Prion Diseases and the Spleen

The role of cellular prion protein in immune system

Numerous studies have investigated the cellular prion protein (PrPC) since its discovery. These investigations have explained that its structure is predominantly composed of alpha helices and short beta sheet segments, and when its abnormal scrapie isoform (PrPSc) is infected, PrPSc transforms the PrPC, leading to prion diseases, including Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle. Given its ubiquitous distribution across a variety of cellular types, the PrPC manifests a diverse range of biological functions, including cell-cell adhesion, neuroprotection, signalings, and oxidative stress response. PrPC is also expressed in immune tissues, and its functions in these tissues include the activation of immune cells and the formation of secondary lymphoid tissues, such as the spleen and lymph nodes. Moreover, high expression of PrPC in immune cells plays a crucial role in the pathogenesis of prion diseases. In addition, it affects inflammation and the development and progression of cancer via various mechanisms. In this review, we discuss the studies on the role of PrPC from various immunological perspectives. [BMB Reports 2023; 56(12): 645-650].

IVIG Delays Onset in a Mouse Model of Gerstmann-Sträussler-Scheinker Disease

Our previous studies showed that intravenous immunoglobulin (IVIG) contained anti-Aβ autoantibodies that might be able to treat Alzheimer's disease (AD). Recently, we identified and characterized naturally occurring autoantibodies against PrP from IVIG. Although autoantibodies in IVIG blocked PrP fibril formation and PrP neurotoxicity in vitro, it remained unknown whether IVIG could reduce amyloid plaque pathology in vivo and be used to effectively treat animals with prion diseases. In this study, we used Gerstmann-Sträussler-Scheinker (GSS)-Tg (PrP-A116V) transgenic mice to test IVIG efficacy since amyloid plaque formation played an important role in GSS pathogenesis. Here, we provided strong evidence that demonstrates how IVIG could significantly delay disease onset, elongate survival, and improve clinical phenotype in Tg (PrP-A116V) mice. Additionally, in treated animals, IVIG could markedly inhibit PrP amyloid plaque formation and attenuate neuronal apoptosis at the age of 120 days in mice. Our results indicate that IVIG may be a potential, effective therapeutic treatment for GSS and other prion diseases.

Molecular dynamics studies on the buffalo prion protein

It was reported that buffalo is a low susceptibility species resisting to transmissible spongiform encephalopathies (TSEs) (same as rabbits, horses, and dogs). TSEs, also called prion diseases, are invariably fatal and highly infectious neurodegenerative diseases that affect a wide variety of species (except for rabbits, dogs, horses, and buffalo), manifesting as scrapie in sheep and goats; bovine spongiform encephalopathy (BSE or "mad-cow" disease) in cattle; chronic wasting disease in deer and elk; and Creutzfeldt-Jakob diseases, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, and Kulu in humans etc. In molecular structures, these neurodegenerative diseases are caused by the conversion from a soluble normal cellular prion protein (PrP(C)), predominantly with α-helices, into insoluble abnormally folded infectious prions (PrP(Sc)), rich in β-sheets. In this article, we studied the molecular structure and structural dynamics of buffalo PrP(C) (BufPrP(C)), in order to understand the reason why buffalo is resistant to prion diseases. We first did molecular modeling of a homology structure constructed by one mutation at residue 143 from the NMR structure of bovine and cattle PrP(124-227); immediately we found that for BufPrP(C)(124-227), there are five hydrogen bonds (HBs) at Asn143, but at this position, bovine/cattle do not have such HBs. Same as that of rabbits, dogs, or horses, our molecular dynamics studies also revealed there is a strong salt bridge (SB) ASP178-ARG164 (O-N) keeping the β2-α2 loop linked in buffalo. We also found there is a very strong HB SER170-TYR218 linking this loop with the C-terminal end of α-helix H3. Other information, such as (i) there is a very strong SB HIS187-ARG156 (N-O) linking α-helices H2 and H1 (if mutation H187R is made at position 187, then the hydrophobic core of PrP(C) will be exposed (L.H. Zhong (2010). Exposure of hydrophobic core in human prion protein pathogenic mutant H187R. Journal of Biomolecular Structure and Dynamics 28(3), 355-361)), (ii) at D178, there is a HB Y169-D178 and a polar contact R164-D178 for BufPrP(C) instead of a polar contact Q168-D178 for bovine PrP(C) (C.J. Cheng, & V. Daggett. (2014). Molecular dynamics simulations capture the misfolding of the bovine prion protein at acidic pH. Biomolecules 4(1), 181-201), (iii) BufPrP(C) owns three 310 helices at 125-127, 152-156, and in the β2-α2 loop, respectively, and (iv) in the β2-α2 loop, there is a strong π-π stacking and a strong π-cation F175-Y169-R164.(N)NH2, has been discovered.

Genetic prion disease: no role for the immune system in disease pathogenesis?

