K cell cytotoxicity against antibody-coated chicken erythrocytes in tumor-bearing mice: its development with progressively growing tumor and the effect of immunization against the tumor.

Antibody-dependent (K cell) cytotoxic activity of spleen cells from mice bearing a chemically induced fibrosarcoma has been studied by using antibody-coated chicken erythrocytes as target cells. Spleen cells from tumor-bearing animals caused a significantly greater degree of target cells destruction than did those from control animals. The elevated cytotoxic activity in tumor-bearing animals increased with time after the tumor inoculation and correlated directly with the size of the tumor. The development of increased cytotoxic activity could be circumvented by surgical removal of the tumor. Mice that received x-irradiated tumor cells of x-irradiated tumor cells followed by a live challenge did not show a tumor growth and also failed to show increased K cell cytotoxic activity. It has been concluded that the increased K cell activity results directly from the active growth of tumor. The role of K cells in immunosurveillance has been discussed.     

The influence of age and sex on phagocyte chemiluminescence

The process of ageing is associated with increased susceptibility to infection. Phagocytes form the primary defence mechanism against infecting microorganisms, but the influence of ageing on phagocyte function remains controversial. In this study we have applied a microtitre plate phagocyte chemiluminescence (CL) assay suitable for clinical use to compare phagocyte oxidative metabolism in younger healthy subjects (age 20–60 years) and healthy older (60–70 years) subjects. Polymorphonuclear leukocytes (PMNL) and monocytes were stimulated using phorbol myristate acetate (PMA), serum opsonized zymosan (SOZ), and non-opsonized zymosan (ZYM) in the presence of both lucigenin and luminol. Monocytes showed a higher luminolenhanced CL response to PMA in males compared with females in the younger age group. No PMNL differences were observed between the sexes. Although no difference were found in relation to age when cells were stimulated with PMA and SOZ, significantly lower background (unstimulated) CL was obtained from PMNL with luminol. PMNL luminol-enhanced CL responses were also lower in response to ZYM. The findings suggest a reduced response of PMNL from older subjects to minimal stimulation. This could be related to abnormalities in the triggering of the respiratory burst or myeloperoxidase release due to ageing. The influence of age and sex should be taken into account in clinical studies of phagocyte CL.     

Generation of cytotoxic lymphocytes to syngeneic tumor by using co-stimulator (Interleukin 2)

Cytotoxic lymphocytes (CL) highly active against the syngeneic mastocytoma, P815, were generated from spleen cells of DBA mice cultured with co-stimulator (Interleukin 2) and P815. More CL activity was generated from spleen cells of P815 tumor-bearing mice than from spleen cells of normal mice. Thymus cells from tumor-bearing mice, however, did not produce increased CL activity. Most of the CL were Thy 1 and Ly 1 positive. The optimal culture conditions and kinetics were similar to those for the generation of allogeneic cytotoxic T lymphocytes. The cytotoxic activity against syngeneic P815 was similar in magnitude to the response of DBA spleen cells to allogeneic tumor lines and to the response of allogeneic CBA spleen cells to P815. Although CL generated from tumor-bearing mice did not lyse normal DBA cells, they did lyse, to a much lesser degree, a number of tumor cell lines other than the sensitizing P815. This nonspecific lysis was not H2 restricted nor was it restricted to tumors of lymphoid origin. Generation of nonspecific cytolytic activity was antigen independent, occurring in the presence of co-stimulator alone.     

Generation of reactive oxygen species by polymorphonuclear leukocytes and its modulation by calcium ions during tumor growth

The generation of reactive oxygen species (ROS) by polymorphonuclear leukocytes (PMNL) and the role of Ca2+ in regulating their activity during Zajdela hepatoma growth in the animal peritoneal cavity were studied. We found a marked increase in the ROS-generating activity of PMNL in circulating blood, the result of increases in both the specific activity of leukocytes and total number of PMNL in circulating blood. The ROS-generating activity of PMNL was substantially activated by Ca2+ ions and a calcium ionophore (ionomycin), but this effect virtually completely disappeared during tumor growth. Perhaps the high ROS-generating activity of PMNL and the lack of the sensitivity to extracellular Ca2+ during tumor growth in the organism are due to an accumulation of intracellular Ca2+ ions.     

