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ABSTRACTIn most human and animal prion diseases the abnormal disease-associated prion protein (PrPSc) is deposited as non-amyloid aggregates in CNS, spleen and lymphoid organs. In contrast, in humans and transgenic mice with PrP mutations which cause expression of PrP lacking a glycosylphosphatidylinositol (GPI)-anchor, most PrPSc is in the amyloid form. In transgenic mice expressing only anchorless PrP (tg anchorless), PrPSc is deposited not only in CNS and lymphoid tissues, but also in extraneural tissues including heart, brown fat, white fat, and colon. In the present paper, we report ultrastructural studies of amyloid PrPSc deposition in extraneural tissues of scrapie-infected tg anchorless mice. Amyloid PrPSc fibrils identified by immunogold-labeling were visible at high magnification in interstitial regions and around blood vessels of heart, brown fat, white fat, colon, and lymphoid tissues. PrPSc amyloid was located on and outside the plasma membranes of adipocytes in brown fat and cardiomyocytes, and appeared to invaginate and disrupt the plasma membranes of these cell types, suggesting cellular damage. In contrast, no cellular damage was apparent near PrPSc associated with macrophages in lymphoid tissues and colon, with enteric neuronal ganglion cells in colon or with adipocytes in white fat. PrPSc localized in macrophage phagolysosomes lacked discernable fibrils and might be undergoing degradation. Furthermore, in contrast to wild-type mice expressing GPI-anchored PrP, in lymphoid tissues of tg anchorless mice, PrPSc was not associated with follicular dendritic cells (FDC), and FDC did not display typical prion-associated pathogenic changes.

ABSTRACTTauopathies are a family of neurodegenerative diseases in which fibrils of human hyperphosphorylated tau (P-tau) are believed to cause neuropathology. In Alzheimer disease, P-tau associates with A-beta amyloid and contributes to disease pathogenesis. In familial human prion diseases and variant CJD, P-tau often co-associates with prion protein amyloid, and might also accelerate disease progression. To test this latter possibility, here we compared progression of amyloid prion disease in vivo after scrapie infection of mice with and without expression of human tau. The mice used expressed both anchorless prion protein (PrP) and membrane-anchored PrP, that generate disease associated amyloid and non-amyloid PrP (PrPSc) after scrapie infection. Human P-tau induced by scrapie infection was only rarely associated with non-amyloid PrPSc, but abundant human P-tau was detected at extracellular, perivascular and axonal deposits associated with amyloid PrPSc. This pathology was quite similar to that seen in familial prion diseases. However, association of human and mouse P-tau with amyloid PrPSc did not diminish survival time following prion infection in these mice. By analogy, human P-tau may not affect prion disease progression in humans. Alternatively, these results might be due to other factors, including rapidity of disease, blocking effects by mouse tau, or low toxicity of human P-tau in this model.

The use of C57BL/6 Rag2−/−γc−/− mice as recipients for xenotransplantation with human immune systems (humanization) has been problematic because C57BL/6 SIRPα does not recognize human CD47, and such recognition is required to suppress macrophage-mediated phagocytosis of transplanted human hematopoietic stem cells (HSCs). We show that genetic inactivation of CD47 on the C57BL/6 Rag2−/−γc−/− background negates the requirement for CD47-signal recognition protein α (SIRPα) signaling and induces tolerance to transplanted human HSCs. These triple-knockout, bone marrow, liver, thymus (TKO-BLT) humanized mice develop organized lymphoid tissues including mesenteric lymph nodes, splenic follicles and gut-associated lymphoid tissue that demonstrate high levels of multilineage hematopoiesis. Importantly, these mice have an intact complement system and showed no signs of graft-versus-host disease (GVHD) out to 29 weeks after transplantation. Sustained, high-level HIV-1 infection was observed via either intrarectal or intraperitoneal inoculation. TKO-BLT mice exhibited hallmarks of human HIV infection including CD4+ T-cell depletion, immune activation, and development of HIV-specific B- and T-cell responses. The lack of GVHD makes the TKO-BLT mouse a significantly improved model for long-term studies of pathogenesis, immune responses, therapeutics, and vaccines to human pathogens.

The use of C57BL/6 Rag2−/−γc−/−mice as recipients for xenotransplantation with human immune systems (humanization) has been problematic because C57BL/6 SIRPα does not recognize human CD47, and such recognition is required to suppress macrophage-mediated phagocytosis of transplanted human hematopoietic stem cells (HSCs). We show that genetic inactivation of CD47on the C57BL/6 Rag2−/−γc−/−background negates the requirement for CD47-signal recognition protein α (SIRPα) signaling and induces tolerance to transplanted human HSCs. These triple-knockout, bone marrow, liver, thymus (TKO-BLT) humanized mice develop organized lymphoid tissues including mesenteric lymph nodes, splenic follicles and gut-associated lymphoid tissue that demonstrate high levels of multilineage hematopoiesis. Importantly, these mice have an intact complement system and showed no signs of graft-versus-host disease (GVHD) out to 29 weeks after transplantation. Sustained, high-level HIV-1 infection was observed via either intrarectal or intraperitoneal inoculation. TKO-BLT mice exhibited hallmarks of human HIV infection including CD4+T-cell depletion, immune activation, and development of HIV-specific B- and T-cell responses. The lack of GVHD makes the TKO-BLT mouse a significantly improved model for long-term studies of pathogenesis, immune responses, therapeutics, and vaccines to human pathogens.

