Immunological protection of the vulnerable marsupial pouch young: two periods of immune transfer during lactation in Trichosurus vulpecula (brushtail possum)

Host reproductive cycle influences the pouch microbiota of wild southern hairy-nosed wombats (Lasiorhinus latifrons)

Background

Marsupials are born much earlier than placental mammals, with most crawling from the birth canal to the protective marsupium (pouch) to further their development. However, little is known about the microbiology of the pouch and how it changes throughout a marsupial’s reproductive cycle. Here, using stringent controls, we characterized the microbial composition of multiple body sites from 26 wild Southern Hairy-nosed Wombats (SHNWs), including pouch samples from animals at different reproductive stages.

Results

Using qPCR of the 16S rRNA gene we detected a microbial community in the SHNW pouch. We observed significant differences in microbial composition and diversity between the body sites tested, as well as between pouch samples from different reproductive stages. The pouches of reproductively active females had drastically lower microbial diversity (mean ASV richness 19 ± 8) compared to reproductively inactive females (mean ASV richness 941 ± 393) and were dominated by gram positive bacteria from the Actinobacteriota phylum (81.7–90.6%), with the dominant families classified as Brevibacteriaceae, Corynebacteriaceae, Microbacteriaceae, and Dietziaceae. Three of the five most abundant sequences identified in reproductively active pouches had closest matches to microbes previously isolated from tammar wallaby pouches.

Conclusions

This study represents the first contamination-controlled investigation into the marsupial pouch microbiota, and sets a rigorous framework for future pouch microbiota studies. Our results indicate that SHNW pouches contain communities of microorganisms that are substantially altered by the host reproductive cycle. We recommend further investigation into the roles that pouch microorganisms may play in marsupial reproductive health and joey survival.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-021-00074-8.

Opossum milk IgG is from maternal circulation and timing of transfer correlates with neonatal immune development

Marsupials, with short gestation times, have more complex and changing patterns of milk composition than eutherians. Maternal immunoglobulins (Ig) that confer immunity on offspring are among the components that change during marsupial lactation. In the present study we quantified the abundance of mammary transcripts encoding Ig heavy chains and their corresponding transporters in the laboratory opossum Monodelphis domestica. IgA transcripts were the most abundant in opossum mammary and, with IgM, increased in abundance linearly from birth to weaning. Similarly, the Fc receptor for IgA, the poly-Ig receptor, also increased in abundance throughout lactation. There were few transcripts for IgG or IgE within the opossum mammaries. This is in contrast with reports for Australian marsupial species. Transcripts for the Neonatal Fc Receptor (FcRN), which transports IgG, were detected throughout lactation, and opossum milk is known to contain IgG. Therefore, milk IgG is likely to be taken from the maternal circulation, rather than resulting from local production. There is a parallel increase in FcRN in the newborn gut that declines around the time when neonates have matured to the point where they can make their own IgG. These results are consistent with a transfer of maternal Ig that is coordinated with the development of the neonatal immune system.

Marsupial and monotreme milk-a review of its nutrient and immune properties

All mammals are characterized by the ability of females to produce milk. Marsupial (metatherian) and monotreme (prototherian) young are born in a highly altricial state and rely on their mother’s milk for the first part of their life. Here we review the role and importance of milk in marsupial and monotreme development. Milk is the primary source of sustenance for young marsupials and monotremes and its composition varies at different stages of development. We applied nutritional geometry techniques to a limited number of species with values available to analyze changes in macronutrient composition of milk at different stages. Macronutrient energy composition of marsupial milk varies between species and changes concentration during the course of lactation. As well as nourishment, marsupial and monotreme milk supplies growth and immune factors. Neonates are unable to mount a specific immune response shortly after birth and therefore rely on immunoglobulins, immunological cells and other immunologically important molecules transferred through milk. Milk is also essential to the development of the maternal-young bond and is achieved through feedback systems and odor preferences in eutherian mammals. However, we have much to learn about the role of milk in marsupial and monotreme mother-young bonding. Further research is warranted in gaining a better understanding of the role of milk as a source of nutrition, developmental factors and immunity, in a broader range of marsupial species, and monotremes.

Age-specific gastrointestinal parasite shedding in free-ranging cheetahs (Acinonyx jubatus) on Namibian farmland

The cheetah (Acinonyx jubatus Brookes 1828) is classified as "vulnerable" by the International Union for Conservation of Nature (IUCN). Threats to cheetah populations are a decrease of suitable habitats, an increase of conflicts with livestock farmers and potentially pathogens. While there is some information on the viral and bacterial pathogens circulating in cheetah populations, information on gastrointestinal parasites is scarce. Here, we investigate the gastrointestinal parasites in 39 free-ranging cheetahs in east-central Namibia using a coproscopical parasitological method. Most cheetahs (82%) shed eggs from Ancylostoma which comprised the majority of the total eggs in feces. Eggs and oocysts from Toxascaris (21% of cheetahs), Coccidia (13%), Physaloptera (8%), Taeniidae (5%), Dipylidium (3%), and Diphyllobothriidae (3%) were present at a lower prevalence. Parasite richness and Ancylostoma egg load were higher in juveniles and adults compared to cubs, but were not associated with sex. To our knowledge, this is the first study that assessed gastrointestinal parasites in free-ranging cheetahs and is a key starting point for future studies on the effect of parasites in this threatened species.

