Tumorigenesis and anti-tumor immune responses in Xenopus

The amphibian's spleen as a source of biomarkers for ecotoxicity assessment: Historical review and trends

Amphibians are very sensitive to many environmental changes, so these animals are considered good bioindicator models for ecotoxicology. Given the importance of the amphibian spleen for hematopoietic and immune responses, this can be a key organ for the evaluation of biomarkers to monitor the health of individuals in nature or in captivity. In this systematic review, we searched databases and summarized the main findings concerning the amphibian spleen as a source of possible biomarkers applied in different scientific fields. The searches resulted in 83 articles published from 1923 to 2022, which applied the use of splenic samples to evaluate the effects of distinct stressors on amphibians. Articles were distributed in more than twenty countries, with USA, Europe, and Brazil, standing out among them. Publications focused mainly on anatomical and histomorphological characterization of the spleen, its physiology, and development. Recently, the use of splenic biomarkers in pathology and ecotoxicology began to grow but many gaps still need to be addressed in herpetological research. About 85 % of the splenic biomarkers showed responses to various stressors, which indicates that the spleen can provide numerous biomarkers to be used in many study fields. The limited amount of information on morphological description and splenic anatomy in amphibians may be a contributing factor to the underestimated use of splenic biomarkers in herpetological research around the world. We hope that this unprecedented review can instigate researchers to refine herpetological experimentation, using the spleen as a versatile and alternative source for biomarkers in ecotoxicology.

The amphibian invitrome: Past, present, and future contributions to our understanding of amphibian immunity

Many amphibian populations are declining worldwide, and infectious diseases are a leading cause. Given the eminent threat infectious diseases pose to amphibian populations, there is a need to understand the host-pathogen-environment interactions that govern amphibian susceptibility to disease and mortality events. However, using animals in research raises an ethical dilemma, which is magnified by the alarming rates at which many amphibian populations are declining. Thus, in vitro study systems such as cell lines represent valuable tools for furthering our understanding of amphibian immune systems. In this review, we curate a list of the amphibian cell lines established to date (the amphibian invitrome), highlight how research using amphibian cell lines has advanced our understanding of the amphibian immune system, anti-ranaviral defence mechanisms, and Batrachochytrium dendrobatidis replication in host cells, and offer our perspective on how future use of amphibian cell lines can advance the field of amphibian immunology.

Bistability in a model of tumor-immune system interactions with an oncolytic viral therapy

A mathematical model of tumor-immune system interactions with an oncolytic virus therapy for which the immune system plays a twofold role against cancer cells is derived. The immune cells can kill cancer cells but can also eliminate viruses from the therapy. In addition, immune cells can either be stimulated to proliferate or be impaired to reduce their growth by tumor cells. It is shown that if the tumor killing rate by immune cells is above a critical value, the tumor can be eradicated for all sizes, where the critical killing rate depends on whether the immune system is immunosuppressive or proliferative. For a reduced tumor killing rate with an immunosuppressive immune system, that bistability exists in a large parameter space follows from our numerical bifurcation study. Depending on the tumor size, the tumor can either be eradicated or be reduced to a size less than its carrying capacity. However, reducing the viral killing rate by immune cells always increases the effectiveness of the viral therapy. This reduction may be achieved by manipulating certain genes of viruses via genetic engineering or by chemical modification of viral coat proteins to avoid detection by the immune cells.

A Highly Complex, MHC-Linked, 350 Million-Year-Old Shark Nonclassical Class I Lineage

Cartilaginous fish, or Chondrichthyes, are the oldest extant vertebrates to possess the MHC and the Ig superfamily-based Ag receptors, the defining genes of the gnathostome adaptive immune system. In this work, we have identified a novel MHC lineage, UEA, a complex multigene nonclassical class I family found in sharks (division Selachii) but not detected in chimaeras (subclass Holocephali) or rays (division Batoidea). This new lineage is distantly related to the previously reported nonclassical class I lineage UCA, which appears to be present only in dogfish sharks (order Squaliformes). UEA lacks conservation of the nine invariant residues in the peptide (ligand)-binding regions (PBR) that bind to the N and C termini of bound peptide in most vertebrate classical class I proteins, which are replaced by relatively hydrophobic residues compared with the classical UAA. In fact, UEA and UCA proteins have the most hydrophobic-predicted PBR of all identified chondrichthyan class I molecules. UEA genes detected in the whale shark and bamboo shark genome projects are MHC linked. Consistent with UEA comprising a very large gene family, we detected weak expression in different tissues of the nurse shark via Northern blotting and RNA sequencing. UEA genes fall into three sublineages with unique characteristics in the PBR. UEA shares structural and genetic features with certain nonclassical class I genes in other vertebrates, such as the highly complex XNC nonclassical class I genes in Xenopus, and we anticipate that each shark gene, or at least each sublineage, will have a unique function, perhaps in bacterial defense.

