Tasmanian devil CD28 and CTLA4 capture CD80 and CD86 from adjacent cells

Immune checkpoint immunotherapy is a pillar of human oncology treatment with potential for non-human species. The first checkpoint immunotherapy approved for human cancers targeted the CTLA4 protein. CTLA4 can inhibit T cell activation by capturing and internalizing CD80 and CD86 from antigen presenting cells, a process called trans-endocytosis. Similarly, CD28 can capture CD80 and CD86 via trogocytosis and retain the captured ligands on the surface of the CD28-expressing cells. The wild Tasmanian devil (Sarcophilus harrisii) population has declined by 77% due to transmissible cancers that evade immune defenses despite genetic mismatches between the host and tumors. We used a live cell-based assay to demonstrate that devil CTLA4 and CD28 can capture CD80 and CD86. Mutation of evolutionarily conserved motifs in CTLA4 altered functional interactions with CD80 and CD86 in accordance with patterns observed in other species. These results suggest that checkpoint immunotherapies can be translated to evolutionarily divergent species.

Relatives of rubella virus in diverse mammals

Since 1814, when rubella was first described, the origins of the disease and its causative agent, rubella virus (Matonaviridae: Rubivirus), have remained unclear1. Here we describe ruhugu virus and rustrela virus in Africa and Europe, respectively, which are, to our knowledge, the first known relatives of rubella virus. Ruhugu virus, which is the closest relative of rubella virus, was found in apparently healthy cyclops leaf-nosed bats (Hipposideros cyclops) in Uganda. Rustrela virus, which is an outgroup to the clade that comprises rubella and ruhugu viruses, was found in acutely encephalitic placental and marsupial animals at a zoo in Germany and in wild yellow-necked field mice (Apodemus flavicollis) at and near the zoo. Ruhugu and rustrela viruses share an identical genomic architecture with rubella virus2,3. The amino acid sequences of four putative B cell epitopes in the fusion (E1) protein of the rubella, ruhugu and rustrela viruses and two putative T cell epitopes in the capsid protein of the rubella and ruhugu viruses are moderately to highly conserved4-6. Modelling of E1 homotrimers in the post-fusion state predicts that ruhugu and rubella viruses have a similar capacity for fusion with the host-cell membrane5. Together, these findings show that some members of the family Matonaviridae can cross substantial barriers between host species and that rubella virus probably has a zoonotic origin. Our findings raise concerns about future zoonotic transmission of rubella-like viruses, but will facilitate comparative studies and animal models of rubella and congenital rubella syndrome.

Immunogenetics of marsupial B-cells

Marsupials and eutherians are mammals that differ in their physiological traits, predominately their reproductive and developmental strategies; eutherians give birth to well-developed young, while marsupials are born highly altricial after a much shorter gestation. These developmental traits also result in differences in the development of the immune system of eutherian and marsupial species. In eutherians, B-cells are the key to humoral immunity as they are found in multiple lymphoid organs and have the unique ability to mediate the production of antigen-specific antibodies in the presence of extracellular pathogens. The development of B-cells in marsupials has been reported and hypothesised to be similar to that of eutherians, except that haematopoiesis occurs in the liver, postpartum, until the bone marrow fully matures. In eutherians, specific genes are linked to specific stages in B-cell development, maturation, and differentiation processes, and have been identified including immunoglobulins (heavy and light chains), cluster of differentiation markers (CD10, 19, 34 and CD79α/β), signal transduction molecules (BTK, Lyn and Syk) and transcriptional regulators (EBF1, E2A, and Pax5). This review aims to discuss the known similarities and differences between marsupial and eutherian B-cells, in regards to their genetic presence, homology, and developmental stages, as well as to highlight the areas requiring further investigation. By enhancing our understanding of the genes that are involved with B-cells in the marsupial lineage, it will, in turn, aid our understanding of the marsupial immune system and support the development of specific immunological reagents for research and wildlife conservation purposes.

Limitations in the isolation and stimulation of splenic mononuclear cells in a dasyurid marsupial, Phascogale calura

OBJECTIVE

Marsupials suffer from an increasing number of stressors in this changing world. Functional studies are thus needed to broaden our understanding of the marsupial immune system. The red-tailed phascogale (Phascogale calura) is a small Australian marsupial previously used in descriptive immunological studies. Here, we aimed to develop functional assays by isolating and stimulating blood and spleen mononuclear cells in vitro.

RESULTS

While peripheral blood mononuclear cell (PBMC) were relatively easy to isolate, only 105 mononuclear cells (> 90% purity and > 75% viability) could be recovered from the spleen, independently of the sex and age of the animal or the centrifugation time and speed tested. The pores of the mesh sieve used for tissue homogenization might have been too big to yield a single cell suspension. Nevertheless, in spite of the overall low number of cells recovered, PBMC and splenic mononuclear cells were successfully activated in preliminary trials with phytohemaglutinin. This activation state was evidenced by a change in shape and the presence of small cell aggregations in the mitogen-stimulated cultures. A non-radioactive colorimetric assay was also performed to confirm cell proliferation in these wells. This work highlights the importance of developing and reporting detailed methodological protocols in non-traditional research species.

Heat shock proteins expressed in the marsupial Tasmanian devil are potential antigenic candidates in a vaccine against devil facial tumour disease

The Tasmanian devil (Sarcophilus harrisii), the largest extant carnivorous marsupial and endemic to Tasmania, is at the verge of extinction due to the emergence of a transmissible cancer known as devil facial tumour disease (DFTD). DFTD has spread over the distribution range of the species and has been responsible for a severe decline in the global devil population. To protect the Tasmanian devil from extinction in the wild, our group has focused on the development of a prophylactic vaccine. Although this work has shown that vaccine preparations using whole DFTD tumour cells supplemented with adjuvants can induce anti-DFTD immune responses, alternative strategies that induce stronger and more specific immune responses are required. In humans, heat shock proteins (HSPs) derived from tumour cells have been used instead of whole-tumour cell preparations as a source of antigens for cancer immunotherapy. As HSPs have not been studied in the Tasmanian devil, this study presents the first characterisation of HSPs in this marsupial and evaluates the suitability of these proteins as antigenic components for the enhancement of a DFTD vaccine. We show that tissues and cancer cells from the Tasmanian devil express constitutive and inducible HSP. Additionally, this study suggests that HSP derived from DFTD cancer cells are immunogenic supporting the future development of a HSP-based vaccine against DFTD.