Antimicrobial Protection of Marsupial Pouch Young

Marsupials diverged from eutherian mammals about 148 million years ago and represent a unique lineage of mammals with distinctive morphological and reproductive characteristics. Marsupials have significantly shorter gestation periods than eutherians. Pregnancy typically ranges from 15 to 35 days, with young being born at a very early developmental stage and lacking differentiated lymphoid tissues and mature effector cells. Recent microbiome studies of the marsupial pouch revealed that marsupial young can face intense microbial challenges after birth, as the pouch contains a broad range of Gram-positive and Gram-negative bacteria. Antimicrobials are believed to play a significant role in the immune protection of marsupial newborns during their pouch life. The skin of the post-reproductive pouch secretes antimicrobial lysozyme and dermcidin, which may contribute to the decreased density of certain bacteria in the pouch. A range of antimicrobial agents, such as immunoglobulins, lysozyme, transferrin, and cathelicidins, have been identified in marsupial milk. Antimicrobial assays have revealed that marsupial cathelicidins have broad-spectrum activity against a variety of bacteria and fungi, including several multi-drug resistant strains. In this article, we will review the action mechanisms of these antimicrobial compounds and discuss how they protect marsupial newborns from potentially pathogenic bacteria inside the pouch. We will also discuss the potential of marsupial antimicrobial compounds as a source of novel antibiotics.

Characterisation of the immune compounds in koala milk using a combined transcriptomic and proteomic approach

Production of milk is a key characteristic of mammals, but the features of lactation vary greatly between monotreme, marsupial and eutherian mammals. Marsupials have a short gestation followed by a long lactation period, and milk constituents vary greatly across lactation. Marsupials are born immunologically naïve and rely on their mother’s milk for immunological protection. Koalas (Phascolarctos cinereus) are an iconic Australian species that are increasingly threatened by disease. Here we use a mammary transcriptome, two milk proteomes and the koala genome to comprehensively characterise the protein components of koala milk across lactation, with a focus on immune constituents. The most abundant proteins were well-characterised milk proteins, including β-lactoglobulin and lactotransferrin. In the mammary transcriptome, 851 immune transcripts were expressed, including immunoglobulins and complement components. We identified many abundant antimicrobial peptides, as well as novel proteins with potential antimicrobial roles. We discovered that marsupial VELP is an ortholog of eutherian Glycam1, and likely has an antimicrobial function in milk. We also identified highly-abundant koala endogenous-retrovirus sequences, identifying a potential transmission route from mother to young. Characterising the immune components of milk is key to understanding protection of marsupial young, and the novel immune compounds identified may have applications in clinical research.

The identification of immune genes in the milk transcriptome of the Tasmanian devil (Sarcophilus harrisii)

Tasmanian devil (Sarcophilus harrisii) pouch young, like other marsupials, are born underdeveloped and immunologically naïve, and are unable to mount an adaptive immune response. The mother’s milk provides nutrients for growth and development as well as providing passive immunity. To better understand immune response in this endangered species, we set out to characterise the genes involved in passive immunity by sequencing and annotating the transcriptome of a devil milk sample collected during mid-lactation. At mid-lactation we expect the young to have heightened immune responses, as they have emerged from the pouch, encountering new pathogens. A total of 233,660 transcripts were identified, including approximately 17,827 unique protein-coding genes and 846 immune genes. The most highly expressed transcripts were dominated by milk protein genes such as those encoding early lactation protein, late lactation proteins, α-lactalbumin, α-casein and β-casein. There were numerous highly expressed immune genes including lysozyme, whey acidic protein, ferritin and major histocompatibility complex I and II. Genes encoding immunoglobulins, antimicrobial peptides, chemokines and immune cell receptors were also identified. The array of immune genes identified in this study reflects the importance of the milk in providing immune protection to Tasmanian devil young and provides the first insight into Tasmanian devil milk.

Immunological Insights into the Life and Times of the Extinct Tasmanian Tiger (Thylacinus cynocephalus)

The thylacine (Thylacinus cynocephalus) was Australia’s largest marsupial carnivore until its extinction within the last century. There remains considerable interest and debate regarding the biology of this species. Studies of thylacine biology are now limited to preserved specimens, and parts thereof, as well as written historical accounts of its biology. This study describes the development of the immune tissues of a pouch young thylacine, one of only eleven in existence, and the only specimen to be histologically sectioned. The appearance of the immune tissue of the developing pouch young thylacine is compared to the immune tissues of extant marsupials, providing insights into the immunity, biology and ecology of the extinct thylacine.

Prolonged transition time between colostrum and mature milk in a bear, the giant panda, Ailuropoda melanoleuca

Bears produce the most altricial neonates of any placental mammal. We hypothesized that the transition from colostrum to mature milk in bears reflects a temporal and biochemical adaptation for altricial development and immune protection. Comparison of bear milks with milks of other eutherians yielded distinctive protein profiles. Proteomic and metabolomic analysis of serial milk samples collected from six giant pandas showed a prolonged transition from colostrum to main-phase lactation over approximately 30 days. Particularly striking are the persistence or sequential appearance of adaptive and innate immune factors. The endurance of immunoglobulin G suggests an unusual duration of trans-intestinal absorption of maternal antibodies, and is potentially relevant to the underdeveloped lymphoid system of giant panda neonates. Levels of certain milk oligosaccharides known to exert anti-microbial activities and/or that are conducive to the development of neonatal gut microbiomes underwent an almost complete changeover around days 20-30 postpartum, coincident with the maturation of the protein profile. A potential metabolic marker of starvation was detected, the prominence of which may reflect the natural postpartum period of anorexia in giant panda mothers. Early lactation in giant pandas, and possibly in other ursids, appears to be adapted for the unique requirements of unusually altricial eutherian neonates.

