Mitogenic effect of prolactin on chicken lymphocytes in vitro

The Role of Chicken Prolactin, Growth Hormone and Their Receptors in the Immune System

Prolactin (PRL) and growth hormone (GH) exhibit important roles in the immune system maintenance. In poultry, PRL mainly plays its roles in nesting, hatching, and reproduction, while GH is primarily responding to body weight, fat formation and feed conversion. In this review, we attempt to provide a critical overview of the relationship between PRL and GH, PRLR and GHR, and the immune response of poultry. We also propose a hypothesis that PRL, GH and their receptors might be used by viruses as viral receptors. This may provide new insights into the pathogenesis of viral infection and host immune response.

dPRLR causes differences in immune responses between early and late feathering chickens after ALV-J infection

To understand the differences in immune responses between early feathering (EF) and late feathering (LF) chickens after infection with avian leukosis virus, subgroup J (ALV-J), we monitored the levels of prolactin, growth hormone and the immunoglobulins IgG and IgM in the serum of LF and EF chickens for 8 weeks. Moreover, we analysed the expression of immune-related genes in the spleen and the expression of PRLR, SPEF2 and dPRLR in the immune organs and DF-1 cells by qRT–PCR. The results showed that ALV-J infection affected the expression of prolactin, growth hormone, IgG and IgM in the serum. Regardless of whether LF and EF chickens were infected with ALV-J, the serum levels of the two hormones and two immunoglobulins in EF chickens were higher than those in LF chickens (P  < 0.05). However, the expression of immune-related genes in the spleen of positive LF chickens was higher than that in the spleen of positive EF chickens. In the four immune organs, PRLR and SPEF2 expression was also higher in LF chickens than in EF chickens. Furthermore, the dPRLR expression of positive LF chickens was higher than that of negative LF chickens. After infection with ALV-J, the expression of PRLR in DF-1 cells significantly increased. In addition, overexpression of PRLR or dPRLR in DF-1 cells promoted replication of ALV-J. These results suggested that the susceptibility of LF chickens to ALV-J might be induced by dPRLR.

Prolactin affects the disappearance of ALV-J viremia in vivo and inhibits viral infection

Based on the RNA-seq data of chicken spleen tissues infected with J subgroup avian leukosis virus (ALV-J), we found that prolactin (PRL) gene was one of differentially expressed gene. We measured ALV-J viremia and PRL levels in the plasma of two groups of ALV-J-infected adult chickens. Furthermore, recombinant chicken PRL (cPRL) was used to assess how cPRL affects ALV-J virus replication both in vivo and in vitro. The results showed that PRL levels in the plasma of adult chickens infected with ALV-J were lower than those of uninfected chickens, and that the difference was more significant in the avian leukemia pathological apparent changes. Notably, the fluctuations in PRL levels might influence the disappearance of ALV-J viremia in chickens. The in vitro results showed that preincubating DF-1 cells with cPRL before ALV-J infection elicited the best antiviral effects. Moreover, these effects were not dose-dependent. in vivo, injection of cPRL into ALV-J-infected chicks could reduce the levels of viremia at the 14 days post infection (dpi). Additionally, the expression of the interferon-stimulated genes oligoadenylate synthetase-like (OSAL) and vasoactive intestinal peptide (VIP) increased, and that of the proinflammatory cytokine-encoding TNTα, IL-1β, and IL-6 genes decreased in the spleens of ALV-J-infected chicks injected with cPRL, leading to inhibition of viral replication at the 7 dpi. Collectively, our data demonstrated that PRL plays an important antiviral role in the immune response to ALV-J infection. This is the first report of the relationship between ALV-J infection and PRL. It is of great significance for the prevention and control of ALV-J.