Subpopulations of neutrophils with increased oxidative product formation in blood of patients with infection

Stimulated human polymorphonuclear leukocytes (PMNL) have a marked increase in oxidative metabolism, producing reduced oxygen species (e.g., H2O2) that mediate bacterial killing. Previously, quantitation of metabolic responses of PMNL from patients with acute infections employed assays that measure mean activity of the entire PMNL population; such studies reported a modest and highly variable increase in oxidative metabolic responses of such "toxic" PMNL compared with normal cells. To assess metabolic capability of PMNL from 51 patients with acute bacterial infection, we employed a quantitative flow cytometric assay of H2O2-dependent oxidative product formation, the intracellular oxidation of 2',7'-dichlorofluorescin (DCFH). After stimulation by phorbol myristate acetate, the PMNL of patients demonstrated an increase in mean DCFH oxidation (315 +/- 14 and 180 +/- 4.5 amol/cell, patients and controls). Hexose monophosphate shunt activation was similarly increased in stimulated PMNL from bacteremic patients. These data are comparable with previous studies of mean metabolic activities of toxic PMNL. However, these mean values underestimate the quantitative responses of the hyperresponsive ("primed") PMNL within a mixture of normal and primed PMNL in the patients' blood. The flow cytometric assay demonstrated that the PMNL of the patients were composed of two populations. One population of PMNL had normal oxidative responses; the other "primed" population had up to 4.6 times the oxidative product formation of normal cells. Similar priming of circulating PMNL was caused by infection with gram-positive or gram-negative staining bacteria or by Candida species. The proportion and oxidative ability of the primed PMNL occurred independently of the number of juvenile neutrophil forms and independently of "toxic" morphologic changes of Wright's-stained PMNL. On the average, 40% of the PMNL of patients were primed, but the size of the primed PMNL population varied widely between patients (range 0 to 80%). This variable subpopulation may explain the variability of mean responsiveness of the PMNL of patients reported previously. Moreover, the marked increase in oxidative metabolic capability of the primed PMNL may be a significant component of the host response to acute infection. It could also contribute to the damage to host tissues such as pulmonary vascular endothelium during bacteremia.     

Synergistic activation of macrophages via CD40 and TLR9 results in T cell independent antitumor effects

We have previously shown that macrophages (Mphi) can be activated by CD40 ligation to become cytotoxic against tumor cells in vitro. Here we show that treatment of mice with agonistic anti-CD40 mAb (anti-CD40) induced up-regulation of intracellular TLR9 in Mphi and primed them to respond to CpG-containing oligodeoxynucleotides (CpG), resulting in synergistic activation. The synergy between anti-CD40 and CpG was evidenced by increased production of IFN-gamma, IL-12, TNF-alpha, and NO by Mphi, as well as by augmented apoptogenic effects of Mphi against tumor cells in vitro. The activation of cytotoxic Mphi after anti-CD40 plus CpG treatment was dependent on IFN-gamma but not TNF-alpha or NO, and did not require T cells and NK cells. Anti-CD40 and CpG also synergized in vivo in retardation of tumor growth in both immunocompetent and immunodeficient mice. Inactivation of Mphi in SCID/beige mice by silica treatment abrogated the antitumor effect. Taken together, our results show that Mphi can be activated via CD40/TLR9 ligation to kill tumor cells in vitro and inhibit tumor growth in vivo even in immunocompromised tumor-bearing hosts, indicating that this Mphi-based immunotherapeutic strategy may be appropriate for clinical testing.     

Characteristics of macrophage activation by gamma interferon for tumor cytotoxicity in peritoneal macrophages and macrophage cell line J774.1

We investigated the characteristics of macrophage-mediated tumor cytotoxicity (MTC) against Meth A target, H2O2 generation and release of effector molecule(s) for MTC, by comparing with those of peritoneal macrophages (PMP) and macrophage cell line J774.1 during stimulation with recombinant gamma interferon (IFN-gamma). In PMP, MTC was demonstrated when they were stimulated with IFN-gamma for 12 hr (short-term stimulation) and was abrogated when they were stimulated for 48 hr (long-term stimulation). Enhanced H2O2 generation was observed in PMP activated by long-term stimulation followed by triggering with PMA, but not observed by triggering with Meth A cells. By contrast, whereas non-treated J774.1 cells have already attained a definite level of MTC, a higher MTC level was demonstrated both by short- and long-term stimulations. Conversely, J774.1 cells were unable to generate H2O2 at any stage of IFN-gamma stimulation followed by triggering both with PMA or Meth A cells. The time course for stimulation of PMP by IFN-gamma for release of cytotoxic factor (CF) corresponded to that for MTC by PMP, and activities of the CF released from both activated PMP and J774.1 cells also closely corresponded to those of MTC by both cells. The serological and physicochemical characteristics of CF released from both activated PMP and J774.1 cells were determined to be closely related to those of tumor necrosis factor (TNF). These results indicate that in contrast to PMP, the J774.1 cell line is free from suppression stage for MTC and CF release during stimulation with IFN-gamma. The results suggest that TNF-like CF plays a crucial role for MTC against Meth A target, and that H2O2 is irrelevant for MTC against Meth A.     