The Mammary Gland in Mucosal and Regional Immunity

The mammary gland (MG) lacks a mucosa but is part of the mucosal immune system because of its role in passive mucosal immunity. The MG is not an inductive site for mucosal immunity. Rather, synthesis of immunoglobulin (Ig)A by plasma cells stimulated at distal inductive sites dominate in the milk of rodents, humans, and swine whereas IgG1 derived from serum predominates in ruminants. Despite the considerable biodiversity in the role of the MG, IgG passively transfers the maternal systemic immunological experience whereas IgA transfers the mucosal immunological experience. Although passive antibodies are protective, they and other lacteal constituents can be immunoregulatory. Immune protection of the MG largely depends on the innate immune system; the monocytes–macrophages group together with intraepithelial lymphocytes is dominant in the healthy gland. An increase in somatic cells (neutrophils) and various interleukins signal infection (mastitis) and a local immune response in the MG. The major role of the MG to mucosal immunity is the passive immunity supplied to the suckling neonate.

Immunohistochemical localization of T-lymphocyte subsets in the developing lymphoid tissues of the tammar wallaby (Macropus eugenii)

Research into marsupial adaptive immunity during ontogeny has been hampered by the lack of antibodies that react to marsupial immunological cell populations. In this study, newly synthesised polyclonal antibodies to the T cell marker, CD8, have been developed and used to investigate the ontogeny and distribution of this T cell population in the tammar wallaby. Immunohistochemical analysis indicated that the distribution of the CD8 lymphocytes in the lymphoid tissues of tammar neonates during the first 144 days of pouch life was similar to that of the eutherian mammals. However, CD8α(+) lymphocytes were observed in the intestines of tammar neonates prior to their first appearance in the cervical thymus, an observation that has not been found in eutherians. A dual labelling immunohistochemical approach was used for the indirect demonstration of CD4 and enabled the simultaneous detection in the tammar wallaby tissues of the two major T-lymphocyte populations, CD4 and CD8 that are associated with adaptive immunity. As in eutherian mammals, CD4(+) cells were the predominant T cell lymphocyte subset observed in the spleen while in the nodal tissues, an age-related decrease in the CD4(+)/CD8(+) ratio was noted. These antibodies provide a new immunological tool to study the role of T cell subsets in marsupial immunity and disease pathogenesis studies.

A review of complementary mechanisms which protect the developing marsupial pouch young

Marsupials are born without a functioning adaptive immune system, into a non-sterile environment where they continue to develop. This review examines the extent of exposure of pouch young to microorganisms and describes the protective mechanisms that are complementary to adaptive immunity in the developing young. Complementary protective mechanisms include the role of the innate immune system and maternal protection strategies, such as immune compounds in milk, prenatal transfer of immunoglobulins, antimicrobial compounds secreted in the pouch, and chemical or mechanical cleaning of the pouch and pouch young.

Evolution of immune functions of the mammary gland and protection of the infant

Abstract The evolution of immunological agents in milk is intertwined with the general aspects of the evolution of the mammary gland. In that respect, mammalian precursors emerged from basal amniotes some 300 million years ago. In contrast to the predominant dinosaurs, proto-mammals possessed a glandular skin. A secondary palate in the roof of the mouth that directed airflow from the nostrils to the oropharynx and thus allowed mammals to ingest and breathe simultaneously first appeared in cynodonts 230 million years ago. This set the stage for mammalian newborns to nurse from the future mammary gland. Interplays between environmental and genetic changes shaped mammalian evolution including the mammary gland from dermal glands some 160 millions of years ago. It is likely that secretions from early mammary glands provided nutrients and immunological agents for the infant. Natural selection culminated in milks uniquely suited to nourish and protect infants of each species. In human milk, antimicrobial, anti-inflammatory, and immunoregulatory agents and living leukocytes are qualitatively or quantitatively different from those in other mammalian milks. Those in human milk compensate for developmental delays in the immunological system of the recipient infant. Consequently, the immune system in human milk provided by evolution is much of the basis for encouraging breastfeeding for human infants.

Immunome database for marsupials and monotremes

BACKGROUND

To understand the evolutionary origins of our own immune system, we need to characterise the immune system of our distant relatives, the marsupials and monotremes. The recent sequencing of the genomes of two marsupials (opossum and tammar wallaby) and a monotreme (platypus) provides an opportunity to characterise the immune gene repertoires of these model organisms. This was required as many genes involved in immunity evolve rapidly and fail to be detected by automated gene annotation pipelines.

DESCRIPTION

We have developed a database of immune genes from the tammar wallaby, red-necked wallaby, northern brown bandicoot, brush-tail possum, opossum, echidna and platypus. The resource contains 2,235 newly identified sequences and 3,197 sequences which had been described previously. This comprehensive dataset was built from a variety of sources, including EST projects and expert-curated gene predictions generated through a variety of methods including chained-BLAST and sensitive HMMER searches. To facilitate systems-based research we have grouped sequences based on broad Gene Ontology categories as well as by specific functional immune groups. Sequences can be extracted by keyword, gene name, protein domain and organism name. Users can also search the database using BLAST.

CONCLUSION

The Immunome Database for Marsupials and Monotremes (IDMM) is a comprehensive database of all known marsupial and monotreme immune genes. It provides a single point of reference for genomic and transcriptomic datasets. Data from other marsupial and monotreme species will be added to the database as it become available. This resource will be utilized by marsupial and monotreme immunologists as well as researchers interested in the evolution of mammalian immunity.

