Involvement of the omental lymphoid organ in the induction of peritoneal immunity against tumor cells.

The omental lymphoid organ (OLO) is a part of the greater omentum composed of vascularized spots containing lymphocytes, plasma cells and macrophages located in a regular pattern between fat cells. To gain insight in the involvement of the OLO in the induction of immunity against tumor cells in the peritoneal cavity, we studied the penetration of tumor cells into the OLO after intraperitoneal, subcutaneous and intravenous injection. Furthermore we analyzed the tumoricidal activity of macrophages isolated from the OLO. Our results indicate that the OLO is only infiltrated by tumor cells directly from the peritoneal cavity, but not by subcutaneously or intravenously injected tumor cells unless they have reached the peritoneal cavity. Bromodeoxy-uridine labelled tumor cells can be detected in the OLO within 10 min after i.p. injection. The penetration is facilitated by the induction of fenestrations between the mesothelial cells (lining the OLO) after intraperitoneal injection of the tumor cells. These fenestrations can also be seen after nonspecific stimulation of the peritoneal cavity. Macrophages isolated from the OLO of mice immunized against syngeneic as well as allogeneic tumor cells express a significant cytotoxicity, which (at least in the syngeneic situation) precedes the cytotoxicity of the macrophages isolated from the peritoneal cavity. In conclusion, our data support the hypothesis that immune reactions against intraperitoneally injected tumor cells are initiated in the OLO leading to 'peritoneal immunity' against these tumor cells.