Functions of mononuclear phagocytes in mice exposed to diethylstilbestrol: a model of aberrant macrophage development

A Brief History of Immunotoxicology and a Review of the Pharmaceutical Guidelines

This article provides a brief history of the development of the field of immunotoxicology from one individual perspective and separates the discussion into five phases: the methods development and validation phase; the compound testing phase; the many meetings and organization of the specialty section phase; the mechanistic studies phase; and the guidelines phase. During the discussion of each phase, major highlights, accomplishments, contributors and key references are provided. The immunotoxicology guidelines for the testing of pharmaceutical chemical entities across the three major geographic axes are also presented and compared, along with some of the concerns raised by industry with these guidelines. The mandatory requirement of functional tests represents the major concern and difference between the Committee of Proprietary Medicinal Products (CPMP) and the new Food and Drug Administration (FDA) guidance. The scientific basis for the recommendation of the functional tests proposed in the guidance documents based on National Institute of Environmental Health and Safety/National Toxicology Program (NIEHS/NTP)-sponsored studies is described. Experience at Sanofi-Synthelabo, with the testing of 29 new chemical drug entities developed across a broad range of therapeutic classes using this testing paradigm and functional tests to define their immunotoxic potential, yielded a low number of compounds (6.8%) that produced any abnormal reaction. The two positive compounds might have been anticipated based on their pharmacology.

Endocrine-disrupting chemical exposure and the American alligator: a review of the potential role of environmental estrogens on the immune system of a top trophic carnivore

Endocrine-disrupting chemicals (EDCs) alter cellular and organ system homeostasis by interfering with the body's normal physiologic processes. Numerous studies have identified environmental estrogens as modulators of EDC-related processes in crocodilians, notably in sex determination. Other broader studies have shown that environmental estrogens dysregulate normal immune function in mammals, birds, turtles, lizards, fish, and invertebrates; however, the effects of such estrogenic exposures on alligator immune function have not been elucidated. Alligators occupy a top trophic status, which may give them untapped utility as indicators of environmental quality. Environmental estrogens are also prevalent in the waters they occupy. Understanding the effects of these EDCs on alligator immunity is critical for managing and assessing changes in their health and is thus the focus of this review.

17 beta-estradiol increases inducible nitric oxide synthase expression in macrophages

In some tissues 17beta-estradiol (E2) is known to increase endothelial NOS expression. In the present study we examined the effects of E2 on estrogen receptors (ERalpha and beta) and inducible nitric oxide synthase (iNOS) expression and analyzed the mechanisms in rat peritoneal macrophages. Reverse-transcription polymerase chain (RT-PCR) and transient transfection experiments using a reporter plasmid that contained a luciferase gene under the transcriptional control of an estrogen-responsive elements revealed that peritoneal macrophages are responsive to E2 and express both ERalpha and ERbeta mRNAs. Incubation with E2 leads to an increased ERbeta mRNA expression. When rat peritoneal macrophages were incubated with physiological concentrations of E2, E2 induced a dose-dependent increase in NO production. E2 significantly affected secretion at concentration levels of more than 10(-11)M, and its maximum effect was at a concentration of 10(-8)M. RT-PCR reactions showed that increases in NO secretion were due to an increase in iNOS mRNA. Coincubation with ICI 182.780, an estrogen-receptor antagonist, inhibited the influence of E2 on NO production and iNOS expression. Thus E2 stimulated iNOS expression by a classic receptor-mediated pathway. We hereby prove that E2 increases the iNOS expression in macrophages and this effect appears to be the consequence of ER activation.

Beta-estradiol-induced decrease in IL-12 and TNF-alpha expression suppresses macrophage functions in the course of Listeria monocytogenes infection in mice

