Inhibition of Friend Leukemia Cell Visceral Métastasesby a New Monoclonal Antibody and Role of the Immune System of the Host in Its Action1

We developed a syngeneic mouse IgG2a monoclonal antibody (MAb) A9D41 directed against the Friend leukemia virus envelope gp70 antigen present on the cell surface membranes of virus producer 3C18 Friend leukemia cells (FLC). A9D41 showed a marked antitumor activity in DBA/2 mice given injections of gp70 positive 3C18 FLC, but it was ineffective in mice given injections of gp70 negative 745 FLC or unrelated tumor cells. A9D41 was particularly effective in inhibiting the develop ment of 3CI8 FLC liver and spleen métastases. MAb was also effective as adjuvant therapy in inhibiting visceral métastasesafter excision of an established s.c. FLC tumor, and combined therapy of A9D41 with mouse interferon a/ßwas more effective than MAb or Interferon ot/ß alone. The immune system of the host played a decisive role in the antimetastatic action of A9D41. Thus, although MAb was cytotoxic for 3C18 FLC in vitro in the presence of rabbit complement, the l-(al>'); fragment was ineffective in vivo, and the antitumor effect of MAb was abolished in mice treated with an antibody to (1)4 and diminished in natural killer cell-deficient beige and athymic nude mice. MAb-treated mice surviving injection of FLC developed an immune response to 3CI8 FLC.