Luminol‐, isoluminol‐ and lucigenin‐enhanced chemiluminescence of rat blood phagocytes stimulated with different activators

Luminol‐, isoluminol‐ or lucigenin‐enhanced chemiluminescence (CL) was used to measure the production of reactive oxygen species by rat blood leukocytes. Opsonized zymosan (OZ), phorbol‐12‐myristate‐13‐acetate (PMA), calcium ionophore A23187 (Ca‐I) or N‐formyl‐Met‐Leu‐Phe (fMLP) were used as activators. The CL signal of isolated blood leukocytes decreased in rank order of luminol > isoluminol > lucigenin. The kinetic pro?les of luminol‐ and isoluminol‐enhanced CL were similar upon stimulation by each activator tested. The remarkably higher luminol and isoluminol CL responses were obtained after OZ stimulation when compared with other activators. However, when lucigenin was used, the PMA‐ and OZ‐stimulated CL were comparable. The presence of plasma increased OZ‐activated CL because of the enhanced phagocytosis of OZ. This was demonstrated by determining the phagocytosis of the ?uorescent OZ using a ?ow cytometer. In contrast, the presence of plasma decreased PMA‐activated CL, due to the antioxidant properties of plasma as determined by the CL method. As far as whole blood is concerned, only OZ activated luminol‐enhanced CL was reliable. Blood volumes over 5 µL decreased CL activity due to the scavenging ability of erythrocytes. The results suggest that 0.5 µL whole blood is suf?cient for routine luminol‐enhanced CL analysis of whole blood oxidative burst in rats. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of glutaraldehyde on NADPH oxidase system of guinea pig polymorphonuclear leukocytes

In an attempt to elucidate properties and activation mechanisms of the NADPH oxidase system, which is known to be responsible for the production of superoxide anion (O2-) in cell membranes of polymorphonuclear leukocytes (PMNL), intact guinea pig PMNL were treated with glutaraldehyde, a protein crosslinking reagent, before or after stimulation with phorbol 12-myristate 13-acetate (PMA). Then, PMNL were disrupted and NADPH oxidase activity was measured. After the treatment of resting PMNL with glutaraldehyde, NADPH oxidase was no longer activated by PMA. On the other hand, the NADPH oxidase activity enhanced by PMA in advance was markedly retained by the glutaraldehyde treatment of such PMA-stimulated PMNL as compared to that in untreated cells. Similar retention by glutaraldehyde of the stimulated NADPH oxidase activity was observed in PMNL stimulated by formyl-methionyl-leucyl-phenylalanine (FMLP) and cytochalasin D. Furthermore, the oxidase activity of glutaraldehyde-treated PMNL was stable during incubation at 37 degrees C, the half life of the oxidase activity of the treated PMNL being more than 90 min whereas that of the untreated PMNL is about 15 min. This ability of the glutaraldehyde treatment to retain the activity was also observed against inactivation by high concentrations of NaCl and by positively charged alkylamine.     

Effects of isolated tumor lymphocytes alone and with adherent cells

Summary The effect on the growth of gradient-isolated mouse mammary tumor cells of different populations of lymphoid cells were evaluated in micrototoxicity assays. Variable effects were obtained with tumor-bearer lymph node and spleen cells: in some experiments growth stimulation occurred, whereas in others inhibition was observed. Mixed effector populations gave more regular results: adherent spleen cells added to lymph node or spleen lymphocytes inhibited tumor cell growth in six of nine experiments; inhibition occurred when either of the effector populations in the mixture was derived from the tumor-bearing mouse. Tumor-associated lymphoid cells (TAL) stimulated growth of the tumor cells in five of seven experiments. However, TAL inhibited tumor growth when combined with adherent spleen cells from tumor-bearing animals. In contrast with the peripheral lymphoid cells, admixture of control adherent cells from normal animals with TAL did not inhibit growth. No natural killer effect was seen in these growth inhibition assays. These data indicate that lymphoid populations capable of inhibiting tumor cell growth can be found in tumor-bearing animals, but such combination of active cells are not present at the tumor site.     