Mice treated with a contraceptive dose of beta-estradiol (E2) demonstrated changes in their macrophage (Mphi) number and functions. While E2 increased and decreased the Mphi number in PBMC and PEC respectively, it enhanced the in vitro phagocytosis of FITC-labeled beads by both cells. E2 treatment also enhanced the phagocytic function of Mphi as assessed by the in vivo carbon clearance assay. In contrast, the in vitro intracellular killing function of adherent cells in peritoneal exudate cells (PEC) against Listeria monocytogenes decreased after E2 treatment. In line with the decrease in the intracellular killing function, the E2-treated mice showed an impaired protection against L. monocytogenes infection. To clarify the mechanism of the E2-mediated suppression of the protective response against L. monocytogenes infection, we next analyzed the cytokine expression by PEC in E2-treated L. monocytogenes-infected mice. On day 5 of the infection, the expression of IL-12, TNF-alpha and IL-10 by adherent PEC from the E2-treated mice was lower than that from the control-infected mice. The decrease in the cytokine expression by adherent PEC of E2-treated mice coincided with the decrease of IFN-gamma expression, and the increase in the IL-4, IL-10 and TGF-beta expressions by non-adherent PEC. These results revealed two aspects of the effects of E2 on Mphi. Even though E2 was found to enhance Mphi phagocytosis, the anti-bacterial function was suppressed. This suppression may be mediated by the inhibition of both IL-12 and TNF-alpha which play important roles in the protective response against intracellular bacteria.

Steroid sex hormones and macrophage function: regulation of chemiluminescence and phagocytosis

PROBLEM

Female sex hormones modulate a variety of humoral and cell-mediated immunologic functions. In this study, the effects of estrogen, progesterone, and testosterone on the chemiluminescence (CL) response and phagocytic ability of male rat peritoneal macrophages (M luminal diameter) were examined.

METHOD

In M luminal diameter pretreated with 10(-2) ng/ml of 17 beta-estradiol (E2) for 20 hours, the CL generated in response to phorbol myristate acetate (PMA), 1,2-dioctanoyl-rac-glycerol (C8:0), or opsonized zymosan (OZ) was significantly increased by 135%, 140%, and 136% of control values, respectively. In addition, M luminal diameter treated with 10(-5) ng/ml or 10 ng/ml of E2 exhibited a significantly greater PMA- or OZ-stimulated CL response than did untreated controls.

RESULTS

At 10(-2) ng/ml, progesterone enhanced and testosterone reduced the CL response, but these changes were not statistically significant. In time course studies, the PMA-stimulated CL response of M luminal diameter treated with 10(-2) ng/ml of E2 or progesterone for 5 h was significantly less than that of the untreated group. In the presence of endotoxin (12 pg/ml), the CL response in M luminal diameter treated with E2 or testosterone was significantly depressed as compared to untreated controls. Phagocytosis of opsonized sheep erythrocytes also was significantly enhanced (140% to 190% of control) when M luminal diameter were pretreated with 10(-12) M to 10(-8) M of either E2 or progesterone.

CONCLUSIONS

These findings suggest that, at physiological concentrations, E2 is capable of modulating both CL generation and phagocytic uptake by M luminal diameter in a manner not shared by other steroid hormones.

Steroid sex hormones and macrophage function: modulation of reactive oxygen intermediates and nitrite release

PROBLEM

In general, females have a more active immune response than do males. The effects of female sex hormones on lymphocytes have been studied extensively but their effects on macrophages are poorly understood.

METHOD

In this study, peritoneal macrophages (M phi) obtained from male rats were treated in vitro with estradiol (E2), progesterone (P), testosterone (TS), or hydrocortisone (HC) and their effects on superoxide, hydrogen peroxide, and nitrite release determined.

RESULTS

At concentrations between 10(-10) and 10(-9) M, female and male sex hormones had no significant effect on superoxide release but, at concentrations above or below that range, these hormones stimulated the release of these reactive oxygen intermediates (ROI). In contrast, M phi treated with HC generally exhibited either unaltered or reduced ROI release.

CONCLUSIONS

These findings suggest that female sex hormones regulate ROI release by M phi in a manner not entirely shared by other steroid hormones. At most concentrations used, E2, P, TS, and HC significantly inhibited nitrite release by M phi. However, with 10(-10) M of E2 or 10(-9)M of P, nitrite release by M phi was not affected. In the presence of anti-TNF antibody, the amounts of superoxide and hydrogen peroxide release were moderately reduced but nitrite release was dramatically inhibited. The sensitivity of M phi to variations in the concentrations of female sex hormones may contribute to gender-related differences in the immune response.

