Inhibition of hyperprogressive cancer disease induced by immune-checkpoint blockade upon co-treatment with meta-tyrosine and p38 pathway inhibitor

Background

Although immune-checkpoint inhibitors (ICI) are overall promissory for cancer treatment, they entail, in some cases, an undesired side-effect called hyperprogressive-cancer disease (HPD) associated with acceleration of tumor growth and shortened survival.

Methods

To understand the mechanisms of HPD we assayed the ICI therapy on two murine tumors widely different regarding immunogenicity and, subsequently, on models of local recurrences and metastases of these tumors. To potentiate the immune response (IR), we combined ICI with meta-tyrosine—that counteracts immune-suppressive signals—and a selective inhibitor of p38 pathway that proved to counteract the phenomenon of tumor-immunostimulation.

Results

ICI were therapeutically effective against both tumor models (proportionally to their immunogenicity) but only when they faced incipient tumors. In contrast, ICI produced acceleration of large and residual tumors. The combined treatment strongly inhibited the growth of large tumors and it managed to cure 80% of mice with local recurrences and 60% of mice bearing residual metastases.

Conclusions

Tumor enhancement was paradoxically correlated to a weak increase of the antitumor IR suggesting that a weak IR – different from a strong tumor-inhibitory one—may produce stimulation of tumor growth, mimicking the HPD observed in some clinical settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09941-2.

Improvement of Antitumor Therapies Based on Vaccines and Immune-Checkpoint Inhibitors by Counteracting Tumor-Immunostimulation

Immune-checkpoint inhibitors and antitumor vaccines may produce both tumor-inhibitory and tumor-stimulatory effects on growing tumors depending on the stage of tumor growth at which treatment is initiated. These paradoxical results are not necessarily incompatible with current tumor immunology but they might better be explained assuming the involvement of the phenomenon of tumor immunostimulation. This phenomenon was originally postulated on the basis that the immune response (IR) evoked in Winn tests by strong chemical murine tumors was not linear but biphasic, with strong IR producing inhibition and weak IR inducing stimulation of tumor growth. Herein, we extended those former observations to weak spontaneous murine tumors growing in pre-immunized, immune-competent and immune-depressed mice. Furthermore, we demonstrated that the interaction of specifical T cells and target tumor cells at low stimulatory ratios enhanced the production of chemokines aimed to recruit macrophages at the tumor site, which, upon activation of toll-like receptor 4 and p38 signaling pathways, would recruit and activate more macrophages and other inflammatory cells which would produce growth-stimulating signals leading to an accelerated tumor growth. On this basis, the paradoxical effects achieved by immunological therapies on growing tumors could be explained depending upon where the therapy-induced IR stands on the biphasic IR curve at each stage of tumor growth. At stages where tumor growth was enhanced (medium and large-sized tumors), counteraction of the tumor-immunostimulatory effect with anti-inflammatory strategies or, more efficiently, with selective inhibitors of p38 signaling pathways enabled the otherwise tumor-promoting immunological strategies to produce significant inhibition of tumor growth.

Concomitant tumor resistance: the role of tyrosine isomers in the mechanisms of metastases control

Concomitant tumor resistance (CR) is a phenomenon in which a tumor-bearing host is resistant to the growth of secondary tumor implants and metastasis. Although previous studies indicated that T-cell-dependent processes mediate CR in hosts bearing immunogenic small tumors, manifestations of CR induced by immunogenic and nonimmunogenic large tumors have been associated with an elusive serum factor. In a recently published study, we identified this factor as meta-tyrosine and ortho-tyrosine, 2 isomers of tyrosine that would not be present in normal proteins. In 3 different murine models of cancer that generate CR, both meta- and ortho-tyrosine inhibited tumor growth. Additionally, we showed that both isoforms of tyrosine blocked metastasis in a fourth model that does not generate CR but is sensitive to CR induced by other tumors. Mechanistic studies showed that the antitumor effects of the tyrosine isomers were mediated in part by early inhibition of the MAP/ERK pathway and inactivation of STAT3, potentially driving tumor cells into a state of dormancy in G(0)-phase. Other mechanisms, putatively involving the activation of an intra-S-phase checkpoint, would also inhibit tumor proliferation by accumulating cells in S-phase. By revealing a molecular basis for the classical phenomenon of CR, our findings may stimulate new generalized approaches to limit the development of metastases that arise after resection of primary tumors or after other stressors that may promote the escape of metastases from dormancy, an issue that is of pivotal importance to oncologists and their patients.

