Natural cytotoxicity towards allogeneic tumour targets in Xenopus mediated by diverse splenocyte populations

Host-parasite interactions during a biological invasion: The fate of lungworms (Rhabdias spp.) inside native and novel anuran hosts

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Rhabdias hylae (frog) lungworms entered cane toads and migrated through the body but were not found in the target tissue, the lungs.

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Larvae of both lungworm species induced inflammation in both types of hosts.

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The immune response (relative numbers of different cell types) differed between hosts and between parasite species.

The cane toad invasion in Australia provides a robust opportunity to clarify the infection process in co-evolved versus de novo host–parasite interactions. We investigated these infection dynamics through histological examination following experimental infections of metamorphs of native frogs (Cyclorana australis) and cane toads (Rhinella marina) with Rhabdias hylae (the lungworm found in native frogs) and Rhabdias pseudosphaerocephala (the lungworm found in cane toads). Cane toads reared under continuous exposure to infective larvae of the frog lungworm were examined after periods of 2, 6, 10 and 15 days. Additionally, both toads and frogs were exposed for 24 h to larvae of either the toad or the frog lungworm, and examined 2, 5, 10 and 20 days post-treatment. R. hylae (frog) lungworms entered cane toads and migrated through the body but were not found in the target tissue, the lungs. Larvae of both lungworm species induced inflammation in both types of hosts, although the immune response (relative numbers of different cell types) differed between hosts and between parasite species. Co-evolution has modified the immune response elicited by infection and (perhaps for that reason) has enhanced the parasite's ability to survive and to reach the host's lungs.

Tumorigenesis and anti-tumor immune responses in Xenopus

Despite intense study, the role of the immune system in detecting (immunosurveillance), controlling and remodeling (immunoediting) neoplasia remains elusive. We present here a comparative view of the complex interactions between neoplasia and the host immune system. We provide evidence, in the amphibian Xenopus laevis, consistent with an evolutionarily conserved and crucial role of the immune system in controlling neoplasia, which involves a striking variety of anti-tumoral immune effectors including conventional CTLs, classical MHC class Ia unrestricted CTLs (CCU-CTLs) that interact with nonclassical MHC class Ib molecules, CD8 NKT-like cells and NK cells. We also review the tumors found in X. laevis with an emphasis on thymic lymphoid tumors and a rare ovarian dysgerminoma. Finally, we consider the use of X. laevis for in vivo study of tumorigenesis. Given our current knowledge, the experimental systems already established in X. laevis, and the rapid accumulation of genetic resources for the sister species Silurana (Xenopus) tropicalis, it is our conviction that these species provide an ideal alternative to the murine system for studying tumorigenesis and tumor immunity.

Avian NK activities, cells and receptors

Natural killer (NK) activity has been examined in birds for over 30 years, but evidence that avian NK activity plays crucial roles in disease is only suggestive. In chickens, NK activity is mediated by TCR0 cells in the intestinal epithelium, but elsewhere subsets of alphabeta and gammadelta T cells (NKT cells) may be more important. There are few lectin-like NK receptor genes, located in the genomic region syntenic with the natural killer complex (NKC) as well as the major histocompatibility complex (MHC). In contrast, a huge number of Ig-like receptor genes are located in a region syntenic with the leukocyte receptor complex (LRC).

Ontogeny of Xenopus NK cells in the absence of MHC class I antigens

This paper explores the ontogeny of NK cells in control and early-thymectomized (Tx) Xenopus laevis through phenotypic analysis of cells expressing the NK cell antigen 1F8 and by performing in vitro cytotoxic assays. Dual color flow cytometry reveals that a few 1F8positive splenocytes first emerge in late larval life, at approximately 7-weeks post-fertilization. This is about 2-weeks after the time when surface MHC class Ia expression can first be detected. The proportion of splenocytes expressing 1F8 remains very low in 3-4 month-old froglets, but by 1 year there is a sizeable 1F8positive population, which is proportionally elevated in Tx frogs. The ontogeny of NK cell function is monitored by a 5 h DNA fragmentation (JAM) assay. Control and Tx larval splenocytes (from either 5- or 7-week-old tadpoles) fail to kill MHC-deficient thymus-derived tumor cell targets. Such in vitro killing is still relatively poor in 3-4 month froglets, compared with high levels of tumor cell cytotoxicity mediated by splenocytes from older frogs. Immunoprecipitation studies identify that the major ligand for the 1F8 mAb is a 55 kDa polypeptide. Finally, further evidence is provided that 1F8positive lymphocytes are indeed bona fide NK cells, distinct from T cells, since purified 1F8positive splenocytes from Tx Xenopus fail to express fully rearranged TCRbeta V region transcripts. We conclude that NK cells fail to develop prior to MHC class I protein expression and, therefore, do not contribute to the larval immune system, whereas they do provide an important backup for T cells in the adult frog by contributing to anti-tumor immunity.