The toll-like receptor ligands Hiltonol® (polyICLC) and imiquimod effectively activate antigen-specific immune responses in Tasmanian devils (Sarcophilus harrisii)

Devil facial tumour disease (DFTD) describes two genetically distinct transmissible tumours that pose a significant threat to the survival of the Tasmanian devil. A prophylactic vaccine could protect devils from DFTD transmission. For this vaccine to be effective, potent immune adjuvants will be required. Toll-like receptors (TLRs) promote robust immune responses in human cancer studies and are highly conserved across mammalian species. In this study, we investigated the proficiency of TLR ligands for immune activation in the Tasmanian devil using in vitro mononuclear cell stimulations and in vivo immunisation trials with a model antigen. We identified two such TLR ligands, polyICLC (Hiltonol^®) (TLR3) and imiquimod (TLR7), that in combination induced significant IFNγ production from Tasmanian devil lymphocytes in vitro. Immunisation with these ligands and the model antigen keyhole limpet haemocyanin activated robust antigen-specific primary, secondary and long-term memory IgG responses. Our results support the conserved nature of TLR signaling across mammalian species. PolyICLC and imiquimod will be trialed as immune adjuvants in future DFTD vaccine formulations.

Sequences and expression of pathway-specific complement components in developing red-tailed phascogale (Phascogale calura)

Marsupials are born immunologically premature, relying on cells and molecules in maternal milk for immune protection. Both immunoglobulin and complement proteins have been identified in marsupial milk, but the expression of specific complement proteins remains largely unexplored. We report partial cDNA sequences for two complement-activating proteins, C3, C1r, CFP and MASP2, in liver tissues from red-tailed phascogale (Phascogale calura). Conservation of functionally relevant motifs were identified in the translated cDNA sequences from phascogale C3, CFP and MASP2 and their eutherian homologues. Gene expression of representative molecules from each of the major complement pathways was also investigated in whole body tissues from 1 to 18 day old animals and liver tissues from 31-day to 14-month old animals. Average complement expression in whole bodies and liver tissues of C1r, CFP, MASP2 and C3 increased significantly in juveniles compared to pouch young, presumably due to the maturation of the young's own complement system. Comparing expression in liver tissues only, we found that the average CFP expression were higher in pouch young compared to juveniles, while results were still statistically similar to the average expression of all tissues for C1r, MASP2 and C3. The average complement expression then significantly decreased as the animals aged into adulthood.

Identification of the mRNA encoding interleukin-6 and its receptor, interleukin-6 receptor α, in five marsupial species

Expressed coding sequences for interleukin-6 (IL-6) and interleukin-6 receptor α (IL-6R) were examined in five marsupial species. Full length expressed coding sequences for IL-6 and IL-6R were identified and characterized in the gray short-tailed opossum (Monodelphis domestica). For IL-6, ∼225 bp fragments of the mRNA sequence were identified in the red-tailed phascogale (Phascogale calura), kultarr (Antechinomys laniger), and stripe-faced dunnart (Sminthopsis macroura), while ∼563 bp fragments of mRNA encoding IL-6R were identified in the red-tailed phascogale, kultarr, stripe-face dunnart and fat-tailed dunnart (Sminthopsis crassicaudata). Relative expression levels of IL-6 and IL-6R were examined in the heart, muscle, lung, liver, spleen and kidney of adult red-tailed phascogales, and IL-6 gene expression was found to be significantly higher in the lung and spleen than the other tissues examined, while the expression of IL-6R was significantly higher in the liver, lung and spleen. These results now serve as a reference point for examining the role and levels of IL-6 and IL-6R in the health and disease of these marsupial species. The pro-tumorigenic nature of IL-6 is of particular interest, and the identification of these IL-6 and IL-6R coding sequences provides a platform for further work to evaluate the potential role of IL-6 in marsupial cancers.

Rapid evolutionary response to a transmissible cancer in Tasmanian devils

Although cancer rarely acts as an infectious disease, a recently emerged transmissible cancer in Tasmanian devils (Sarcophilus harrisii) is virtually 100% fatal. Devil facial tumour disease (DFTD) has swept across nearly the entire species' range, resulting in localized declines exceeding 90% and an overall species decline of more than 80% in less than 20 years. Despite epidemiological models that predict extinction, populations in long-diseased sites persist. Here we report rare genomic evidence of a rapid, parallel evolutionary response to strong selection imposed by a wildlife disease. We identify two genomic regions that contain genes related to immune function or cancer risk in humans that exhibit concordant signatures of selection across three populations. DFTD spreads between hosts by suppressing and evading the immune system, and our results suggest that hosts are evolving immune-modulated resistance that could aid in species persistence in the face of this devastating disease.

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.

Toll-like receptor signaling is functional in immune cells of the endangered Tasmanian devil

Devil facial tumour disease (DFTD) is a fatally transmissible cancer that threatens the Tasmanian devil population. As Tasmanian devils do not produce an immune response against DFTD cells, an effective vaccine will require a strong adjuvant. Activation of innate immune system cells through toll-like receptors (TLRs) could provide this stimulation. It is unknown whether marsupials, including Tasmanian devils, express functional TLRs. We isolated RNA from peripheral blood mononuclear cells and, with PCR, detected transcripts for TLRs 2, 3, 4, 5, 6, 7, 8, 9, 10 and 13. Stimulation of the mononuclear cells with agonists to these TLRs increased the expression of downstream TLR signaling products (IL1α, IL6, IL12A and IFNβ). Our data provide the first evidence that TLR signaling is functional in the mononuclear cells of the Tasmanian devil. Future DFTD vaccination trials will incorporate TLR agonists to enhance the immune response against DFTD.

