Opposing influence of age on the growth and colony-forming ability of mouse melanoma B16 and mammary adenocarcinoma: correlation with natural killer activity

A role for NK cells in greater susceptibility of young rats to metastatic formation

Cancer is the second leading cause of death in children (after accidents) and is more prevalent in the first 5 years of life than in the subsequent 10 years. Very young animals have been shown to be more susceptible to malignant growth and whether such increased susceptibility is attributable to reduced resistance of the host to tumor development or to increased incidence of cancerous cells is, as yet, unclear. In the current study, we used 36 day old male and female rats and adult rats to specifically study the role of natural killer (NK) cell activity, as well as hormones known to regulate their activity, in mediating reduced resistance to tumor metastasis at prepubescence. A mammary adenocarcinoma cell line (MADB106) syngeneic to the Fischer 344 rat was used. Following i.v. injection, MADB106 tumor cells seed and colonize only in the lungs, a process shown in adult rats to be controlled by NK cells during the first 24 hours after tumor inoculation. As was found in our previous studies, young rats demonstrated a 10-fold higher percentage of lung tumor cell retention compared to adult rats. Importantly, this higher percentage of tumor cell retention was evident using the same number of tumor cells per kg of body weight in young and adult rats, and maintained even when young rats were challenged with 10- and 100-fold fewer MADB106 cells per kg than adults. Selective depletion of NK cells markedly increased tumor cell retention in all rats, indicating that NK cells play a crucial role in resistance against MADB106 retention in both young and adult rats. Employing in vitro assessment of whole blood NK cytotoxicity. young animals exhibited markedly less specific killing compared to the mature animals. Taken together, these findings indicate a reduced resistance of the young rats against MADB106 retention that is mediated by diminished NK activity in these rats. Factors other than NK cells appear to play a minor role determining age differences in this model. Age- and sex-related differences in plasma beta-endorphin and corticosterone levels were also found, suggesting different activation levels of the HPA axis. These differences, however, seen unlikely to underlie the reduced NK activity in young rats.

The development of sexual dimorphism in natural killer cell activity and resistance to tumor metastasis in the Fischer 344 rat

The development of sexual dimorphism in the number and activity level of natural killer (NK) cells was studied in the inbred Fischer 344 rat from prepubescence to maturity. Additionally, in view of the biological significance of NK cells in controlling cancer, especially the metastatic process, we used a syngeneic mammary tumor (MADB106) to assess the host anti-metastatic activity. This tumor model was used because NK cells control the lung clearance of i.v.-injected MADB106 tumor cells, a process that critically affects the metastatic colonization of these tumor cells in the lungs. The results indicated that although prepubescent (36 days of age) males and females exhibited greater NK cytotoxicity (assessed in vitro) and higher anti-metastatic activity, evidenced by fewer tumor cells retained in the lungs. On the other hand, the mature males (140-170 days of age) displayed greater LGL/NK number and activity per ml blood, retained fewer tumor cells, and developed fewer lung tumor colonies compared to the females. During early postpubescence (63 days of age), a transitional stage between prepubescence and maturity, females and males exhibited equivalent numbers of circulating LGL/NK cells, and females displayed slightly greater NK cytotoxicity per ml blood yet retained somewhat greater numbers of tumor cells compared to the males. Overall, whereas the males exhibited increasing levels of NK number and activity throughout the age span tested, the females, despite displaying greater NK function compared to the males at prepubescence and slight improvement at postpubescence, fell behind the males in these indices of NK function at maturity.

Further characterization of a clinically relevant model of melanoma metastasis and an effective vaccine

A major problem in evaluating the effectiveness of tumor cell vaccination and other biological therapies is the variability of experimental models. In this study we have further developed and characterized a model for metastatic melanoma that approximates the major clinical stages of metastatic dissemination: stage I--growth of the primary (local) tumor, stage II--dissemination to regional lymph nodes, and stage III--metastasis to distant organs (lungs). C57BL/6 mice were challenged subcutaneously with B16 F10 murine melanoma cells in the midtail, and within 3 weeks 100% of the mice had local tumors growing in their tails. By 5-7 weeks after challenge, most of the mice had developed metastases to the inguinal lymph nodes and subsequently had metastatic colonies in the lungs and in the bone marrow. Preimmunization of mice with a formalinized extracellular antigen vaccine, derived from B16 F10 melanoma cells, provided partial inhibition of the growth of the primary melanoma tumors, as well as reducing the number of metastases to the regional (inguinal) lymph nodes and lungs along with concomitantly increasing survival time. This model for melanoma metastasis provides a reasonable and reproducible test system for the study of anti-melanoma immunity and the different cellular and humoral mechanisms involved.