Larval antigen molecules recognized by adult immune cells of inbred Xenopus laevis: partial characterization and implication in metamorphosis

It has been shown that larval skin (LS) grafts are rejected by an inbred strain of adult Xenopus, which suggests a mechanism of metamorphosis by which larval cells are recognized and attacked by the newly differentiating immune system, including T lymphocytes. In an attempt to define the larval antigenic molecules that are targeted by the adult immune system, anti-LS antibodies (IgY) were produced by immunizing adult frogs with syngeneic LS grafts. The antigen molecules that reacted specifically with this anti-LS antiserum were localized only in the larval epidermal cells. Of 53 and 59-60 kDa acidic proteins that were reactive with anti-LS antibodies, a protein of 59 kDa and with an isoelectric point of 4.5 was selected for determination of a 19 amino acid sequence (larval peptide). The rat antiserum raised against this peptide was specifically reactive with the 59 kDa molecules of LS lysates. Immunofluorescence studies using these antisera revealed that the larval-specific molecules were localized in both the tail and trunk epidermis of premetamorphic larvae, but were reduced in the trunk regions during metamorphosis, and at the climax stage of metamorphosis were detected only in the regressing tail epidermis. Culture of splenocytes from LS-immunized adult frogs in the presence of larval peptide induced augmented proliferative responses. Cultures of larval tail pieces in T cell-enriched splenocytes from normal frogs or in natural killer (NK)-cell-enriched splenocytes from early thymectomized frogs both resulted in significant destruction of tail pieces. Tissue destruction in the latter was enhanced when anti-LS antiserum was added to the culture. These results indicate that degeneration of tail tissues during metamorphosis is induced by a mechanism such that the larval-specific antigen molecules expressed in the tail epidermis are recognized as foreign by the newly developing adult immune system, and destroyed by cytotoxic T lymphocytes and/or NK cells.

Minor histocompatibility antigen-specific MHC-restricted CD8 T cell responses elicited by heat shock proteins

In mammals, the heat shock proteins (HSP) gp96 and hsp70 elicit potent specific MHC class I-restricted CD8(+) T cell (CTL) response to exogenous peptides they chaperone. We show in this study that in the adult frog Xenopus, a species whose common ancestors with mammals date back 300 million years, both hsp70 and gp96 generate an adaptive specific cellular immune response against chaperoned minor histocompatibility antigenic peptides that effects an accelerated rejection of minor histocompatibility-locus disparate skin grafts in vivo and an MHC-specific CD8(+) cytotoxic T cell response in vitro. In naturally class I-deficient but immunocompetent Xenopus larvae, gp96 also generates an antitumor immune response that is independent of chaperoned peptides (i.e., gp96 purified from normal tissue also generates a significant antitumor response); this suggests a prominent contribution of an innate type of response in the absence of MHC class I Ags.

Amphibian declines: an immunological perspective

Many, but not all, amphibian populations have been declining on all six continents on which they live. Although habitat destruction, direct application of toxicants, and introduction of predators/competitors are obvious causes of amphibian declines, many amphibians are dying of infectious diseases in relatively pristine habitats on several continents. In this paper, we review the patterns of these disease outbreaks and the characteristics of amphibian immune systems. Hypotheses are presented to explain the apparent susceptibility of amphibians to these pathogens. Natural and man-made factors that can alter amphibian immune responses to pathogens are discussed. Additional research is needed on the biology of the specific pathogens, the pattern of immune responses they elicit, and the nature of environmental stressors that may increase susceptibility to infectious disease.

Evolution of immune surveillance and tumor immunity: studies in Xenopus

We have developed a novel experimental model of cancer immunity in the frog, Xenopus, which may provide a useful alternative to murine tumor models and a way to assess whether the control of tumor development is a fundamental function of the immune system of vertebrates. In Xenopus, tumor immunity can be studied in two developmentally distinct immune systems. The larval immune system reflects characteristics of an ancestral system that appears to function without classical MHC class I antigen presentation and an efficient effector mechanism. The adult system appears more highly evolved in that it is remarkably similar to that of mammals and is able to generate a potent antitumor response. This amphibian model also provides a unique system with which to investigate a postulated role of heat shock proteins as components of an ancestral system of antigen presentation and/or immune surveillance that predates the antigen presentation pathway that exclusively involves MHC molecules.

T-cell and natural killer cell development in thymectomized Xenopus

The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) cells. Loss of T-cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor beta transcripts in spleen, liver and intestine, indicate that T-cell development in minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T-cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non-T/non-B population and also two T-cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.

Metamorphosis and the amphibian immune system

Studies of the ontogeny of immunity in a limited number of representative amphibians have shown that while the immune systems of the larval forms are competent to defend against potential pathogens in the temporary ponds they inhabit, they are not equivalent to the mature immune systems that develop after metamorphosis. Metamorphosis is a critical time of transition when increased concentrations of metamorphic hormones, principally thyroid hormones (TH) and corticosteroid hormones (CH), orchestrate the loss or reorganization of many tissues and organ systems, including the immune system. Immune system reorganization may serve to eliminate unnecessary lymphocytes that could be destructive if they recognized newly emerging adult-specific antigens on the adult tissues. Increased corticosteroids during metamorphosis appear to induce apoptosis of susceptible lymphocytes. This cell death can be inhibited in vitro or in vivo by the corticosteroid receptor antagonist, RU486. A coordinate increase in both TH and CH at metamorphosis may be common to all amphibians that undergo metamorphosis. Current evidence suggests that the central hypothalamic mediator that induces pituitary production of both thyroid-stimulating hormone and adrenocorticotropic hormone in larval amphibians is corticotropin-releasing hormone. Most amphibians probably survive the temporary immunosuppression associated with metamorphosis with no deleterious effects. However, it is hypothesized that if environmental stressors result in the induction of metamorphosis at a less than optimal body size and state of immune maturation, the immune system destruction would be more significant, and the amphibians could be at greater risk of infection and death.