A Glimpse of the Peptide Profile Presentation by Xenopus laevis MHC Class I: Crystal Structure of pXela-UAA Reveals a Distinct Peptide-Binding Groove

The African clawed frog, Xenopus laevis, is a model species for amphibians. Before metamorphosis, tadpoles do not efficiently express the single classical MHC class I (MHC-I) molecule Xela-UAA, but after metamorphosis, adults express this molecule in abundance. To elucidate the Ag-presenting mechanism of Xela-UAA, in this study, the Xela-UAA structure complex (pXela-UAAg) bound with a peptide from a synthetic random peptide library was determined. The amino acid homology between the Xela-UAA and MHC-I sequences of different species is <45%, and these differences are fully reflected in the three-dimensional structure of pXela-UAAg. Because of polymorphisms and interspecific differences in amino acid sequences, pXela-UAAg forms a distinct peptide-binding groove and presents a unique peptide profile. The most important feature of pXela-UAAg is the two-amino acid insertion in the α2-helical region, which forms a protrusion of ∼3.8 Å that is involved in TCR docking. Comparison of peptide-MHC-I complex (pMHC-I) structures showed that only four amino acids in β2-microglobulin that were bound to MHC-I are conserved in almost all jawed vertebrates, and the most unique feature in nonmammalian pMHC-I molecules is that the AB loop bound β2-microglobulin. Additionally, the binding distance between pMHC-I and CD8 molecules in nonmammals is different from that in mammals. These unique features of pXela-UAAg provide enhanced knowledge of T cell immunity and bridge the knowledge gap regarding the coevolutionary progression of the MHC-I complex from aquatic to terrestrial species.

The Role of Heritable Tumors in Evolution of Development: a New Theory of Carcino-evo-devo

The hypothesis of evolution by tumor neofunctionalization (the "main hypothesis") describes the possible role of hereditary tumors in evolution. The present article examines the relationship of the main hypothesis to other biological theories. As shown in this paper, the main hypothesis does not contradict to the existing biological theories, but fills the lacunas between them and explains some unexplained (or not completely understood) questions. Common features of embryonic development and tumorigenesis are described by several recognized theories. Similarities between normal development and tumorigenesis suggest that tumors could participate in the evolution of ontogenesis and in the origin of new cell types, tissues and organs. A wide spectrum of non-trivial explanations and non-trivial predictions in different fields of biology, suggested by the main hypothesis, is an indication of its fundamental nature and the potential to become a new biological theory, a theory of the role of tumors in evolution of development, or carcino-evo-devo.

Characteristics of in Vivo Model Systems for Ovarian Cancer Studies

An understanding of the molecular pathogenesis and heterogeneity of ovarian cancer holds promise for the development of early detection strategies and novel, efficient therapies. In this review, we discuss the advantages and limitations of animal models available for basic and preclinical studies. The fruit fly model is suitable mainly for basic research on cellular migration, invasiveness, adhesion, and the epithelial-to-mesenchymal transition. Higher-animal models allow to recapitulate the architecture and microenvironment of the tumor. We discuss a syngeneic mice model and the patient derived xenograft model (PDX), both useful for preclinical studies. Conditional knock-in and knock-out methodology allows to manipulate selected genes at a given time and in a certain tissue. Such models have built our knowledge about tumor-initiating genetic events and cell-of-origin of ovarian cancers; it has been shown that high-grade serous ovarian cancer may be initiated in both the ovarian surface and tubal epithelium. It is postulated that clawed frog models could be developed, enabling studies on tumor immunity and anticancer immune response. In laying hen, ovarian cancer develops spontaneously, which provides the opportunity to study the genetic, biochemical, and environmental risk factors, as well as tumor initiation, progression, and histological origin; this model can also be used for drug testing. The chick embryo chorioallantoic membrane is another attractive model and allows the study of drug response.

