Creation of a large genomic deletion at the T-cell antigen receptor beta-subunit locus in mouse embryonic stem cells by gene targeting

Card9 and MyD88 differentially regulate Th17 immunity to the commensal yeast Malassezia in the murine skin

The fungal community of the skin microbiome is dominated by a single genus, Malassezia. Besides its symbiotic lifestyle at the host interface, this commensal yeast has also been associated with diverse inflammatory skin diseases in humans and pet animals. Stable colonization is maintained by antifungal type 17 immunity. The mechanisms driving Th17 responses to Malassezia remain, however, unclear. Here, we show that the C-type lectin receptors Mincle, Dectin-1, and Dectin-2 recognize conserved patterns in the cell wall of Malassezia and induce dendritic cell activation in vitro, while only Dectin-2 is required for Th17 activation during experimental skin colonization in vivo. In contrast, Toll-like receptor recognition was redundant in this context. Instead, inflammatory IL-1 family cytokines signaling via MyD88 were also implicated in Th17 activation in a T cell-intrinsic manner. Taken together, we characterized the pathways contributing to protective immunity against the most abundant member of the skin mycobiome. This knowledge contributes to the understanding of barrier immunity and its regulation by commensals and is relevant considering how aberrant immune responses are associated with severe skin pathologies.

The TCR assigns naive T cells to a preferred lymph node

Naive T cells recirculate between the spleen and lymph nodes where they mount immune responses when meeting dendritic cells presenting foreign antigen. As this may happen anywhere, naive T cells ought to visit all lymph nodes. Here, deep sequencing almost-complete TCR repertoires led to a comparison of different lymph nodes within and between individual mice. We find strong evidence for a deterministic CD4/CD8 lineage choice and a consistent spatial structure. Specifically, some T cells show a preference for one or multiple lymph nodes, suggesting that their TCR interacts with locally presented (self-)peptides. These findings are mirrored in TCR-transgenic mice showing localized CD69 expression, retention, and cell division. Thus, naive T cells intermittently sense antigenically dissimilar niches, which is expected to affect their homeostatic competition.

MHC heterozygosity limits T cell receptor variability in CD4 T cells

αβ T cell receptor (TCR) V(D)J genes code for billions of TCR combinations. However, only some appear on peripheral T cells in any individual because, to mature, thymocytes must react with low affinity but not high affinity with thymus expressed major histocompatibility (MHC)/peptides. MHC proteins are very polymorphic. Different alleles bind different peptides. Therefore, any individual might express many different MHC alleles to ensure that some peptides from an invader are bound to MHC and activate T cells. However, most individuals express limited numbers of MHC alleles. To explore this, we compared the TCR repertoires of naïve CD4 T cells in mice expressing one or two MHC alleles. Unexpectedly, the TCRs in heterozygotes were less diverse that those in the sum of their MHC homozygous relatives. Our results suggest that thymus negative selection cancels out the advantages of increased thymic positive selection in the MHC heterozygotes.

An axon-T cell feedback loop enhances inflammation and axon degeneration

Inflammation is closely associated with many neurodegenerative disorders. Yet, whether inflammation causes, exacerbates, or responds to neurodegeneration has been challenging to define because the two processes are so closely linked. Here, we disentangle inflammation from the axon damage it causes by individually blocking cytotoxic T cell function and axon degeneration. We model inflammatory damage in mouse skin, a barrier tissue that, despite frequent inflammation, must maintain proper functioning of a dense array of axon terminals. We show that sympathetic axons modulate skin inflammation through release of norepinephrine, which suppresses activation of γδ T cells via the β2 adrenergic receptor. Strong inflammatory stimulation-modeled by application of the Toll-like receptor 7 agonist imiquimod-causes progressive γδ T cell-mediated, Sarm1-dependent loss of these immunosuppressive sympathetic axons. This removes a physiological brake on T cells, initiating a positive feedback loop of enhanced inflammation and further axon damage.

Enhancer dependent repositioning of TCRb locus with respect to the chromosome territory

Intranuclear position of several genes is dynamically altered during development concordant with their activation. To understand this dynamic, but non-random, nuclear organization, it is important to identify the relevant regulatory elements and trans acting factors. Murine TCRb locus gets activated during thymic development. Enhancer Eb is important for VDJ recombination at TCRb locus as it is critically required establishment of recombination center. Our analysis revealed that TCRb locus gets located out of the chromosome territory specifically in developing thymocytes. Further, CRISPR/Cas9 based deletion mutagenesis established an unambiguous role of enhancer Eb in defining TCRb location relative to chromosome territory. The ability to reposition the target locus relative to chromosome territory highlights a novel aspect pertaining to activity of enhancers which may contribute to their ability to regulate gene expression. Additionally, our observations have implications for understanding the role of enhancers in three-dimensional genome organization and function.

A wave of monocytes is recruited to replenish the long-term Langerhans cell network after immune injury

A dense population of embryo-derived Langerhans cells (eLCs) is maintained within the sealed epidermis without contribution from circulating cells. When this network is perturbed by transient exposure to ultraviolet light, short-term LCs are temporarily reconstituted from an initial wave of monocytes but thought to be superseded by more permanent repopulation with undefined LC precursors. However, the extent to which this process is relevant to immunopathological processes that damage LC population integrity is not known. Using a model of allogeneic hematopoietic stem cell transplantation, where alloreactive T cells directly target eLCs, we have asked whether and how the original LC network is ultimately restored. We find that donor monocytes, but not dendritic cells, are the precursors of long-term LCs in this context. Destruction of eLCs leads to recruitment of a wave of monocytes that engraft in the epidermis and undergo a sequential pathway of differentiation via transcriptionally distinct EpCAM⁺ precursors. Monocyte-derived LCs acquire the capacity of self-renewal, and proliferation in the epidermis matched that of steady-state eLCs. However, we identified a bottleneck in the differentiation and survival of epidermal monocytes, which, together with the slow rate of renewal of mature LCs, limits repair of the network. Furthermore, replenishment of the LC network leads to constitutive entry of cells into the epidermal compartment. Thus, immune injury triggers functional adaptation of mechanisms used to maintain tissue-resident macrophages at other sites, but this process is highly inefficient in the skin.