Effects of exposure of different skin areas to low-power laser light

The effect of He-Ne laser light of extremely low power (632.8 nm, 0.2 mW/cm²) on the immune status of mice bearing solid tumors was studied. The state of tumor-bearing animals was assessed taking into account the number of immunocompetent cells, concentration of cytokines (tumor necrosis factor and interleukin-2), production of nitric oxide, expression of heat shock proteins 70 and 90, and the activity of natural killer cells. The model of a solid tumor was formed by subcutaneous transplantation of Ehrlich ascite carcinoma cells to mice; the average lifespan of animals was approximately 55 days. Different areas of skin of tumor-bearing mice were irradiated with laser light either singly (1 min; dose, 0.012 J/cm²) or repeatedly (1 min every 3 days over 30 days; total dose, 0.1 J/cm²). It was established that long-term chronic exposure of mice bearing Ehrlich ascite carcinoma cells to low-power laser light in the thymus projection area and especially in the tumor projection area leads to a decrease in the natural antitumor potential, which is manifested in acceleration of tumor growth and a tendency to decrease in the lifespan of tumor-bearing mice. Conversely, stimulation of antitumor immunity was observed over several days after a single exposure to low-power laser radiation. The results suggest that it is expedient to continue studies of the immunomodulating effects of low-power laser light and demonstrate the necessity of monitoring the immune system in the course of laser therapy.     

Induction of activated natural killer cells from murine spleen cells primed in vivo and subsequently challenged in vitro with the streptococcal preparation OK432

Summary The present study shows that natural killer cell-mediated cytotoxicity of BALB/c mouse spleen cells to syngeneic tumor cells was augmented by in vivo priming or in vitro stimulation with the streptococcal preparation OK432. The augmentation of spleen cell cytotoxicity to syngeneic tumor cells by in vivo priming alone with OK432 was lower than that obtained by in vitro stimulation alone with OK432. When the murine spleen cells primed in vivo with OK432 were rechallenged in vitro with OK432 at various intervals, the natural cytotoxicity was more strongly enhanced than that seen with in vitro stimulation alone. The cell surface phenotype of killer cells activated with OK432 was Thy 1⁺ and asialo GM _inf1 ^sup+ , suggesting the activated natural killer cell. Next, mice were transplanted with syngeneic colon adenocarcinoma cells, and primed in vivo with OK432. These spleen cells were subsequently challenged in vitro with OK432. These spleen cells displayed a strong cytotoxic activity not only to the transplanted adenocarcinoma cells but also to other syngeneic tumor cells.     

Identification of tumor-infiltrating macrophages as the killers of tumor cells after immunization in a rat model system.

Immunization can prevent tumor growth, but the effector cells directly responsible for tumor cell killing in immunized hosts remain undetermined. The present study compares tumor grafts that progress in naive syngeneic rats with the same grafts that completely regress in hosts preimmunized with an immunogenic cell variant. The progressive tumors contain only a few macrophages that remain at the periphery of the tumor without direct contact with the cancer cells. These macrophages do not kill tumor cells in vitro. In contrast, tumors grafted in immunized hosts and examined at the beginning of tumor regression show a dramatic infiltration with mature macrophages, many of them in direct contact with the cancer cells. These macrophages are strongly cytotoxic for the tumor cells in vitro. In contrast to macrophages, tumor-associated lymphocytes are not directly cytotoxic to the tumor cells, even when obtained from tumor-immune rats. However, CD4(+) and CD8(+) T cells prepared from the regressing tumors induce tumoricidal activity in splenic macrophages from normal or tumor-bearing rats and in macrophages that infiltrate progressive tumors. These results strongly suggest that the main tumoricidal effector cells in preimmunized rats are macrophages that have been activated by adjacent tumor-immune lymphocytes.     

Caspase 3-mediated stimulation of tumor cell repopulation during cancer radiotherapy

In cancer treatment, apoptosis is a well-recognized cell death mechanism through which cytotoxic agents kill tumor cells. Here we report that dying tumor cells use the apoptotic process to generate potent growth-stimulating signals to stimulate the repopulation of tumors undergoing radiotherapy. Furthermore, activated caspase 3, a key executioner in apoptosis, is involved in the growth stimulation. One downstream effector that caspase 3 regulates is prostaglandin E(2) (PGE(2)), which can potently stimulate growth of surviving tumor cells. Deficiency of caspase 3 either in tumor cells or in tumor stroma caused substantial tumor sensitivity to radiotherapy in xenograft or mouse tumors. In human subjects with cancer, higher amounts of activated caspase 3 in tumor tissues are correlated with markedly increased rate of recurrence and death. We propose the existence of a cell death-induced tumor repopulation pathway in which caspase 3 has a major role.     

