterotoxin B to Neutralize Pulmonary Metastases

Abstract

T-cell antitumor activities are limited by the requirement of two specific major histocompatibility complex restricted steps, T-helper cell activation and cytotoxic T-lymphocyte targeting. The aim of this study was to investigate whether bypassing these major histocompatibility complex restricted steps using nonspecific in vivo activation of T-cells with staphylococcal enterotoxin-B (SE-B) and retargeting with antitumor × anti-CD3 bifunctional antibody (BFA) could provide an effective antitumor response.

C3H/HeN mice were injected i.v. with CL-62 melanoma cells, which express the human melanoma antigen p97, and were treated 10 min later with SE-B and/or anti-CD3 (500A2) × anti-p97 (96.5) BFA. Pulmonary metastases were counted 14 days following injections. SE-B alone induced a dose-dependant activation of T-cells as measured by increased interleukin-2 receptor expression and enhanced proliferative responses. SE-B doses greater than 10 µg significantly reduced the number of pulmonary metastases versus control (P < 0.01). Combined treatment with SE-B (50 µg) and BFA (5 to 50 µg) significantly decreased pulmonary metastases compared to treatment with SE-B alone (P < 0.05). Similar reductions in metastases were observed with the F(ab′)2 BFA but not with the unconjugated antitumor component of the BFA. Combined treatments with SE-B plus BFA accomplished better tumor neutralization than adoptively transferred in vitro activated splenocytes (4 × 10⁷) retargeted with BFA (5–100 µg; P < 0.05).

These studies demonstrate that T-cells can be activated in vivo by SE-B and retargeted with small doses of BFA. In this immunocompetent syngeneic pulmonary metastasis model, SE-B plus BFA provided a dramatic antitumor response.