Biparental mucus feeding: a unique example of parental care in an Amazonian cichlid

Vertebrates display a wide variety of parental care behaviours, including the guarding of offspring pre and post nutritional independence as well as the direct provision of nutrients during the early development period. The Amazonian cichlid Symphysodon spp. (discus fish) is unusual among fish species, in that both parents provide offspring with mucus secretions to feed from after hatching. This extensive provision of care, which can last up to a month, imposes a physiological demand on both parents and gives rise to conflict between the parent and offspring. Here, we investigated the relationship between parents and offspring during a breeding cycle, determining both mucus composition (total protein, cortisol, immunoglobulin, and Na+, K+ and Ca2+ concentrations) and the behavioural dynamics of the parent–offspring relationship. Over the course of a breeding cycle, a significant increase in offspring bite rate was recorded, with a concomitant increase in the frequency of turns the male and female parent took at caring for their young. A peak in mucus antibody provision was seen as offspring reached the free-swimming stage, suggesting a role analogous to colostrum provision in mammals. Mucus protein content was lowest during the second and third weeks of free swimming, and a weaning period, similar to that seen in mammalian parental care, occurred when the offspring had been free swimming for ∼3 weeks. In many ways, the parental behaviour of discus fish is more similar to mammalian and avian parental care than other fish species, and represents an exciting aquatic model for studying the parent–offspring conflict.

Developmental changes in the electroencephalogram and responses to a noxious stimulus in anaesthetized tammar wallaby joeys (Macropus eugenii eugenii)

The tammar wallaby joey is born extremely immature and most of its neurological development occurs in the maternal pouch. It is not known at what in-pouch age functions such as conscious sensory perception commence. We determined the electroencephalographic (EEG) responses to noxious stimulation in lightly anaesthetized tammar wallaby joeys. Baseline median (F50) and spectral edge (F95) frequencies, total power (Ptot) and frequency spectra between 1 and 30 Hz of the EEG power spectrum were determined. Joeys aged less than 127 days showed little or no EEG activity. Prolonged periods of spontaneous EEG activity were present by 142 days. This activity increased, as did the power in all frequencies, while the duration of any intervening isoelectric periods decreased with increasing in-pouch age. EEG responses to a noxious stimulus (toe clamping) changed with increasing in-pouch age as there was no response from joeys aged 94-127 days (no EEG), a minimal decrease in the F50 in those aged between 142 and 181 days (P = 0.052) and a greater decrease in the F50 in those aged between 187 and 261 days (P < 0.001). The pattern of these changes, which presumably reflects anatomical and functional maturation of the cerebral cortex, is similar to, but develops more slowly than, that reported in the rat. The opening of the eyes and development of the pelage are discussed as markers of when brain development may be sufficient for joeys to consciously perceive noxious sensations including pain.

A longitudinal study of the protein components of marsupial milk from birth to weaning in the tammar wallaby (Macropus eugenii)

The major milk whey proteins of the tammar wallaby (Macropus eugenii) have been identified over the total period of lactation using proteomic analysis techniques comprising two-dimensional electrophoresis, comparative image analysis, matrix assisted laser desorption ionisation mass spectrometry (MALDI MS), de novo peptide sequencing and cross species protein matching. Samples were collected at the periods coinciding with major milestones of immunological development in the developing marsupial and in the four phases of milk production, specifically, Days 0, 5 (Phase 1); 27, 68 (Phase 2A); 137, 174 (Phase 2B) and 250 (Phase 3). Major changes in the protein content of marsupial milk whey correlated with the changing needs of the pouch young for stages in growth and development. We have shown that the levels of milk whey proteins vary with the developmental stage of the young animal, with a high number of proteins detected in early and late milk compared with the middle phases of lactation. Over 41 proteins were confidently identified, of which most had known roles in immunological protection. Proteins providing immunological protection across the lactation period included transferrin, beta2 microglobulin, haptoglobulin and a 78kDa glucose regulated protein. Immunoglobulin IgJ linker chain and a known antimicrobial cathelicidin, were only detected for the first 100-137 days, after which time Complement B factor was found to be present (Phase 2B). The changes which correlated with development and growth in the pouch young were reflected by the presence of proteins such as an alpha-fetoprotein like protein and clusterin found in early milk (Phase 1-2A) and two unknown proteins which were apparent in very early mammary gland secretions. This is the first comprehensive proteomic study of the major whey proteins of a marsupial across the entire period of lactation and provides fundamental data on proteins secreted by the mammary gland during key stages of immunological development of the young animal.

Molecular analysis of tammar (Macropus eugenii) mammary epithelial cells stimulated with lipopolysaccharide and lipoteichoic acid

The immunological function of the metatherian mammary gland plays a crucial part in neonatal survival of the marsupial young. Marsupial pouch young do not develop adult like immune responses until just prior to leaving the pouch. The immune components of the maternal milk secretions are important during this vulnerable early post-partum period. In addition, infection of the mammary gland has not been recognized in metatherians, despite the ready availability of pathogens in the pouch. Regardless of which, little is known about the immunobiology of the mammary gland and the immune responses of mammary epithelial cells in metatherians. In this study, a molecular approach was utilized to examine the response of tammar (Macropus eugenii) mammary epithelial cells to Escherichia coli derived lipopolysaccharide (LPS) and Staphylococcus aureus derived lipoteichoic acid (LTA). Using custom-made cDNA microarrays, candidate genes were identified in the transciptome, which were involved in antigen presentation, inflammation, cell growth and proliferation, cellular damage and apoptosis. Quantification of mRNA expression of several of these candidate genes, along with seven other genes (TLR4, CD14, TNF-alpha, cathelicidin, PRDX1, IL-5 and ABCG2) associated with innate immunity in LPS and LTA challenged mammary epithelial cells and leukocytes, was assessed for up to 24 h. Differences in genes associated with cellular damage and pro-inflammatory cytokine production were seen between stimulated mammary epithelial cells and leukocytes. LTA challenge tended to result in lower level induction of pro-inflammatory cytokines, increased PRDX1 mRNA levels, suggesting increased oxidative stress, and increased CD14 expression, but in a non-TLR4-dependent manner. The use of functional genomic tools in the tammar identified differences in the response of tammar mammary epithelial cells (MEC) and leukocytes to challenge with LPS and LTA, and validates the utility of the approach. The results of this study are consistent with a model in which tammar mammary epithelial cells have the capacity to elicit a complex and robust immune response to pathogens.