Comparison of in vitro bioactivity of chicken prolactin and mammalian lactogenic hormones

Recombinant chicken prolactin, expressed in Escherichia coli as an unfolded protein, was successfully refolded and purified to homogeneity as a monomeric protein. Its biological activity was evidenced by its ability to interact with rabbit prolactin receptor extracellular domain and stimulate prolactin receptor-mediated proliferation in three cell types possessing mammalian prolactin receptors. Chicken prolactin activity in those assays was 20-100-fold lower than that of mammalian lactogenic hormones, likely due to lower affinity for mammalian prolactin receptors and not to improper refolding, because in two homologous bioassays, chicken prolactin activity was equal to or higher than that of ovine prolactin and the CD spectra of chicken and human prolactin were almost identical. Our results using seven mammalian lactogenic hormones from five species in three bioassays revealed the major role of species specificity in testing biological activity in vitro. Heterologous bioassays may be misleading and homologous assays are strongly recommended for predicting the activity of species-specific lactogenic hormones in vivo.

Extra-pituitary prolactin (PRL) and prolactin-like protein (PRL-L) in chickens and zebrafish

It is generally believed that in vertebrates, prolactin (PRL) is predominantly synthesized and released by pituitary lactotrophs and plays important roles in many physiological processes via activation of PRL receptor (PRLR), including water and electrolyte balance, reproduction, growth and development, metabolism, immuno-modulation, and behavior. However, there is increasing evidence showing that PRL and the newly identified 'prolactin-like protein (PRL-L)', a novel ligand of PRL receptor, are also expressed in a variety of extra-pituitary tissues, such as the brain, skin, ovary, and testes in non-mammalian vertebrates. In this brief review, we summarize the recent research progress on the structure, biological activities, and extra-pituitary expression of PRL and PRL-L in chickens (Gallus gallus) and zebrafish (Danio rerio) from our and other laboratories and briefly discuss their potential paracrine/autocrine roles in non-mammalian vertebrates, which may promote us to rethink the broad spectrum of PRL actions previously attributed to pituitary PRL only.

Molecular characterization of prolactin receptor (cPRLR) gene in chickens: gene structure, tissue expression, promoter analysis, and its interaction with chicken prolactin (cPRL) and prolactin-like protein (cPRL-L)

In this study, gene structure, tissue expression, and promoter usage of prolactin receptor (PRLR) and its interaction with prolactin (PRL) and the newly identified prolactin-like protein (PRL-L) were investigated in chickens. The results showed that (1) PRLR gene was found to consist of at least 25 exons by 5'-RACE and RT-PCR assays; (2) multiple PRLR 5'-UTR sequences different in exon composition were isolated from chicken liver or intestine by 5'-RACE and could be subdivided into type I and type II transcripts according to the first exon used (exon 1G or exon 1A); (3) PRLR Type I transcripts with exon 1G were detected to be predominantly expressed in adult kidney and small intestine by RT-PCR, implying their expression is likely controlled by a tissue-specific promoter (P1). By contrast, PRLR type II transcripts containing exon 1A are widely expressed in adult and embryonic tissues examined and their expression is controlled by a generic promoter (P2) near exon 1A, which was demonstrated to display promoter activities in cultured DF-1, HEK293 and LoVo cells by the dual-luciferase reporter assay; (4) Using a 5×STAT5-luciferase reporter system, cPRLR expressed in HepG2 cells was shown to be activated by recombinant cPRL and cPRL-L via interaction with PRLR membrane-proximal ligand-binding domain, suggesting that like cPRL, cPRL-L is also a functional ligand of cPRLR. Collectively, characterization of cPRLR gene helps to elucidate the roles of PRLR and its ligands in birds and provides insights into the regulatory mechanisms of PRLR expression conserved in birds and mammals.