Impacts of the MHC class I-like XNC10 and innate-like T cells on tumor tolerance and rejection in the amphibian Xenopus

The conditions that lead to antitumor or protumor functions of natural killer T (NKT) cells against mammalian tumors are only partially understood. Therefore, insights into the evolutionary conservation of NKT and their analogs-innate-like T (iT) cells-may reveal factors that contribute to tumor eradication. As such, we investigated the amphibian Xenopus laevis iT cells and interacting MHC class I-like (XNC or mhc1b.L) genes against ff-2 thymic lymphoid tumors. Upon ff-2 intraperitoneal transplantation into syngeneic tadpoles, two iT cell subsets iVα6 and iVα22, characterized by an invariant T-cell receptor α chain rearrangement (Vα6-Jα1.43 and Vα22-Jα1.32 respectively), were recruited to the peritoneum, concomitant with a decreased level of these transcripts in the spleen and thymus. To address the hypothesize that different iT cell subsets have distinct, possibly opposing, roles upon ff-2 tumor challenge, we determined whether ff-2 tumor growth could be manipulated by impairing Vα6 iT cells or by deleting their restricting element, the XNC gene, XNC10 (mhc1b10.1.L), on ff-2 tumors. Accordingly, the in vivo depletion of Vα6 iT cells using XNC10-tetramers enhanced tumor growth, indicating Vα6 iT cell-mediated antitumor activities. However, XNC10-deficient transgenic tadpoles that also lack Vα6 iT cells were resistant to ff-2 tumors, uncovering a potential new function of XNC10 besides Vα6 iT cell development. Furthermore, the CRISPR/Cas9-mediated knockout of XNC10 in ff-2 tumors broke the immune tolerance. Together, our findings demonstrate the relevance of XNC10/iT cell axis in controlling Xenopus tumor tolerance or rejection.

Xenopus Models of Cancer: Expanding the Oncologist's Toolbox

The use of the Xenopus model system has provided diverse contributions to cancer research, not least because of the striking parallels between tumour pathogenesis and early embryo development. Cell cycle regulation, signalling pathways, and cell behaviours such as migration are frequently perturbed in cancers; all have been investigated using Xenopus, and these developmental events can additionally act as an assay for drug development studies. In this mini-review, we focus our discussion primarily on whole embryo Xenopus models informing cancer biology; the contributions to date and future potential. Insights into tumour immunity, oncogene function, and visualisation of vascular responses during tumour formation have all been achieved with naturally occurring tumours and induced-tumour-like-structures in Xenopus. Finally, as we are now entering the era of genetically modified Xenopus models, we can harness genome editing techniques to recapitulate human disease through creating embryos with analogous genetic abnormalities. With the speed, versatility and accessibility that epitomise the Xenopus system, this new range of pre-clinical Xenopus models has great potential to advance our mechanistic understanding of oncogenesis and provide an early in vivo model for chemotherapeutic development.

Use of genetically encoded, light-gated ion translocators to control tumorigenesis

It has long been known that the resting potential of tumor cells is depolarized relative to their normal counterparts. More recent work has provided evidence that resting potential is not just a readout of cell state: it regulates cell behavior as well. Thus, the ability to control resting potential in vivo would provide a powerful new tool for the study and treatment of tumors, a tool capable of revealing living-state physiological information impossible to obtain using molecular tools applied to isolated cell components. Here we describe the first use of optogenetics to manipulate ion-flux mediated regulation of membrane potential specifically to prevent and cause regression of oncogene-induced tumors. Injection of mutant-KRAS mRNA induces tumor-like structures with many documented similarities to tumors, in Xenopus tadpoles. We show that expression and activation of either ChR2^D156A, a blue-light activated cation channel, or Arch, a green-light activated proton pump, both of which hyperpolarize cells, significantly lowers the incidence of KRAS tumor formation. Excitingly, we also demonstrate that activation of co-expressed light-activated ion translocators after tumor formation significantly increases the frequency with which the tumors regress in a process called normalization. These data demonstrate an optogenetic approach to dissect the biophysics of cancer. Moreover, they provide proof-of-principle for a novel class of interventions, directed at regulating cell state by targeting physiological regulators that can over-ride the presence of mutations.