Prion diseases, which can manifest by transmissible, sporadic or genetic etiologies, share several common features, such as a fatal neurodegenerative outcome and the aberrant accumulation of proteinase K (PK)-resistant PrP forms in the CNS. In infectious prion diseases, such as scrapie in mice, prions first replicate in immune organs, then invade the CNS via ascending peripheral tracts, finally causing death. Accelerated neuroinvasion and death occurs when activated prion-infected immune cells infiltrate into the CNS, as is the case for scrapie-infected mice induced for experimental autoimmune encephalomyelitis (EAE), a CNS inflammatory insult. To establish whether the immune system plays such a central role also in genetic prion diseases, we induced EAE in TgMHu2ME199K mice, a line mimicking for late onset genetic Creutzfeldt Jacob disease (gCJD), a human prion disease. We show here that EAE induction of TgMHu2ME199K mice neither accelerated nor aggravated prion disease manifestation. Concomitantly, we present evidence that PK-resistant PrP forms were absent from CNS immune infiltrates, and most surprisingly also from lymph nodes and spleens of TgMHu2ME199K mice at all ages and stages of disease. These results imply that the mechanism of genetic prion disease differs widely from that of the infectious presentation, and that the conversion of mutant PrPs into PK resistant forms occurs mostly/only in the CNS. If the absence of pathogenic PrP forms form immune organs is also true for gCJD patients, it may suggest their blood is devoid of prion infectivity.

PrP(Sc)-specific antibodies do not induce prion disease or misfolding of PrP(C) in highly susceptible Tga20 mice

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders caused by misfolding of a cellular protein PrP(C) into an infectious conformation PrP(Sc). Previously our group demonstrated induction of PrP(Sc)-specific antibodies with a SN6b vaccine that targets regions of the protein that are exposed upon misfolding. There are concerns that these antibodies could function as templates to promote misfolding and cause disease. To evaluate the consequences of prolonged exposure to PrP(Sc)-specific antibodies in a prion sensitized animal, tga20 mice were vaccinated with the SN6b vaccine. No clinical signs of disease were detected up to 255 d post-vaccination, and postmortem assay of brains and spleens revealed no proteinase-K resistant PrP. These results suggest that vaccinating against TSEs with the SN6b antigen is safe from the standpoint of prion disease induction.

Targeting of prion-infected lymphoid cells to the central nervous system accelerates prion infection

Background

Prions, composed of a misfolded protein designated PrP^Sc, are infectious agents causing fatal neurodegenerative diseases. We have shown previously that, following induction of experimental autoimmune encephalomyelitis, prion-infected mice succumb to disease significantly earlier than controls, concomitant with the deposition of PrP^Sc aggregates in inflamed white matter areas. In the present work, we asked whether prion disease acceleration by experimental autoimmune encephalomyelitis results from infiltration of viable prion-infected immune cells into the central nervous system.

Methods

C57Bl/6 J mice underwent intraperitoneal inoculation with scrapie brain homogenates and were later induced with experimental autoimmune encephalomyelitis by inoculation of MOG_35-55 in complete Freund's adjuvant supplemented with pertussis toxin. Spleen and lymph node cells from the co-induced animals were reactivated and subsequently injected into naïve mice as viable cells or as cell homogenates. Control groups were infected with viable and homogenized scrapie immune cells only with complete Freund's adjuvant. Prion disease incubation times as well as levels and sites of PrP^Sc deposition were next evaluated.

Results

We first show that acceleration of prion disease by experimental autoimmune encephalomyelitis requires the presence of high levels of spleen PrP^Sc. Next, we present evidence that mice infected with activated prion-experimental autoimmune encephalomyelitis viable cells succumb to prion disease considerably faster than do mice infected with equivalent cell extracts or other controls, concomitant with the deposition of PrP^Sc aggregates in white matter areas in brains and spinal cords.

Conclusions

Our results indicate that inflammatory targeting of viable prion-infected immune cells to the central nervous system accelerates prion disease propagation. We also show that in the absence of such targeting it is the load of PrP^Sc in the inoculum that determines the infectivity titers for subsequent transmissions. Both of these conclusions have important clinical implications as related to the risk of prion disease contamination of blood products.

Chemically induced accumulation of GAGs delays PrP(Sc) clearance but prolongs prion disease incubation time

Prion diseases are a group of fatal neurodegenerative diseases affecting humans and animals. The only identified component of the infectious prion is PrP(Sc), an aberrantly folded isoform of PrP(C). Glycosaminoglycans, which constitute the main receptor for prions on cells, play a complex role in the pathogenesis of prion diseases. For example, while agents inducing aberrant lysosomal accumulation of GAGs such as Tilorone and Quinacrine significantly reduced PrP(Sc) content in scrapie-infected cells, administration of Quinacrine to prion-infected subjects did not improve their clinical status. In this study, we investigated the association of PrP(Sc )with cells cultured with Tilorone. We found that while the initial incorporation of PrP(Sc) was similar in the treated and untreated cells, clearance of PrP(Sc) from the Tilorone-treated cells was significantly impaired. Interestingly, prolonged administration of Tilorone to mice prior to prion infection resulted in a significant delay in disease onset, concomitantly with in vivo accumulation of lysosomal GAGs. We hypothesize that GAGs may complex with newly incorporated PrP(Sc) in lysosomes and further stabilize the prion protein conformation. Over-stabilized PrP(Sc) molecules have been shown to comprise reduced converting activity.