A novel prolactin-like protein (PRL-L) gene in chickens and zebrafish: cloning and characterization of its tissue expression

In this study, a full-length cDNA encoding a prolactin-like protein (PRL-L) was cloned from chicken brain tissues using RT-PCR. This putative PRL-L precursor has 225 amino acids in length and shares 30-35% amino acid sequence identity with prolactin (PRL) of chicken, zebrafish, Xenopus, rat and human. Using RT-PCR, the mRNA expression of PRL-L in chicken tissues was further examined. Unlike the predominant expression of PRL in pituitary, PRL-L was found to be widely expressed in adult chicken extra-pituitary tissues with only minimal expression detected in pituitary. In day-7 chicken embryos, the expression of PRL-L, but not PRL, was also detected in all extra-pituitary tissues examined. In line with this finding, the 5'-flanking region of chicken PRL-L (cPRL-L) gene, but not PRL gene, displayed a strong promoter activity in cultured DF-1 cell (a chicken embryo fibroblast cell line), suggesting that the basal expression of PRL-L gene is controlled by a transcriptional regulatory mechanism different from that of PRL gene. As the same findings in chickens, PRL-like protein(s), which share high amino acid sequence (42-86%) identity with chicken PRL-L, was identified in several non-mammalian vertebrate species including zebra finch, tiger puffer, green puffer and zebrafish. RT-PCR assay demonstrated that zebrafish PRL-L, similar to chicken PRL-L, is expressed in extra-pituitary tissues including brain, gill, muscle, ovary and testis. Taken together, these findings strongly suggest that a novel PRL-like protein exists in some non-mammalian vertebrates and may play an important role in target tissues, such as extra-pituitary tissues of chickens and zebrafish.

Expression patterns of the prolactin receptor gene in chicken lymphoid tissues during embryogenesis and posthatch period

Prolactin (PRL) is a pituitary hormone with multiple homeostatic roles among vertebrates. Although it has mainly been studied in relation to its role during the initiation and maintenance of incubation behavior in avian species, it has also been shown to act on the immune system. In this study, levels of PRL receptor (PRLR) mRNA were quantified by real-time PCR, and tissue expression was localized by in situ hybridization in primary and secondary lymphoid organs. Prolactin receptor was shown to be expressed in the bursa follicles, thymus lobules, and splenic pulp at all stages of development examined. Levels of PRLR expression were consistently higher in the bursa of Fabricius when compared with other lymphoid organs, suggesting that PRL acts primarily on bursal development. Furthermore, levels of PRLR mRNA appeared to fluctuate during embryogenesis, with a significant increase observed at embryonic day 19 in the bursa, at 7 d of age in the thymus, and on hatching day in the spleen. Thus, PRL might play an important role during the development of the immune system in chickens.

Enhanced expression of CD69 and CD25 antigen on human peripheral blood mononuclear cells by prolactin

Several clinical reports have suggested that prolactin (PRL) plays an important role in the pathogenesis of autoimmune diseases such as rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE). We have investigated the influence of PRL on immune system, by evaluating the effects of PRL on the expression of CD69 and CD25 on human peripheral blood mononuclear cells (PBMCs). Human PBMCs obtained from healthy female volunteers were incubated with phytohemagglutinin (PHA) in the presence or absence of various concentrations of PRL. The expression of CD69 and CD25 was monitored using immunofluorescence staining and flow cytometry. PRL significantly enhanced the expression of CD69 and CD25 on activated PBMCs compared with that in the absence of PRL (p<0.05, paired t-test). Increasing doses of PRL enhanced the expression of CD69 up to 2 microg/ml and CD25 up to 1 microg/ml. The enhanced expression of CD69 was observed on CD8+ T lymphocytes but not on CD4+ T lymphocytes. Our data suggest that PRL can significantly enhance the expression of CD69 and CD25 molecule on human PBMCs when induced by PHA. However, PRL would have to be at optimal concentration in order to enhance their expression.