Anti-inflammatory pretreatment enables an efficient dendritic cell-based immunotherapy against established tumors

Although animals can be immunized against the growth of some tumor implants, most of the attempts to use immunotherapy to cause the regression of animal and human tumors once they have become established have been disappointing even when strongly immunogenic tumors were used as target. In this paper, we demonstrate that the failure to achieve an efficient immunological treatment against an established strongly immunogenic murine fibrosarcoma was paralleled with the emergence of a state of immunological unresponsiveness (immunological eclipse) against tumor antigens observed when the tumor surpassed the critical size of 500 mm(3). In turn, the onset of the immunological eclipse was coincidental with the onset of a systemic inflammatory condition characterized by a high number of circulating and splenic polymorphonucleated neutrophils (PMN) displaying activation and Gr1(+)Mac1(+) phenotype and an increasing serum concentration of the pro-inflammatory cytokines TNF-alpha, IL-1beta and IL-6 cytokines and C-reactive protein (CRP) and serum A amyloid (SAA) phase acute proteins. Treatment of tumor-bearing mice with a single low dose (0.75 mg/kg) of the synthetic corticoid dexamethasone (DX) significantly reduced all the systemic inflammatory parameters and simultaneously reversed the immunological eclipse, as evidenced by the restoration of specific T-cell-dependent concomitant immunity, ability of spleen cells to transfer anti-tumor activity and recovery of T-cell signal transduction molecules. Two other anti-inflammatory treatments by using indomethacin or dimeric TNF-alpha receptor, also partially reversed the immunological eclipse although the effect was not as striking as that observed with DX. The reversion of the immunological eclipse was not enough on its own to inhibit the primary growing tumor. However, when we used the two-step strategy of inoculating DX to reverse the eclipse and then dendritic cells loaded with tumor antigens (DC) as an immunization booster, a significant inhibition of the growth of both established tumors and remnant tumor cells after excision of large established tumors was observed, despite the fact that the vaccination alone (DC) had no effect or even enhanced tumor growth in certain circumstances. The two-step strategy of tumor immunotherapy that we present is based on the rationale that it is necessary to eliminate or ameliorate the immunological eclipse as a precondition to allow an otherwise ineffective anti-tumor immunological therapy to have a chance to be successful.

Therapeutic anti-tumor vaccines: from tumor inhibition to enhancement

Numerous immunization trials have proved successful in preventing the growth of experimental animal tumors and human hepatocarcinomas induced by hepatitis B virus. These results have prompted researchers and physicians to use vaccines in a therapeutic mode but the results have, in general, been disappointing even when strongly immunogenic murine tumors were concerned. Data presented herein suggest that immunotherapy induced by a single dose of a dendritic cell-based vaccine against a murine established tumor or against residual tumor cells after debulking the primary tumor, can render not only inhibitory or null but also stimulatory effects on tumor growth. These different effects might be dependent on where the system is located in the immune response curve that relates the quantity of the immune response to the quantity of target tumor cells. We suggest that high ratios render tumor inhibition, medium and very low ratios render null effects and low ratios-between medium and very low ones-render tumor stimulation. Since the magnitude of these ratios would depend on the antigenic profile of the tumor, the immunogenic strength of the vaccine used and the immunological state of the host, studies aimed to determine the magnitude of these variables in each particular case, seem to be necessary as a pre-condition to design rational immunotherapeutic approaches to cancer. In contrast, if these studies are neglected, the worst thing that an immunotherapist could face is not merely a null effect but enhancement of tumor growth.