Characterisation of non-classical MHC class I genes in the Tasmanian devil (Sarcophilus harrisii)

The Tasmanian devil (Sarcophilus harrisii) is a carnivorous marsupial that is under threat of extinction due to an unusual transmissible disease called Devil Facial Tumour Disease (DFTD). Previous studies on the classical MHC genes have provided important insights into immune responses in this endangered species; however, so far, very little is known about the non-classical MHC genes of this species, which can also play significant roles in the immune system. Here, we report characterisation of five non-classical class I genes in the Tasmanian devil, including Saha-UD, -UK, -UM, -MR1 and -CD1. Saha-UD has been isolated previously and is known to have low genetic polymorphism, though its categorisation as classical or non-classical gene has remained undetermined. In this study, we observed tissue-specific expression of Saha-UD, suggesting that it is more characteristic of a non-classical gene. Restricted tissue expression patterns were also observed for other genes, with an exception of Saha-MR1 being ubiquitously expressed in all examined tissues. Saha-UK, -UM and -MR1 were found to be genetically monomorphic, while four alleles were found at Saha-CD1 with signs of positive selection detected within the α1 domain. Among the four Saha-CD1 alleles, one predominant allele (Saha-CD1*01) showed a high allele frequency of 0.906 in the Tasmanian devil population, resulting in a low heterozygosity (0.188) at this locus. Alternative splicing takes place in Saha-CD1, giving rise to a full-length transcript and a splice variant lacking intact antigen-binding, β2m-binding, transmembrane and cytoplasmic domains.

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.

Reduced effect of Tasmanian devil facial tumor disease at the disease front

Pathogen-driven declines in animal populations are increasingly regarded as a major conservation issue. The Tasmanian devil (Sarcophilus harrisii) is threatened with extinction by devil facial tumor disease, a unique transmissible cancer. The disease is transmitted through direct transfer of tumor cells, which is possible because the genetic diversity of Tasmanian devils is low, particularly in the major histocompatibility complex genes of the immune system. The far northwest of Tasmania now holds the last remaining disease-free wild devil populations. The recent discovery of unique major histocompatibility complex genotypes in the northwestern region of Tasmania has raised the possibility that some animals may be resilient to the disease. We examined the differences in the epidemiology and population effects of devil facial tumor disease at 3 well-studied affected sites in eastern Tasmania and 1 in western Tasmania (West Pencil Pine). In contrast to the 3 eastern sites, there has been no rapid increase in disease prevalence or evidence of population decline at West Pencil Pine. Moreover, this is the only onsite at which the population age structure has remained unaltered 4 years after the first detection of disease. The most plausible explanations for the substantial differences in population effects and epidemiology of the disease between eastern and western sites are geographic differences in genotypes or phenotypes of devils and functional differences between tumor strains in the 2 regions. We suggest that conservation efforts focus on identifying whether either or both these explanations are correct and then, if resistance alleles exist, to attempt to spread the resistant alleles into affected populations. Such assisted selection has rarely been attempted for the management of wildlife diseases, but it may be widely applicable.

Natural killer cell mediated cytotoxic responses in the Tasmanian devil

The Tasmanian devil (Sarcophilus harrisii), the world's largest marsupial carnivore, is under threat of extinction following the emergence of an infectious cancer. Devil facial tumour disease (DFTD) is spread between Tasmanian devils during biting. The disease is consistently fatal and devils succumb without developing a protective immune response. The aim of this study was to determine if Tasmanian devils were capable of forming cytotoxic antitumour responses and develop antibodies against DFTD cells and foreign tumour cells. The two Tasmanian devils immunised with irradiated DFTD cells did not form cytotoxic or humoral responses against DFTD cells, even after multiple immunisations. However, following immunisation with xenogenic K562 cells, devils did produce cytotoxic responses and antibodies against this foreign tumour cell line. The cytotoxicity appeared to occur through the activity of natural killer (NK) cells in an antibody dependent manner. Classical NK cell responses, such as innate killing of DFTD and foreign cancer cells, were not observed. Cells with an NK-like phenotype comprised approximately 4 percent of peripheral blood mononuclear cells. The results of this study suggest that Tasmanian devils have NK cells with functional cytotoxic pathways. Although devil NK cells do not directly recognise DFTD cancer cells, the development of antibody dependent cell-mediated cytotoxicity presents a potential pathway to induce cytotoxic responses against the disease. These findings have positive implications for future DFTD vaccine research.

Allorecognition in the Tasmanian devil (Sarcophilus harrisii), an endangered marsupial species with limited genetic diversity

Tasmanian devils (Sarcophilus harrisii) are on the verge of extinction due to a transmissible cancer, devil facial tumour disease (DFTD). This tumour is an allograft that is transmitted between individuals without immune recognition of the tumour cells. The mechanism to explain this lack of immune recognition and acceptance is not well understood. It has been hypothesized that lack of genetic diversity at the Major Histocompatibility Complex (MHC) allowed the tumour cells to grow in genetically similar hosts without evoking an immune response to alloantigens. We conducted mixed lymphocyte reactions and skin grafts to measure functional MHC diversity in the Tasmanian devil population. The limited MHC diversity was sufficient to produce measurable mixed lymphocyte reactions. There was a wide range of responses, from low or no reaction to relatively strong responses. The highest responses occurred when lymphocytes from devils from the east of Tasmania were mixed with lymphocytes from devils from the west of Tasmania. All of the five successful skin allografts were rejected within 14 days after surgery, even though little or no MHC I and II mismatches were found. Extensive T-cell infiltration characterised the immune rejection. We conclude that Tasmanian devils are capable of allogeneic rejection. Consequently, a lack of functional allorecognition mechanisms in the devil population does not explain the transmission of a contagious cancer.

MHC gene copy number variation in Tasmanian devils: implications for the spread of a contagious cancer

Tasmanian devils face extinction owing to the emergence of a contagious cancer. Devil facial tumour disease (DFTD) is a clonal cancer spread owing to a lack of major histocompatibility complex (MHC) barriers in Tasmanian devil populations. We present a comprehensive screen of MHC diversity in devils and identify 25 MHC types and 53 novel sequences, but conclude that overall levels of MHC diversity at the sequence level are low. The majority of MHC Class I variation can be explained by allelic copy number variation with two to seven sequence variants identified per individual. MHC sequences are divided into two distinct groups based on sequence similarity. DFTD cells and most devils have sequences from both groups. Twenty per cent of individuals have a restricted MHC repertoire and contain only group I or only group II sequences. Counterintuitively, we postulate that the immune system of individuals with a restricted MHC repertoire may recognize foreign MHC antigens on the surface of the DFTD cell. The implication of these results for management of DFTD and this endangered species are discussed.