Inflammation-induced IgE promotes epithelial hyperplasia and tumour growth

IgE is the least abundant circulating antibody class but is constitutively present in healthy tissues bound to resident cells via its high-affinity receptor, FcεRI. The physiological role of endogenous IgE antibodies is unclear but it has been suggested that they provide host protection against a variety of noxious environmental substances and parasitic infections at epithelial barrier surfaces. Here we show, in mice, that skin inflammation enhances levels of IgE antibodies that have natural specificities and a repertoire, VDJ rearrangements and CDRH3 characteristics similar to those of IgE antibodies in healthy tissue. IgE-bearing basophils are recruited to inflamed skin via CXCL12 and thymic stromal lymphopoietin (TSLP)/IL-3-dependent upregulation of CXCR4. In the inflamed skin, IgE/FcεRI-signalling in basophils promotes epithelial cell growth and differentiation, partly through histamine engagement of H1R and H4R. Furthermore, this IgE response strongly drives tumour outgrowth of epithelial cells harbouring oncogenic mutation. These findings indicate that natural IgE antibodies support skin barrier defences, but that during chronic tissue inflammation this role may be subverted to promote tumour growth.

Bone marrow chimeras-a vital tool in basic and translational research

Bone marrow chimeras are used routinely in immunology research as well as in other fields of biology. Here, we provide a concise state-of-the-art review about the types of chimerisms that can be achieved and the type of information that each model generates. We include separate sections for caveats and future developments. We provide examples from the literature in which different types of chimerism were employed to answer specific questions. While simple bone marrow chimeras allow to dissect the role of genes in distinct cell populations such as the hematopoietic cells versus non-hematopoietic cells, mixed bone marrow chimeras can provide detailed information about hematopoietic cell types and the intrinsic and extrinsic roles of individual genes. The advantages and caveats of bone marrow chimerism for the study of microglia are addressed, as well as alternatives to irradiation that minimize blood-brain-barrier disruption. Elementary principles are introduced and their potential is exemplified through summarizing recent studies.

Epithelial damage and tissue γδ T cells promote a unique tumor-protective IgE response

IgE is an ancient and conserved immunoglobulin isotype with potent immunological function. Nevertheless, the regulation of IgE responses remains an enigma, and evidence of a role for IgE in host defense is limited. Here we report that topical exposure to a common environmental DNA-damaging xenobiotic initiated stress surveillance by γδTCR+ intraepithelial lymphocytes that resulted in class switching to IgE in B cells and the accumulation of autoreactive IgE. High-throughput antibody sequencing revealed that γδ T cells shaped the IgE repertoire by supporting specific variable-diversity-joining (VDJ) rearrangements with unique characteristics of the complementarity-determining region CDRH3. This endogenous IgE response, via the IgE receptor FcεRI, provided protection against epithelial carcinogenesis, and expression of the gene encoding FcεRI in human squamous-cell carcinoma correlated with good disease prognosis. These data indicate a joint role for immunosurveillance by T cells and by B cells in epithelial tissues and suggest that IgE is part of the host defense against epithelial damage and tumor development.

The somatically generated portion of T cell receptor CDR3α contributes to the MHC allele specificity of the T cell receptor

Mature T cells bearing αβ T cell receptors react with foreign antigens bound to alleles of major histocompatibility complex proteins (MHC) that they were exposed to during their development in the thymus, a phenomenon known as positive selection. The structural basis for positive selection has long been debated. Here, using mice expressing one of two different T cell receptor β chains and various MHC alleles, we show that positive selection-induced MHC bias of T cell receptors is affected both by the germline encoded elements of the T cell receptor α and β chain and, surprisingly, dramatically affected by the non germ line encoded portions of CDR3 of the T cell receptor α chain. Thus, in addition to determining specificity for antigen, the non germline encoded elements of T cell receptors may help the proteins cope with the extremely polymorphic nature of major histocompatibility complex products within the species.

Cutting Edge: Dual TCRα Expression Poses an Autoimmune Hazard by Limiting Regulatory T Cell Generation

Despite accounting for 10-30% of the T cell population in mice and humans, the role of dual TCR-expressing T cells in immunity remains poorly understood. It has been hypothesized that dual TCR T cells pose an autoimmune hazard by allowing self-reactive TCRs to escape thymic selection. We revisited this hypothesis using the NOD murine model of type 1 diabetes. We bred NOD mice hemizygous at both TCRα and β (TCRα^+/- β^+/-) loci, rendering them incapable of producing dual TCR T cells. We found that the lack of dual TCRα expression skewed the insulin-specific thymocyte population toward greater regulatory T (T_reg) cell commitment, resulting in a more tolerogenic T_reg to conventional T cell ratio and protection from diabetes. These data support a novel hypothesis by which dual TCR expression can promote autoimmunity by limiting agonist selection of self-reactive thymocytes into the T_reg cell lineage.

Establishment and growth responses of Nile tilapia embryonic stem-like cell lines under feeder-free condition

Embryonic stem (ES) cells provide an invaluable tool for molecular analysis of vertebrate development and a bridge linking genomic manipulations in vitro and functional analysis of target genes in vivo. Work towards fish ES cells so far has focused on zebrafish (Danio renio) and medaka (Oryzias latipes). Here we describe the derivation, pluripotency, differentiation and growth responses of ES cell lines from Nile tilapia (Oreochromis niloticus), a world-wide commercial farmed fish. These cell lines, designated as TES1-3, were initiated from blastomeres of Nile tilapia middle blastula embryos (MBE). One representative line, TES1, showed stable growth and phenotypic characteristics of ES cells over 200 days of culture with more than 59 passages under feeder-free conditions. They exhibited high alkaline phosphatase activity and expression of pluripotency genes including pou5f3 (the pou5f1/oct4 homologue), sox2, myc and klf4. In suspension culture together with retinoic acid treatment, TES1 cells formed embryoid bodies, which exhibited expression profile of differentiation genes characteristics of all three germ cell layers. Notably, PKH26-labeled TES1 cells introduced into Nile tilapia MBE could contribute to body compartment development and led to hatched chimera formation with an efficacy of 13%. These results suggest that TES1 cells have pluripotency and differentiation potential in vitro and in vivo. In the conditioned DMEM, all of the supplements including the fetal bovine serum, fish embryonic extract, fish serum, basic fibroblast growth factor and non-protein supplement combination 5N were mitogenic for TES1 cell growth. This study will promote ES-based biotechnology in commercial fish.