[Unveiling antigens in a non-immunogenic spontaneous murine tumor using a dendritic cell based vaccine]

Up to date, most attempts to use immunotherapy to cause the regression of animal and human established tumors have not been successful. Former experiments have postulated that this failure could be attributed, at least in part, to a lack of immunogenicity of spontaneous tumors. In this paper, we have investigated whether this lack of immunogenicity can be attributed to the absence of tumor antigens or to the existence of tolerogenic mechanisms preventing such antigens from initiating an antitumor immune response. We have used two murine tumors a non-immunogenic spontaneous lymphoma (LB) and a strongly immunogenic methylcholanthrene-induced fibrosarcoma (MC-C) together with a vaccination strategy based on the inoculation of dendritic cells (DC) loaded with a tumor lysate. When DC were pulsed with LB lysate (DC+LB), no maturation of DC was achieved in vitro and no protection against LB implants after DC+LB inoculation was observed in vivo. On the other hand, when DC were pulsed with MC-C lysate (DC+MC-C), maturation of DC was observed along with a strong protection against MC-C implants after DC+MC-C inoculaton. Finally, when DC were pulsed with both LB and MC-C lysates (DC+LB+MC-C), maturation of DC and protection against LB implants were achieved. Since no immune cross reaction between MC-C and LB was ever observed, the most likely interpretation is that LB bears specific tumor antigens but lacks other signals to achieve DC maturation. These signals would be provided by MC-C which would enable DC to mature and to initiate an effective anti-LB immune response.

[Systemic inflammation and experimental cancer in a murine model]

The link between cancer and inflammation in an organ or tissue has firmly been established on the basis that cancer tends to occur at sites of chronic inflammation and that local inflammatory processes can accelerate the growth of preexisting tumors in both animals and human beings. In contrast, the relationship between cancer and systemic inflammation has been less studied. In this work, we demonstrated that the growth of the murine fibrosarcoma MC-C, was accompanied by manifestations of systemic inflammation, as demonstrated by an increase in both the number of circulating polymorphonuclear neutrophils (PMN) and the serum concentration of the proinflammatory cytokines interleukin-1beta (IL-1beta), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) and the acute phase proteins C reactive (CRP) and serum A amyloid (SAA). Two temporally separate peaks of systemic inflammation were detected during tumor development. The first was displayed during the first week after tumor inoculation. The second peak began around day 14 and its intensity was proportional to tumor size. In mice bearing a large MC-C tumor, a high number of circulating PMN and myeloid precursors were evident. Most of these cells exhibited activation evidenced by an increased reactive oxygen species generation and high expression of the Gr1+/Mac1+ markers. Inoculation of thioglycolate -which generates a transient systemic inflammation-accelerated the growth of MC-C tumor and reciprocally, inhibition of such systemic inflammation by using indomethacin, prevented that enhancing effect. This suggests that the systemic inflammation that the tumor generates on its own, could be part of its growth strategy.

[Reversion of the immunological eclipse and therapeutic vaccination against cancer in an experimental model]

Although animals can be prophylactically immunized against the growth of tumor implants, most of the attempts to use immunotherapy to cause the regression of animal and human tumors once they become established have been unsuccessful. To understand the nature of this refractoriness we have studied a methylcholanthrene-induced and strongly immunogenic murine fibrosarcoma. In our model, the onset of this refractoriness was associated with the beginning of an immunosuppressive state known as "immunological eclipse" characterized by a loss of the antitumor immune response when tumor grows beyond a critical size. This immunological eclipse was accompanied by the emergence of a systemic inflammatory condition. Treatment of tumor-bearing mice with a single dose of a synthetic corticosteroid, dexamethasone (DX), reduced significantly all parameters of systemic inflammation and simultaneously reversed the immunological eclipse. The reversion of the eclipse upon DX treatment was not curative itself, but allowed an immunological therapy based in dendritic cells pulsed with tumor antigens, which was itself absolutely ineffective, to exert a significant inhibitory effect against an established growing tumor. The two-step schedule using an anti-inflammatory treatment to reverse the immunological eclipse plus a dendritic cell-based vaccination strategy aimed to stimulate the antitumor immune response, could serve eventually as a model of immunotherapy against animal and human tumors.

The biological sense of cancer: a hypothesis

BACKGROUND

Most theories about cancer proposed during the last century share a common denominator: cancer is believed to be a biological nonsense for the organism in which it originates, since cancer cells are believed to be ones evading the rules that control normal cell proliferation and differentiation. In this essay, we have challenged this interpretation on the basis that, throughout the animal kingdom, cancer seems to arise only in injured organs and tissues that display lost or diminished regenerative ability.