Vesicle-associated protein 1: a novel ovarian immunocontraceptive target in the common brushtail possum, Trichosurus vulpecula

Ovarian-based immunological research is currently restricted to proteins of the zona pellucida. This study examined the immunocontraceptive potential of a novel vesicle-associated protein, VAP1, previously isolated from the vesicle-rich hemisphere of the brushtail possum oocyte. Seven female possums were immunized against recombinant glutathione S-transferase-VAP1 fusion protein. Control animals (n=3) received antigen-free vaccinations. Following immunization, regular blood sampling determined the level and duration of immune response. Animals were monitored daily, pre- and post-immunization, to determine estrous cycling activity and the percentage of reproductive cycles yielding viable young. The reproductive tracts and somatic organs of VAP1-immunized (n=7), control-immunized (n=3) and non-immunized (n=5) animals were collected and examined by histology and transmission electron microscopy. VAP1 immunization caused a strong and sustained immune response. Elevated levels of VAP1 antibody binding were detected in sera following initial injections, and immune titers rose as boosters were administered. Immunization had no adverse effect upon animal behavior or body condition. Immunized females demonstrated no major change in annual estrous cycling activity; however, the percentage of reproductive cycles resulting in pouch young decreased significantly (P<0.05) by 40%. Histological and ultrastructural analyses revealed an abundance of lipid-like degradation bodies within the ooplasm of developing oocytes and the cytoplasm of failing uterine zygotes. Active macrophage invasion of enlarged endometrial glands was observed in the uteri of two females. Reproductive tract changes are discussed in relation to observed fertility decline. The results of this study indicate that VAP1 has exciting potential as an immunocontraceptive target for possum control in New Zealand.

CD14 and TLR4 are expressed early in tammar (Macropus eugenii) neonate development

Marsupials are born in a relatively underdeveloped state and develop during a period of intensive maturation in the postnatal period. During this period, the young marsupial lacks a competent immune system, but manages to survive despite the potential of exposure to environmental pathogens. Passive immune transfer via the milk is one well-recognised strategy to compensate the neonate, but there also may be innate immune mechanisms in place. In this study, CD14 and Toll-like receptor 4 (TLR4), integral molecular components of pathogen recognition, were identified and characterised for the first time in a marsupial, the tammar wallaby (Macropus eugenii). Functional motifs of tammar CD14 and the toll/interleukin receptor (TIR) domain of TLR4 were highly conserved. The lipopolysaccharide (LPS) binding residues and the TLR4 interaction site of CD14 were conserved in all marsupials. The TIR signalling domain had 84% identity within marsupials and 77% with eutherians. Stimulation of adult tammar leukocytes resulted in the induction of a biphasic pattern of CD14 and TLR4 expression, and coincided with increased production of the pro-inflammatory cytokine TNF-alpha. Differential patterns of expression of CD14 and TLR4 were observed in tammar pouch young early in development, suggesting that early maturation of the innate immune system in these animals may have developed as an immune survival strategy to protect the marsupial neonate from exposure to microbial pathogens.

Identification, characterization and expression of cathelicidin in the pouch young of tammar wallaby (Macropus eugenii)

Antimicrobial peptides, such as cathelicidin, are an evolutionarily old defense system. However they have more complex actions than just simply their antimicrobial effects, including immunoregulation and interaction with the adaptive immune system. In this study we have characterized several novel cathelicidin-like peptides from the tammar wallaby (Macropus eugenii). The tammar cathelicidin-like (MaeuCath) mRNA were isolated based on the conservation of the cathelin-like amino terminus. Mature MaeuCath peptides were positively charged with hydrophobic carboxyl tails, features that are fundamental for antimicrobial function. MaeuCath1 was induced in tammar leukocytes in response to pathogen-associated molecular patterns from both gram positive and negative bacteria. In addition, we also examined the expression of MaeuCath1 in the primary and secondary lymphoid organs of the tammar neonate throughout early pouch life. The results from this study demonstrate the importance that MaeuCath1 may play in innate defense of the marsupial young, especially in the mucosal organs. Such expression of antimicrobial peptides may form part of the immune strategies of marsupials for neonatal survival during their post-partum development.

Analysis of the expression of immunoglobulins throughout lactation suggests two periods of immune transfer in the tammar wallaby (Macropus eugenii)

Marsupial young are born in an under-developed state without mature immune responses. Prior to the maturation of an immune system, marsupial young are heavily reliant upon immune factors secreted in the milk to defend them against potential microbial pathogens in the environment. In this study, we identified and characterized the immunoglobulin heavy chain constant regions, light chains, polymeric Ig receptor (pIgR), J chain, neonatal Fc receptor (alpha chain) (FcRn) and the chemokine CCL28 from the model marsupial species, the tammar wallaby (Macropus eugenii). Low levels of conservation were seen in motifs in C alpha and C gamma associated with receptor binding and or transcytosis, and this may have potential implications for functionality. We evaluated the expression of immunoglobulin genes in the tammar mammary gland throughout lactation and found that two periods of increased expression of immunoglobulin genes occur. These two periods coincide with the birth of the young, and with its first emergence from the pouch. This increased expression may represent a strategy for maternal immunological protection of the pouch young.