The Zahn drawings: new illustrations of Xenopus embryo and tadpole stages for studies of craniofacial development

The embryos and tadpoles of the frog Xenopus are increasingly important subjects for studies of the development of the head and face - studies that are providing novel and crucial insight into the causes and prevention of a suite of devastating birth defects, as well as basic evolutionary and developmental biology. However, many studies are conducted on a range of embryonic stages that are not fully represented in the beloved Xenopus resource, Nieuwkoop and Faber's classic Normal Table of Xenopus laevis (Daudin) The lack of standardized images at these stages acts as a barrier to the efficient and accurate representation and communication of experimental methodology and expression data. To fill this gap, we have created 27 new high-quality illustrations. Like their oft-used predecessors from Nieuwkoop and Faber, these drawings can be freely downloaded and used, and will, we hope, serve as an essential resource for this important model system.

Exploring the functions of nonclassical MHC class Ib genes in Xenopus laevis by the CRISPR/Cas9 system

A large family of highly related and clustered Xenopus nonclassical MHC class Ib (XNC) genes influences Xenopus laevis immunity and potentially other physiological functions. Using RNA interference (RNAi) technology, we previously demonstrated that one of XNC genes, XNC10.1, is critical for the development and function of a specialized innate T (iT) cell population. However, RNAi limitation such as a variable and unstable degree of gene silencing in F0 and F1 generations is hampering a thorough functional analysis of XNC10.1 and other XNC genes. To overcome this obstacle, we adapted the CRISPR/Cas9-mediated gene editing technique for XNC genes. We efficiently and specifically generated single gene knockouts of XNC10.1, XNC11, and XNC1 as well as double gene knockouts of XNC10.1 and XNC11 in X. laevis. In single XNC10.1 knockout X. laevis tadpoles, the absence of XNC10.1 and Vα6-Jα1.43 invariant T cell receptor rearrangement transcripts indicated XNC10.1 loss-of-function and deficiency in Vα6-Jα1.43 iT cells. Notably, targeting XNC10.1 did not affect neighboring XNC genes exhibiting high sequence similarity. Furthermore, XNC1 gene disruption induced mortality during developmental stage 47, suggesting some non-immune but essential function of this gene. These data demonstrate that the CRISPR/Cas9 system can be successfully adapted for genetic analysis in F0 generation of X. laevis.

TALENs and CRISPR/Cas9 fuel genetically engineered clinically relevant Xenopus tropicalis tumor models

The targeted nuclease revolution (TALENs, CRISPR/Cas9) now allows Xenopus researchers to rapidly generate custom on-demand genetic knockout models. These novel methods to perform reverse genetics are unprecedented and are fueling a wide array of human disease models within the aquatic diploid model organism Xenopus tropicalis (X. tropicalis). This emerging technology review focuses on the tools to rapidly generate genetically engineered X. tropicalis models (GEXM), with a focus on establishment of genuine genetic and clinically relevant cancer models. We believe that due to particular advantageous characteristics, outlined within this review, GEXM will become a valuable alternative animal model for modeling human cancer. Furthermore, we provide perspectives of how GEXM will be used as a platform for elucidation of novel therapeutic targets and for preclinical drug validation. Finally, we also discuss some future prospects on how the recent expansions and adaptations of the CRISPR/Cas9 toolbox might influence and push forward X. tropicalis cancer research.

Use of Xenopus Frogs to Study Renal Development/Repair

The Xenopus genus includes several members of aquatic frogs native to Africa but is perhaps best known for the species Xenopus laevis and Xenopus tropicalis. These species were popularized as model organisms from as early as the 1800s and have been instrumental in expanding several biological fields including cell biology, environmental toxicology, regenerative biology, and developmental biology. In fact, much of what we know about the formation and maturation of the vertebrate renal system has been acquired by examining the intricate genetic and morphological patterns that epitomize nephrogenesis in Xenopus. From these numerous reports, we have learned that the process of kidney development is as unique among organs as it is conserved among vertebrates. While development of most organs involves increases in size at a single location, development of the kidney occurs through a series of three increasingly complex nephric structures that are temporally distinct from one another and which occupy discrete spatial locales within the body. These three renal systems all serve to provide homeostatic, osmoregulatory, and excretory functions in animals. Importantly, the kidneys in amphibians, such as Xenopus, are less complex and more easily accessed than those in mammals, and thus tadpoles and frogs provide useful models for understanding our own kidney development. Several descriptive and mechanistic studies conducted with the Xenopus model system have allowed us to elucidate the cellular and molecular mediators of renal patterning and have also laid the foundation for our current understanding of kidney repair mechanisms in vertebrates. While some species-specific responses to renal injury have been observed, we still recognize the advantage of the Xenopus system due to its distinctive similarity to mammalian wound healing, reparative, and regenerative responses. In addition, the first evidence of renal regeneration in an amphibian system was recently demonstrated in Xenopus laevis. As genetic and molecular tools continue to advance, our appreciation for and utilization of this amphibian model organism can only intensify and will certainly provide ample opportunities to further our understanding of renal development and repair.