HYPOTHESIS

According to our hypothesis, a tumor cell would be the only one able to respond to the demand to proliferate in the organ of origin. It would be surrounded by "normal" aged cells that cannot respond to that signal. According to this interpretation, cancer would have a profound biological sense: it would be the ultimate way to attempt to restore organ functions and structures that have been lost or altered by aging or noxious environmental agents. In this way, the features commonly associated with tumor cells could be reinterpreted as progressively acquired adaptations for responding to a permanent regenerative signal in the context of tissue injury. Analogously, several embryo developmental stages could be dependent on cellular damage and death, which together disrupt the field topography. However, unlike normal structures, cancer would have no physiological value, because the usually poor or non-functional nature of its cells would make their reparative task unattainable.

CONCLUSION

The hypothesis advanced in this essay might have significant practical implications. All conventional therapies against cancer attempt to kill all cancer cells. However, according to our hypothesis, the problem might not be solved even if all the tumor cells were eradicated. In effect, if the organ failure remained, new tumor cells would emerge and the tumor would reinitiate its progressive growth in response to the permanent regenerative signal of the non-restored organ. Therefore, efficient anti-cancer therapy should combine an attack against the tumor cells themselves with the correction of the organ failure, which, according to this hypothesis, is fundamental to the origin of the cancer.

Tumor Transition Zone: A Putative New Morphological and Functional Hallmark of Tumor Aggressiveness

A small primary or secondary tumor load can occasionally induce more deleterious effects than a histologically identical larger one. In the four murine models studied herein this enhanced tumor aggressiveness could not be attributed to NRAS mutations or other hereditary changes, differential vascularization of live tumor tissues, or necrosis content. Instead, the main tumor feature associated with a more aggressive behavior was the presence of a high number of vessels, sometimes filled with inflammatory cells, inside a tumor area, which we have identified and designated as the transition zone between the live and the necrotic zones. Our experiments suggest that during tumor growth, different cachectic factors are produced within the transition and necrotic zones by dying tumor cells and by tumor infiltrating macrophages only reaching the general circulation through the vessels present in the transition zone. Therefore, a small tumor displaying high vascularization of its transition area could be harmful to its host, while, in contrast, a large tumor could behave as a relatively benign one if its transition zone exhibited little or no vascularization, and in consequence its cachectic factors remained "trapped." Similar histological images to those observed in mice were seen in a significant percentage of human cancer biopsies, raising the possibility that such images might have a prognostic value.

Embryonal mass and hormone-associated effects of pregnancy inducing a differential growth of four murine tumors

A differential effect of pregnancy on the growth of subcutaneous implants of four murine tumors has been observed. Two tumors lacking receptors for progesterone and estrogen [methylcholanthrene-induced fibrosarcoma (MC-C) and spontaneous lymphoid leukemia (LB)] exhibited slow kinetics throughout the course of pregnancy, although inhibition was stronger beyond day 10. On the other hand, one of two tumors bearing receptors for progesterone and estrogen [medroxyprogesterone (MPA)-induced mammary adenocarcinoma (C7HI)] exhibited three phases: up to days 8-10 of gestation the tumor grew faster than in virgins, between days 8-10 and 15 it reached a plateau, and beyond day 15 a sharp reduction in tumor mass was observed. The other tumor [mouse mammary tumor virus (MMTV)-induced mammary carcinoma(T2280)] behaved as a typical pregnancy-dependent tumor (i.e., it grew in pregnant but not in virgin mice, regressed soon after delivery, and reassumed its growth at the middle of a second round of pregnancy). Neither MPA nor estrogen affected MC-C and LB tumor growth. On the other hand, MPA-treated mice enhanced C7HI tumor and reciprocally C7HI tumor-bearing mice treated with estrogen strongly inhibited tumor growth. As for T2280, neither MPA nor estrogen alone could promote tumor growth and, in consequence, no tumor developed. However, when MPA plus estrogen was administered in a schedule simulating the successive appearance of these hormones in pregnancy, T2280 grew even faster than in pregnant mice. When the four tumors were implanted in mice bearing grafts of embryonal tissues (teratomas), all of them were inhibited. This antitumor effect was similar to that observed in pregnancy when tumors unresponsive to progesterone and estrogen were tested. On the other hand, with tumors bearing progesterone and estrogen receptors, differences in tumor growth were detected in pregnant and teratoma-bearing mice. This suggested the existence during pregnancy of two factors potentially acting on tumor growth. First, a progesterone and estrogen-mediated hormonal component, which would exert either inhibitory or stimulatory effects only evidenced with tumors bearing hormonal receptors. Secondly, an antitumor effect proportional to the growing embryonal mass, inhibiting all tumors independently of their origin or hormone responsiveness. This antitumor effect could be attributed to a beat-resistant serum factor (1,000-1,200 Da molecular weight) presumably associated with the pathway of the arachidonic acid metabolism. The interplay between the hormonal component and the serum factor associated with embryonal mass could account for some of the largely heterogeneous and otherwise unexplained effects of pregnancy on tumor growth reported in the literature and illustrated by the four tumors studied here.