Intracellular second messengers involved in melatonin signal transduction in chicken splenocytes in vitro

The pineal hormone melatonin exhibits immunomodulatory activity well documented in mammals and birds. The mechanism of melatonin action within the immune system is, however, poorly understood. In mammalian immune cells in vitro, melatonin acts mainly as an antiapoptotic, oncostatic and antiproliferative agent, and these effects are exerted via specific receptors or are related to its free radical scavenging activity. In previous studies we have found that in short-term chicken splenocyte cultures in vitro melatonin stimulated basil proliferation and inhibited that stimulated with phytohemagglutinin, a T-cell mitogen. This paper is devoted to the involvement of membrane receptors, previously characterised by us as MT2 (Mel(1b)) and Mel(1c) subtypes, in the above mentioned melatonin effects in chicken splenocyte cultures. For this purpose, in present study a nonselective melatonin receptor antagonist, luzindole, and the selective MT2 blocker, 4P-PDOT, were used. The effect of melatonin on second messengers, cyclic adenosine-3',5'-monophosphate (cAMP) and inositol-1,4,5-trisphosphate (IP(3)), involved in the regulation of proliferation, was examined. We have found that the stimulation of proliferation occurs via Mel(1c) receptor and is associated with the changes in intracellular second messengers concentration: a decrease in cAMP and an increase in IP(3). In contrast, in mitogen-activated splenocytes, melatonin-induced inhibition of proliferation is mediated by MT2 receptors and is related to cAMP accumulation, as well as a decrease in IP(3). In conclusion, we have demonstrated that the stimulatory and inhibitory effect of melatonin on chicken splenocytes in vitro, dependent on the magnitude of cell stimulation, resulted from two different subtypes of membrane receptors.

Analysis of anti-prolactin autoantibodies in systemic lupus erythematosus

Evidence has shown that prolactin is an essential component of an effective immune response. In systemic lupus erythematosus, clinical trials have produced controversial information about the role of PRL. Some results find association between serum PRL levels and disease activity. In contrast, other authors did not find this. Recently, autoantibodies against prolactin in SLE patients have been described. One hundred percent of SLE patients with anti-PRL autoantibodies had hyperprolactinemia (hPRL) and 31.7% of the SLE patients classified with idiopathic hPRL had anti-prolactin antibodies. A similar result was found in 103 pediatric SLE patients. The patients with idiopathic hyperprolactinemia and anti-PRL autoantibodies had less clinical and serological lupus activity than the SLE patients with idiopathic hyperprolactinemia, but without anti-PRL autoantibodies. This evidence suggests that anti-PRL autoantibodies or the complex with any other molecule, like macroprolactinemia (big-big PRL) could have attenuated biological activity and this could explain why some clinical studies did not find any association between serum PRL levels and disease activity in SLE patients. However, studies in vitro have shown normal or elevated biological activity in Nb2 cell lines using PRL from serum with anti-PRL autoantibodies from patients with or without autoimmune diseases. Several conclusions could be drawn. One is that while a set of hyperprolactinemic SLE patients display autoantibodies against PRL, it is not clear what role these autoantibodies play in the whole system. However, until now, we knew that the patients with antibodies to PRL lacked the clinical symptoms of hyperprolactinemia such as menstrual disturbances and/or galactorrhea and show less clinical and serological lupus activity.

The roles of prolactin, growth hormone, insulin-like growth factor-I, and thyroid hormones in lymphocyte development and function: insights from genetic models of hormone and hormone receptor deficiency

An extensive literature suggesting that PRL, GH, IGF-I, and thyroid hormones play an important role in immunity has evolved. Because the use of one or more of these hormones as immunostimulants in humans is being considered, it is of critical importance to resolve their precise role in immunity. This review addresses new experimental evidence from analysis of lymphocyte development and function in mice with genetic defects in expression of these hormones or their receptors that calls into question the presumed role played by some of these hormones and reveals unexpected effects of others. These recent findings from the mutant mouse models are integrated and placed in context of the wider literature on endocrine-immune system interactions. The hypothesis that will be developed is that, with the exception of a role for thyroid hormones in B cell development, PRL, GH, and IGF-I are not obligate immunoregulators. Instead, they apparently act as anabolic and stress-modulating hormones in most cells, including those of the immune system.

Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Mitogenic effect of naturally occurring elevated plasma prolactin on ring dove lymphocytes

3H-thymidine incorporation into isolated ring dove lymphocytes in vitro was used as a measure of lymphocyte proliferation. Lymphocytes taken from doves with increased plasma concentrations of prolactin demonstrated significantly increased 3H-thymidine incorporation. In vitro incubation with mitogens significantly increased incorporation of 3H-thymidine into lymphocytes from non-breeding doves. However, similar treatment of lymphocytes taken from doves which had elevated levels of plasma prolactin failed to induce any further increase in the stimulation index. Antigen caused a significant increase in 3H-thymidine incorporation in non-breeding doves. Antigen administration also led to the production of specific antibodies. The titre of specific anti-human red blood cell (HRBC) agglutinins was greatest in those birds which also had elevated levels of plasma prolactin, reaching significance in the group of incubating doves with naturally occurring increased concentrations of plasma prolactin. The results presented here may be relevant to our understanding of the role of hormones such as prolactin on lymphocyte activation.

Effect of prolactin, in vivo and in vitro, upon heterophil phagocytic function in the ring dove (Streptopelia risoria)

The purpose of this study was to investigate the effects of prolactin, both in vitro and in vivo, upon heterophil phagocytic activity of the ring dove, Streptopelia risoria. In vitro incubation of heterophils with either 0.1 or 100 micrograms/mL of ovine prolactin for 2 h-significantly increased both latex bead phagocytosis and Nitroblue tetrazolium (NBT) reduction, an index of phagocytic metabolic activity. Ring doves given antigen demonstrated an increase in latex bead phagocytosis and NBT reduction. The greatest increase in both of these parameters was observed in those birds which possessed elevated plasma prolactin concentrations, either through exogenous administration or naturally through being in the later stages of incubation. These results suggest that during the incubation period of the avian breeding cycle there is an increase in phagocytic activity which is a direct consequence of the elevation shown by these birds in the concentration of plasma prolactin.

The effect of age and sex on the expression of prolactin binding activity in the chicken bursa of Fabricius

The binding of 125I-labeled prolactin (PRL) to membranes from the bursa of Fabricius of male and female chicks of different ages (15-30-45 and 60 days) was studied. In male chicks the binding was very low in 15 day-old animals and slightly increased in more aged animals. In female chicks the binding was more evident in young animals and decreased in 60 day-old animals. The binding showed a hormonal specificity and Scatchard analysis of the binding revealed the presence of binding sites with low capacity and high affinity. The presence of PRL receptors in the bursa of the chick, a structure that confers immunological competence to birds, gives further support to the involvement of the hormone in the immune processes.

Mitogenic effect of growth hormone and prolactin on chum salmon Oncorhynchus keta leukocytes in vitro

Chum salmon, Oncorhynchus keta, growth hormone and prolactin induced a direct proliferative response on chum salmon peripheral blood leukocytes in vitro. Maximal responses were obtained using 10-100 ng ml-1 of both hormones and were equivalent to the proliferation reference obtained with 100 micrograms concanvalin A ml-1.

Development and validation of a homologous radioimmunoassay using a biologically active recombinant turkey prolactin

1. A new homologous radioimmunoassay has been developed for the measurement of turkey prolactin. 2. A 25000 kDa purified recombinant derived turkey prolactin (rtPRL), the biological activity of which was tested using a crop sac assay, was used as immunogen for the production of rabbit antiserum. In this biological test, the rtPRL was as active as the ovinePRL. 3. The radioligand (rtPRL) was labelled with 125I and the assay allowed the detection of standard doses of rtPRL ranging from 400 pg/tube to 50 ng/tube. 4. No cross reaction with chicken luteinising hormone and recombinant chicken growth hormone was detected. 5. The within and between assay coefficients of variability were 5.0 +/- 2.7% and 16.3%, respectively. The overall mean recovery ratio was 1.01. 6. The dose-response curves obtained with serial dilution of plasma and pituitary from turkey hens at different physiological stages and from male turkeys were parallel to those obtained with standard rtPRL. 7. The measured concentration of prolactin was 5 times higher in plasma from incubating than laying turkey hens, and the pituitaries from incubating hens contained 2 and 4 times more prolactin than those of laying and out of lay hens or males, respectively. 8. To further assess the validity of the assay, we measured changes in plasma concentration of prolactin in turkeys following stimulation with chicken vasointestinal peptide (cVIP). A single injection of 1 or 10 micrograms/kg body weight of cVIP to laying hens produced a large and rapid increase in plasma prolactin. 9. This new radioimmunoassay appears to be high for the measurement of turkey prolactin.