Prion protein (PrP) is a cell surface glycoprotein which is required for susceptibility to prion infection and disease. However, PrP is expressed in many different cell types located in numerous organs. Therefore, in addition to its role in prion diseases, PrP may have a large variety of other biological functions involving the nervous system and other systems. We recently showed that susceptibility to kainate-induced seizures differed in Prnp−/−and Prnp+/+ mice on the C57BL/10SnJ background. However, in a genetic complementation experiment a PrP expressing transgene was not able to rescue the Prnp+/+phenotype. Thus the apparent effect of PrP on seizures was actually due to genes flanking the Prnp−/−gene rather that the Prnp deletion itself. We discuss here several pitfalls in the use of Prnp−/− genotypes expressed in various mouse genetic backgrounds to determine the functions of PrP. In particular, the use of Prnp−/−mice with heterogeneous mixed genetic backgrounds may have weakened the conclusions of many previous experiments. Use of either co-isogenic mice or congenic mice with more homogeneous genetic backgrounds is now feasible. For congenic mice, the potential problem of flanking genes can be mitigated by the use of appropriate transgene rescue experiments to confirm the conclusions.

ABSTRACTThe current studies demonstrate complex and seemingly contradictory effects by gamma interferon (IFN-?) on Friend virus (FV) infection. Both temporal and tissue-specific effects were observed. During the first week of infection, IFN-?-deficiency caused increased levels of FV infection in multiple tissues. Surprisingly, however, by 2 weeks postinfection, IFN-?-deficient mice had significantly lower levels of infection in both the spleen and bone marrow compared to wild-type mice. The rapid reduction of virus in the IFN-?-deficient mice correlated with a more rapid virus-neutralizing antibody response than was observed in the wild-type mice. Furthermore, the virus-neutralizing antibody response in wild-type mice could be accelerated by ablation of their IFN-? response. Although the IFN-?-deficient mice developed an accelerated virus-neutralizing antibody response, they did not class-switch to immunoglobulin G class immunoglobulins nor could they maintain long-term virus-neutralizing antibody titers. Eventually, all of the IFN-?-deficient mice failed to keep persistent virus in check and developed fatal FV-induced erythroleukemia.

ABSTRACTIt is known that both animal and human retroviruses typically cause immunosuppression in their respective hosts, but the mechanisms by which this occurs are poorly understood. The present study uses Friend virus (FV) infections of mice as a model to determine how major histocompatibility complex (MHC) genes influence immunosuppression. Previously, MHC-I genes were shown to influence antibody responses to potent antigenic challenges given during acute FV infection. The mapping of an immune response to an MHC-I gene implicated CD8+T cells in the mechanism, so we directly tested for their role by using in vivo CD8+T-cell depletions. Mice resistant to FV-induced immunosuppression became susceptible when they were depleted of CD8+T cells. Resistance also required gamma interferon (IFN-?), as in vivo neutralization of IFN-? converted mice from a resistant to susceptible phenotype. On the other hand, susceptibility to FV-induced immunosuppression was dependent on the immunosuppressive cytokine, interleukin-10 (IL-10), as antibody responses were restored in susceptible mice when IL-10 function was blocked in vivo. Thus, FV-induced immunosuppression of antibody responses involves complex mechanisms controlled at least in part by CD8+T cells.

It is known that both animal and human retroviruses typically cause immunosuppression in their respective hosts, but the mechanisms by which this occurs are poorly understood. The present study uses Friend virus (FV) infections of mice as a model to determine how major histocompatibility complex (MHC) genes influence immunosuppression. Previously, MHC-I genes were shown to influence antibody responses to potent antigenic challenges given during acute FV infection. The mapping of an immune response to an MHC-I gene implicated CD8+ T cells in the mechanism, so we directly tested for their role by using in vivo CD8+ T-cell depletions. Mice resistant to FV-induced immunosuppression became susceptible when they were depleted of CD8+ T cells. Resistance also required gamma interferon (IFN-gamma), as in vivo neutralization of IFN-gamma converted mice from a resistant to susceptible phenotype. On the other hand, susceptibility to FV-induced immunosuppression was dependent on the immunosuppressive cytokine, interleukin-10 (IL-10), as antibody responses were restored in susceptible mice when IL-10 function was blocked in vivo. Thus, FV-induced immunosuppression of antibody responses involves complex mechanisms controlled at least in part by CD8+ T cells.