The humoral immune response of the Tasmanian devil (Sarcophilus harrisii) against horse red blood cells

The Tasmanian devil (Sarcophilus harrisii) is under threat of extinction due to a fatal infectious neoplastic disease, named Devil Facial Tumour Disease. Tumours are transferred as allografts between animals and no effective immune response or host resistance to the disease has been detected, raising interest in the immune function of the species. To investigate whether Tasmanian devils had a competent humoral immune response, four devils were immunised with horse red blood cells (HRBC) either intraperitoneally or subcutaneously. Antibody responses were measured by direct and indirect haemagglutination assays for a period of up to 40 weeks. Primary responses were well defined, but secondary responses were prominent only in the devils immunised subcutaneously. All devils showed evidence for a memory antibody response following a booster given 32 weeks after the first injection and this was more evident with the subcutaneous route. Tasmanian devils tested were capable of mounting a humoral immune response against HRBC and the subcutaneous injection in the presence of the adjuvant Montanide was a safe and effective route.

Assessment of cellular immune responses of healthy and diseased Tasmanian devils (Sarcophilus harrisii)

The Tasmanian devil (TD) (Sarcophilus harrisii) is under threat from devil facial tumour disease (DFTD), a cancer that is transmitted between devils by direct cell implantation. As no devil is known to have rejected the tumour allograft, an understanding of the immune status of this species is essential to help explain the unique infectious nature of this cancer. We analysed differential white blood cell counts, the phagocytic response of neutrophils as well as mitogen-induced lymphocyte proliferation. Devils analysed included healthy TDs kept in captivity, healthy devils from disease-free and diseased areas as well as diseased devils. Neutrophils isolated from the peripheral blood of healthy devils showed competent phagocytosis and peripheral blood mononuclear cells from healthy and diseased devils proliferated in response to Con A, PHA and PWM stimulation. Although a wide range of responses was observed and relatively high doses of mitogens were required, there was no significant difference between males and females, adults and juveniles or between normal and diseased animals, suggesting that transmission of DFTD is not a consequence of a severely impaired immune system. As lymphocytes from all TDs appear to require strong stimulation for activation, this threshold may contribute to all devils being susceptible to DFTD.

Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial

A fatal transmissible tumor spread between individuals by biting has emerged in the Tasmanian devil (Sarcophilus harrisii), a carnivorous marsupial. Here we provide genetic evidence establishing that the tumor is clonal and therefore foreign to host devils. Thus, the disease is highly unusual because it is not just a tumor but also a tissue graft, passed between individuals without invoking an immune response. The MHC plays a key role in immune responses to both tumors and grafts. The most common mechanism of immune evasion by tumors is down-regulation of classical cell surface MHC molecules. Here we show that this mode of immune escape does not occur. However, because the tumor is a graft, it should still be recognized and rejected by the host's immune system due to foreign cell surface antigens. Mixed lymphocyte responses showed a lack of alloreactivity between lymphocytes of different individuals in the affected population, indicating a paucity of MHC diversity. This result was verified by genotyping, providing a conclusive link between a loss of MHC diversity and spread of a disease through a wild population. This novel disease arose as a direct result of loss of genetic diversity and the aggressive behavior of the host species. The neoplastic clone continues to spread although the population, and, without active disease control by removal of affected animals and the isolation of disease-free animals, the Tasmanian devil faces extinction.

Characterization of major histocompatibility complex class I and class II genes from the Tasmanian devil (Sarcophilus harrisii)

The Tasmanian devil (Sarcophilus harrisii) is currently threatened by an emerging wildlife disease, devil facial tumour disease. The disease is decreasing devil numbers dramatically and may lead to the extinction of the species. At present, nothing is known about the immune genes or basic immunology of the devil. In this study, we report the construction of the first genetic library for the Tasmanian devil, a spleen cDNA library, and the isolation of full-length MHC Class I and Class II genes. We describe six unique Class II beta chain sequences from at least three loci, which belong to the marsupial Class II DA gene family. We have isolated 13 unique devil Class I sequences, representing at least seven Class I loci, two of which are most likely non-classical genes. The MHC Class I sequences from the devil have little heterogeneity, indicating recent divergence. The MHC genes described here are most likely involved in antigen presentation and are an important first step for studying MHC diversity and immune response in the devil.

A unique T cell receptor discovered in marsupials

T cells recognize antigens by using T cell receptors (TCRs) encoded by gene segments, called variable (V), diversity (D), and joining (J), that undergo somatic recombination to create diverse binding specificities. Four TCR chains (alpha, beta, gamma, and delta) have been identified to date, and, as T cells develop in the thymus, they express exclusively either an alphabetaTCR or a gammadeltaTCR heterodimer. Here, we show that marsupials have an additional TCR (TCRmu) that has V, D, and J that are either somatically recombined, as in conventional TCRs, or are already prejoined in the germ-line DNA in a manner consistent with their creation by retrotransposition. TCRmu does not have a known homolog in eutherian mammals but has features analogous to a recently described TCRdelta isoform in sharks. TCRmu is expressed in at least two mRNA isoforms that appear capable of encoding a full-length protein, both of which are transcribed in the thymus and spleen. One contains two variable domains: a somatically recombined V and a prejoined V. This appears to be the dominant isoform in peripheral lymphoid tissue. The other isoform contains only the prejoined V and is structurally more similar to conventional TCR chains, however invariant. Unlike other TCRs, TCRmu uses prejoined gene segments and is likely present in all marsupials. Its similarity to a TCR isoform in sharks suggests that it, or something similar, may be present in other vertebrate lineages and, therefore, may represent an ancient receptor system.