Bi-Allelic TCRα or β Recombination Enhances T Cell Development but Is Dispensable for Antigen Responses and Experimental Autoimmune Encephalomyelitis

Dual TCRα-expressing T cells outnumber dual TCRβ-expressing cells by ~10:1. As a result, efforts to understand how dual TCR T cells impact immunity have focused on dual TCRα expression; dual TCRβ expression remains understudied. We recently demonstrated, however, that dual TCRβ expression accelerated disease in a TCR transgenic model of autoimmune arthritis through enhanced positive selection efficiency, indicating that dual TCRβ expression, though rare, can impact thymic selection. Here we generated mice hemizygous for TCRα, TCRβ, or both on the C57BL/6 background to investigate the impact bi-allelic TCR chain recombination has on T cell development, repertoire diversity, and autoimmunity. Lack of bi-allelic TCRα or TCRβ recombination reduced αβ thymocyte development efficiency, and the absence of bi-allelic TCRβ recombination promoted γδ T cell development. However, we observed no differences in the numbers of naïve and expanded antigen-specific T cells between TCRα^+/-β^+/- and wildtype mice, and TCR repertoire analysis revealed only subtle differences in Vβ gene usage. Finally, the absence of dual TCR T cells did not impact induced experimental autoimmune encephalomyelitis pathogenesis. Thus, despite more stringent allelic exclusion of TCRβ relative to TCRα, bi-allelic TCRβ expression can measurably impact thymocyte development and is necessary for maintaining normal αβ/γδ T cell proportions.

Influence of a CTCF-Dependent Insulator on Multiple Aspects of Enhancer-Mediated Chromatin Organization

Developmental stage-specific enhancer-promoter-insulator interactions regulate the chromatin configuration necessary for transcription at various loci and additionally for VDJ recombination at antigen receptor loci that encode immunoglobulins and T-cell receptors. To investigate these regulatory interactions, we analyzed the epigenetic landscape of the murine T-cell receptor β (TCRβ) locus in the presence and absence of an ectopic CTCF-dependent enhancer-blocking insulator, H19-ICR, in genetically manipulated mice. Our analysis demonstrated the ability of the H19-ICR insulator to restrict several aspects of enhancer-based chromatin alterations that are observed during activation of the TCRβ locus for transcription and recombination. The H19-ICR insulator abrogated enhancer-promoter contact-dependent chromatin alterations and additionally prevented Eβ-mediated histone modifications that have been suggested to be independent of enhancer-promoter interaction. Observed enhancer-promoter-insulator interactions, in conjunction with the chromatin structure of the Eβ-regulated domain at the nucleosomal level, provide useful insights regarding the activity of the regulatory elements in addition to supporting the accessibility hypothesis of VDJ recombination. Analysis of H19-ICR in the heterologous context of the developmentally regulated TCRβ locus suggests that different mechanisms proposed for CTCF-dependent insulator action might be manifested simultaneously or selectively depending on the genomic context and the nature of enhancer activity being curtailed.

A Knockout Experiment: Disciplinary Divides and Experimental Skill in Animal Behaviour Genetics

In the early 1990s, a set of new techniques for manipulating mouse DNA allowed researchers to 'knock out' specific genes and observe the effects of removing them on a live mouse. In animal behaviour genetics, questions about how to deploy these techniques to study the molecular basis of behaviour became quite controversial, with a number of key methodological issues dissecting the interdisciplinary research field along disciplinary lines. This paper examines debates that took place during the 1990s between a predominately North American group of molecular biologists and animal behaviourists around how to design, conduct, and interpret behavioural knockout experiments. Drawing from and extending Harry Collins's work on how research communities negotiate what counts as a 'well-done experiment,' I argue that the positions practitioners took on questions of experimental skill reflected not only the experimental traditions they were trained in but also their differing ontological and epistemological commitments. Different assumptions about the nature of gene action, eg., were tied to different positions in the knockout mouse debates on how to implement experimental controls. I conclude by showing that examining representations of skill in the context of a community's knowledge commitments sheds light on some of the contradictory ways in which contemporary animal behaviour geneticists talk about their own laboratory work as a highly skilled endeavour that also could be mechanised, as easy to perform and yet difficult to perform well.

Requirement for MyD88 signaling in B cells and dendritic cells for germinal center anti-nuclear antibody production in Lyn-deficient mice

The intracellular tyrosine kinase Lyn mediates inhibitory receptor function in B cells and myeloid cells, and Lyn(-/-) mice spontaneously develop an autoimmune and inflammatory disease that closely resembles human systemic lupus erythematosus. TLR-signaling pathways have been implicated in the production of anti-nuclear Abs in systemic lupus erythematosus and mouse models of it. We used a conditional allele of Myd88 to determine whether the autoimmunity of Lyn(-/-) mice is dependent on TLR/MyD88 signaling in B cells and/or in dendritic cells (DCs). The production of IgG anti-nuclear Abs, as well as the deposition of these Abs in the glomeruli of the kidneys, leading to glomerulonephritis in Lyn(-/-) mice, were completely abolished by selective deletion of Myd88 in B cells, and autoantibody production and glomerulonephritis were delayed or decreased by deletion of Myd88 in DCs. The reduced autoantibody production in mice lacking MyD88 in B cells or DCs was accompanied by a dramatic decrease in the spontaneous germinal center (GC) response, suggesting that autoantibodies in Lyn(-/-) mice may depend on GC responses. Consistent with this view, IgG anti-nuclear Abs were absent if T cells were deleted (TCRβ(-/-) TCRδ(-/-) mice) or if T cells were unable to contribute to GC responses as the result of mutation of the adaptor molecule SAP. Thus, the autoimmunity of Lyn(-/-) mice was dependent on T cells and on TLR/MyD88 signaling in B cells and in DCs, supporting a model in which DC hyperactivity combines with defects in tolerance in B cells to lead to a T cell-dependent systemic autoimmunity in Lyn(-/-) mice.

Mast cells are required for full expression of allergen/SEB-induced skin inflammation

Atopic dermatitis is a chronic pruritic inflammatory skin disease. We recently described an animal model in which repeated epicutaneous applications of a house dust mite extract and staphylococcal enterotoxin B induced eczematous skin lesions. In this study we showed that global gene expression patterns are very similar between human atopic dermatitis skin and allergen/staphylococcal enterotoxin B-induced mouse skin lesions, particularly in expression of genes related to epidermal growth/differentiation, skin-barrier, lipid/energy metabolism, immune response, or extracellular matrix. In this model, mast cells and T cells, but not B cells or eosinophils, were shown to be required for the full expression of dermatitis, as revealed by reduced skin inflammation and reduced serum IgE levels in mice lacking mast cells or T cells (TCRβ^−/− or Rag1^−/−). The clinical severity of dermatitis correlated with the numbers of mast cells, but not eosinophils. Consistent with the idea that Th2 cells play a predominant role in allergic diseases, the receptor for the Th2-promoting cytokine thymic stromal lymphopoietin and the high-affinity IgE receptor, FcεRI, were required to attain maximal clinical scores. Therefore, this clinically relevant model provides mechanistic insights into the pathogenic mechanism of human atopic dermatitis.