A characterization of the in vivo immunomodulation by Met-enkephalin in mice

Single intraperitoneal injections of Met-enkephalin (MENK) into CBA mice decreased the phagocytic activity of peritoneal macrophages, Con-A-induced proliferation and NK-activity of spleen cells. Conversely, in mice which had been immunized with sheep erythrocytes, treatment with MENK was associated with enhancement of phagocytosis, and no effect on lymphoid proliferation and NK-cytotoxicity of spleen cells. MENK-induced inhibition of cellular functions in nonimmunized mice was associated with a decrease of plasma ACTH level, whereas MENK-induced stimulation of phagocytosis in immunized mice was paralleled with an elevation of ACTH, suggesting a role of corticoids in immunomodulation by MENK. MENK-induced modulation (suppression and stimulation) of phagocytosis, as well as inhibition of spleen cell proliferation, was not observable in adrenalectomized mice, although a reduced NK-cytotoxicity was still present.

Preimmunization of mice with formalinized extracellular antigens of melanoma in combination with IL-2 and surgical resection increased survival and tumor control in metastatic melanoma model

Recently we found that immunization with formalized extracellular antigens (FECAs) could induce the production of specific antimelanoma antibodies and increase the defense mechanisms of antimelanoma cellular and humoral immunity. In experiments we used pathogen-free female mice C57BL/6 18-20 g. We injected FECA (0.02 mg of protein/per S.C.--subcutaneous injection) for 1 month, once per week. Concurrently we injected S.C. human recombinant IL-2: 100 U/g of weight (2,000 U/per mouse). Interleukin-2 (IL-2) was injected for 1 month, 5 days/week. On days 7, 14, 21, and 28 we took retroorbital blood from mice for the study of anti-FECA and anti-IL-2 antibody production with ELISA. Control and experimental mice were then given a subcutaneous injection with 0.5 x 10(6) cells B16-F10 melanoma in 25 microliters into the middle of the tail. By 18 days 100% developed local melanoma tumors. We resected tails of all control and experimental animals 5 mm distal the base of the tail under metaphan anesthesia. The production of antibodies to FECA and IL-2 started after the 21st day and was higher in the group of mice immunized with FECA and with IL-2 than in control animals. Combining preimmunization with FECA and IL-2 and resection of local melanoma tumors decreased the mortality and the number of mice with local recurrence and metastatic melanoma tumors to the lungs.

Alteration of NK- and ADCC-activities in rats genetically selected for low or high platelet serotonin level

By selective breeding we have recently obtained two discrete sublines of rats that differ in serotonin content in their platelets. As both serotonin and platelets may influence, or even take part, in immune reactions, we tested in this work the natural cytotoxicity in rats with constitutionally different platelet serotonin levels (PSL). Rats with low platelet serotonin level (mean +/- SD, 1.26 +/- 0.14 micrograms 5HT/mg protein; 81% vs. controls) had significantly higher (P less than 0.001) natural killer (NK) activity (mean +/- SD, 9.1 +/- 3.9%) than control rats with average PSL (1.57 +/- 0.18 micrograms 5HT/mg protein). On the contrary, rats with constitutionally high PSL (2.42 +/- 0.21 micrograms 5HT/mg protein, 154% vs. controls) had somewhat lower (P less than 0.02) NK activity (4.1 +/- 1.7%) than control animals (5.7 +/- 1.9%). Antibody-dependent cellular cytotoxicity (ADCC) against nucleated targets of the RCH line, detecting lymphoid effectors, as well as ADCC against chicken red blood cells (CRBC), detecting predominantly non-lymphoid effectors, were also significantly higher (P less than 0.001) in rats with low PSL (19.6 +/- 6.8% vs. 6.6 +/- 3.1% in controls for lymphoid effectors, and 71.8 +/- 6.1% vs. 48.7 +/- 8.8% in control rats for non-lymphoid effectors). However, no significant alteration of either ADCC was determined in rats with high PSL. The results suggest in vivo regulation of natural cytotoxicity by serotonin.

In vivo bidirectional regulation of mouse natural killer (NK) cell cytotoxic activities by Leu-enkephalin: reversibility by naloxone

Intraperitoneal injection of Leu-enkephalin (LENK, 10 or 7.5 mg/kg) induced bidirectional modulation of natural cytotoxic activities in spleens of CBA mice (suppression followed by enhancement). NK-cytotoxic activity was more affected than the ADCC. Early suppression of NK activity could be reversed by 4 x M excess of naloxone injected 20 min before LENK, suggesting that the suppression was mediated by opioid receptors. Subsequent increase of NK activity could not be abrogated by naloxone, at least not completely. Naloxone itself decreased NK activity 12 hours after treatment, but enhanced ADCC at 24 and 48 hours. This increase was abrogated by LENK. In addition to functional alterations, LENK also induced phenotypic changes of spleen cells, i.e. a decrease in the percentage of asialo-GM-1+ cells 24 hours posttreatment. There was no correlation between LENK-induced alterations of cytotoxic function and the percentage of cells with NK phenotype (GM-1+). Thus, LENK modulates cytolytic functions and the phenotype of NK cells in vivo in a complex way, which besides opioid mechanisms may also include non-opioid ones.