Synergetic effect of interleukin-2 and cellular cytotoxicity against a novel tumor-associated carbohydrate antigen Le(a)/Le(a) (dimeric Le(a)) mediated by monoclonal antibody NCC-ST-421 in adoptive immunization using SCID mice.

The murine IgG3 monoclonal antibody NCC-ST-421, raised against a human gastric cancer, shows strong reactivity with dimeric Le(a) (Le(a)/Le(a); V4FucIII4FucLc6Cer) expressed on gastrointestinal cancer cells. ST-421 reacted minimally with non-dimeric or simple Le(a) expressed on normal tissues. ST-421 is capable of mediating both antibody-dependent cellular cytotoxicity (ADCC) with human peripheral blood lymphocytes, and complement-dependent cytotoxicity with human complement. Interleukin-2 (IL-2) modulates the function of immunocytes, in particular inducing lymphokine-activated killer (LAK) cell activity and enhancing ADCC. We therefore employed combination immunotherapy with IL-2, LAK, and ST-421-induced ADCC in vitro and in mice with severe combined immunodeficiency (SCID), using target tumor cells expressing Le(a)/Le(a) antigen. ADCC against human colon cancer cell lines in vitro was enhanced three to four times after preincubation with IL-2. Addition of IL-2 reduced the amount of ST-421 required for efficient ADCC 10- to 100-fold. ADCC was activated by IL-2 earlier (1 day) than the generation of LAK cells (3-4 days), and at lower concentration of IL-2. These effects were specific for ST-421, as demonstrated by experiments with irrelevant antibody or irrelevant target cells. An anti-(Fc receptor) antibody blocked the ADCC but not the LAK activity in vitro. The enhancement of ADCC by IL-2 may be caused by activation of effector cells expressing Fc receptors. In vivo experiments using SCID mice inoculated with human colon cancer showed a significant tumor-growth-suppressive effect after combined therapy using human peripheral blood lymphocytes, LAK, IL-2, and ST-421. In summary, adoptive immunization with human lymphocytes activated by IL-2 and ST-421 effectively suppressed growth of gastrointestinal cancer cells expressing Le(a)/Le(a).