Tumor-associated immunosuppressive factors

Host response to myeloma: I. Induction of cytotoxic and suppressor T cells by in vivo immunization with MOPC 104E plasmacytoma

Spleen cells from BALB/c mice immunized with mitomycin C-treated MOPC 104E plasmacytoma cells demonstrated negligible cytotoxic activity (less than 10% specific cytotoxic activity) in the 51Cr release assay. These cells exhibited increased cytotoxic activity when they were secondarily sensitized in vitro with mitomycin C-treated MOPC 104E cells. Spleen cells from normal mice showed tumor-specific cytotoxic activity when cocultured with mitomycin C-treated tumor cells at the optimal responder to stimulator ratio of 25:1. The level of cytotoxic activity obtained by in vivo primed and secondarily in vitro sensitized spleen cells did not exceed the level of activity obtained by in vitro primary sensitized cells. Significant suppression of the cytotoxic activity of in vitro primary sensitized cells was observed when cocultured with in vivo primed spleen cells during primary sensitization in vitro at a responder to suppressor cell ratio of 1:1. Suppressor cells of in vivo primed mice were removed by treatment with anti-Thy 1.2 and complement. These results suggest that spleen cells from in vivo primed mice consisted of at least two subpopulations of cells, a cytotoxic (prekiller) and suppressor T cells. Attempts to induce cytotoxic cells in vivo might have failed because of the appearance of suppressor T cells.

Spontaneous tumor rejection is not always due to a complete cellular rejection

In this paper we have studied whether there is a correlation between efficacy of hosts to reject tumor cells spontaneously and the induction of cytotoxic macrophages in these recipients due to injection (immunization) with tumor cells. Results show that there is no cause-effect relationship between macrophage cytotoxicity and tumor rejection in vivo. This lack of cause-effect relationship is greatly influenced by differences in immunogenicity between the various tumor cell lines as well as by the release of tumor-factors by some of these tumors.

Amplification by macrophages of prostaglandin-mediated immunosuppression in mice bearing syngeneic tumors

The role of macrophages in tumor-mediated immunosuppression was examined, using C57B1/6 strain mice bearing four different immunosuppressive transplantable syngeneic tumors (Lewis Lung Carcinoma, B16 Melanoma, and two fibrosarcomas induced by methylcholanthrene in our laboratory). When tested for immunosuppressive activity, in inhibiting the induction of antibody formation by normal spleen cells in response to SRBC in vitro, the splenic and peritoneal macrophages from tumor-bearing mice were all significantly suppressive. The degree of suppression correlated with immunosuppression in tumor-bearing mice challenged in vivo with SRBC. Direct action of tumor cells on normal splenic macrophages in vitro caused them to become suppressive, the extent of suppression dependent on the time of interaction and on the immunosuppressive activity of the tumor cells in vivo. Pretreatment of suppressive splenic macrophages with indomethacin, a potent inhibitor of the synthesis of prostaglandins (PG), reduced significantly their immunosuppressive activity. Also, peritoneal macrophages from tumor-bearing mice produced significantly more PGE in culture than control macrophages. Thus, tumor-activated macrophages, presumably those macrophages that infiltrate the tumor in a host reaction against the tumor, serve to amplify the level of immunosuppression in the host by producing relatively large amounts of PGE that is a key physiological mediator in the activation and function of suppressor T lymphocytes. The stimulation of PGE synthesis in macrophages, as a result of their interaction with syngeneic tumors, is initiated by PGE produced in relatively large amount by the tumor cells.

Tumor immunity to murine plasma-cell tumors. VIII. Immunosuppression of the generation of cytotoxic T cells by murine plasma-cell tumor lines

Previous studies have clearly established that murine plasmacytomas suppress B-cell responses, but no effects on T-cell responses have been noted by most investigators. However, in our investigation of the tumor-associated antigens of plasmacytomas, we have found that immunosuppression plays an important role in the complex interaction between host immune T cells and tumor cells. In these investigations, varying the number of plasmacytoma cells added to in vitro induction systems separated two different effects which these cells had on the generation of cytotoxic T cells. On the one hand, when appropriate numbers were present, tumor cells acted as immunogens and stimulated the generation of specific killer cells against tumor-associated antigen or alloantigen which they expressed. In contrast, greater numbers of tumor cells exerted a profound inhibitory effect on the generation of cytotoxic activity when cells inactivated with irradiation were used. Mitomycin-C inactivation of plasmacytoma cells abrogated this inhibitory activity. In further experiments, MPC-II cells that had been inactivated with Mitomycin C were able to prime T cells in vivo, whereas priming by live tumor cells could not be detected in the same situation. It is suggested that immunosuppression by the live tumor cells may account for their inability to prime T cells in vivo. These results indicate that immunosuppression by plasmacytomas may be one of the important variables which influence the generation of cytotoxic T cells against tumor-associated antigens both in vivo and in vitro.