Sex hormones and the immune system--Part 2. Animal data

Sex hormones have physiological and pathological (autoimmune conditions) effects on the immune system. Studies in experimental animal models of human autoimmune diseases have clearly shown that sex hormones regulate the expression, severity and course of autoimmune diseases. Sex hormones affect the function of T, B and NK cells, and macrophages. Precisely how sex hormones affect lymphocytes is a highly complex question. Sex hormones can modulate the immune system, perhaps directly (e.g. thymic reticular tissue), or indirectly via host and many oestrogen target tissues, including the central nervous system hypothalamic-pituitary axis (the neuroendocrine tissues). The effects of sex hormones on the immune system (immunosuppression or immunopotentiation) may vary, even with the same hormone. For example, oestrogen can increase IgA levels in the uterus, but decrease IgA levels in the vagina or have no effect in lacrimal tissues (Sullivan, 1989). Therefore the effects of sex hormones on the immune system cannot be generalized but must be evaluated independently. Some of the reasons for variability in results have been reviewed in detail elsewhere (Steinberg et al, 1979; Ansar Ahmed et al, 1985b). These include, dose of hormones, age and sex-hormonal status of animals, route and time of administration, the immunocompetence of the host, stress, the metabolism of hormones (e.g. metabolism of testosterone to oestrogen) resulting in alteration of biological activity, and differential response to various antigens. The initial encounter of sex hormones with the type of target cells, the variability of secondary messengers and gene activation events are other important considerations. The effects of sex hormones on the immune system to modulate immune responses are unequivocal. The burgeoning advances in cellular immunology, endocrinology and molecular biology, should provide a better understanding of: (1) the interactions of hormones with the immune system; (2) how hormones activate specific genes; and (3) how hormones influence intracellular communication. In a clinical situation, it is hoped that androgenic compounds which lack virilizing effects, but possessing the desired immunomodulatory effects, will eventually be synthesized. These hormone analogues, in combination with specific (non-toxic) oestrogen antagonists, may offer new therapeutic avenues.

Murine macrophage activation with resorcyclic acid lactones (RALs): comparison with diethylstilbestrol and 17 beta-estradiol

Zearalane, a resorcyclic acid lactone (RAL) derivative, activates murine macrophages (M phi) in vivo and in vitro. Mouse M phi incubated in vitro with different concentrations of zearalane released superoxide anion (O2-) on stimulation with either opsonized zymosan (Op-zym), phorbol myristate acetate (PMA), or Salmonella typhimurium. The levels of O2- released were similar to those released from M phi incubated in vitro with supernatants enriched in macrophage-activating factors from concanavalin A-stimulated mouse spleen cells. In contrast, 17 beta-estradiol (E2) or diethylstilbestrol (DES) induced little or no enhancement of O2- release. Zearalane, dideoxyzearalane, DES and E2 were tested for induction of host resistance to S. typhimurium infection in mice. Treatment of mice with zearalane (9 mg/kg) resulted in at least 65% survival 4 days post-infection, compared to 10% survival of mice with vehicle alone, DES, or E2. Peritoneal and alveolar M phi from the zearalane-treated mice released up to six times as much O2- on stimulation with Op-zym as M phi from the other treatment groups. M phi activation was observed for up to 7 days after intraperitoneal or subcutaneous administration of zearalane in either aqueous or organic vehicles. These results suggest that zearalane may enhance resistance to infection by increasing bactericidal activity due to increased release of toxic oxygen radicals by M phi and mononuclear phagocytes. These effects differ from the immunosuppressive effects reported for DES and E2.

Immunological aspects of toxicology: premises not promises

The immune system is gaining increasing attention as a target of toxicant action. Many compounds, including metals, drugs, and pesticides, are able to alter immune functions. Additionally, the ability of toxicants to experimentally alter resistance to a variety of bacterial, viral, and tumor challenges is suggestive for the possible involvement of toxicants in morbidity and mortality. However, the implications of immunotoxicology as a subject of study may not be fully realized in some laboratories due to unfamiliarities into the workings of immunity by toxicologists. In an attempt to bridge this toxicology-immunology gap, this paper presents the major concepts of the immune system by reviewing specific examples of immune responses and their environmental interactions. Data from selected toxins are then used to illustrate how these responses may be altered.