Analysis of gene function in lymphocytes by RAG-2-deficient blastocyst complementation

Humanized Immunoglobulin Mice: Models for HIV Vaccine Testing and Studying the Broadly Neutralizing Antibody Problem

A vaccine that can effectively prevent HIV-1 transmission remains paramount to ending the HIV pandemic, but to do so, will likely need to induce broadly neutralizing antibody (bnAb) responses. A major technical hurdle toward achieving this goal has been a shortage of animal models with the ability to systematically pinpoint roadblocks to bnAb induction and to rank vaccine strategies based on their ability to stimulate bnAb development. Over the past 6 years, immunoglobulin (Ig) knock-in (KI) technology has been leveraged to express bnAbs in mice, an approach that has enabled elucidation of various B-cell tolerance mechanisms limiting bnAb production and evaluation of strategies to circumvent such processes. From these studies, in conjunction with the wealth of information recently obtained regarding the evolutionary pathways and paratopes/epitopes of multiple bnAbs, it has become clear that the very features of bnAbs desired for their function will be problematic to elicit by traditional vaccine paradigms, necessitating more iterative testing of new vaccine concepts. To meet this need, novel bnAb KI models have now been engineered to express either inferred prerearranged V(D)J exons (or unrearranged germline V, D, or J segments that can be assembled into functional rearranged V(D)J exons) encoding predecessors of mature bnAbs. One encouraging approach that has materialized from studies using such newer models is sequential administration of immunogens designed to bind progressively more mature bnAb predecessors. In this review, insights into the regulation and induction of bnAbs based on the use of KI models will be discussed, as will new Ig KI approaches for higher-throughput production and/or altering expression of bnAbs in vivo, so as to further enable vaccine-guided bnAb induction studies.

Mouse chimeras as a system to investigate development, cell and tissue function, disease mechanisms and organ regeneration

Chimeras are organisms composed of at least two genetically distinct cell lineages originating from different zygotes. In the laboratory, mouse chimeras can be produced experimentally; various techniques allow combining different early stage mouse embryos with each other or with pluripotent stem cells. Identification of the progeny of the different lineages in chimeras permits to follow cell fate and function, enabling correlation of genotype with phenotype. Mouse chimeras have become a tool to investigate critical developmental processes, including cell specification, differentiation, patterning, and the function of specific genes. In addition, chimeras can also be generated to address biological processes in the adult, including mechanisms underlying diseases or tissue repair and regeneration. This review summarizes the different types of chimeras and how they have been generated and provides examples of how mouse chimeras offer a unique and powerful system to investigate questions pertaining to cell and tissue function in the developing and adult organism.

Lessons learned from Myc/Max/Mad knockout mice

The past two decades of gene targeting experiments have allowed us to make significant strides towards understanding how the Myc/Max/Mad network influences multiple aspects of cellular behavior during development. Here we summarize the findings obtained from the myc/max/mad knockout mice generated to date, namely those in which the N-myc, c-myc, L-myc, mad1, mxi1, mad3, mnt, or max genes have been targeted. A compilation of lessons we have learned from these myc/max/mad knockout mouse models, and suggestions as to where future efforts could be focused, are also presented.

Chromatin dynamics and locus accessibility in the immune system

Development in vertebrates follows distinctive pathways of cellular differentiation. Starting from the zygote, newly formed cells continually differentiate until they reach a final mature fate. Whether differentiating into a neuron, a hepatocyte or a myofibril, every normal cell, with the exception of developing lymphocytes, carries the same genetic information enclosed within its nucleus. To acquire distinct cellular identities, cells need to control gene expression in a very regulated way. Genes encoding factors required for identity at a particular developmental stage need to be appropriately activated, whereas genes required for identity during the previous developmental stage are often silenced. Moreover, once a cell becomes terminally differentiated, 'heritable' gene expression must be maintained in all daughter cells and, thus, faithfully recapitulated after each cellular division.

Use of recombinase activation gene-2 deficient mice to ascertain the role of cellular and humoral immune responses in the development of chronic rejection

INTRODUCTION

Given its multifactorial etiology, the relative contribution of anti-donor cellular and humoral immune responses in the pathogenesis of chronic rejection is as yet ambiguous. We hypothesized that alloreactive T and B cells play a seminal role in the development of this lesion.

METHODS

To address this hypothesis, RAG-2(-/-) mice were used as donors and recipients in a well-established murine model of aortic transplantation. Grafts were transplanted across the following groups: Group I: C3H --> C3H; Group II: Wild-type [WT] 129Sv (H-2(b)) --> C3H (H-2(k)); Group III: C3H --> WT 129Sv; Group IV: 129SvEv RAG-2(-/-) --> C3H; and Group V: C3H --> 129SvEv RAG-2(-/-). Grafts were harvested at d40 to 146 post-transplantation for morphologic and immunohistochemical analyses and semi-quantitative RT-PCR was employed to evaluate the intragraft mRNA expression of various immune mediators. Mixed lymphocyte reaction and complement-mediated alloantibody cytotoxicity assays were performed to determine anti-donor proliferative and humoral responses, respectively.

RESULTS

Unlike that across the syngeneic combination (Group I), marked intimal thickening with corresponding luminal narrowing was observed in the majority of the aortic allografts (Groups II-IV). On the contrary, the morphology of C3H aortic allografts harvested from the majority of the RAG-2(-/-) was remarkably preserved. Correspondingly, anti-donor proliferative and humoral immune responses were undetectable in C3H --> RAG-2(-/-) recipients as was the intragraft mRNA expression of the Th(1) and the Th(2)-type cytokines.