ABSTRACTThe immunological resistance of a host to viral infections may be strongly influenced by cytokines such as interleukin-12 (IL-12) and gamma interferon (IFN-?), which promote T helper type 1 responses, and IL-4, which promotes T helper type 2 responses. We studied the role of these cytokines during primary and secondary immune responses against Friend retrovirus infections in mice. IL-4- and IL-12-deficient mice were comparable to wild-type B6 mice in the ability to control acute and persistent Friend virus infections. In contrast, more than one-third of the IFN-?-deficient mice were unable to maintain long-term control of Friend virus and developed gross splenomegaly with high virus loads. Immunization with a live attenuated vaccine virus prior to challenge protected all three types of cytokine-deficient mice from viremia and high levels of spleen virus despite the finding that the vaccinated IFN-?-deficient mice were unable to class switch from immunoglobulin M (IgM) to IgG virus-neutralizing antibodies. The results indicate that IFN-? plays an important role during primary immune responses against Friend virus but is dispensable during vaccine-primed secondary responses.

ABSTRACTVaccination of mice with a live attenuated vaccine virus induces potent protection against subsequent challenge with pathogenic Friend retroviral complex. The kinetic studies presented here demonstrate protection from acute splenomegaly as early as 1 week postvaccination. At this time point virus-specific cytotoxic T lymphocytes (CTL) were demonstrable in direct chromium release assays. However, during the first 2 weeks after vaccination protection was incomplete since the mice were not protected against establishment of low-level persistent infections in the spleen. By 3 weeks postvaccination the animals were protected against the establishment of persistent virus as well as acute splenomegaly. The timing of this complete protection correlated with the presence of both virus-neutralizing antibodies and primed CTL in the immunized mice. Within 3 days of virus challenge, vaccinated mice showed high levels of activated B cells and CD4+and CD8+T cells, indicating an efficient priming of all lymphocyte subsets. Despite very limited replication of the vaccine virus, the protective effect was long lived and was still present 6 months after immunization.

ABSTRACTTo assess prospects for early diagnosis of prion disease based on prion seeding activity in cerebrospinal fluid (CSF), we measured the activity over time in scrapie-infected hamsters by real-time quaking-induced conversion (RT-QuIC). After intracerebral inoculation, activity appeared in CSF within 1 day and plateaued weeks before the onset of clinical signs. However, after intratongue inoculation, activity first appeared in CSF with the onset of clinical signs, well after higher-level accumulation of seeding activity in brain.

ABSTRACTAggregation of misfolded host proteins in the central nervous system is believed to be important in the pathogenic process in several neurodegenerative diseases of humans, including prion diseases, Alzheimer's disease, and Parkinson's disease. In these diseases, protein misfolding and aggregation appear to expand through a process of seeded polymerization. Prion diseases occur in both humans and animals and are experimentally transmissible orally or by injection, thus providing a controllable model of other neurodegenerative protein misfolding diseases. In rodents and ruminants, prion disease has a slow course, lasting months to years. Although prion infectivity has been detected in brain tissue at 3 to 4 weeks postinfection (p.i.), the details of early prion replication in the brain are not well understood. Here we studied the localization and quantitation of PrPSc generation in vivostarting at 30 min postmicroinjection of scrapie into the brain. In C57BL mice at 3 days p.i., generation of new PrPSc was detected by immunohistochemistry and immunoblot assays, and at 7 days p.i., new generation was confirmed by real-time quaking-induced conversion assay. The main site of new PrPSc generation was near the outer basement membrane of small and medium blood vessels. The finding and localization of replication at this site so early after injection have not been reported previously. This predominantly perivascular location suggested that structural components of the blood vessel basement membrane or perivascular astrocytes might act as cofactors in the initial generation of PrPSc. The location of PrPSc replication at the basement membrane also implies a role for the brain interstitial fluid drainage in the early infection process.IMPORTANCENeurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and prion diseases, of humans are characterized by misfolding and aggregation of certain proteins, resulting in the destruction of brain tissue. In these diseases, the damage process spreads progressively within the central nervous system, but only prion diseases are known to be transmissible between individuals. Here we used microinjection of infectious prion protein (PrPSc) into the mouse brain to model early events of iatrogenic prion transmission via surgical instruments or tissue grafts. At 3 and 7 days postinjection, we detected the generation of new PrPSc, mostly on the outer walls of blood vessels near the injection site. This location and very early replication were surprising and unique. Perivascular prion replication suggested the transport of injected PrPSc via brain interstitial fluid to the basement membranes of blood vessels, where interactions with possible cofactors made by astrocytes or endothelia might facilitate the earliest cycles of prion infection.