Peter Mombaerts

RAG-1-deficient mice have no mature B and T lymphocytes

The V(D)J recombination activation gene RAG-1 was isolated on the basis of its ability to activate V(D)J recombination on an artificial substrate in fibroblasts. This property and the expression pattern in tissues and cell lines indicate that RAG-1 either activates or catalyzes the V(D)J recombination reaction of immunoglobulin and T cell receptor genes. We here describe the introduction of a mutation in RAG-1 into the germline of mice via gene targeting in embryonic stem cells. RAG-1-deficient mice have small lymphoid organs that do not contain mature B and T lymphocytes. The arrest of B and T cell differentiation occurs at an early stage and correlates with the inability to perform V(D)J recombination. The immune system of the RAG-1 mutant mice can be described as that of nonleaky scid mice. Although RAG-1 expression has been reported in the central nervous system of the mouse, no obvious neuroanatomical or behavioral abnormalities have been found in the RAG-1-deficient mice.

Visualizing an Olfactory Sensory Map

We have developed a genetic approach to visualize axons from olfactory sensory neurons expressing a given odorant receptor, as they project to the olfactory bulb. Neurons expressing a specific receptor project to only two topographically fixed loci among the 1800 glomeruli in the mouse olfactory bulb. Our data provide direct support for a model in which a topographic map of receptor activation encodes odor quality in the olfactory bulb. Receptor swap experiments suggest that the olfactory receptor plays an instructive role in the guidance process but cannot be the sole determinant in the establishment of this map. This genetic approach may be more broadly applied to visualize the development and plasticity of projections in the mammalian nervous system.

Spontaneous development of inflammatory bowel disease in T cell receptor mutant mice

We describe the spontaneous development of inflammatory bowel disease (IBD) in several immunodeficient mouse strains created via gene targeting in embryonic stem cells. Chronic colitis was observed in T cell receptor (TCR) alpha mutant, TCR beta mutant, TCR beta x delta double mutant, or class II major histocompatibility complex (MHC) mutant mice, but not in recombination-activating gene RAG-1 mutant mice or nude mice kept in the same specific pathogen-free animal facility. This clinical pattern suggests that the disease requires the presence of B lymphocytes and the absence of class II MHC-restricted CD4+ alpha beta T cells. IBD in the mutant mice has some of the features of the human disease ulcerative colitis. Based on these results, we suggest that dysfunction of the mucosal immune system may underly the pathogenesis of some types of IBD in humans.

T cell receptor δ gene mutant mice: Independent generation of αβ T cells and programmed rearrangements of γδ TCR genes

Abstract T cells bearing T cell receptor (TCR) γ and δ chain heterodimers are first generated early in ontogeny. They form distinct subsets that differ in their TCR repertoires and tissue distribution. Disruption of the mouse TCR Cδ gene segment by a gene targeting method caused the complete loss of T cells bearing TCR γδ chains, but had little or no effect on the development of T cells bearing TCR αβ chains. The analyses of TCR γ and δ genes in the mutant mice suggest that intracellular mechanisms acting at the level of DNA rearrangement play key roles in the differential γ and δ gene rearrangements and in the generation of the highly restricted junctional sequences during fetal thymic development.

An activated Ick transgene promotes thymocyte development in rag-1 mutant mice

Expression of the T cell receptor beta (TCR beta) chain is necessary for the transition from the CD4CD8- stage in the major alpha beta thymocyte lineage. The protein tyrosine kinase p56lck has been implicated in the regulation of early thymocyte differentiation and of allelic exclusion at the TCR beta locus. Using mice overexpressing an activated lck transgene and mice with a disruption of the lck gene, we demonstrate that p56lck participates in a pathway that regulates the expansion of the pool of CD4+CD8+ thymocytes to wild-type levels. In addition, p56lck may be involved in the down-regulation of the putative pre-TCR on CD4+CD8+ thymocytes.

Odorant receptors instruct functional circuitry in the mouse olfactory bulb

The mammalian olfactory system detects and discriminates thousands of odorants using many different receptors expressed by sensory neurons in the nasal epithelium. Axonal projections from these neurons to the main olfactory bulbs form reproducible patterns of glomeruli in two widely separated regions of each bulb, creating two mirror-symmetric maps of odorant receptor projections. To investigate whether odorant receptors organize neural circuitry in the olfactory bulb, we have examined a genetically modified mouse line, rI7 → M71, in which a functionally characterized receptor, rI7, has been substituted into the M71 receptor locus. Here we show that despite their ectopic location the resulting glomeruli are responsive to known ligands of the rI7 receptor, attract postsynaptic innervation by mitral/tufted cell dendrites, and endow these cells with responses that are characteristic of the rI7 receptor. External tufted cells receiving input from rI7 → M71 glomeruli form precise intrabulbar projections that link medial and lateral rI7 → M71 glomeruli anatomically, thus providing a substrate for coordinating isofunctional glomeruli. We conclude that odorant receptor identity in epithelial neurons determines not only glomerular convergence and function, but also functional circuitry in the olfactory bulb.

Regulation of thymocyte development through CD3: Functional dissociation between p56lck and CD3ζ in early thymic selection

We studied the extent of functional linkage between CD3 sigma and p56lck in pre-TCR-dependent thymocyte development. Differentiation of DN to DP cells was examined by treatment of RAG2/CD3 sigma and RAG1/p56lck double-deficient mice with anti-CD3 epsilon antibodies. The results suggest that CD3 sigma has no specific role in this maturation step, but may be important for amplification of signaling through the pre-TCR. In contrast, p56lck is the main protein tyrosine kinase associated with signaling through the pre-TCR-CD3 complex. In DP thymocytes, the Ca2+ response to anti-CD3 epsilon was totally abolished in CD3 sigma-I-but only reduced in p56lck-I-mice, and in vivo responses to anti-CD3 epsilon differed from one another. Thus, CD3 sigma and p56lck are functionally not tightly associated and their deficiencies cause distinct developmental defects.