Two different types of concomitant resistance induced by murine tumors: morphological aspects and intrinsic mechanisms

Concomitant resistance (CR) is the phenomenon according to which a tumor-bearing host inhibits the growth of a secondary implant of the same tumor at a distant site. Confirming and extending previous results of our laboratory, histological studies have revealed that two temporally separate peaks of CR can be detected throughout tumor evolution. The first peak induced by immunogenic small tumors, in euthymic but not in nude mice, is associated with extensive necrosis of the secondary tumor implant and a profuse infiltration of polymorphonuclear granulocytes and mononuclear cells resulting in its final destruction; these features correspond to a typical immunological rejection. The second peak of CR induced by both immunogenic and non-immunogenic large tumors, in euthymic as well as in nude mice, is characterized by a dormant tumor stage with scarce or null mononuclear infiltration, associated with a significant reduction of tumor mitotic index and of the number of PCNA+ cells along with an increase in apoptosis and an arrest in S phase. In previous reports we suggested that a 1000 D serum fraction from mice bearing large tumors could be responsible for the induction of this dormant tumor stage. In this study tumor cells incubated in vitro with that serum factor mimicked the inhibition and cellular alterations observed in vivo in the secondary tumor inhibited by the second peak of CR. Moreover, the passive transfer of this factor by the intra-peritoneal (i.p.) route induced an in vivo inhibition of an i.p. tumor reproducing the image characteristic of the second peak of CR. This represents a direct proof that this serum factor can restrain tumor growth in vivo and that it is, most probably, the effector of the second peak of CR.

Inhibition of metastases by a serum factor associated to concomitant resistance induced by unrelated murine tumors

Murine lung metatases growing undisturbed by the primary tumor were significantly inhibited by the concomitant resistance induced by a secondary subcutaneous implant of two unrelated tumors. Such inhibition was T-independent since it was also observed in nude mice; its full expression was dependent on the presence of the secondary tumor implant and it was exerted on both macroscopic and microscopic established metastases and not on the process of tumor cell dissemination from the primary tumor. Direct and indirect mechanisms seemed to be involved, the former affecting the metastatic cells per se by causing a decrease in proliferation and an increase in apoptosis while the latter affected neo-vascularization. These antitumor and antiangiogenic effects could be attributed to a serum factor induced by the unrelated tumors generating concomitant resistance. This factor proved to be heat, acid and alkaline resistant and dialysable; it was recovered in an HPLC column with maximum absorption at 215 and 266 nm; it was anionic at neutral pH, exhibiting free carboxil groups and one or more molecules of tyrosine, with a molecular weight between 870 and 1300 Dalton. Intravenous administration of this factor significantly inhibited lung metastases, decreasing mitosis and increasing apoptosis similar to that observed in the presence of the unrelated tumors.