T cells and their eons-old obsession with MHC

T cells bearing receptors made up of α and β chains (TCRs) usually react with peptides bound to major histocompatibility complex proteins (MHC). This bias could be imposed by positive selection, the phenomenon that selects thymocytes to mature into T cells only if the TCRs they bear react with low but appreciable affinity with MHC + peptide combinations in the thymus cortex. However, it is also possible that the polypeptides of TCRs themselves do not have random specificities but rather are biased toward reaction with MHC. Evolution would therefore have selected for a collection of TCR variable elements that are prone to react with MHC. If this were to be so, positive selection would act on thymocytes bearing a pre biased collection of TCRs to pick out those that react to some extent, but not too well, with self MHC + self-peptides. A problem with studies of this evolutionary idea is the fact that there are many TCR variable elements and that these differ considerably in the amino acids with which they contact MHC. However, recent experiments by our group and others suggest that one group of TCR variable elements, those related to the mouse Vβ8 family, has amino acids in their CDR2 regions that consistently bind a particular site on an MHC α-helix. Other groups of variable elements may use different patterns of amino acids to achieve the same goal. Mutation of these amino acids reduces the ability of T cells and thymocytes to react with MHC. These amino acids are present in the variable regions of distantly related species such as sharks and human. Overall the data indicate that TCR elements have indeed been selected by evolution to react with MHC proteins. Many mysteries about TCRs remain to be solved, including the nature of auto-recognition, the basis of MHC allele specificity, and the very nature and complexity of TCRs on mature T cells.

A molecular genetic approach to uncovering the differential functions of dopamine D2 receptor isoforms

Alterations in the activity of the dopamine D2 receptor (D2R) have been implicated in several neurological and psychiatric disorders, including schizophrenia, Parkinson's disease, Huntington's disease, Tourette syndrome, attention-deficit hyperactivity disorder (ADHD), and drug addiction. Two isoforms of D2R, long form (D2LR) and short form (D2SR), have been identified. The specific function of each D2R isoform is poorly understood, primarily because isoform-selective pharmacological agents are not available. Using homologous recombination, we have generated D2LR knockout (KO) mice. D2LR KO mice are completely deficient in D2LR, but still express functional D2SR at a level similar to the total D2R level in wild-type (WT) mice. D2LR is generally the predominant isoform expressed in WT mice. We showed that D2LR KO mice displayed a number of robust behavioral phenotypes distinct from WT mice, indicating that D2LR and D2SR have differential functions. In this chapter we describe the generation and characterization of the D2LR KO mouse. This genetic approach provides a valuable research tool to investigate the functional role of individual D2R isoforms in the mammalian central nervous system (CNS).

An ectopic CTCF-dependent transcriptional insulator influences the choice of Vβ gene segments for VDJ recombination at TCRβ locus

Insulators regulate transcription as they modulate the interactions between enhancers and promoters by organizing the chromatin into distinct domains. To gain better understanding of the nature of chromatin domains defined by insulators, we analyzed the ability of an insulator to interfere in VDJ recombination, a process that is critically dependent on long-range interactions between diverse types of cis-acting DNA elements. A well-established CTCF-dependent transcriptional insulator, H19 imprint control region (H19-ICR), was inserted in the mouse TCRβ locus by genetic manipulation. Analysis of the mutant mice demonstrated that the insulator retains its CTCF and position-dependent enhancer-blocking potential in this heterologous context in vivo. Remarkably, the inserted H19-ICR appears to have the ability to modulate cis-DNA interactions between recombination signal sequence elements of the TCRβ locus leading to a dramatically altered usage of Vβ segments for Vβ-to-DβJβ recombination in the mutant mice. This reveals a novel ability of CTCF to govern long range cis-DNA interactions other than enhancer-promoter interactions and suggests that CTCF-dependent insulators may play a diverse and complex role in genome organization beyond transcriptional control. Our functional analysis of mutated TCRβ locus supports the emerging role of CTCF in governing VDJ recombination.

Catalytic activity control of restriction endonuclease--triplex forming oligonucleotide conjugates

Targeting of individual genes in complex genomes requires endonucleases of extremely high specificity. To direct cleavage at the unique site(s) in the genome, both naturally occurring and artificial enzymes have been developed. These include homing endonucleases, zinc-finger nucleases, transcription activator-like effector nucleases, and restriction or chemical nucleases coupled to a triple-helix forming oligonucleotide (TFO). The desired cleavage has been demonstrated both in vivo and in vitro for several model systems. However, to limit cleavage strictly to unique sites and avoid undesired reactions, endonucleases with controlled activity are highly desirable. In this study we present a proof-of-concept demonstration of two strategies to generate restriction endonuclease-TFO conjugates with controllable activity. First, we combined the restriction endonuclease caging and TFO coupling procedures to produce a caged MunI-TFO conjugate, which can be activated by UV-light upon formation of a triple helix. Second, we coupled TFO to a subunit interface mutant of restriction endonuclease Bse634I which shows no activity due to impaired dimerization but is assembled into an active dimer when two Bse634I monomers are brought into close proximity by triple helix formation at the targeted site. Our results push the restriction endonuclease-TFO conjugate technology one step closer to potential in vivo applications.

The intraepithelial T cell response to NKG2D-ligands links lymphoid stress surveillance to atopy

Epithelial cells respond to physicochemical damage with up-regulation of major histocompatibility complex-like ligands that can activate the cytolytic potential of neighboring intraepithelial T cells by binding the activating receptor, NKG2D. The systemic implications of this lymphoid stress-surveillance response, however, are unknown. We found that antigens encountered at the same time as cutaneous epithelial stress induced strong primary and secondary systemic, T helper 2 (T(H)2)-associated atopic responses in mice. These responses required NKG2D-dependent communication between dysregulated epithelial cells and tissue-associated lymphoid cells. These data are germane to uncertainty over the afferent induction of T(H)2 responses and provide a molecular framework for considering atopy as an important component of the response to tissue damage and carcinogenesis.