The role of prolactin in the pathogenesis of autoimmune disease

Prolactin has emerged in recent years as a major regulator of both the maturation and the function of lymphocytes. Prolactin abnormalities, which include elevated serum levels, decreased bioactivity, abnormal circadian rhythm, and exaggerated secretion after stimulation by TRH, are associated with various autoimmune conditions in humans. Some animal experiments and observations in humans indicate that proiactin has an important role in the pathogenesis of autoimmune disease. There are several mechanisms through which prolactin could promote the development of autoimmunity. It is concluded that prolactin abnormalities alone are not likely to cause autoimmunity, but rather additional regulatory defects are perhaps also required for disease to develop.

Differential effects of prolactin upon activation and differentiation of human B lymphocytes

The effects of human prolactin on enriched peripheral B lymphocytes obtained from healthy males were examined. Immunoregulation by prolactin was studied in B cells activated with either anti-IgM alone, or anti-IgM antibodies and recombinant interleukin-2 (r-IL-2), as well as control resting B cells. Expression of IL-2 receptors (IL-2R), and IgM and IgG in the culture supernatants were used as a measure of B cell activation and differentiation. Prolactin significantly synergized with IL-2 in the enhancement of surface expression of IL-2R on anti-IgM treated B cells. Although no differentiating effect was observed on resting B cells, prolactin (0.2-100 ng/ml) exhibited a dose-dependent enhancement upon both IgM and IgG secretion from B cells treated with anti-IgM and IL-2. In the absence of exogenously added IL-2 similar differentiating effect were observed in B cells treated with anti-IgM at prolactin concentrations of 0.2-10 ng/ml, but not 100 ng/ml. Thus, the present results demonstrate the modulatory effect of prolactin on activation and differentiation of anti-IgM triggered human B cells, and emphasize the importance of co-stimulatory signal mediated by IL-2 in B cell responses to high prolactin levels. These findings extend the immunoregulatory effects of prolactin, previously confirmed for T cells, to the B cell arm of the immune response, and suggest an important role of prolactin in mediating adaptation and communication between the nerve and immune systems.

Prolactin as an immunoregulatory hormone in mammals and birds

The immunoregulatory function of prolactin (PRL) and the mechanism of its action in mammals seem to be well documented. Reciprocal interdependence between PRL secretion and immune system function is essential for normal ontogeny, development and aging. PRL receptors in lymphocytes participate in the transduction of its regulatory signal into the intracellular enzymatic machinery including that of the nucleus, leading to the expression of some genes and to the synthesis of new proteins. Activation of phosphoinositide turnover and subsequent increase in protein kinase-C activity seems to be a possible mechanism acting in the regulatory influence of PRL on mammalian immune cells. These cells in turn, under mitogen or antigen stimulation, secrete a substance with PRL-like activity. The regulatory function of PRL within the avian immune system is less well known, but it seems to have some features in common with those in mammals. Direct mitogenic action on thymocytes and splenocytes in the chicken might indicate the existence of PRL receptors in these cells and could explain the immunostimulatory effect of PRL observed in vivo, which is dependent on the time of hormone administration. As the avian PRL stimulates mitogenesis of rat Nb2 lymphoma cells, the mechanism of direct PRL action on immune cells in mammals and birds seems to be similar. PRL in chickens also modifies the level and the diurnal rhythm of corticosterone which, in turn, influences the immunoregulatory effect exerted by PRL. Thus, PRL seems to be an important factor, influencing directly or indirectly the avian immune system.