A unique T cell receptor discovered in marsupials

T cells recognize antigens by using T cell receptors (TCRs) encoded by gene segments, called variable (V), diversity (D), and joining (J), that undergo somatic recombination to create diverse binding specificities. Four TCR chains (alpha, beta, gamma, and delta) have been identified to date, and, as T cells develop in the thymus, they express exclusively either an alphabetaTCR or a gammadeltaTCR heterodimer. Here, we show that marsupials have an additional TCR (TCRmu) that has V, D, and J that are either somatically recombined, as in conventional TCRs, or are already prejoined in the germ-line DNA in a manner consistent with their creation by retrotransposition. TCRmu does not have a known homolog in eutherian mammals but has features analogous to a recently described TCRdelta isoform in sharks. TCRmu is expressed in at least two mRNA isoforms that appear capable of encoding a full-length protein, both of which are transcribed in the thymus and spleen. One contains two variable domains: a somatically recombined V and a prejoined V. This appears to be the dominant isoform in peripheral lymphoid tissue. The other isoform contains only the prejoined V and is structurally more similar to conventional TCR chains, however invariant. Unlike other TCRs, TCRmu uses prejoined gene segments and is likely present in all marsupials. Its similarity to a TCR isoform in sharks suggests that it, or something similar, may be present in other vertebrate lineages and, therefore, may represent an ancient receptor system.

Evolution of mammalian CD1: marsupial CD1 is not orthologous to the eutherian isoforms and is a pseudogene in the opossum Monodelphis domestica

CD1 is a member of the major histocompatibility complex (MHC) class I family of proteins that present lipid antigens to T cells and natural killer (NK) T cells; it is found in both eutherian mammals and birds. In eutherians, duplication of the CD1 gene has resulted in multiple isoforms. A marsupial CD1 homologue was identified in a set of expressed sequence tags from the thymus of the bandicoot Isoodon macrourus. Southern blot and genomic sequence analyses revealed that CD1 is a single copy gene in both I. macrourus and a distantly related marsupial, the opossum Monodelphis domestica, which is currently the only marsupial species for which a whole genome sequence is available. We found that the opossum CD1 is located in a genomic region with a high degree of conserved synteny to the chromosomal regions containing human and mouse CD1. A phylogenetic analysis of mammalian CD1 revealed that marsupial CD1 is not orthologous to the eutherian CD1 isoforms, consistent with the latter having emerged by duplication after the separation of marsupials and eutherians 170-180 million years ago. The I. macrourus CD1 gene is actively transcribed and appears to encode a functional protein. In contrast, transcription of the M. domestica CD1 was not detected in any tissue and the predicted CD1 gene sequence contains a number of deletions that appear to render the locus a pseudogene.

The immune tissues of the endangered red-tailed phascogale (Phascogale calura)

The lymphoid tissues of the red-tailed phascogale (Phascogale calura) were examined using histological and immunohistochemical techniques. The distribution of immune cells in the tissue beds was documented using antibodies to surface markers CD3 and an MHC Class II antigen (equivalent to HLA DRII). Spleen, gut-associated lymphoid tissues (GALT), lung, bronchus-associated lymphoid tissue (BALT) and liver were examined. The spleen had defined areas of red and white pulp, with follicles containing tingible-bodied macrophages. Anti-CD3 and anti-HLA DRII antibodies revealed the presence of T cells in areas of white pulp and around the peri-arterial lymphatic sheaths. GALT and BALT were detected and appeared as scattered areas of lymphocytes in the tissues beds. This is the first study to report on the lymphoid tissues of this endangered species of marsupial and the first report of the capacity of anti-human antibodies to a surface MHC molecule to react with Dasyurid cells.

Allograft theory: transmission of devil facial-tumour disease

The Tasmanian devil, a large carnivorous Australian marsupial, is under threat from a widespread fatal disease in which a malignant oral-facial tumour obstructs the animal's ability to feed. Here we show that the chromosomes in these tumours have undergone a complex rearrangement that is identical for every animal studied. In light of this remarkable finding and of the known fighting behaviour of the devils, we propose that the disease is transmitted by allograft, whereby an infectious cell line is passed directly between the animals through bites they inflict on one another.

Reconstructing an ancestral mammalian immune supercomplex from a marsupial major histocompatibility complex

The first sequenced marsupial genome promises to reveal unparalleled insights into mammalian evolution. We have used the Monodelphis domestica (gray short-tailed opossum) sequence to construct the first map of a marsupial major histocompatibility complex (MHC). The MHC is the most gene-dense region of the mammalian genome and is critical to immunity and reproductive success. The marsupial MHC bridges the phylogenetic gap between the complex MHC of eutherian mammals and the minimal essential MHC of birds. Here we show that the opossum MHC is gene dense and complex, as in humans, but shares more organizational features with non-mammals. The Class I genes have amplified within the Class II region, resulting in a unique Class I/II region. We present a model of the organization of the MHC in ancestral mammals and its elaboration during mammalian evolution. The opossum genome, together with other extant genomes, reveals the existence of an ancestral "immune supercomplex" that contained genes of both types of natural killer receptors together with antigen processing genes and MHC genes.

Assessment of anti-bovine IL4 and IFN gamma antibodies to label IL4 and IFN gamma in lymphocytes of the koala and brushtail possum

We assess anti-bovine IL4 and IFN gamma (IFNg) antibodies for their ability to label IL4 and IFNg in koala (Phascolarctos cinereus), common brushtail possum (Trichosurus vulpecula) and mountain brushtail possum (Trichosurus caninus) lymphocytes using flow cytometry and immunohistochemistry to determine their applicability to studies of host response to intracellular pathogens. Anti-IFNg labelled a product of PMA-ionomycin stimulated sheep, koala and possum lymphocytes. High intensity labelling was not reduced by blocking non-specific binding with 10% FCS; and non-permeabilised koala lymphocytes labelled less, demonstrating that the labelled product was intracellular. The anti-IL4 antibody labelled variably more cells than the irrelevant antibody in some stimulated and non-stimulated preparations in all species but intensity of this labelling was similar to that of cells labelled with the irrelevant antibody. In this study, the antibodies did not label frozen or formalin-fixed tissues in a range of immunohistochemical techniques. We expect the anti-IFNg antibody to be effective in evaluating Th1 responses of koalas and possums exposed to various host, pathogen and environmental factors and add to the limited tools available for investigating the pathogenesis of marsupial diseases, especially those caused by intracellular organisms, such as tuberculosis of brushtail possums and chlamydial disease of koalas.