Overview: generation of gene knockout mice

The technique of gene targeting allows for the introduction of engineered genetic mutations into a mouse at a determined genomic locus. The process of generating mouse models with targeted mutations was developed through both the discovery of homologous recombination and the isolation of murine embryonic stem cells (ES cells). Homologous recombination is a DNA repair mechanism that is employed in gene targeting to insert a designed mutation into the homologous genetic locus. Targeted homologous recombination can be performed in murine ES cells through electroporation of a targeting construct. These ES cells are totipotent and, when injected into a mouse blastocyst, they can differentiate into all cell types of a chimeric mouse. A chimeric mouse harboring cells derived from the targeted ES cell clone can then generate a whole mouse containing the desired targeted mutation. The initial step for the generation of a mouse with a targeted mutation is the construction of an efficient targeting vector that will be introduced into the ES cells.

Protective role of gammadelta T cells in spontaneous ocular inflammation

PURPOSE

A role for gammadelta T cells in immunoregulation has been shown in a number of studies, but in the absence of infection or induced disease, mice lacking gammadelta T cells generally appear to be healthy. That certain mice lacking gammadelta T cells often spontaneously develop keratitis, characterized by a progressive and destructive inflammation of the cornea is reported here.

METHODS

The keratitis developing in these mice was characterized in terms of prevalence in males versus females, age of onset, and histologic features. Attempts were made to understand the underlying causes of the disease by removing alphabeta T cells, altering sex hormones, and reconstituting gammadelta T cells.

RESULTS

The development of keratitis in these mice depended on the C57BL/10 genetic background, and was much more common among females than males. The incidence of the disease increased with age, exceeding 80% in females greater than 18 weeks old. Evidence that the keratitis in these mice is at least partly autoimmune in nature, and that despite its prevalence in females, male hormones do not protect against the disease is presented.

CONCLUSIONS

These findings indicate an important role for gammadelta T cells in maintaining immune balance in the eye. The mice described in this study represent a potential new small animal model of keratitis.

Id3 restricts the developmental potential of gamma delta lineage during thymopoiesis

Most T cell progenitors develop into the alphabeta T cell lineage with the exception of a small fraction contributing to the gammadelta lineage throughout postnatal life. T cell progenitors usually commit to the alphabeta lineage upon the expression of a fully rearranged and functional TCRbeta gene, and most cells that fail to produce a functional TCRbeta-chain will die instead of adopting the alternative gammadelta T cell fate. What prevents these cells from continuing TCRgamma rearrangement and adopting the gammadelta T cell fate is not known. In this study, we show that functional loss of Id3 results in a significant increase of gammadelta T cell production from progenitor cells undergoing TCRbeta rearrangement. The enhanced gammadelta T cell development correlated with increased TCRgamma gene rearrangement involving primarily Vgamma1.1 in Id3 deficient mice. We further show that Id3 deficiency promotes gammadelta T cell production in a manner independent of TCRbeta-chain expression. Our data indicates that Id3 suppresses Vgamma1.1 rearrangement and gammadelta lineage potential among T cell progenitors that have completed TCRbeta gene rearrangement without producing a functional TCRbeta protein.

Overview of gene targeting by homologous recombination

The analysis of mutant organisms and cell lines is important in determining the function of specific proteins. Recent technological advances in gene targeting by homologous recombination in mammalian systems enable the production of mutants in any desired gene, and can be used to produce mutant mouse strains and mutant cell lines. The yeast Flp/FRT recombinase system and bacteriophage recombinases such as Cre and its recognition sequence, loxP, allow spatial and temporal control of knockouts. This unit discusses crucial issues for homologous recombination experiments, including requirements for the source of DNA, criteria for the targeting constructs, methods of enrichment for homologous recombinants, (positive and negative selection, and the use of endogenous promoters), and the types of mutations that can be created.

Overview of gene targeting by homologous recombination

Formerly UNIT 9.15, this unit has been moved to the opening spot of our new chapter on Embryonic Stem Cell technology. The unit has also been updated, and now includes information about the Cre-lox and FlP/FRT recombinase systems.

Different composition of the human and the mouse gammadelta T cell receptor explains different phenotypes of CD3gamma and CD3delta immunodeficiencies

The γδ T cell receptor for antigen (TCR) comprises the clonotypic TCRγδ, the CD3 (CD3γε and/or CD3δε), and the ζζ dimers. γδ T cells do not develop in CD3γ-deficient mice, whereas human patients lacking CD3γ have abundant peripheral blood γδ T cells expressing high γδ TCR levels. In an attempt to identify the molecular basis for these discordant phenotypes, we determined the stoichiometries of mouse and human γδ TCRs using blue native polyacrylamide gel electrophoresis and anti-TCR–specific antibodies. The γδ TCR isolated in digitonin from primary and cultured human γδ T cells includes CD3δ, with a TCRγδCD3ε₂δγζ₂ stoichiometry. In CD3γ-deficient patients, this may allow substitution of CD3γ by the CD3δ chain and thereby support γδ T cell development. In contrast, the mouse γδ TCR does not incorporate CD3δ and has a TCRγδCD3ε₂γ₂ζ₂ stoichiometry. CD3γ-deficient mice exhibit a block in γδ T cell development. A human, but not a mouse, CD3δ transgene rescues γδ T cell development in mice lacking both mouse CD3δ and CD3γ chains. This suggests important structural and/or functional differences between human and mouse CD3δ chains during γδ T cell development. Collectively, our results indicate that the different γδ T cell phenotypes between CD3γ-deficient humans and mice can be explained by differences in their γδ TCR composition.

The TCRbeta enhancer is dispensable for the expression of rearranged TCRbeta genes in thymic DN2/DN3 populations but not at later stages

The Ebeta enhancer has been shown to be dispensable for germline transcription of nonrearranged TCRbeta segments but appears to be required for TCRbeta V to DJ rearrangement. Ebeta dependency of the subsequent expression of VDJ-rearranged TCRbeta genes in thymic subpopulations has so far not been analyzed. We generated transgenic mice, using a Vbeta8.2Dbeta1Jbeta1.3-rearranged TCRbeta bacterial artificial chromosome, which lacked Ebeta, and monitored transgene expression by flow cytometry using Vbeta-specific mAbs and an IRES-eGFP reporter. Transgene expression was found in double negative (DN)2 and DN3 but not at later stages of thymopoesis. There was no toxicity associated with the transgene given that apoptosis in DN3, DN4 was not increased, and the number of DN4 cells generated from DN3 cells in reaggregate thymic organ cultures was not diminished. The transgenic TCRbeta gave rise to a pre-TCR, as suggested by its ability to suppress endogenous TCRbeta rearrangement, to facilitate beta-selection on a TCRbeta-deficient background and to inhibit gammadelta T cell lineage development. The results suggest that the Vbeta8.2 promoter is sufficient to drive expression of rearranged TCRbeta VDJ genes Ebeta independently in DN2/DN3 but not at later stages.