Innate immunity in vertebrates: an overview

Innate immunity is a semi-specific and widely distributed form of immunity, which represents the first line of defence against pathogens. This type of immunity is critical to maintain homeostasis and prevent microbe invasion, eliminating a great variety of pathogens and contributing with the activation of the adaptive immune response. The components of innate immunity include physical and chemical barriers, humoral and cell-mediated components, which are present in all jawed vertebrates. The understanding of innate defence mechanisms in non-mammalian vertebrates is the key to comprehend the general picture of vertebrate innate immunity and its evolutionary history. This is also essential for the identification of new molecules with applications in immunopharmacology and immunotherapy. In this review, we describe and discuss the main elements of vertebrate innate immunity, presenting core findings in this field and identifying areas that need further investigation.

Collagenoma in an African Clawed Frog (Xenopus laevis)

A 3-y-old female Xenopus laevis was reported for a gray mass on the abdomen. The frog was used for egg collection and was otherwise experimentally naïve. On physical exam, the frog was bright and active and had a firm, gray, lobulated mass (1.5 cm × 0.5 cm × 0.5 cm) in the cutaneous tissue of the left lateral abdomen. An excisional biopsy was performed under anesthesia, and the entire mass was removed and processed for histopathology. Microscopically, the dermis was greatly expanded by connective tissue with a marked decrease in the number of glands, and occasional degenerative glands were present. When stained with Masson trichrome, the excessive connective tissue stained blue, indicating that it was composed of collagen. With Verhoeff-van Gieson staining, the connective tissue stained bright red with an absence of black-staining material, demonstrating the presence of collagen and ruling out elastic fibers. In light of the morphology of the mass and the results of the special stains, the mass was diagnosed as a collagenoma. To our knowledge, this report is the first description of a collagenoma in X. laevis.

Semi-solid tumor model in Xenopus laevis/gilli cloned tadpoles for intravital study of neovascularization, immune cells and melanophore infiltration

Tumors have the ability to grow as a self-sustaining entity within the body. This autonomy is in part accomplished by the tumor cells ability to induce the formation of new blood vessels (angiogenesis) and by controlling cell trafficking inside the tumor mass. These abilities greatly reduce the efficacy of many cancer therapies and pose challenges for the development of more effective cancer treatments. Hence, there is a need for animal models suitable for direct microscopy observation of blood vessel formation and cell trafficking, especially during early stages of tumor establishment. Here, we have developed a reliable and cost effective tumor model system in tadpoles of the amphibian Xenopus laevis. Tadpoles are ideally suited for direct microscopy observation because of their small size and transparency. Using the thymic lymphoid tumor line 15/0 derived from, and transplantable into, the X. laevis/gilli isogenic clone LG-15, we have adapted a system that consists in transplanting 15/0 tumor cells embedded into rat collagen under the dorsal skin of LG-15 tadpole recipients. This system recapitulates many facets of mammalian tumorigenesis and permits real time visualization of the active formation of the tumor microenvironment induced by 15/0 tumor cells including neovascularization, collagen rearrangements as well as infiltration of immune cells and melanophores.

An oncologist׳s friend: How Xenopus contributes to cancer research

One of the most striking features of the Xenopus system is the versatility in providing a unique range of both in vitro and in vivo models that are rapid, accessible and easily manipulated. Here we present an overview of the diverse contribution that Xenopus has made to advance our understanding of tumour biology and behaviour; a contribution that goes beyond the traditional view of Xenopus as a developmental model organism. From the utility of the egg and oocyte extract system to the use of whole embryos as developmental or induced tumour models, the Xenopus system has been fundamental to investigation of cell cycle mechanisms, cell metabolism, cell signalling and cell behaviour, and has allowed an increasing appreciation of the parallels between early development and the pathogenesis of tumour progression and metastasis. Although not the prototypical oncological model system, we propose that Xenopus is an adaptable and multifunctional tool in the oncologist׳s arsenal.