The use of P815: a warning

It is generally known that many malignancies are accompanied by forms of impairment of the immune system of the host. Also P815 mastocytoma cells can exert suppressive effects on the immune response reaction in vivo and in vitro. However, we have found a product released by P815 cells, which is able to activate macrophages non-specifically; macrophages normally not cytotoxic against tumor cells become cytotoxic against various tumor cell lines upon incubation with the P815 product. Nevertheless the same product which induces macrophage activation in vitro, seems to cause enhancement of tumor growth in vivo. We feel, if the activity of a tumor product depends so strongly on the experimental conditions used, one should be very careful in interpreting the results of experiments in which P815 cells are used as target cells.

Augmentation of the anti-tumor therapeutic efficacy of long-term cultured T lymphocytes by in vivo administration of purified interleukin 2

Spleen cells from C57BL/6 mice immunized in vivo with a syngeneic Friend virus-induced leukemia, FBL-3, were specifically activated by culture for 7 d with FBL-3, then nonspecifically induced to proliferate in vitro for 12 d by addition of supernatants from concanavalin A-stimulated lymphocytes containing interleukin 2 (IL-2). Such long-term cultured T lymphocytes have previously been shown to specifically lyse FBL-3 and to mediate specific adoptive therapy of advanced disseminated FBL-3 when used as an adjunct to cyclophosphamide (CY) in adoptive chemoimmunotherapy. Because the cultured cells are dependent upon IL-2 for proliferation and survival in vitro, their efficacy in vivo is potentially limited by the availability of endogenous IL-2. Thus, the aim of the current study was to determine whether exogenously administered purified IL-2 could augment the in vivo efficacy of long-term cultured T lymphocytes. Purified IL-2 alone or as an adjunct to CY as ineffective in tumor therapy. However, IL-2 was extremely effective in augmenting the efficacy of IL-2-dependent long-term cultured T lymphocytes in adoptive chemoimmunotherapy. The mechanism by which IL-2 functions in vivo is presumably by promoting in vivo growth and/or survival of adoptively transferred cells. This assumption was supported by the findings that IL-2 did not enhance the modest therapeutic efficacy of irradiated long-term cultured cells that were incapable of proliferating in the host and was ineffective in augmenting the in vivo efficacy of noncultured immune cells that are not immediately dependent upon exogenous IL-2 for survival.

Murine macrophage cytotoxicity induced by mastocytoma cells, cell-free mastocytoma exudates, and extracts

Peritoneal macrophages obtained from normal CBA mice expressed significant cytotoxicity against the DBA/2-derived P815 mastocytoma but not against DBA/2-derived SL2 tumor or the C57BL-derived tumors TLX9 and EL4. The macrophages also expressed some cytotoxicity against the DBA/2-derived L5178Y tumor. Incubation of normal CBA macrophages with cell-free exudate of intraperitoneally growing P815 cells resulted in cytotoxicity against the SL2 and against the EL4 lymphosarcoma. Incubation of SL2 tumor cells with P815 ascitic fluid before adding the SL2 tumor cells to normal CBA macrophage monolayers also resulted in inhibition of SL2 tumor cell growth on these monolayers. Trypsinization of the macrophages after incubation with ascitic fluid (or tumor extract) but before challenge with tumor cells abolished cytotoxic activity of the macrophages. Incubation of normal macrophages with a comparable amount of trypsin before tumor cells were added had no influence on their activity. Cytotoxicity could be induced after 7 days storage of the exudate at 5 degrees C, but this ability was lost within 72 hr when kept at room temperature. Storage at -20 degrees C had no influence. Gel fractionation of the cell-free exudate showed that the product responsible for the effect is a small molecular weight product (mol wt less than 1000). Furthermore the "product" was dialyzable. The "factor" could not be shown in the supernatant of P815 cell cultures unless the cultures comprised greater than or equal to 40% dead cells.

The effect of 6-keto-prostaglandin E1 on human lymphocyte cAMP levels

6-keto-PGE1 was capable of increasing cyclic AMP levels of peripheral human blood lymphocytes in a dose and time dependent fashion. The response was maximal after 30 min of incubation at 37 degrees C; 1 microgram per ml of 6-keto-PGE1 elevated cyclic AMP levels 5 fold which compares with a 10 fold increase due to PGE1. Thus 6-keto-PGE1 may play a role as a modulator of lymphocyte function.

Involvement of peritoneal macrophages in cellular responses to mastocytoma in resistant and susceptible mice

The involvement of peritoneal macrophages in rejection of mastocytoma cells in the C57BL/6 mice was examined in comparison to similar cell responses in susceptible DBA/2 mice. By means of scanning and transmission electron microscopy it was found that macrophages constituted the major cell class responding to the mastocytoma cells in the peritoneum of both mouse strains. However, in the resistant mouse strain macrophages formed the predominant cell type during the course of tumor growth. Furthermore, tissue culture of peritoneal exudate cells from this resistant mouse strain injected with mastocytoma cells five days earlier failed to grow out tumor cells. On the other hand, macrophages decreased in number in the susceptible DBA/2 mouse strain and tumor cells did grow readily in vitro when peritoneal cells containing tumor cells and macrophages were cultured in vitro. These results indicate that macrophages constitute an important cell class in resistance of a mouse strain which is now susceptible to mastocytoma cells. The ultrastructural study provided some insight into the nature of the cell types involved and their interaction with the tumor cell.