CONCLUSIONS

Taken together, these data suggest that in this murine model of aortic allotransplantation, donor-specific cellular and humoral responses play a dominant role in the initiation and perpetuation of chronic rejection.

Requirement of Shp-2 tyrosine phosphatase in lymphoid and hematopoietic cell development

Shp-1 and Shp-2 are cytoplasmic phosphotyrosine phosphatases with similar structures. Mice deficient in Shp-2 die at midgestation with defects in mesodermal patterning, and a hypomorphic mutation at the Shp-1 locus results in the moth-eaten viable (me(v)) phenotype. Previously, a critical role of Shp-2 in mediating erythroid/myeloid cell development was demonstrated. By using the RAG-2-deficient blastocyst complementation, the role of Shp-2 in lymphopoiesis has been determined. Chimeric mice generated by injecting Shp-2(-/-) embryonic stem cells into Rag-2-deficient blastocysts had no detectable mature T and B cells, serum immunoglobulin M, or even Thy-1(+) and B220(+) precursor lymphocytes. Collectively, these results suggest a positive role of Shp-2 in the development of all blood cell lineages, in contrast to the negative effect of Shp-1 in this process. To determine whether Shp-1 and Shp-2 interact in hematopoiesis, Shp-2(-/-):me(v)/me(v) double-mutant embryos were generated and the hematopoietic cell development in the yolk sacs was examined. More hematopoietic stem/progenitor cells were detected in Shp-2(-/-):me(v)/me(v) embryos than in Shp-2(-/-) littermates. The partial rescue by Shp-1 deficiency of the defective hematopoiesis caused by the Shp-2 mutation suggests that Shp-1 and Shp-2 have antagonistic effects in hematopoiesis, possibly through a bidirectional modulation of the same signaling pathway(s).

RAG2-/-, I kappa B-alpha-/- chimeras display a psoriasiform skin disease

Nuclear factor-kappa B, a ubiquitous transcription factor involved in inflammatory and immune responses, is inappropriately activated in several immuno-related diseases, such as allograft rejection, or bronchial asthma. As nuclear factor-kappa B activity is regulated by inhibitor of kappa B (I kappa B), the gene encoding I kappa B-alpha was disrupted in mice to observe the in vivo effects of hyperactivation of nuclear factor-kappa B. I kappa B-alpha-/- mice have constitutive nuclear factor-kappa B activity, severe skin disease, and neonatal lethality. To determine the role of I kappa B-alpha deficient immunocytes in the pathogenesis of the skin disease in adult mice, we utilized the RAG2-deficient blastocyst complementation system to generate RAG2-/-, I kappa B-alpha-/- chimeras. These animals display a psoriasiform dermatitis characterized by hyperplastic epidermal keratinocytes and dermal infiltration of immunocytes, including lymphocytes. Skin grafts transferred from diseased chimeras to recipient nude mice produce hyperproliferative psoriasiform epidermal keratinocytes in response to stimulation. Furthermore, adoptive transfer of lymph node cells from diseased chimeras to RAG2-/- recipient mice recapitulates the disease. Taken together, these characterizations provide evidence to suggest that constitutive activation of nuclear factor-kappa B, due to deficiency in I kappa B-alpha, can invoke severe psoriasiform dermatitis in adult mice. J Invest Dermatol 115:1124-1133 2000

RAG2 is regulated differentially in B and T cells by elements 5' of the promoter

To study RAG2 gene regulation in vivo, we developed a blastocyst complementation method in which RAG2-deficient embryonic stem cells were transfected with genomic clones containing RAG2 and then assessed for their ability to generate lymphocytes. A RAG2 genomic clone that contained only the RAG2 promoter sequences rescued V(D)J recombination in RAG2-deficient pro-B cell lines, but did not rescue development of RAG2-deficient lymphocytes in vivo. However, inclusion of varying lengths of sequences 5' of the RAG2 promoter generated constructs capable of rescuing only in vivo B cell development, as well as other constructs that rescued both B and T cell development. In particular, the 2-kb 5' region starting just upstream of the RAG2 promoter, as well as the region from 2-7 kb 5', could independently drive B cell development, but not efficient T cell development. Deletion of the 2-kb 5' region from the murine germ line demonstrated that this region was not required for RAG expression sufficient to generate normal B or T cell numbers, implying redundancy among 5' elements. We conclude that RAG2 expression in vivo requires elements beyond the core promoter, that such elements contribute to differential regulation in the B vs. T lineages, and that sequences sufficient to direct B cell expression are located in the promoter-proximal 5' region.

Jak2 deficiency defines an essential developmental checkpoint in definitive hematopoiesis

Janus kinases (Jaks) play an important role in signal transduction via cytokine and growth factor receptors. A targeted inactivation of Jak2 was performed. Jak2-/- embryos are anemic and die around day 12.5 postcoitum. Primitive erythrocytes are found, but definitive erythropoiesis is absent. Compared to erythropoietin receptor-deficient mice, the phenotype of Jak2 deficiency is more severe. Fetal liver BFU-E and CFU-E colonies are completely absent. However, multilineage hematopoietic stem cells (CD34low, c-kit(pos)) can be found, and B lymphopoiesis appears intact. In contrast to IFNalpha stimulation, Jak2-/- cells do not respond to IFNgamma. Jak2-/- embryonic stem cells are competent for LIF signaling. The data provided demonstrate that Jak2 has pivotal functions for signal transduction of a set of cytokine receptors required in definitive erythropoiesis.