Pancreatic carcinoma in an African clawed frog (Xenopus laevis)

This report describes the histologic features of a pancreatic carcinoma in an adult female African clawed frog (Xenopus laevis). The animal was found to be in poor body condition and subsequently euthanized for a complete necropsy. Histologically, the pancreas was effaced by packets of polyhedral cells consistent with a pancreatic islet cell carcinoma. Metastatic disease was not identified. Pancreatic tumors are uncommon in amphibians, and this report is the first to describe a pancreatic carcinoma in an African clawed frog.

The genus Xenopus as a multispecies model for evolutionary and comparative immunobiology of the 21st century

The Xenopus model for immunological research offers a collection of invaluable research tools including MHC-defined clones, inbred strains, cell lines, and monoclonal antibodies. Further, the annotated full genome sequence of Xenopus tropicalis and its remarkable conservation of gene organization with mammals, as well as ongoing genome mapping and mutagenesis studies in X. tropicalis, add a new dimension to the study of immunity. In this paper, we review uses of this amphibian model to study: the development of the immune system; vascular and lymphatic regeneration; immune tolerance; tumor immunity; immune responses to important emerging infectious diseases; and the evolution of classical and non-classical MHC class I genes. We also discuss the rich potential of the species with different degrees of polypoidy resulting from whole genome-wide duplication of the Xenopodinae subfamily as a model to study regulation at the genome level.

The phylogenetic origins of natural killer receptors and recognition: relationships, possibilities, and realities

Natural killer (NK) cells affect a form of innate immunity that recognizes and eliminates cells that are infected with certain viruses or have undergone malignant transformation. In mammals, this recognition can be mediated through immunoglobulin- (Ig) and/or lectin-type NK receptors (NKRs). NKR genes in mammals range from minimally polymorphic single-copy genes to complex multigene families that exhibit high levels of haplotypic complexity and exhibit significant interspecific variation. Certain single-copy NKR genes that are present in one mammal are present as expanded multigene families in other mammals. These observations highlight NKRs as one of the most rapidly evolving eukaryotic gene families and likely reflect the influence of pathogens, especially viruses, on their evolution. Although well characterized in human and mice, cytotoxic cells that are functionally similar to NK cells have been identified in species ranging from birds to reptiles, amphibians and fish. Although numerous receptors have been identified in non-mammalian vertebrates that share structural relationships with mammalian NKRs, functionally defining these lower vertebrate molecules as NKRs is confounded by methodological and interpretive complexities. Nevertheless, several lines of evidence suggest that NK-type function or its equivalent has sustained a long evolutionary history throughout vertebrate species.

Comparative and developmental study of the immune system in Xenopus

Xenopus laevis is the model of choice for evolutionary, comparative, and developmental studies of immunity, and invaluable research tools including MHC-defined clones, inbred strains, cell lines, and monoclonal antibodies are available for these studies. Recent efforts to use Silurana (Xenopus) tropicalis for genetic analyses have led to the sequencing of the whole genome. Ongoing genome mapping and mutagenesis studies will provide a new dimension to the study of immunity. Here we review what is known about the immune system of X. laevis integrated with available genomic information from S. tropicalis. This review provides compelling evidence for the high degree of similarity and evolutionary conservation between Xenopus and mammalian immune systems. We propose to build a powerful and innovative comparative biomedical model based on modern genetic technologies that takes take advantage of X. laevis and S. tropicalis, as well as the whole Xenopus genus. Developmental Dynamics 238:1249-1270, 2009. (c) 2009 Wiley-Liss, Inc.

Novel nonclassical MHC class Ib genes associated with CD8 T cell development and thymic tumors

In jawed vertebrates, the heterogeneous nonclassical MHC class Ib (class Ib) gene family encodes molecules structurally similar to classical MHC class Ia (class Ia) but with more limited tissue distribution and lower polymorphism. In mammals, class Ib gene products are involved in stress responses, malignancy and differentiation of intrathymic CD8 T cells. The frog Xenopus laevis possesses at least 20 class Ib genes (XNCs), and 9 subfamilies have been defined so far. We have characterized two novel subfamilies, XNC10 and XNC11. XNC10 is phylogenetically and structurally distinct from both class Ia and other XNC genes. Besides thymic lymphoid tumors, XNC10 is preferentially expressed by circulating T cells and thymocytes of the CD8 lineage both in adult and in larvae from the onset of thymus organogenesis. XNC11 is expressed only by thymocytes and upregulated by several thymic lymphoid tumors. These data provide the first evidence of the expression of any class Ib genes in Xenopus larvae, and suggests evolutionary relationships between certain class Ib genes, malignancy and CD8 T cell ontogeny.