The appearance and distribution of mature T and B cells in the developing immune tissues of the stripe-faced dunnart (Sminthopsis macroura)

This paper describes the initial appearance and distribution of mature T and B cells in the developing immune tissues of the stripe-faced dunnart (Sminthopsis macroura) based on the use of species cross-reactive antibodies to the lymphocyte cell surface markers CD3, CD5 and CD79b. At birth no mature T or B cells were detected in the liver or bone marrow using anti-CD3, anti-CD5 or anti-CD79b antibodies. T cells were detected in the thymus with anti-CD3 by day 12 and anti-CD5 by day 50 postpartum, and T cells in the spleen were detected by day 43 and day 80 postpartum using anti-CD3 and anti-CD5, respectively. B cells were observed in the dunnart spleen by 43 days after birth. CD3- and CD79b-positive cells were detected in the lymph nodes by 50 days and CD5 by day 15 after birth, and in the gut-associated lymphoid tissues by day 50 and anti-CD5 by day 57 postpartum. The development and distribution of T and B cells in the immune tissues of dunnart pouch young is similar to that described in other marsupial species. Low numbers or absence of mature lymphocytes in immune tissues of early pouch young dunnarts further support the proposition that young marsupials are reliant on non-specific defence strategies and/or maternal strategies for a significant period of their time of development in the pouch.

Development of Lymphoid Tissues of the Stripe-Faced Dunnart (Sminthopsis macroura)

The development of the lymphoid tissues of a model marsupial, the stripe-faced dunnart, has been described from birth to weaning, a period of 2.5 months. At birth the lymphoid tissues, including the thymus, lymph nodes and mucosa-associated lymphoid tissues, were undeveloped. A thoracic thymus consisting primarily of stromal tissue was observed by day 4 after birth but by day 12, lymphocytes were observed in the thymus and some cortico-medullary differentiation was apparent. Lymph nodes were histologically mature by day 31, the earliest day investigated for this tissue. In gut tissue, lymphoid follicles were first observed by day 57 post-partum. No bronchus-associated lymphoid tissue was observed in any lung samples. The thymus, lymph nodes and gut-associated lymphoid tissues were all distinguishable before weaning (day 70) but not all were histologically mature. The sequence of development of the lymphoid tissues in the stripe-faced dunnart was similar to those observed in other marsupial species.

Primary structure and variation of the T-cell receptor delta-chain from a marsupial, Macropus eugenii

Although gammadelta T-cells form only a small portion of circulating T-cells in mice and humans, they are more frequent in many other types of mammals and this has lead to speculation regarding their roles and the evolutionary significance of their relative abundance. Moreover, whilst clear homologues of four types of T-cell receptor (TCR) chains (alpha, beta, delta and gamma) have been identified in vertebrates as distantly related as eutherian mammals and cartilaginous fish, there are still many gaps in our knowledge of these TCR components from various taxa. Such knowledge would further illuminate the evolution and function of these receptors and of gammadelta T-cells. Here, we report the molecular cloning of a TCR-delta chain cDNA from the tammar wallaby (Macropus eugenii) which represents the first component of the gammadelta TCR to be characterised from a marsupial. A PCR-based survey of variable (V) segment usage in tammar wallaby mammary-associated lymph node indicated that, although gammadelta T-cells may be sparse in this type of tissue, this species has at least three subfamilies of V genes that have been broadly conserved across vertebrate evolution. Two V subfamilies found in the tammar wallaby were relatively similar and may have diverged more recently, an event that probably occurred at some point in the marsupial lineage.

Cellular response in the dermis of common wombats (Vombatus ursinus) infected with Sarcoptes scabiei var. wombati

The cellular response in the dermis of common wombats (Vombatus ursinus) with sarcoptic mange exhibited some typical aspects of an immune response to Sarcoptes scabiei. There was an induction phase for wombats experimentally infected with S. scabiei represented by absence of a dermal inflammatory infiltrate for at least 12 days after infection. T lymphocytes, plasma cells, mast cells, and neutrophils then entered the dermis, consistent with a type IV (delayed) hypersensitivity response. In free-living wombats with severe parakeratotic sarcoptic mange eosinophils were also present in the dermis suggesting that a type I (immediate) hypersensitivity response may develop after a type IV hypersensitivity response. Absence of plasma cells and B lymphocytes in free-living wombats with severe parakeratotic sarcoptic mange compared with their presence in wombats experimentally infected with S. scabiei suggested that some immune tolerance may develop with severe infections. A large proportion of cells in the dermal response were not identified but were possibly cells of connective tissue. The thickness of the epidermis increased within 4 days in response to S. scabiei infection. Some antibodies raised against human leucocyte antigens CD3, CD5, HLA-DP, DQ, DR, and CD79b cross-reacted with leucocyte antigens of common wombats and were used to identify cell types in inflammatory infiltrates using immunohistochemistry.

The gut-associated lymphoid tissues of the northern brown bandicoot (Isoodon macrourus)

The gut associated lymphoid tissues (GALT) of a juvenile bandicoot has been examined using histological and immunohistochemical techniques. The mesenteric lymph nodes were hyperfollicular and had well defined paracortical and medullary areas. Lymphocytes were densely packed throughout the cortex and paracortex and the mantles of the follicles. The GALT contained two distinct areas of tissue organisation. One consisted of large areas of aggregated follicles, whilst the other consisted of more linearly distributed follicles. The distribution of T and B cells in the tissue beds was documented using antibodies to surface markers CD3, CD5 and CD79b. T-cells were present in high numbers in the cortical region of the lymph node, whilst B-cells were predominant in the mantle of the follicles. Dispersed CD3 positive T-cells were abundant in the villi lacteals and present in high numbers in follicular areas of gut. CD79b positive B-cells were not observed in the lacteals but were abundant in the mantles of follicles. This is similar to that observed in other metatherians.