Antiangiogenesis and apoptosis as mediators of concomitant tumor resistance induced by Calu-6, a human lung carcinoma cell line, in nude mice

Concomitant resistance (CR), the phenomenon by which tumor-bearing hosts are able to inhibit secondary implants of the same tumor at distant sites of the body, has been previously observed by us and others in different murine tumor models. Here, we verified the generation of CR in nude mice by tumors induced by SC inoculation of Calu-6, a human lung carcinoma cell line. Histological analysis of secondary tumors subject to CR did not reveal macrophage infiltration nor cytotoxic signs. Although serum from tumor-bearing mice inhibited in vitro [3H]thymidine uptake by Calu-6 cells, no significant differences in [3H]thymidine labeling index of tumors implanted in the right flank of mice with and without a primary tumor in the left flank were detected. In our model, the presence of a primary tumor hindered remote tumor angiogenesis, as well as serum from tumor-bearing mice inhibited in vitro proliferation of an endothelial cell line derived from a murine hemangioendothelioma. Conversely, an enhancement of the apoptotic index was observed in secondary tumor implants carried out in tumor-bearing mice. The results reported herein show that human tumor cells are capable of inducing CR, and that this phenomenon would be a consequence of an impaired neovascularization as well as an increased programmed cell death at sites distant from the primary tumor.

A serum-mediated mechanism for concomitant resistance shared by immunogenic and non-immunogenic murine tumours

Resistance of tumour-bearing mice to a second tumour challenge, that is concomitant resistance, was evaluated in euthymic and nude mice using nine tumours with widely different degrees of immunogenicity. Two temporally separate peaks of concomitant resistance were detected during tumour development. The first one was exhibited only by small immunogenic tumours; it was tumour specific and mediated by classical immunological T-cell-dependent mechanisms. The second peak was shared by both immunogenic and non-immunogenic large tumours; it was non-specific, thymus independent and correlated with the activity of a serum factor (neither antibody nor complement) that inhibited the in vitro proliferation of tumour cells. This factor was eluted from a Sephadex G-15 column at fractions corresponding to a molecular weight of approximately 1000 Da and it was recovered from a high-performance liquid chromatography column in one peak presenting maximum absorption at 215 and 266 nm. The data presented in this paper suggest for the first time, to our knowledge, that in spite of the differences between immunogenic and non-immunogenic tumours, a common serum-mediated mechanism seems to underlie the concomitant resistance induced by both types of tumours at late stages of tumour development.

The role of apoptosis in the inhibition of a secondary tumor by concomitant resistance in a mouse model of metastases

Resistance of tumor-bearing mice to a second tumor challenge, that is, concomitant resistance, was studied using the LB tumor model. In a secondary LB tumor implant inhibited by concomitant resistance an increase in the percentage or apoptotic cells and alterations in cell cycle distribution were observed. Similar alterations were observed in LB tumor cells incubated with serum from tumor-bearing mice. The data presented in this paper suggest that apoptosis is one of the mechanisms involved in tumor dormancy due to concomitant resistance.

[Role of connective tissue in the tumor-host relationship]

In the embryo, both differentiation and temporospatial organization are regulated by the mesoderm. Some of these functions are expressed by the connective tissue during wound or organ repair and regeneration. The normal development of the latter depends on the epithelium-mesenchyme interrelationship and the formation of an adequate amount of stroma and a certain type of collagen or proteoglycans. Our hypothesis proposes that cancer is a regenerative process which has failed as a consequence of alterations in the connective tissue. The object of this paper was to investigate whether the connective tissue and the amorphous fundamental substance (SFA) are capable of regulating the proliferation and death of normal and tumor cells and to duplicate the mechanisms involved. The results obtained in vivo, ex-vivo and in vitro experiments indicate that following: 1) SFA exerts a direct and selective cytotoxic effect on tumor cells; 2) SFA reduces the proliferative capacity of normal and tumor cells; 3) both the cytotoxic and antiproliferative effects of SFA are independent of cellular and humoral immune responses but are dependent on the chemical integrity of its component since its denaturalization reduces its antitumoral activity; 4) the tumor cells modulate the regulatory effects of SFA through endocellular enzymes liberated by cell death induced by the cytotoxic action of SFA itself. These results suggest that in the absence of the inhibitory effect of SFA, the tumor cells which remain viable con now proliferate actively due to enzyme stimulation. In conclusion, the regulatory function of the connective tissue on the proliferation and viability of tumor cells would depend on the molecular constitution of SFA.

