Lance Miller

Sex steroid hormones and macrophage function

Macrophages are versatile cells whose activities are programmed by environmental signals. In this review, we discuss the potential impact of sex steroid hormones on macrophage activation and production of various effector molecules. The evidence accumulated to date supports the postulate that estrogens, progesterone, androgens and testosterone profoundly influence host defense by controlling the ability of macrophages to participate in immune responses.

Progesterone inhibits inducible nitric oxide synthase gene expression and nitric oxide production in murine macrophages.

The purpose of this study was to determine whether the female hormones estradiol‐17β (E2) and progesterone (P4) influence inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) by interferon‐γ(IFN‐γ)‐ and lipopolysaccharide (LPS)‐activated mouse macrophages. Treatment with P4 alone caused a time‐ and dose‐dependent inhibition of NO production by macrophage cell lines (RAW 264.7, J774) and mouse bone marrow culture‐derived macrophages as assessed by nitrite accumulation. RAW 264.7 cells transiently transfected with an iNOS gene promoter/luciferase reportergene construct that were stimulated with IFN‐γ/LPS in the presence of P4 displayed reduced luciferase activity and NO production. Analysis of RAW 264.7 cells by Northern blot hybridization revealed concurrent P4‐mediated reduction in iNOS mRNA. These observations suggest that P4‐mediated inhibition of NO may be an important gender‐based difference within females and males that relates to macrophage‐mediated host defense.

Female steroid hormones regulate production of pro-inflammatory molecules in uterine leukocytes

Estrogens and progesterone could be among the environmental signals that govern uterine immune cell synthesis of pro-inflammatory substances. In order to investigate this possibility, we first mapped expression of the inducible nitric oxide synthase (iNOS) and tumor necrosis factor-alpha (TNF-alpha) genes in the leukocytes of cycling and pregnant mouse uteri, then tested the ability of estradiol-17 beta (E2) and progesterone to influence gene expression. Immunohistochemistry, in situ hybridization, and other experimental approaches, revealed that the iNOS and TNF-alpha genes are expressed in mouse uterine mast cells, macrophages and natural killer cells (uNK). Gene expression in each cell type was noted to be dependent upon stage of the cycle or stage of gestation, implying potential relationships with levels of female hormones and state of cell differentiation or activation. Further in vivo and in vitro experiments showed that individual hormones have cell type-specific effects on synthesis of iNOS and TNF-alpha that are exerted at the level of transcription. In uterine mast cells, iNOS and TNF-alpha are promoted by E2 whereas preliminary studies in macrophages suggest that transcription and translation of the two genes are unaffected by E2 but are inhibited by progesterone. Uterine NK cell production of iNOS and TNF-alpha is strongly related to cell differentiation, which is initiated and sustained by progesterone. Collectively, the results indicate that regulation of synthesis of pro-inflammatory molecules by hematopoietic cells in cycling and pregnant uterus comprises a new and potentially critical role for female steroid hormones.

Hormonal Regulation of Uterine Macrophages

Macrophages are major cellular inhabitants of cycling and pregnant mammalian uteri. Their densities and patterns of tissue distribution in this organ fluctuate in concert with levels of circulating female sex steroid hormones, estrogens and progesterone, and their production of various effector molecules also may be hormonally regulated. Hormonal control may be achieved by direct binding to receptors or by indirect pathways where hormones modulate production of various autocrine and paracrine cytokines and growth factors that then target to resident macrophages and influence their secretory profiles. In this paper, we marshall evidence supporting the concept that progesterone acts as a powerful negative regulator of these versatile cells, reducing their migration into the uterus and impairing their ability to produce potent effector molecules such as nitric oxide that could interfere with the success of pregnancy.

Nonspecific immunity in the uterus and placenta

Summary The studies described above clearly illustrate the principle that preventing the mother from rejecting her “foreign” fetus takes precedence over protecting the pregnancy from infection or invasion by foreign cells. In a recent commentary on the conflicting needs of pregnancy, Nahmias and Kourtis (1997) concluded that this causes no unacceptable risk to perpetuation of the species. Our experiments would support this conclusion; the pathways uncovered thus far are cleverly designed so that the negative effects are likely to be comparatively inconsequential. Regarding production of soluble HLA-G in macrophages, synthesis is linked to cell activation, a state wherein the phagocytic and degradative functions of the macrophages are improved. No conflict thus ensues; innate immunity provided by the macrophages would be enhanced rather than impaired by conditions associated with successful pregnancy. It will be extremely interesting to learn whether soluble HLA-G does stimulate programmed cell death in T lymphocytes and what concentrations of soluble HLA-G are required to inhibit NK cells. As for reduced production of NO and TNF, tight control is highly desirable; excessive production of either substance could interrupt pregnancy. It is therefore satisfying to find that progesterone, the dominant hormone of pregnancy, provides the control. Furthermore, the elegant dose-dependent system uncovered in our experiments, where only high concentrations of progesterone inhibit NO and TNF production, assures that synthesis of these two powerful molecules will be unimpaired in macrophages residing in tissues and organs distant from the progesterone-producing ovaries and placeta. As illustrated in the studies described above and recently summarized ( Hunt and Hutter, 1996 ), the mechanisms protecting the fetus from destruction by the maternal immune system are redundant and complex. Significant progress is being made in identifying and elucidating the specific features of each system. Concurrently, related issues such as the risks of chimerism in the mother and fetus and mechanisms underlying immune privilege are under intensive investigation ( Nelson, 1996 ; Runic et al., 1996 ; Green and Ware, 1997 ; Hunt et al., 1997b ).