Neutralizing antirabies antibodies in urban terrestrial wildlife in Brazil

The prevalence of rabies neutralizing antibodies (NA) in sera of wild animals from São Paulo City (Brazil) was investigated using the Rapid Fluorescent Focus Inhibition Test between 1994 and 1997. Sera from 547 specimens were examined. Marsupials represented 45% of the sample and primates 37%; carnivores, rodents, deer and edentates represented 6, 6, 3 and 2%, respectively. The overall prevalence of NA was 14%. The prevalence of NA was 18% in primates; whereas in marsupials, carnivores, edentates and rodents it was 13, 9, 8 and 6%, respectively. The stratification according to sex, age, and site of capture of the marsupials and primates showed a small predominance in males versus females and a large predominance of adults versus juveniles. The same relationship was seen in specimens captured near human habitations versus specimens captured in their own habitat. It is evident that there is circulation of rabies virus in wild animals, which are not recommended as pets since they represent a potential risk of exposure to rabies virus for both humans and domestic animals.

Characterisation of the kappa light chain of the brushtail possum (Trichosurus vulpecula)

Two full length cDNA sequences encoding the kappa light chain of the Australian marsupial, Trichosurus vulpecula, the brushtail possum, were isolated from a mesenteric lymph node cDNA library. The constant regions (Ckappa) of the two light chains were identical, but the variable (Vkappa) and joining (Jkappa) regions were different. At the amino acid level, possum Ckappa was most similar to Ckappa of an American marsupial, Monodelphis domestica (75%), with similarity to eutherian Ckappa ranging from 47 to 63%. The availability of molecular data will enable the development of immunological reagents for studying immune responses and disease in marsupials, thereby aiding conservation strategies and veterinary medicine.

Immune system cell markers in the northern brown bandicoot, Isoodon macrourus

This is the first study to report the presence of T and B lymphocyte markers and antigen presenting-like molecules in a marsupial bandicoot. Intra-cytoplasmic markers for CD3 and CD5, as well as surface Thy-1.1 and CD1a molecules were located in lymphocytes of T dependent regions of immuno-lymphoid tissue in the northern brown bandicoot using immunohistochemical techniques. Similarly, intra-cytoplasmic domains of CD79a, CD79b molecules and surface IgG molecules enabled characterisation of B lymphocytes and plasma cells. The phenotypic expression of these molecules parallels findings in eutherians, suggesting firstly the conservation of lineage epitopes for T and B subsets and secondly, the potential for similar functional properties of immune system cells between marsupials and eutherians. In addition, the presence of MHC class II and CD1a molecules on dendritic-like cells may indicate similar mechanisms for antigen processing and presentation as reported in eutherians. The use of such immune system cell markers will enable functional studies to characterise the marsupial immune system as well as ontogeny studies of immune competence.

Immunoglobulin genetics of marsupials

Until recently, studies of marsupial immunoglobulins were limited to primarily protein analyses, such as Protein A binding and immunological cross-reactivity to eutherian immunoglobulins to draw conclusions about the isotypes present in metatherians. This left an interesting gap in our knowledge of the evolution of vertebrate, more specifically mammalian, antibodies and provided little insight into the diversity of marsupial antibodies. Recently, however, there has been a flurry of papers from multiple laboratories describing, at the molecular level, the heavy and light chain classes present in marsupials with some analysis of the expressed repertoires. These studies have provided the evidence to determine when some of the uniquely mammalian isotypes, e.g. IgG and IgE, appeared in evolution, and are a first look at the complexity of heavy and light chain variable regions in a metatherian. Here we review what was known prior to the cloning of marsupial Ig genes and what we have learned recently.

Marsupial cytokines. Structure, function and evolution

The cytokines are an important group of molecules involved in coordinating the many and varied components of the immune system. These molecules have been extensively studied in model eutherian mammals such as mice but comparatively little is known about the cytokine network of marsupials. Such information will be invaluable in elucidating fundamental aspects of the marsupial immune system and will also highlight parallels and differences between the immune systems of marsupials and eutherians. Given the importance of these goals, our groups have recently begun to tackle this lack of knowledge of the marsupial cytokine system and have met with considerable success in the face of the rapid rate of change of these proteins. This has led to the isolation of the full-length sequences encoding marsupial orthologues of tumour necrosis factor (TNF), lymphotoxins alpha and beta (LT-alpha and beta), interleukin-1 beta (IL-1beta), and interleukin-10 (IL-10). Here we review what has been learnt about structural, functional and evolutionary aspects of these marsupial cytokines as well as briefly describing more recent work in progress and future directions in this field.

Immunobiology of mycobacterial infections in marsupials

Mycobacterial infections of marsupials are important for two reasons. Firstly, the Australian brushtail possum (Trichosurus vulpecula) serves as the major wildlife reservoir for Mycobacterium bovis in New Zealand and secondly, M. avium is a significant cause of disease in endangered marsupial species held in captivity. Marsupials are highly susceptible to specific mycobacterial infections which may be linked to deficiencies in their cellular immunity. Histopathological inspection of affected tissues indicates that, unlike most eutherians, marsupials are unable to wall off infection sites, resulting in formation of satellite lesions and generalised disease. This review examines possible reasons for the high susceptibility of marsupials to mycobacterial infections and investigates the prospects for developing vaccines to control these diseases.

Development of the immune system and immunological protection in marsupial pouch young

At birth the tissues of marsupial immune system are underdeveloped. The young animal is not immunocompetent. Histological and immunohistochemical studies of pouch young epithelial tissues provide a clear picture of tissue development but the timing of onset of immunocompetence awaits definition. The survival of the neonatal marsupial in a microbially rich environment is dependent on maternal strategies, including immunoglobulin transfer via milk and, in some species, prenatally via the yolk sac placenta. It is also likely that pouch secretions play a role. This review summarizes our current knowledge of the pathway of immunological development in marsupials and the protection and threats afforded by the pouch environment.

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.

Isolation and sequence of a cDNA coding for the heavy chain constant region of IgG from the Australian brushtail possum, Trichosurus vulpecula

A brushtail possum (Trichosurus vulpecula) mesenteric lymph node cDNA library was screened with a South American short-tailed opossum (Monodlelphis domestica) immunoglobulin gamma heavy chain constant region (Cgamma) probe, resulting in the isolation of a 1518 nucleotide cDNA clone. The sequence corresponds to exons 1-3 of Cgamma. The Australian marsupial (T. vulpeculla) sequence is 70% identical at the amino acid level with the American marsupial (M. domestica) sequence, but less similar to the eutherian mammals (45-50%). These data provide the opportunity to compare the evolution of IgG between orders of marsupials separated by at least 75 million years and confirm the appearance of IgG prior to the metatherian/eutherian divergence.