Genomic deletions of the Drosophila melanogaster Hsp70 genes

Homologous recombination can produce directed mutations in the genomes of a number of model organisms, including Drosophila melanogaster. One of the most useful applications has been to delete target genes to generate null alleles. In Drosophila, specific gene deletions have not yet been produced by this method. To test whether such deletions could be produced by homologous recombination in D. melanogaster we set out to delete the Hsp70 genes. Six nearly identical copies of this gene, encoding the major heat-shock protein in Drosophila, are found at two separate but closely linked loci. This arrangement has thwarted standard genetic approaches to generate an Hsp70-null fly, making this an ideal test of gene targeting. In this study, ends-out targeting was used to generate specific deletions of all Hsp70 genes, including one deletion that spanned approximately 47 kb. The Hsp70-null flies are viable and fertile. The results show that genomic deletions of varied sizes can be readily generated by homologous recombination in Drosophila.

Ionizing radiation-induced foci formation of mammalian Rad51 and Rad54 depends on the Rad51 paralogs, but not on Rad52

Homologous recombination is of major importance for the prevention of genomic instability during chromosome duplication and repair of DNA damage, especially double-strand breaks. Biochemical experiments have revealed that during the process of homologous recombination the RAD52 group proteins, including Rad51, Rad52 and Rad54, are involved in an essential step: formation of a joint molecule between the broken DNA and the intact repair template. Accessory proteins for this reaction include the Rad51 paralogs and BRCA2. The significance of homologous recombination for the cell is underscored by the evolutionary conservation of the Rad51, Rad52 and Rad54 proteins from yeast to humans. Upon treatment of cells with ionizing radiation, the RAD52 group proteins accumulate at the sites of DNA damage into so-called foci. For the yeast Saccharomyces cerevisiae, foci formation of Rad51 and Rad54 is abrogated in the absence of Rad52, while Rad51 foci formation does occur in the absence of the Rad51 paralog Rad55. By contrast, we show here that in mammalian cells, Rad52 is not required for foci formation of Rad51 and Rad54. Furthermore, radiation-induced foci formation of Rad51 and Rad54 is impaired in all Rad51 paralog and BRCA2 mutant cell lines tested, while Rad52 foci formation is not influenced by a mutation in any of these recombination proteins. Despite their evolutionary conservation and biochemical similarities, S. cerevisiae and mammalian Rad52 appear to differentially contribute to the DNA-damage response.

Delayed and restricted expression limits putative instructional opportunities of Vgamma1.1/Vgamma2 gammadelta TCR in alphabeta/gammadelta lineage choice in the thymus

Development of alphabeta and gammadelta T cells depends on productive rearrangement of the appropriate TCR genes and their subsequent expression as proteins. TCRbeta and TCRgammadelta proteins first appear in DN3 and DN4 thymocytes, respectively. So far, it is not clear whether this is due to a delayed expression of TCRgammadelta proteins or to a more rapid progression to DN4 of thymocytes expressing TCRgammadelta. The answer to this question bears on the distinction between instructive and stochastic models of alphabeta/gammadelta lineage decision. To study this question, we first monitored initial TCR protein expression in wild-type and TCR transgenic mice in reaggregate thymic organ cultures. A TCRbeta transgene was expressed in nearly all DN3 and DN4 cells, accelerated DN3 to DN4 transition, and strongly diminished the number of cells that express TCRgammadelta proteins. In contrast, TCRgammadelta transgenes were expressed only in a fraction of DN4 cells, did not accelerate DN3 to DN4 transition, and did not reduce the number of DN4 cells expressing TCRbeta proteins. The TCRbeta transgene partially inhibited endogenous TCRgamma rearrangements, whereas the TCRgammadelta transgenes did not inhibit endogenous TCRbeta rearrangements. Second, we analyzed frequencies of productive TCRbeta and TCRgammadelta V(D)J junctions in DN3 and DN4 subsets. Most importantly, frequencies of productive TCRgammadelta rearrangements (Vdelta5, Vgamma1.1, and Vgamma2) appeared unselected in DN3. The results suggest a late and restricted expression of the corresponding gammadeltaTCR, severely limiting their putative instructional opportunities in alphabeta/gammadelta divergence.

Total Deletion ofin VivoTelomere Elongation Capacity: An Ambitious but Possibly Ultimate Cure for All Age-Related Human Cancers

Despite enormous effort, progress in reducing mortality from cancer remains modest. Can a true cancer "cure" ever be developed, given the vast versatility that tumors derive from their genomic instability? Here we consider the efficacy, feasibility, and safety of a therapy that, unlike any available or in development, could never be escaped by spontaneous changes of gene expression: the total elimination from the body of all genetic potential for telomere elongation, combined with stem cell therapies administered about once a decade to maintain proliferative tissues despite this handicap. We term this therapy WILT, for whole-body interdiction of lengthening of telomeres. We first argue that a whole-body gene-deletion approach, however bizarre it initially seems, is truly the only way to overcome the hypermutation that makes tumors so insidious. We then identify the key obstacles to developing such a therapy and conclude that, while some will probably be insurmountable for at least a decade, none is a clear-cut showstopper. Hence, given the absence of alternatives with comparable anticancer promise, we advocate working toward such a therapy.

Salmonella typhimurium strains carrying independent mutations display similar virulence phenotypes yet are controlled by distinct host defense mechanisms

The outcome of Salmonella infection in the mammalian host favors whoever succeeds best in disturbing the equilibrium between coordinate expression of bacterial (virulence) genes and host defense mechanisms. Intracellular persistence in host cells is critical for pathogenesis and disease, because Salmonella typhimurium strains defective in this property are avirulent. We examined whether similar host defense mechanisms are required for growth control of two S. typhimurium mutant strains. Salmonella pathogenicity island 2 (SPI2) and virulence plasmid-cured Salmonella mutants display similar virulence phenotypes in immunocompetent mice, yet their gene loci participate in independent virulence strategies. We determined the role of TNF-alpha and IFN-gamma as well as different T cell populations in infection with these Salmonella strains. After systemic infection, IFN-gamma was essential for growth restriction of plasmid-cured S. typhimurium, while SPI2 mutant infections were controlled in the absence of IFN-gamma. TNFRp55-deficiency restored systemic virulence to both Salmonella mutants. After oral inoculation, control of plasmid-cured bacteria substantially relied on both IFN-gamma and TNF-alpha signaling while control of SPI2 mutants did not. However, for both mutants, ultimate clearance of bacteria from infected mice depended on alphabeta T cells.

