Modulation of murine lymphoma growth by MDP, MDP(D-D) and cyclophosphamide. 1. Inhibition of growth in vivo

Production and enhanced anti-tumor activity of tumor necrosis factor in mice treated with cyclophosphamide

We demonstrated recently that the production of tumor necrosis factor (TNF) is induced in normal mice and in the immunosuppressed nude mouse model by the administration of muramyl dipeptide (MDP) derivatives followed by endotoxin (lipopolysaccharide). In the present study, the ability of this treatment to induce the production of TNF in mice receiving cyclophosphamide (CY) was examined. Two days following treatment with high-dose CY (250 mg/kg), mice exhibited leukocytopenia and drastically reduced splenic weight. However, these animals remained capable of producing TNF, albeit at lower levels, when treated with MDP derivatives and lipopolysaccharide (LPS), particularly when the lipophilic analogue MDP-dipalmitoyl glycerol (GDP) was utilized. TNF was also induced by the administration of MDP-GDP and LPS to Meth A sarcoma-bearing mice treated with this dose of CY. Furthermore, in all animals receiving this combination therapy, sarcoma necrosis and complete regression were obtained without any sign of tumor regrowth. A dose of 100 mg/kg CY was not effective for inhibiting tumor regrowth under the same experimental conditions. These results demonstrated that the anti-tumor activity of endogenously induced TNF is potentiated by combined therapy with a high dose of CY.

Kupffer cells and liver metastasis. Optimization and limitation of activation of tumoricidal activity

Kupffer cells, tissue-fixed macrophages located in the sinusoids of the liver, represent the highest concentration of mononuclear phagocytes in the body. Their ability to act as scavengers of particulate material in the blood has given rise to speculation that they play a role in controlling hepatic metastases derived from blood-borne tumor cells. Circumstantial evidence for such a role has been obtained from animal studies where Kupffer cell function has been compromised or inhibited, and from anecdotal clinical observations. Current evidence suggests that Kupffer cells are capable of nonspecifically eliminating some circulating tumor cells from the circulation via phagocytosis. This surveillance mechanism would appear to be limited in capacity, and subject to a number of external factors. Recent studies have demonstrated that Kupffer cells can be activated to a tumoricidal state via the administration of biological response modifiers such as gamma interferon or muramyl peptides. The localization of liposomes within Kupffer cells after systemic administration has provided a considerable stimulus for the efficient targeting of macrophage-activating compounds to these cells. Such therapeutic intervention, while capable of inducing Kupffer cell tumoricidal activity in situ and inhibiting tumor growth, is limited with respect to the location of the tumor cells (sinusoidal versus parenchymal) and to the size of the metastatic nodule. Therapeutic intervention using liposomes containing macrophage-activating agents may only be of benefit in patients with minimal tumor load who are at risk for hepatic metastases, rather than those patients who already have clinically detectable liver tumors.

Modulation of pulmonary macrophage superoxide release and tumoricidal activity following activation by biological response modifiers

Following immunologic activation, pulmonary macrophages may prevent or cause regression of lung metastases by mechanisms which remain largely unknown. The studies described here were designed to determine if enhanced oxygen metabolite release was related to postactivation tumoricidal activity. We have shown that in vitro activation of Fischer 344 rat pulmonary macrophages by either free or liposome-encapsulated muramyl dipeptide leads to both enhanced release of superoxide anions and marked tumoricidal activity against syngenic (Fischer 13762), allogeneic (Schmidt-Ruppin RR 1022) and xenogeneic (Fibrosarcoma MCA-F) 125I-deoxyuridine-labeled target cells. This immune modulator did not, however, metabolically activate pulmonary macrophages as effectively as liposome-encapsulated lipopolysaccharide. A 24-h in vitro incubation with either 150 U or 300 U of interferon-gamma (3 X 10(6) U/mg) or 30 U, 150 U or 300 U of interferon-alpha (6 X 10(5) U/mg) caused a significant elevation in superoxide release above controls, whereas short-term exposure (2 or 4 h) had little or no effect. Free or encapsulated 6-O-stearoyl muramyl dipeptide, on the other hand, did increase superoxide levels at all 3 time periods. When either interferon-gamma or free or encapsulated muramyl dipeptide derivative were administered to intact rats by either i.v. injection, intratracheal instillation or osmotic minipump infusion, pulmonary macrophage tumoricidal activity was observed 96 h after cell harvesting. Zymosan-stimulated superoxide release, however, was not consistently elevated above control or empty liposome treatment following this course of in vivo activation. The data collectively suggest that in vivo pulmonary macrophage activation to a tumoricidal state and metabolic activation resulting in enhanced superoxide may be separable events.