Expression of the spermatid-specific Hsp70 antigen is conserved in mammals including marsupials

The anatomical location of testes in mammals ranges from a location close to that observed in the embryo to a lower position usually involving a pendant scrotum. In scrotal mammals, the abdominal position of the cryptorchid testis, which elevates its temperature, is detrimental to spermatogenesis and causes infertility. Spermatocytes are sensitive but late spermatids are relatively resistant to thermal stress suggesting that the latter might be protected in some way. In general, most organisms express Hsp70 proteins, which play a crucial role in the protection of cells against thermal stress. We have found previously that the Hsc70t protein, a member of the Hsp70 family of proteins, is constitutively expressed in the late spermatids of mice. Here, we have utilized immunohistochemistry with anti-mouse Hsc70t antiserum to examine the expression of the spermatid-specific Hsp70 antigen in the testes of several mammalian species with different degrees of testes migration. Our data indicate that the antigen is conserved in the mammals including marsupials. We also examined whether antigens of Hsp70-related proteins were expressed in non-mammalian vertebrates including not only homoiothermal but also poikilothermal animals. The spermatid-specific Hsp70 antigens were not detectable in the testes of the animals examined. From results of immunohistochemistry with BRM22 monoclonal antibody which reacts broadly with Hsp70 family proteins, however, we revealed constitutive expression of antigens of Hsp70-related proteins in spermatogenic cells of the vertebrates. These results suggest that the expression of spermatid-specific Hsp70 protein may be involved in the developmental pathway during spermiogenesis in mammals rather than in thermotolerance.

Serum antibody response to Cryptococcus neoformans in cats, dogs and koalas with and without active infection

Anti-cryptococcal antibodies were measured in normal cats, dogs, horses and koalas, and cats, dogs and koalas with cryptococcosis using an enzyme immunoassay. Antibody levels were expressed as absorbance readings. Over 80% of cats and dogs with cryptococcal infection had elevated antibody levels at the time of diagnosis, during or after successful therapy. Antibody levels in these patients either remained elevated or declined slowly after treatment. For cats, anti-cryptococcal antibody levels were higher in C. neoformans var. gattii than var. neoformans infections, and lower in mild than in moderate or severe infections. The persistence of increased anti-cryptococcal antibody levels in over half of the feline and canine cases following active infection suggested the use of antibody determinations as a seroepidemiologic marker of previous infection. Consequently, antibody measurements from 'normal' animals indicated a prevalence of previous cryptococcal infection of 10% in cats and dogs, compared with 3% in horses and 5% in koalas. Preliminary studies of young animals suggested that anti-cryptococcal antibody levels were substantially lower in the young cats but not the young dogs surveyed, compared with their mature counterparts. The cut-offs used in the present work may thus be erroneously high, with a corresponding underestimation of the prevalence of inapparent cryptococcosis.

Characterisation of fibrous sheath and midpiece fibre network polypeptides of marsupial spermatozoa with a monoclonal antibody

In this study cytoskeletal antigens common to brushtail possum and tammar wallaby spermatozoa were characterised using a monoclonal antibody (PSA-10). Using indirect immunofluorescence, the PSA-10 antibody detected antigens predominantly associated with the midpiece and principal piece of mature, permeabilised marsupial spermatozoa. The principal piece determinant, shared by a variety of other species, was found to arise in the marsupial testis. Midpiece localisation of the PSA-10 epitope was detected only in marsupial spermatozoa and shown to arise in the epididymis. Immunogold labelling demonstrated that the PSA-10 antigens were predominantly associated with the fibrous sheath and midpiece fibre network of both possum and wallaby spermatozoa. Western blotting suggested that two major possum and wallaby sperm polypeptides of 158 and 182 kDa were associated with the midpiece fibre network, a cytoskeletal structure unique to marsupial spermatozoa. A 32 kDa polypeptide was associated with the principal piece fibre network and/or fibrous sheath. The finding that these marsupial sperm cytoskeletal proteins share a common linear epitope suggests that they share some sequence similarity. The midpiece fibre network of marsupial sperm, like the fibrous sheath, has been proposed to have a structural role in providing passive stiffening for the flagellum (Harding et al., 1975, 1979; Olsen, 1975). The PSA-10 monoclonal antibody may provide a tool for comparative studies of mammalian sperm cytoskeletal proteins, particularly the marsupial midpiece fibre network. It may also allow the formation of this unique marsupial cytoskeletal structure, and its fate during the fertilisation process, to be followed by immunological means.

The complement system of the marsupial Monodelphis domestica

The activation requirements and pathways of the serum C' system of the marsupial Monodelphis domestica were characterized using standard hemolytic procedures. The existence of distinct classical and alternative activation pathways was established on the basis of their ionic requirements, hemolytic capacity at different temperatures, kinetics of hemolysis, and differential susceptibility to the classical pathway inhibitor carrageenan. For the most part, the activities of these pathways were influenced by factors and conditions in a manner similar to the way they affect the activity of eutherian complement. These observations provide further support for the idea that Monodelphis domestica would serve as a useful model for comparative immunological studies in mammals.

Histological differences between gravid and non-gravid uteri in the dasyurid marsupial, Sminthopsis macroura (Spencer)

Uterine samples from pregnant Sminthopsis macroura representing the first 10 days of its 11 day gestation period and samples from non-pregnant animals were compared histologically and examined for differences in the following characteristics: thickness of the endometrial stroma, luminal epithelium, myometrium and glandular epithelium, and the density of stromal glands and number of lymphocytes at the endometrial basal lamina. A highly significant difference between gravid and non-gravid uteri with respect to thickness of the endometrial epithelium was found on day 3, when lineage divergence occurs between the pluriblast and trophoblast. The endometrial stroma was significantly thicker in pregnant animals on day 8, when the epiblast differentiates into ectoderm, endoderm and mesoderm. Other differences between gravid and non-gravid uteri were detected in myometrial thickness on days 1 and 5. Taken together, these results indicate that despite similar endocrinological profiles of pregnant and non-pregnant marsupials, there are subtle, but significant, differences in uterine histology. The observed concordance between histological differences and differentiative events in embryogenesis is considered is indicative of embryo-maternal signalling.