Immunologic significance of nonspecific suppressor cells in spleens of tumor-bearing mice.

Spleen cells from mice bearing a progressively growing syngeneic tumor failed to respond to stimulation with mitogens in vitro. This lack of reactivity was due to the presence of nylon wool-adherent cells in the population that could inhibit the mitogen response of normal lymphocytes. Paradoxically, at times when strong suppressor cell activity could be detected in tumor-bearing mice, the animals responded normally to in vivo immunization with sheep erythrocytes and allogeneic tumors, and to in vitro sensitization with allogeneic tumor cells. Regression of a highly antigenic syngeneic tumor also was unaffected by the presence of these suppressor cells. Thus, the occurrence of nonspecific suppressor cells in the spleens of tumor-bearing mice did not influence the overall immunologic competence of these animals.     

Eradication of tumor cells after injection into immunized hosts compared with the eradication of tumor cells after transfer of immune peritoneal exudates into tumor-bearing recipients

Summary DBA/2 mice were immunized against the syngeneic SL2 lymphoma by two or five injections with irradiated lymphoma cells given IP or SC, respectively. The antitumor efficacy induced in immunized mice was tested by (a) IP injection of the immunized mice with nonirradiated tumor cells and (b) transfer of the total immune peritoneal exudate, the cellular fraction only, or the cell-free fraction only, IP into tumor-bearing recipients, or (c) tumor neutralization tests (Winn assay). It was shown that immunized mice were able to reject 5×10⁷ SL2 tumor cells (8 of 14 mice survived >100 days), while in most transfer experiments 2×10⁵ SL2 cells could be eradicated. In the tumor neutralization experiments a number of 10⁶ SL2 cells were eradicated. When the immune exudates were given before the inoculations of SL2 tumor cells the number of survivors increased significantly. Further, it was shown that the cellular fraction is the major contributor to the antitumor effect in the transfer experiments, since there was no significant difference in tumor eradication after injection of a complete immune exudate and after injection of the isolated cellular fraction. Injection of the noncellular fraction had no measurable antitumor effect. An increase in the number of injections with total peritoneal exudates from immunized mice did not result in an increase in tumor eradication in the tumor-bearing recipients.Extra stimulation (IP) of immunized mice with 10⁴ nonirradiated cells 6 days after the last immunization resulted in an increase of the antitumor efficacy of these peritoneal exudates of these mice when collected 4–24 h after this stimulation. Extra stimulation with 10⁶ irradiated cells had no measurable effect.     

Priming of the respiratory burst of neutrophils by diacylglycerol. Independence from activation or translocation of protein kinase C

Preincubation of neutrophils with certain agonists may "prime" the cells to cause increased responses to a second stimulus ("primed stimulation"). We used two approaches to examine the role of protein kinase C (Ca2+/phospholipid-dependent enzyme) in priming and stimulation by 1-oleoyl-2-acetylglycerol (OAG), phorbol 12-myristate 13-acetate (PMA), and N-formyl-Met-Leu-Phe (fMLP): inhibition of protein kinase C by 1-(5-isoquinolinesulfonyl)-piperazine (C-I) and measurement of protein kinase C translocation induced by priming and stimulatory concentrations of OAG. C-I had little effect on stimulation or primed stimulation by fMLP, suggesting that fMLP invokes events independent of protein kinase C. C-I equally inhibited stimulation and primed stimulation by PMA. Direct stimulation by OAG was inhibited, but priming and primed stimulation by OAG was unaltered by C-I. OAG concentrations greater than or equal to 100 microM caused translocation of protein kinase C, in correlation with direct stimulation of the respiratory burst. Lower OAG concentrations (10-30 microM) primed to stimulation by fMLP and, conversely, stimulated neutrophils primed with fMLP, yet did not cause translocation of protein kinase C. The data are compatible with previous assumptions that PMA and OAG directly stimulate polymorphonuclear neutrophil leukocytes by translocation and activation of protein kinase C. However, priming and primed stimulation by OAG apparently invoke distinct transduction mechanisms other than protein kinase C translocation.     