Fatal neurological disease in scrapie-infected mice induced for experimental autoimmune encephalomyelitis

During the years or decades of prion disease incubation, at-risk individuals are certain to encounter diverse pathological insults, such as viral and bacterial infections, autoimmune diseases, or inflammatory processes. Whether prion disease incubation time and clinical signs or otherwise the pathology of intercurrent diseases can be affected by the coinfection process is unknown. To investigate this possibility, mice infected with the scrapie agent at both high and low titers were subsequently induced for experimental autoimmune encephalomyelitis, an immune system-mediated model of central nervous system (CNS) inflammation. We show here that co-induced mice died from a progressive neurological disease long before control mice succumbed to classical scrapie. To investigate the mechanism of the co-induced syndrome, we evaluated biochemical and pathological markers of both diseases. Brain and spleen PrP(Sc) levels in the dying co-induced mice were comparable to those observed in asymptomatic scrapie-infected animals, suggesting that co-induced disease is not an accelerated form of scrapie. In contrast, inflammatory markers, such as demyelination, immune cell infiltrates, and gliosis, were markedly increased in co-induced mouse spinal cords. Activated astrocytes were especially elevated in the medulla oblongata. Furthermore, PrP(sc) depositions were found in demyelinated white matter areas in co-induced mouse spinal cords, suggesting the presence of activated infected immune cells that infiltrate into the CNS to facilitate the process of prion neuroinvasion. We hypothesize that inflammatory processes affecting the CNS may have severe clinical implications in subjects incubating prion diseases.

PRNP promoter polymorphisms are associated with BSE susceptibility in Swiss and German cattle

BACKGROUND

Non-synonymous polymorphisms within the prion protein gene (PRNP) influence the susceptibility and incubation time for transmissible spongiform encephalopathies (TSE) in some species such as sheep and humans. In cattle, none of the known polymorphisms within the PRNP coding region has a major influence on susceptibility to bovine spongiform encephalopathy (BSE). Recently, however, we demonstrated an association between susceptibility to BSE and a 23 bp insertion/deletion (indel) polymorphism and a 12 bp indel polymorphism within the putative PRNP promoter region using 43 German BSE cases and 48 German control cattle. The objective of this study was to extend this work by including a larger number of BSE cases and control cattle of German and Swiss origin.

RESULTS

Allele, genotype and haplotype frequencies of the two indel polymorphisms were determined in 449 BSE cattle and 431 unaffected cattle from Switzerland and Germany including all 43 German BSE and 16 German control animals from the original study. When breeds with similar allele and genotype distributions were compared, the 23 bp indel polymorphism again showed a significant association with susceptibility to BSE. However, some additional breed-specific allele and genotype distributions were identified, mainly related to the Brown breeds.

CONCLUSION

Our study corroborated earlier findings that polymorphisms in the PRNP promoter region have an influence on susceptibility to BSE. However, breed-specific differences exist that need to be accounted for when analyzing such data.

Bovine prion protein gene (PRNP) promoter polymorphisms modulate PRNP expression and may be responsible for differences in bovine spongiform encephalopathy susceptibility

The susceptibility of humans to the variant Creutzfeldt-Jakob disease is greatly influenced by polymorphisms within the human prion protein gene (PRNP). Similar genetic differences exist in sheep, in which PRNP polymorphisms modify the susceptibility to scrapie. However, the known coding polymorphisms within the bovine PRNP gene have little or no effect on bovine spongiform encephalopathy (BSE) susceptibility in cattle. We have recently found a tentative association between PRNP promoter polymorphisms and BSE susceptibility in German cattle (Sander, P., Hamann, H., Pfeiffer, I., Wemheuer, W., Brenig, B., Groschup, M., Ziegler, U., Distl, O., and Leeb, T. (2004) Neurogenetics 5, 19-25). A plausible hypothesis explaining this observation could be that the bovine PRNP promoter polymorphisms cause changes in PRNP expression that might be responsible for differences in BSE incubation time and/or BSE susceptibility. To test this hypothesis, we performed a functional promoter analysis of the different bovine PRNP promoter alleles by reporter gene assays in vitro and by measuring PRNP mRNA levels in calves with different PRNP genotypes in vivo. Two variable sites, a 23-bp insertion/deletion (indel) polymorphism containing a RP58-binding site and a 12-bp indel polymorphism containing an SP1-binding site, were investigated. Band shift assays indicated differences in transcription factor binding to the different alleles at the two polymorphisms. Reporter gene assays demonstrated an interaction between the two postulated transcription factors and lower expression levels of the ins/ins allele compared with the del/del allele. The in vivo data revealed substantial individual variation of PRNP expression in different tissues. In intestinal lymph nodes, expression levels differed between the different PRNP genotypes.