Towards a ZP-based contraceptive for marsupials: Comparative analysis and developmental expression of marsupialZP genes

Fertility control in the form of a zona pellucida (ZP)-based immunocontraceptive has shown potential as a humane form of control for overabundant marsupials including the brushtail possum and macropods. Further refinement and development of a ZP-based vaccine requires detailed knowledge of the protein structure and expression in order to ensure maximum efficacy and specificity. Sequencing and comparative analysis of the ZP3 protein from three marsupial orders in this study found a high overall level of conservation; within order Diprotodontia, the ZP3 protein is 86.9-98.9% identical. ZP3 identity falls to 56.6-57.2%, when the grey, short-tailed opossum (a Didelphimorphian) is compared to dasyurid and diprotodontan marsupials. This is similar to its amino acid identity with ZP3 from eutherian species (50.7-52.8%). Comparison of a 21 amino acid epitope in marsupial ZP3 that has shown contraceptive effects, reveals 95-100% identity between the four macropodid species, 81-86% amino acid identity between brushtail possum and the macropods and 67-71% identity between the diprotodontans and the fat-tailed dunnart (a dasyurid). This is comparable to the level of identity between related eutherian mammals. The expression pattern of three ZP genes during brushtail possum and tammar wallaby pouch young development was examined by RT-PCR. This analysis of ZP gene expression has confirmed that ZP mRNA transcription begins in the ovary during pouch young development by about 51 days of age. The presence of ZP transcripts at this stage in pouch young development suggests that marsupial ZP gene transcription begins before the onset of follicular development.

Changes in tissue nucleic acid content and mucosal morphology during intestinal development in pouch young of the tammar wallaby (Macropus eugenii eugenii)

DNA and RNA content and the timing of development of various histological features in the small and large intestine of in-pouch tammar wallabies (Macropus eugenii eugenii) of various ages were measured. A significant decline in gut tissue DNA concentrations and increase in the RNA/DNA ratios over 300 days postpartum indicated that the early postnatal increase in gut tissue mass resulted largely from hypertrophy. Mean duodenal and ileal villus height and crypt depth were significantly greater for in-pouch young aged >100 days compared with those <100 days and were significantly greater in the duodenum than in the ileum. Goblet cells appeared more slowly during development and were fewer in number in the duodenal than in the colonic mucosa. The numbers of mucin-secreting duodenal goblet cells were greater in pouch young aged >100 days than in young aged <100 days. The colonic mucosa exhibited no villi or villus-like folds. Colonic crypt depth increased uniformly with age.

A quantitative study of the morphological development and bacterial colonisation of the gut of the tammar wallaby Macropus eugenii eugenii and brushtail possum Trichosurus vulpecula during in-pouch development

We compared the rates of change of various morphological parameters of the stomach, small intestine, caecum and colon of tammar wallabies and brushtail possums with body mass during in-pouch development. These were correlated with changes in the numbers of bacterial species in the various gut segments. In the pouch-young of both species, the wet tissue masses of all gut segments increased with body mass in a positively allometric manner (i.e. with a body mass exponent > 1), suggesting that the mass of each component was disproportionately low at birth, but increased disproportionately rapidly postnatally. However, the lengths of the wallaby stomach and small intestine scaled isometrically with respect to body mass (i.e. with a body mass exponent around 0.33), which may indicate that the shape of these components changes to the adult form during early neonatal development. Conversely, the length of the caecum and colon of both wallabies and possums scaled in a positively allometric manner with respect to body mass, showing area to volume compensation. This may indicate a more general pattern of disproportionately rapid postnatal enlargement in areas that are distal to the principal sites of neonatal digestion (i.e. the stomach). The numbers of bacterial species present in the various gastrointestinal segments of both species were low in animals aged 100 days or less but there was a significant increase in microbial diversity in the caecum of brushtail possums aged over 100 days. The possum caecum also showed the greatest rate of increase in wet tissue mass relative to body mass. It is postulated that caecal development may act as a nidus for establishment of communities of commensal microflora in the developing marsupial.

An immunomodulator used to protect young in the pouch of the Tammar wallaby, Macropus eugenii

Eugenin [pGluGlnAspTyr(SO(3))ValPheMetHisProPhe-NH(2)] has been isolated from the pouches of female Tammar wallabies (Macropus eugenii) carrying young in the early lactation period. The sequence of eugenin has been determined using a combination of positive and negative ion electrospray mass spectrometry. This compound bears some structural resemblance to the mammalian neuropeptide cholecystokinin 8 [AspTyr(SO(3))MetGlyTrpMetAspPhe-NH(2)] and to the amphibian caerulein peptides [caerulein: pGluGlnAspTyr(SO(3))ThrGlyTrpMetAspPhe-NH(2)]. Eugenin has been synthesized by a route which causes only minor hydrolysis of the sulfate group when the peptide is removed from the resin support. Biological activity tests with eugenin indicate that it contracts smooth muscle at a concentration of 10(-9) M, and enhances the proliferation of splenocytes at 10(-7) M, probably via activation of CCK(2) receptors. The activity of eugenin on splenocytes suggests that it is an immunomodulator peptide which plays a role in the protection of pouch young.

Molecular cloning and characterization of the polymorphic MHC class II DBB from the tammar wallaby (Macropus eugenii)

Genes of the major histocompatibility complex (MHC) have been characterized in all extant lineages of mammals. The tammar wallaby (Macropus eugenii) is well established as a model marsupial species; however, no classical MHC sequences have been described from this species. We have isolated two MHC class II beta-chain sequences from a tammar wallaby spleen cDNA library using a tammar MHC class II beta probe. These sequences belong to the marsupial MHC class II DBB gene family. Two additional DBB sequences were amplified from tammar wallaby genomic DNA. All four sequences were obtained from the same individual, indicating that there are at least two DBB loci in the tammar wallaby.