Enhanced superoxide release and tumoricidal activity by a postlavage, in situ pulmonary macrophage population in response to activation by Mycobacterium bovis BCG exposure

The monocytic phagocyte population of rat lungs is heterogeneous. In addition to the freely lavagable alveolar macrophages, there is a fixed in situ tissue-associated subpopulation of pulmonary macrophages. The response of this subpopulation to classical macrophage activation by Mycobacterium bovis BCG exposure was monitored. Results indicate that this population can be activated both metabolically and functionally, as evidenced by enhanced release of superoxide anions and demonstrable tumoricidal activity against syngeneic and xenogeneic target cells. The pattern of metabolic activation of in situ tissue-associated macrophages differed somewhat from that of alveolar macrophages and was observed only after subsequent exposure of the cells to either zymosan particles or phorbol myristate acetate. Upon such exposure, the activated zymosan-treated tissue macrophages released approximately twice as much superoxide as the nonactivated cells and amounts comparable to the amounts released by activated alveolar macrophages. The tissue macrophages also displayed greater levels of cytotoxicity toward xenogenic targets than the alveolar cells and may have an important role in preventing microbial or tumor cell colonization of respiratory systems.

In vitro inhibition of murine B-cell tumor growth by MDP, MDP(D-D) and Vaccin is mediated by macrophages

The effect of two muramyl dipeptides, N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and N-acetylmuramyl-D-alanyl-D-isoglutamine (MDP(D-D)), on the in vitro growth of two murine ascitic plasmacytomas, MOPC 173 and TEPC 15, and on an ascitic lymphoma cell line, ABPL2, was studied. The ability of the muramyl dipeptides to inhibit tumor cell growth was compared with a sonicated antigenic preparation of Mycobacterium vaccae (Vaccin), known to have macrophage stimulation activity. The growth of all three ascitic cell lines was inhibited by both muramyl dipeptides and Vaccin. Macrophage-depletion of the ascitic cell populations led to an increase in cell growth of TEPC 15 and MOPC 173, and a decrease in ABPL2. A reduction or loss of the inhibitory activity of the muramyl dipeptides or Vaccin was also observed, and no inhibitory activity was found when the tumor cell lines were cultured in vitro to render them macrophage-free. The inhibitory activity of MDP or MDP(D-D) was restored when purified ascitic macrophages were added to the in vitro cultured cell lines. It was demonstrated that a minimum number of macrophages were necessary for the expression of inhibitory activity. Indomethacin, a PG-synthetase inhibitor, was found to act in a synergistic manner with MDP and MDP(D-D) in inhibiting TEPC 15, but antagonized the effect of these two agents on ABPL2. The lymphoma cell line ABPL2 appeared to be the most sensitive to inhibition by MDP(D-D), a nonpyrogenic, adjuvant-inactive stereoisomer of MDP.

Inhibition of tumor growth in mice treated with synthetic muramyl dipeptide

Treatment with synthetic MDP inhibited growth of transplantable, chemically induced tumors in syngeneic mice. The tumor-inhibitory effect was dependent on the schedule of MDP administration. Growth of SC transplants of a nonmetastasizing, MC-induced fibrosarcoma, MC11, was inhibited by local treatment with 200 micrograms and 1,000 micrograms MDP given SC 5-7 weeks before challenge. Treatment with lower (10 micrograms and 100 micrograms) doses of MDP and shorter (1-4 weeks) time intervals was not effective. Single doses of MDP (10-1,000 micrograms) 1-3 weeks after challenge had no effect. Growth of IV-inoculated, metastasizing AAT-induced hepatoma A was inhibited by IV injections of 20 micrograms MDP given 1 and 2 days prior to the challenge. Significant increases in the survival of hepatoma-bearing mice were observed only after injections of MDP incorporated in multilamellar liposomes.

Modulation of the growth of murine thymoma cell lines having different Lyt-phenotypes by MDP and MDP(D-D): macrophage-mediated inhibition of in vitro cell growth

The ability of N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and its adjuvant-inactive stereoisomer, N-acetylmuramyl-D-alanyl-D-isoglutamine, (MDP(D-D] to inhibit the in vitro growth of 3 murine ascitic thymoma lines, expressing different Lyt-phenotypes, was studied. MDP inhibited the growth of all 3 cell lines. MDP(D-D) inhibited the 2 cell lines expressing Lyt-1+2- or Lyt-1+2+ phenotypes, but not the third cell line which expressed the Lyt-1-2+ phenotype. The ability of MDP or MDP(D-D) to inhibit thymoma growth was lost when the ascitic cell populations were depleted of macrophages. MDP could be replaced by a supernatant derived from an ascitic Lyt-1+2- cell population exposed to MDP. The supernatant required the presence of macrophages for activity. The inhibition by MDP of the growth of the Lyt-1-2+ cell line was prostaglandin synthetase dependent: indomethacin antagonized the inhibitory activity of MDP. The inhibition by MDP of the Lyt-1+2- cell line was partially antagonized by indomethacin, and no antagonism was observed with the Lyt-1+2+ cell line.