Effector mechanisms of the anti-cancer immune responses of macrophages in SR/CR mice.

SR/CR (spontaneous regression/complete resistance) mice resist multiple types of cancer cells injected at numbers that are lethal to wild type (WT) mice. When the anti-tumor response was examined, leukocytes of the innate immune system, including neutrophils (PMN), macrophages and NK cells, infiltrated the tumor site for a multipronged killing response. Each cell type had independent killing activity against the cancer cells. A second aspect of this multipronged response was that cancer cells could be killed either via necrosis in vivo or via apoptosis by purified macrophages. Lymphoid cells displayed perforin (pfp) and granzymes (gzm) as effector molecules, but macrophages produced reactive oxygen species (ROS) and secreted serine proteases to kill the cancer cells. However, SR/CR macrophages did not use the well-studied tumoricidal mechanism of reactive nitrogen species (RNS) production. We previously demonstrated that macrophages tightly bound cancer cells in rosettes, and we show here that macrophages required contact with the target cells in order to unleash their cytotoxic mechanisms. Once SR/CR mice survived challenge with cancer cells, they produced antibodies that recognized the cancer cells. However, the antibodies were not required for killing by SR/CR macrophages through antibody-dependent cell-mediated cytotoxicity (ADCC) and did not enable wild type macrophages to kill target cells. In summary, purified SR/CR macrophages killed cancer cells in a non-ADCC manner via apoptosis induced by ROS and serine proteases.