Murine models of ulcerative colitis

Ulcerative colitis (UC) is an inflammatory bowel disease of unknown etiology limited to the large intestine. The disease is prevalent in industrial societies and is associated with specific ethnic populations. A number of murine models, each focused on distinct aspects of the disease process, were developed over the past 20 years to further our understanding of the pathogenesis of UC. These models have been and remain our best resource for the study of the disorder as a result of their homology to human UC and the ease in which they can be manipulated and examined. This review examines and distills what has been leamed from these models and how this information is related back to human UC.

Use of a simple, general targeting vector for replacing the DNA of the heavy chain constant region in mouse hybridoma cells

It is often necessary to modify the constant region of the immunoglobulin (Ig) heavy chain in order to produce Ig with optimal properties. In the case of Ig production by mouse hybridoma cells, it is possible to modify the Ig heavy chain (IgH) locus by gene targeting to achieve the desired changes. DNA segments from the JH-S micro region and from the region 3' of Calpha are normally present in the functional IgH gene of all hybridomas, regardless of the heavy chain class which is expressed. Consequently, these DNA segments could in principle serve as 5' and 3' homology regions to create a "universal" targeting vector for replacing the constant region exons in the IgH locus of any hybridoma cell. The practicality of this vector design has been uncertain. That is, the extent of the chromosomal DNA which would be replaced by a universal targeting vector would be as little as 5 kb (in a cell producing the alpha heavy chain) and as much as 180 kb (in a micro -producing cell), and it has been uncertain whether it would be practical to generate such long chromosomal deletions by gene targeting. Using a vector of this design, we found (a) that correctly targeted recombinant cells lacking the 180 kb DNA segment occurred at a low but usable frequency, (b) that these recombinants expressed the modified IgH locus at the same rate as the original hybridoma and (c) that IgH expression in these cell lines was stable. Our results thus indicate that this vector design is suitable for modifying IgH loci expressing any heavy chain, provided that an efficient selection or screening for targeted recombinants is available.

Aberrant transcription of unrearranged T-cell receptor ? gene in mouse brain

The nervous system and the immune system share several functional molecules involved in various cell-cell interaction events. In this study, we used in situ hybridization to identify immune molecules that are expressed by a restricted population of neurons in the mouse brain and found that mRNA for the beta subunit of T-cell receptor (TCRbeta) was predominantly and strongly localized to neurons in deep layers of the cerebral neocortex and weakly expressed in the thalamus. Developmentally, TCRbeta mRNA expression started at embryonic day 15 in the thalamic nuclei and at postnatal day 1 in the cerebral neocortex. The level of TCRbeta mRNA in the neocortex subsequently increased until postnatal day 21, and it remained high in the adult. Detailed analysis revealed that only the Cbeta2 segment of TCRbeta, not the Cbeta1 or Vbeta segments, was expressed by the brain neurons. By the 5' rapid amplification of cDNA ends method, we determined a brain-specific transcription start site in the Jbeta2 region locus, not in the Vbeta region locus. Furthermore, we confirmed that the aberrant transcription around the Jbeta2 region took place only in neurons and lymphocytes in transgenic mice. These results demonstrate that the transcriptional machinery for unrearranged TCRbeta expression is shared by the nervous and immune systems and raise a possibility of gene rearrangement in neurons under certain circumstances.

Contemporary gene targeting strategies for the novice

Gene targeting in mouse embryonic stem (ES) cells is a fundamental methodology for generating mice with precise genetic modifications. Although there are many complex gene targeting strategies for creating a variety of diverse mutations in mice, most investigators initially choose to generate a null allele. Here we provide a guide for the novice to generate a null allele for a protein coding gene using a fundamental gene targeting strategy. Ultimately, a well considered gene targeting strategy saves significant amounts of time, money, and research animal lives. The straightforward strategy presented here bypasses many of the pitfalls associated with gene knockouts generated by novices. This guide also serves as a foundation for subsequently designing more complex gene targeting strategies.

Cutting edge: germinal centers can be induced in the absence of T cells

Immunization of mice containing mutations that inactivate the TCR Cbeta and Cdelta genes with the T cell-independent (TI) type 2 Ag (4-hydroxy-3-nitrophenyl)acetyl-Ficoll induces clusters of peanut agglutinin-binding B cells in the spleen. These clusters are histologically indistinguishable from germinal centers (GCs) typical of T cell-dependent immune responses. They are located in follicles, and contain mature follicular dendritic cells, immune complex deposits, and B cells that display the phenotypic qualities of conventional GC B cells. However, the kinetics of this TI GC response differ from T cell-dependent GC responses in being rapidly induced and of short duration. Moreover, the Ab V genes expressed in TI GCs have not undergone somatic hypermutation. Therefore, T cells may be required for B cell differentiation processes associated with the intermediate and latter stages of the GC reaction, but they are dispensable for the induction and initial development of this response.

Self-limiting systemic autoimmune disease during reconstitution of T cell-deficient mice with syngeneic T cells: support for a multifaceted role of T cells in the maintenance of peripheral B cell tolerance

The T cell compartment can be partially reconstituted in mice with targeted inactivation of the TCR C(beta) and C(delta) genes by injection of mature, syngeneic T cells. Surprisingly, during this reconstitution high titers of IgG anti-nuclear antibodies and symptoms of systemic autoimmune disease develop. However, this autoimmune response is transient and aged, reconstituted mice show no overt signs of disease. The autoantibody response appears to be derived from a pre-existing population of host self-reactive B cells and requires CD40 ligand-mediated co-stimulation from donor cells. Diminution of this response is coincident with a vigorous germinal center reaction and the disappearance of a subpopulation of activated B cells that expresses elevated levels of Fas. Collectively, our data support the idea that T cells play a multifaceted role in the maintenance of peripheral B cell tolerance that includes mediating the activation-induced death of autospecific B cells.