Expression of the FcRn receptor (alpha and beta) gene homologues in the intestine of suckling brushtail possum (Trichosurus vulpecula) pouch young

The neonatal IgG transporter FcRn consists of two chains, FcRn alpha and beta (also known as beta(2) microglobulin), and is involved in transferring IgG molecules across both mammary and intestinal epithelial cells. Developmental changes in FcRn IgG alpha and beta chain mRNA levels were investigated in the gut of brushtail possum (Trichosurus vulpecula) pouch young (PY) using Northern hybridisation. FcRn alpha transcripts were detected in the PY proximal intestine at all times examined, between days 1 and 195 of post-natal life, with increased levels detected from around day 110. The beta(2) microglobulin transcript levels in the PY proximal intestine were low to undetectable until day 110 of post-natal life and then increased dramatically after day 159. Both the FcRn alpha and beta gene transcripts were detected in a wide range of tissues in the adult possum (>365 days). Genomic sequences located 5' to the start of transcription of the FcRn alpha and beta(2) microglobulin genes were cloned and analysed for predicted cis-acting transcription control elements. Both the FcRn alpha and beta(2) microglobulin genomic sequences contained STAT5 binding motifs consistent with the transcription of both genes being modulated by prolactin. Using in situ hybridisation, the FcRn alpha and beta(2) microglobulin transcripts were localised to the epithelial cells of the PY intestine. However, no prolactin receptor transcripts were detected in the same epithelial cells suggesting that the observed changes in FcRn alpha and beta(2) microglobulin gene expression in the proximal intestine are not modulated directly by prolactin. The results are consistent with the hypothesis that changes in FcRn alpha and beta(2) microglobulin gene expression take place in the possum PY intestine to accommodate changes in maternal milk composition to meet the changing immunological demands of the PY.

Characterisation of antisera to recombinant IgA of the common brushtail possum (Trichosurus vulpecula)

One of the limiting factors in understanding immune responses in marsupials is the scarcity of marsupial specific immunological reagents. This paper describes the characterisation of an antiserum raised against a recombinant protein of the constant region of the heavy chain of IgA (C(alpha)) of the common brushtail possum (Trichosurus vulpecula). The availability of a marsupial specific anti-IgA provides a useful tool for the characterisation of mucosal immune responses in possums. Anti-C(alpha) specifically detects IgA in possum serum and secretions using ELISAs, immuno-dot blots and Western blots without any cross-reactivity to IgG. The possum anti-C(alpha) cross-reacts with IgA of koala (Phascolarctos cinereus), tammar wallaby (Macropus eugenii) and eastern grey kangaroo (Macropus giganteus), demonstrating the potential for use in other marsupials.

Immunohistochemistry of the lymphoid tissues of the tammar wallaby, Macropus eugenii

The lymphoid tissues of the metatherian mammal, the adult tammar wallaby, Macropus eugenii, were investigated using immunohistochemical techniques. Five cross-reactive antibodies previously shown to recognize surface markers in marsupial tissues and five previously untested antibodies were used. The distribution of T-cells in the tissue beds of spleen, lymph node, thymus, gut-associated lymphoid tissue (GALT) and bronchus-associated lymphoid tissue (BALT) was documented using antibodies to CD3 and CD5. Similarly, B-cells were identified in the same tissues using anti-CD79b. Antibodies to CD8, CD31, CD79a and CD68 failed to recognize cells in these tissue beds. In general the pattern of cellular distribution identified using these antibodies was similar to that observed in other marsupial and eutherian lymphoid tissues. This study provides further information on the commonality of lymphoid tissue structure in the two major groups of extant mammals, metatherians and eutherians.

Mucosal immunity in the brushtail possum (Trichosurus vulpecula): detection of antibody in serum and at female reproductive sites after intranasal immunization

Vaccination strategies for the brushtail possum, which rely upon stimulation of mucosal immunity, are being developed for biocontrol purposes. As little is known about how to stimulate possum immune responses via a mucosal site, groups of possums were immunized intranasally with keyhole limpet haemocyanin (KLH) alone or in combination with known or novel mucosal adjuvants. Antigen-specific antibody titres in female reproductive secretions were measured by ELISA and compared with antibody titres in the serum. Antigen-induced lymphocyte proliferative responses were measured as an indicator of cell-mediated responses. Intranasal immunization with KLH alone stimulated a weak serum antibody response that was significantly increased when KLH was given with cholera toxin subunit B (CTB), recombinant possum tumour necrosis factor alpha (TNF alpha) or live Mycobacterium bovis bacillus Calmette Guerin (BCG). Antibody titres in secretions from ovarian follicles and the uterus were very low in animals administered KLH alone. Significantly higher antibody titres to KLH were present in the reproductive secretions of possums immunized with KLH plus CTB, BCG or heat-killed Mycobacterium vaccae. Antibody titres were lower in mucosal secretions than in the serum, but there was a significant correlation between the two. In addition, coadministration of live BCG with KLH produced a strong antigen-specific cell-mediated response to KLH. This study has shown that an immune response to a protein antigen can be stimulated in possums by intranasal immunization and that antigen-specific antibodies can be detected in secretions from the female reproductive tract.

Evolution of the mammary gland defense system and the ontogeny of the immune system

A decisive event in the evolution of mammals from synapsid reptiles was the modification of ventral thoracic-abdominal epidermal glands to form the mammary gland. The natural selection events that drove the process may have been the provision of certain immunological agents in dermal secretions of those nascent mammals. This is mirrored by similar innate immune factors in mammalian sebum and in protherian and eutherian milks. On the basis of studies of existing mammalian orders, it is evident that immune agents in milk such as immunoglobulins, iron-binding proteins, lysozyme, oligosaccharides, and leukocytes compensate for developmental delays in early postnatal production of antimicrobial factors. At least in human milk, anti-inflammatory and immunomodulating agents also evolved to provide different types of protection for the offspring. In addition, investigations reveal that the types or concentrations of immunological agents in milk vary depending upon the type of placenta, lactation pattern, and environment of the species.