[Concomitant antitumor resistance]

Concomitant resistance of tumor-bearing mice against a second tumor challenge was evaluated in euthymic and athymic mice using 17 tumors with different degrees of immunogenicity. Two temporarily separated peaks of concomitant resistance were detected during tumor development: the first peak was only observed associated with small immunogenic tumors (< 500 m3., it was tumor-specific and mediated by T cell-dependent immunological mechanisms. The second peak was exhibited by large tumors (> 2000 mm3) independently of their immunogenicity; it was non-tumor specific, thymus-independent and correlated with a serum-activity (neither antibodies nor complement) which inhibited the in vitro proliferation of tumor cells. Out of 17 tumors studied, 15 tumors exhibited a moderate or strong concomitant resistance. The remaining two, which exhibited a weak or undetectable concomitant resistance and correlatively, a low or absent serum-inhibitory activity were the only tumors which included lung metastases. This fact suggested a correlation between concomitant resistance, absence of metastases and the existence of an inhibitory factor(s) in the serum. This inhibitory factor was partially characterized: it was resistant to boiling (5-10' at 100 degrees C) and to variations of pH; its molecular weight was estimated between 850 and 1200 D; it was recovered in only one fraction from HPLC (high power liquid chromatography) columns presenting maximum absorption at 215 and 266 nm; amino acid analysis and magnetic nuclear resonance studies suggested the presence of a molecule of thyrosine and one or two molecules of carbohydrates in its structure.

Stimulation of angiogenesis as an explanation of Matrigel-enhanced tumorigenicity

Matrigel, a reconstituted extract of basement membrane, enhances the growth of different human cancer cell lines when transplanted into nude mice. Here that stimulation was confirmed in the BALB/c murine mammary-tumor cell line M3MC, as well as in human colon (SW948) and mammary (MDA-MB-468) carcinoma cell lines transplanted in nude and SCID mice, respectively. Subcutaneous and intra-mammary fat-pad inoculations of Matrigel alone generated an angiogenic response which was macroscopically evident by day 9. Histological analysis of the local host reaction occurring at the site of injection revealed an early peripheral fibroblast response, followed by mononuclear cell infiltration, solid and hollow fibroblast cords projections from the edge to the center of the Matrigel plug, and finally capillary ingrowths. Conditioned media obtained from the gels generated in vivo, acted as very strong chemoattractants for mouse lung capillary endothelial cells, stimulating their motility between 38 and 82 times with respect to the control. Our results suggest an important role of host cells recruited by Matrigel, which could favor angiogenesis of the area and thus facilitate the growth of tumor cells co-inoculated with the basement membrane extract.

Expression of gelatinase/type IV collagenase in tumor necrosis correlates with cell detachment and tumor invasion

We have previously observed that acellular extracts from necrotic areas (NE) of the non-metastatic murine mammary adenocarcinoma M3, enhance in vitro cell detachment and spontaneous lung metastases. In the present study, using different proteinase inhibitors along with NE, only the calcium chelator EDTA could significantly abrogate the enhanced cell detachment from M3 produced by NE. The typical cleavage products of type IV collagenase were detected inside the tumor necrotic area, mainly in association with necrobiotic cells, as evaluated by Western blot analysis and immunohistochemical assays. Zymography revealed the presence of 72- and 92-kDa gelatinase/type IV collagenase in NE. Moreover, NE increased the in vitro invasive ability of cultured M3 cells. The use of specific antibodies against both 72- and 92-kDa type IV collagenases in the invasion assay showed that only the latter was able to revert the enhanced invasiveness to the baseline. It can be concluded that tumor necrosis is an important source of gelatinase/type IV collagenase, mainly in its 92 kDa form, and plays a major role in tumor invasion.

Correlation between seric antitumor activity and concomitant resistance in mice bearing nonimmunogenic tumors

Serum from mice bearing five weakly immunogenic or nonimmunogenic tumors inducing concomitant resistance exhibited a growth-inhibitory activity on in vitro proliferation of the tumor cells. This activity was proportional to the intensity of concomitant resistance and correlated with the capacity to restrain metastatic development. It was not attributable to cytotoxic antibodies, was relatively nonspecific, and operated through a cytostatic and reversible mechanism. All attempts to transfer antitumor resistance in vivo by serum inoculation have failed, but this could be attained by parabiosis. Physical and chemical serum treatments suggest that heat-, acid-, and alkali-resistant peptide(s) with molecular weights ranging from 1000 to 3000 could account for this inhibitory effect.

