The proper function of immune surveillance requires well-coordinated mechanisms in order to guide the patrolling immune cells through peripheral tissues and into secondary lymphoid organs. Analyzing gene-targeted mice, we identified the chemokine receptor CCR7 as an important organizer of the primary immune response. CCR7-deficient mice show severely delayed kinetics regarding the antibody response and lack contact sensitivity and delayed type hypersensitivity reactions. Due to the impaired migration of lymphocytes, these animals reveal profound morphological alterations in all secondary lymphoid organs. Upon activation, mature skin dendritic cells fail to migrate into the draining lymph nodes. Thus, in order to bring together lymphocytes and dendritic cells to form the characteristic microarchitecture of secondary lymphoid organs, CCR7 is required to rapidly initiate an adoptive immune response.

Megalin is an endocytic receptor highly expressed in the proximal tubules of the kidney. Recently, we demonstrated that this receptor is essential for the renal uptake and conversion of 25-OH vitamin D3 to 1,25-(OH)2 vitamin D3, a central step in vitamin D and bone metabolism. Unfortunately, the perinatal lethality of the conventional megalin knockout mouse model precluded the detailed analysis of the significance of megalin for calcium homeostasis and bone turnover in vivo. Here, we have generated a new mouse model with conditional inactivation of the megalin gene in the kidney by using Cre recombinase. Animals with a renal-specific receptor gene defect were viable and fertile. However, lack of receptor expression in the kidney results in plasma vitamin D deficiency, in hypocalcemia and in severe bone disease, characterized by a decrease in bone mineral content, an increase in osteoid surfaces, and a lack of mineralizing activity. These features are consistent with osteomalacia (softening of the bones) as a consequence of hypovitaminosis D and demonstrate the crucial importance of the megalin pathway for systemic calcium homeostasis and bone metabolism.

O-mannosylation is an important protein modification in eukaryotes that is initiated by an evolutionarily conserved family of protein O -mannosyltransferases. The first mammalian protein O -mannosyltransferase gene described was the human POMT1 . Mutations in the hPOMT1 gene are responsible for Walker–Warburg syndrome (WWS), a severe recessive congenital muscular dystrophy associated with defects in neuronal migration that produce complex brain and eye abnormalities. During embryogenesis, the murine Pomt1 gene is prominently expressed in the neural tube, the developing eye, and the mesenchyme. These sites of expression correlate with those in which the main tissue alterations are observed in WWS patients. We have inactivated a Pomt1 allele by gene targeting in embryonic stem cells and produced chimeras transmitting the defect allele to offspring. Although heterozygous mice were viable and fertile, the total absence of Pomt1 – / – pups in the progeny of heterozygous intercrosses indicated that this genotype is embryonic lethal. An analysis of the mutant phenotype revealed that homozygous Pomt1 – / – mice suffer developmental arrest around embryonic day (E) 7.5 and die between E7.5 and E9.5. The Pomt1 – / – embryos present defects in the formation of Reichert's membrane, the first basement membrane to form in the embryo. The failure of this membrane to form appears to be the result of abnormal glycosylation and maturation of dystroglycan that may impair recruitment of laminin, a structural component required for the formation of Reichert's membrane in rodents. The targeted disruption of mPomt1 represents an example of an engineered deletion of a known glycosyltransferase involved in O -mannosyl glycan synthesis.

Prion diseases are neurodegenerative infectious disorders for which no prophylactic regimens are known. In order to induce antibodies/auto-antibodies directed against surface-located PrP c , we used a covalently linked dimer of mouse prion protein expressed recombinantly in Escherichia coli. Employing dimeric PrP as an immunogen we were able to effectively overcome autotolerance against murine PrP in PrP wild-type mice without inducing obvious side effects. Treatment of prion-infected mouse cells with polyclonal anti-PrP antibodies generated in rabbit or auto-antibodies produced in mice significantly inhibited endogenous PrP Sc synthesis. We show that polyclonal antibodies are binding to surface-located PrP c , thereby interfering with prion biogenesis. This effect is much more pronounced in the presence of full IgG molecules, which, unlike Fab fragments, seem to induce a significant cross-linking of surface PrP. In addition, we found immune responses against different epitopes when comparing antibodies induced in rabbits and PrP wild-type mice. Only in the auto-antibody situation in mice an immune reaction against a region of PrP is found that was reported to be involved in the PrP Sc conversion process. Our data point to the possibility of developing means for an active immunoprophylaxis against prion diseases.

SePP (selenoprotein P) is central for selenium transport and distribution. Targeted inactivation of the Sepp gene in mice leads to reduced selenium content in plasma, kidney, testis and brain. Accordingly, activities of selenoenzymes are reduced in Sepp(-/-) organs. Male Sepp(-/-) mice are infertile. Unlike selenium deficiency, Sepp deficiency leads to neurological impairment with ataxia and seizures. Hepatocyte-specific inactivation of selenoprotein biosynthesis reduces plasma and kidney selenium levels similarly to Sepp(-/-) mice, but does not result in neurological impairment, suggesting a physiological role of locally expressed SePP in the brain. In an attempt to define the role of liver-derived circulating SePP in contrast with locally expressed SePP, we generated Sepp(-/-) mice with transgenic expression of human SePP under control of a hepatocyte-specific transthyretin promoter. Secreted human SePP was immunologically detectable in serum from SEPP1-transgenic mice. Selenium content and selenoenzyme activities in serum, kidney, testis and brain of Sepp(-/-;SEPP1) (SEPP1-transgenic Sepp(-/-)) mice were increased compared with Sepp(-/-) controls. When a selenium-adequate diet (0.16-0.2 mg/kg of body weight) was fed to the mice, liver-specific expression of SEPP1 rescued the neurological defects of Sepp(-/-) mice and rendered Sepp(-/-) males fertile. When fed on a low-selenium diet (0.06 mg/kg of body weight), Sepp(-/-;SEPP1) mice survived 4 weeks longer than Sepp(-/-) mice, but ultimately developed the neurodegenerative phenotype. These results indicate that plasma SePP derived from hepatocytes is the main transport form of selenium supporting the kidney, testis and brain. Nevertheless, local Sepp expression is required to maintain selenium content in selenium-privileged tissues such as brain and testis during dietary selenium restriction.