Growth hormone and prolactin--molecular and functional evolution

Growth hormone, prolactin, the fish hormone, somatolactin, and related mammalian placental hormones, including placental lactogen, form a family of polypeptide hormones that share a common tertiary structure. They produce their biological effects by interacting with and dimerizing specific single transmembrane-domain receptors. The receptors belong to a superfamily of cytokine receptors with no intrinsic tyrosine kinase, which use the Jak-Stat cascade as a major signalling pathway. Hormones and receptors are thought to have arisen as a result of gene duplication and subsequent divergence early in vertebrate evolution. Mammalian growth hormone and prolactin show a slow basal evolutionary rate of change, but with episodes of accelerated evolution. These occurred for growth hormone during the evolution of the primates and artiodactyls and for prolactin in lineages leading to rodents, elephants, ruminants, and man. Placental lactogen has probably evolved independently on three occasions, from prolactin in rodents and ruminants and from growth hormone in man. Receptor sequences also show variable rates of evolution, corresponding partly, but not completely, with changes in the ligand. A principal biological role of growth hormone, the control of postnatal growth, has remained quite consistent throughout vertebrate evolution and is largely mediated by insulin-like growth factors. Prolactin has many and diverse roles. In relation to lactation, the relative roles of growth hormone and prolactin vary between species. Correlation between the molecular and functional evolution of these hormones is very incomplete, and it is likely that many important functional adaptations involved changes in regulatory elements, for example, altering tissue of origin or posttranscriptional processing, rather than change of the structures of the proteins themselves.

Differential expression of the whey acidic protein gene during lactation in the brushtail possum (Trichosurus vulpecula)

The whey acidic protein (WAP) is a whey protein found in the milk of a number of species. We have isolated and characterised a WAP cDNA clone from the brushtail possum (Trichosurus vulpecula) and examined its expression in the mammary gland. The amino acid sequences of WAP from the possum and another marsupial, the tammar wallaby, share 69% identity, however, less sequence identity exists between the marsupial and eutherian WAP sequences (30-37%). The possum and tammar WAP genes consist of three four-disulphide core (4-DSC) domains, with a WAP motif at the beginning of each domain. In contrast, the eutherian WAP sequences consist of two 4-DSC domains with the WAP motif only present in the second domain. This WAP motif is also present in a number of protease inhibitors found in a wide range of species. Phylogenetic analysis of marsupial and eutherian WAP sequences suggests that the ancestral WAP gene has three domains and that one of the domains has been deleted from the eutherian gene. The profile of WAP gene expression in the possum mammary gland changed throughout lactation, with WAP mRNA levels reaching a peak between days 106 and 177 of lactation. The level of WAP mRNA in the mammary gland appeared to be correlated with the level of circulating prolactin in the lactating female and was different to that observed for several other whey protein genes. Overlapping expression of the WAP and early lactation protein genes, both of which are putative protease inhibitors, may provide protection of milk immunoglobulins that are required for the prolonged period of passive immune transfer to the marsupial pouch young.

Purification of secretory immunoglobulin A from milk of the brushtail possum (Trichosurus vulpecula)

AIM

To identify and purify secretory immunoglobulin A (sIgA), a key effecter molecule in mucosal immune responses, from milk of the brushtail possum (Trichosurus vulpecula).

METHODS

Milk samples were collected from female possums with pouch young, and clarified by centrifugation and precipitation methods. The clarified fraction was purified by gel filtration and affinity chromatography to yield sIgA. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDSPAGE) and immunoblotting techniques were used to assess the purity of the final product, and to identify the heavy (H) chain, light (L) chain and secretory component (SC) of possum sIgA.

RESULTS

Immunoblotting, using antibodies raised against cloned possum sIgA SC and H-chain, and a synthetic peptide fragment of the H-chain, confirmed the identity of the purified protein. The N-terminal amino acid sequence of purified possum sIgA showed strong homology to reported sequences of H-chain variable regions of marsupial immunoglobulins.

CONCLUSIONS

Milk was shown to be a convenient source of mucosal secretion containing sIgA, and a process involving 2 precipitation and 2 chromatography steps produced purified sIgA. This IgA preparation will prove useful for the generation of sIgA-specific immunological reagents for measurement of immune responses in the development of mucosal-based vaccines for biological control of possums.

Expression of the Fc receptor in the mammary gland during lactation in the marsupial Trichosurus vulpecula (brushtail possum)

One of several functions described for the Fc receptor is regulation of IgG isotype transport into milk. The first marsupial homologues of the Fc receptor heavy and light chains, FcRn and beta-2 microglobulin, from the brushtail possum have been cloned and characterised. The level of FcRn mRNA in the possum mammary gland was highest at the start of lactation, and decreased slowly thereafter. Expression of FcRn mRNA did not increase during the switch phase when the concentration of IgG in milk is highest. In contrast, the level of beta-2 microglobulin mRNA in the mammary gland increased during the switch phase when milk IgG concentration also increases. This correlation between beta-2 microglobulin mRNA expression in the mammary gland with the time of active IgG-transfer into milk was also observed in the bovine and murine mammary gland. This suggests that expression of the Fc receptor in the mammary gland is controlled by the expression of beta-2 microglobulin and that its expression is upregulated during the period of highest IgG-transfer into milk.