Population biology of lymphocytes: the flight for survival

In this essay we suggest that the primary goal of the cells of the immune system is to ensure their own growth and survival. In adults, in steady-state conditions, the number and distribution of lymphocyte populations is under homeostatic control. New lymphocytes that are continuously produced in primary and secondary lymphoid organs must compete with resident cells for survival. We discuss recent findings supporting lymphocyte survival as a continuous active process and implicating cognate receptor engagement as fundamental survival signals for both T and B lymphocytes. The conflict of survival interests between different cell types gives rise to a pattern of interactions that mimics the behavior of complex ecological systems. In their flight for survival and in response to competition, lymphocytes use different survival signals within different ecological niches during cell differentiation. This is the case for T and B lymphocytes and also for naive and memory/activated T and B cells. We discuss how niche differentiation allows the co-existence of different cell types and guarantees both repertoire diversity and efficient immune responses.

Dual renin gene targeting by Cre-mediated interchromosomal recombination

This study describes a new approach to targeting clustered genes. Our study began with the establishment of two lines of mice carrying different mutations in either Ren1 or Ren2. These two genes, both encoding renin, span over 40 kb in tandem on chromosome 1. Each gene was mutated by gene targeting to contain loxP sites. These two mutants and Cre transgenic mice were mated to produce offspring carrying the mutant Ren1 and Ren2 genes, as well as the Cre transgene concurrently. Initially, two mutant Ren genes were located on separate chromosomes. Southern analysis of mice from the second generation revealed that the mutant Ren1 and Ren2 were interchromosomally recombined at the loxP sites to produce a new dually mutated allele on the chromosome at the rate of 9.6% (7/73). Thus, interchromosomal recombination can be efficiently programmed by mating as designed using the Cre-loxP system.

Proinflammatory roles of T-cell receptor (TCR)gammadelta and TCRalphabeta lymphocytes in a murine model of asthma

The role of lymphocytes bearing alphabeta or gammadelta T-cell receptors (TCRs) was assessed during the acute allergic response in a mouse model of asthma. The inflammatory immune response to ovalbumin (OVA) was characterized in wild-type C57BL/6J mice and congenic TCRbeta(-/-) and TCRdelta(-/-) mice by evaluation of airway eosinophilia, histopathology, serum immunoglobulin (Ig)E levels, and in vivo airway responsiveness to methacholine. OVA-challenged wild-type mice demonstrated marked pulmonary inflammation, evidenced by airway eosinophilia (68 +/- 7 x 10(4) cells), peribronchial lympho-plasmocytic infiltration, and elevated serum IgE (4.9 +/- 0.6 microg/ml). These responses were markedly attenuated in TCRdelta(-/-) animals (5.0 +/- 1.0 x 10(4) eosinophils and 1.6 +/- 0. 3 microg/ml IgE) and were completely absent in TCRbeta(-/-) mice (< 1 x 10(3) eosinophils and 0.38 +/- 0.21 microg/ml IgE). Similar results were observed in mice treated with anti-TCRgammadelta or anti-TCRalphabeta monoclonal antibodies. Airway responsiveness to aerosolized methacholine was also reduced in challenged TCRdelta(-/-) animals relative to challenged wild-type mice. These results demonstrate that acute allergic airway responses are dependent upon intact TCRalphabeta and TCRgammadelta lymphocyte function and that TCRgammadelta cells promote acute airway sensitization.

Mice cloned from embryonic stem cells

Cloning allows the asexual reproduction of selected individuals such that the offspring have an essentially identical nuclear genome. Cloning by nuclear transfer thus far has been reported only with freshly isolated cells and cells from primary cultures. We previously reported a method of cloning mice from adult somatic cells after nuclear transfer by microinjection. Here, we apply this method to clone mice from widely available, established embryonic stem (ES) cell lines at late passage. With the ES cell line R1, 29% of reconstructed oocytes developed in vitro to the morula/blastocyst stage, and 8% of these embryos developed to live-born pups when transferred to surrogate mothers. We thus cloned 26 mice from R1 cells. Nuclei from the ES cell line E14 also were shown to direct development to term. We present evidence that the nuclei of ES cells at G(1)- or G(2)/M-phases are efficiently able to support full development. Our findings demonstrate that late-passage ES cells can be used to produce viable cloned mice and provide a link between the technologies of ES cells and animal cloning. It thus may be possible to clone from a single cell a large number of individuals over an extended period.

Enrichment and efficient screening of ES cells containing a targeted mutation: the use of DT-A gene with the polyadenylation signal as a negative selection maker

Gene targeting in embryonic stem (ES) cells via homologous recombination can occur at very low frequency. In order to enrich homologous recombinants before screening, a negative selection marker, such as thymidine kinase (TK) and diphtheria toxin A fragment (DT-A), has been commonly used. In this study, we developed a negative selection marker using DT-A gene with polyadenylation signal and it was designated DT-ApA. To determine the difference in targeting efficiency of the negative selections, we constructed three different targeting vectors for each negative selection (first, TK at the 3' end, second, TK at both the 5' and 3' ends < 2 X TK >, and third, DT-ApA at the 5' end of the homologous sequences). Gene targeting experiments using these constructs clearly showed that negative selection using DT-ApA was more efficient than that using TK for homologous recombination and that negative selection using DT-ApA was as efficient as that using 2 X TK. Considering the fact that the use of DT-ApA is more convenient for construction of targeting vectors than that of 2 X TK, DT-ApA is an efficient negative selection marker. In addition, we examined long and accurate PCR (LA-PCR) for screening gene targeted clones. The use of LA-PCR with genomic DNAs from ES cell clones facilitated simple detection of homologous recombinants, suggesting that the screening with LA-PCR is compatible with the use of longer homologous sequences of both arms in vector design. Our results indicate that the use of DT-ApA for negative selection together with the application of LA-PCR for screening ensures efficient and time-saving screening for homologous recombinants.

Gene targeting of retinoid receptors

Gene targeting in embryonic stem (ES) cells has been employed to investigate the role of the retinoid receptors and binding proteins both in the mouse as well as in embryocarcinoma cells. It is a powerful technique for the modification of the mouse genome. With more recent refinements in gene targeting technology, it is now possible to introduce more subtle mutations in the murine genome, as well as to investigate gene function in a tissue and temporally-restricted manner. It should also be possible to modify genes in diverse diploid cell lines, to generate diverse model systems for analysis of retinoid receptor function. In this article, some of the basic principles for gene targeting are described.