Generation from tumor-bearing mice of lymphocytes with in vivo therapeutic efficacy

Systemic transfer of sensitized lymphocytes can effectively mediate the regression of established tumors. However, virtually all prior experimental applications of this approach have utilized lymphocytes from animals that have been immunized to reject tumor challenge. A similar source of cells is not available in the human. With the use of a weakly immunogenic murine tumor, MCA 105, we demonstrate here that following in vitro sensitization (IVS) with viable tumor cells and interleukin 2, the nontherapeutic lymphoid cells from mice bearing a progressively growing tumor acquired antitumor reactivity capable of mediating the regression of established pulmonary metastases. Although the IVS system induced nonspecific lymphokine-activated killer-like cytotoxic activity from lymphoid cells of normal as well as tumor-bearing mice, therapeutically active cells could only be generated from cultures initiated with lymphoid cells from tumor-bearing animals, indicating that the IVS was a secondary in vitro immune response. Without other treatment, the IVS cells could mediate antitumor effects. However, low doses of exogenous interleukin 2 administration could enhance their therapeutic efficacy. By in vivo T cell subset depletion with monoclonal antibodies, the primary effector cells were identified as belonging to cytotoxic/suppressor T cell lineage expressing the Lyt-2 phenotype. In addition, these therapeutic effector cells could be further expanded in numbers in vitro with continuous stimulation by tumor cells in the presence of interleukin 2. Compared to the number of cells initiating the culture, as many as 126 times the number of cells were obtained after 9 days of IVS followed by in vitro expansion for an additional 5 days. Studies on the kinetics of the occurrence of the pre-effector lymphocytes during tumor growth revealed that they were readily obtained from draining lymph nodes of mice with a broad range of tumor burdens as well as durations of tumor growth. The ability to generate and expand, in vitro, therapeutically active lymphocytes from tumor-bearing hosts has important implications for cellular therapy of human cancers.     

Suppression of the in vitro secondary response to syngeneic tumor and of in vivo tumor therapy with immune cells by culture-induced suppressor cells.

Mice with advanced disseminated syngeneic tumor can be successfully treated with a combination of chemotherapy and adoptively transferred syngeneic immune cells. We have previously demonstrated that in vivo primed cells secondarily sensitized in vitro became more effective in tumor therapy, whereas primed cells cultured for 5 days without tumor stimulation became less effective than an equal number of uncultured fresh primed cells. Therefore, we examined stimulated and unstimulated cultures of tumor-primed cells for the presence of culture-induced suppressor cells, and determined whether in vivo tumor therapy with immune cells could be inhibited by concurrent inoculation of immune effector cells and cultured normal spleen cells, which contain culture-induced suppressor cells but are devoid of additional effector cells. The in vitro primary allogeneic response was suppressed by cultured normal spleen cells, or tumor-primed spleen cells previously cultured for 5 days with or without tumor stimulation. In vitro secondary sensitization to syngeneic tumor was suppressed by normal or tumor-primed cells that had previously been cultured for 5 days without stimulation. The majority of this suppression was mediated by T cells in the cultured populations. The efficacy of fresh tumor-primed cells, as well as primed cells secondarily sensitized in vitro, in adoptive chemoimmunotherapy of advanced tumor was diminished by concurrent inoculation of cultured normal cells. The cells mediating suppression of in vivo therapy required previous in vitro culture for induction, and were radiation sensitive.     

Tumoricidal activity of syngeneic macrophages from melanoma-bearing mice: Changes with progressive tumor growth

Summary Changes in the cytostatic and cytotoxic activity of macrophages from tumor-bearing (TBM) and control mice were studied in a murine model of malignant melanoma. Syngeneic macrophages from TBM were initially noncytotoxic, but became cytotoxic and achieved their maximum destructive ability after 14 days of tumor growth. With continued tumor growth these macrophages either lost or had reduced cytotoxic activity. In contrast, macrophages from the same melanoma-bearing animals were significantly cytostatic at an earlier stage of tumor growth, but with continued melanoma growth these macrophages were no more cytostatic than controls. Moreover, melanomas grew slowly during the time when macrophages were observed to be cytostatic but grew rapidly at those stages when macrophages had a reduced ability to inhibit melanoma DNA synthesis. When these effector cells became cytotoxic melanomas were growing rapidly and changes in cytotoxicity had little effect on tumor mass. Thus, macrophages do not completely suppress melanoma proliferation and, although exhibiting cytotoxicity they were relatively ineffective in controlling a large mass of tumor cells.