Anti-inflammation induced by counter-irritation or by treatment with non-steroidal agents inhibits the growth of a tumour of non-detected immunogenicity

Counter-irritation (CI) triggered by different non-specific irritant stimuli delayed the growth of a murine tumour of non-detected immunogenicity. The syngeneic LB tumour transplant by itself also induced CI and decreased the number of leukocytes migrating to a secondary s.c. irritant stimulus, e.g. sponge or carrageenan. On the other hand, partial inhibition of cell migration by treatment with either 0.5 mg kg-1 indomethacin or 0.3 mg kg-1 piroxicam retarded LB tumour growth, presumably by a mechanism unrelated to inhibition of immune responses by PGE2. It is suggested that CI may play a role in the early stages of concomitant resistance.

Role of concomitant resistance in the development of murine lung metastases

An attempt was made to explain the distinct lung metastatic patterns of 2 mammary adenocarcinomas with a common BALB/c origin: M3, which does not induce spontaneous metastases, and MM3 with an almost 100% incidence. No difference between the 2 tumors was detected with respect to host mononuclear cell content, degree of immunogenicity or lung-colony-forming ability. Conversely, there was a marked difference in the capacity to induce concomitant resistance: M3-bearing mice induced stronger and earlier resistance against i.v. challenge of both M3 and MM3 tumor cells than MM3-bearing mice; this resistance was expressed as lower number of lung metastases and lower tumor-cell proliferation in metastatic nodules. M3 was also able to control the development of spontaneous metastases: metastases developed in all M3-excised mice, compared with none in M3-bearing mice, while MM3-bearing mice also bearing a secondary M3 tumor developed fewer metastases than mice bearing MM3 only. This anti-metastatic effect does not appear to depend on classical immunological mechanisms since no difference could be detected between the 2 tumors in response to T cells, NK, macrophages or antibodies.

Tumor necrosis can facilitate the appearance of metastases

The non-metastatic murine mammary adenocarcinoma M3 and its metastatic variant MM3 were used to evaluate the role of intratumoral necrosis in cell detachment and metastasis. Accelular extracts from necrotic areas of both tumors increased in vitro cellular detachment from M3 but not from MM3 fragments. Furthermore, the in vivo inoculation of the necrotic extracts within non-metastatic M3 tumors gave rise to pulmonary metastases. Histological studies revealed in M3 a central necrosis limited by an uninterrupted peripheral ring of well preserved cells, while in MM3 necrotic and non-necrotic areas alternated. It is concluded that the distribution of necrosis within the primary tumor by facilitating cell detachment is, at least in part, responsible for the development of metastases.

Macrophages and tumor growth. II. Cell kinetics at the site of allogeneic tumor growth

The sponge matrix allograft model made possible the growth of an AKR lymphoma in a certain percentage of BALB/c mice making progressors (tumor-bearing) and regressors (tumor-rejecting) simultaneously available. Mice bearing either an AKR kidney allograft or a sponge alone were used as controls. The cell population infiltrating the sponge was evaluated 2, 5, 10, 15, and 21 days after subcutaneous implantation. It consisted of macrophages, lymphocytes, and neutrophils. There was no difference between groups on Day 2 and Day 5. From Day 10 onwards, tumor growth was evident with a clear cut separation between progressors and regressors. The latter behaved like the two control groups except for a significant increase in lymphocytes on Day 21. The progressors showed a striking increase in total cell count from Day 10 onwards. Macrophages were the major population with a maximum value of 201 X 10(6) as compared to 8 X 10(6) in the regressors; their phagocytic and lysosomal activity remained similar in all groups. The lymphocytes showed no variation in absolute numbers but, because of the high cell count in progressors, their ratio to macrophages reached 1:50 on Day 10, making up only 2% of the cell population. Neutrophils were significantly increased in progressors as compared with the other groups. It can be concluded that in the presence of a foreign body reaction a marked influx of macrophages accompanies allogeneic tumor growth.