Alzheimer's disease (AD), the most common neurodegenerative disorder, goes along with extracellular amyloid-β (Aβ) deposits. The cognitive decline observed during AD progression correlates with damaged spines, dendrites and synapses in hippocampus and cortex. Numerous studies have shown that Aβ oligomers, both synthetic and derived from cultures and AD brains, potently impair synaptic structure and functions. The cellular prion protein (PrPC) was proposed to mediate this effect. We report that ablation or overexpression of PrPC had no effect on the impairment of hippocampal synaptic plasticity in a transgenic model of AD. These findings challenge the role of PrPC as a mediator of Aβ toxicity.

PrPSc, a misfolded and aggregated form of the cellular prion protein PrPC, is the only defined constituent of the transmissible agent causing prion diseases. Expression of PrPC in the host organism is necessary for prion replication and for prion neurotoxicity. Understanding prion diseases necessitates detailed structural insights into PrPC and PrPSc. Towards this goal, we have developed a comprehensive collection of monoclonal antibodies denoted POM1 to POM19 and directed against many different epitopes of mouse PrPC. Three epitopes are located within the N-terminal octarepeat region, one is situated within the central unstructured region, and four epitopes are discontinuous within the globular C-proximal domain of PrPC. Some of these antibodies recognize epitopes that are resilient to protease digestion in PrPSc. Other antibodies immunoprecipitate PrPC, but not PrPSc. A third group was found to immunoprecipitate both PrP isoforms. Some of the latter antibodies could be blocked with epitope-mimicking peptides, and incubation with an excess of these peptides allowed for immunochromatography of PrPC and PrPSc. Amino-proximal antibodies were found to react with repetitive PrPC epitopes, thereby vastly increasing their avidity. We have also created functional single-chain miniantibodies from selected POMs, which retained the binding characteristics despite their low molecular mass. The POM collection, thus, represents a unique set of reagents allowing for studies with a variety of techniques, including western blotting, ELISA, immunoprecipitation, conformation-dependent immunoassays, and plasmon surface plasmon resonance-based assays.

Progressive accumulation of PrPSc, a hallmark of prion diseases, occurs when conversion of PrPC into PrPSc is faster than PrPSc clearance. Engulfment of apoptotic bodies by phagocytes is mediated by Mfge8 (milk fat globule epidermal growth factor 8). In this study, we show that brain Mfge8 is primarily produced by astrocytes. Mfge8 ablation induced accelerated prion disease and reduced clearance of cerebellar apoptotic bodies in vivo, as well as excessive PrPSc accumulation and increased prion titers in prion-infected C57BL/6 × 129Sv mice and organotypic cerebellar slices derived therefrom. These phenotypes correlated with the presence of 129Sv genomic markers in hybrid mice and were not observed in inbred C57BL/6 Mfge8−/− mice, suggesting the existence of additional strain-specific genetic modifiers. Because Mfge8 receptors are expressed by microglia and depletion of microglia increases PrPSc accumulation in organotypic cerebellar slices, we conclude that engulfment of apoptotic bodies by microglia may be an important pathway of prion clearance controlled by astrocyte-borne Mfge8.

Serological assays can detect anti-SARS-CoV-2 (SARS2) antibodies, but their sensitivity often comes at the expense of specificity. Here we developed a Tripartite Automated Blood Immunoassay (TRABI) to assess the IgG response against SARS2. Calibration was per-formed with 90 prepandemic and 55 virologically and clinically confirmed COVID-19 sam-ples. Posterior probabilities of seropositivities were calculated from 3x8 measurements of logarithmically diluted samples against the ectodomain and the receptor-binding domain of the spike protein and the nucleoprotein. We then performed 760'320 assays on 5'503 prepandemic and 26'177 copandemic samples from hospital patients and healthy blood donors. We found 176 seropositive samples between December 2019 and May 2020. The seroprevalence increased conspicuously in March 2020 but plateaued in late April at 0.8-1.6% in both cohorts, indicating an equilibrium between new infections and the waning of immunity. This points to a high effectiveness of containment measures and/or to unex-pectedly rapid loss of humoral responses.

The enumeration of the interaction partners of the cellular prion protein, PrPC, may help clarifying its elusive molecular function. Here we added a carboxy proximal myc epitope tag to PrPC. When expressed in transgenic mice, PrPmyc carried a GPI anchor, was targeted to lipid rafts, and was glycosylated similarly to PrPC. PrPmyc antagonized the toxicity of truncated PrP, restored prion infectibility of PrPC-deficient mice, and was physically incorporated into PrPSc aggregates, indicating that it possessed all functional characteristics of genuine PrPC. We then immunopurified myc epitope-containing protein complexes from PrPmyc transgenic mouse brains. Gentle differential elution with epitope-mimetic decapeptides, or a scrambled version thereof, yielded 96 specifically released proteins. Quantitative mass spectrometry with isotope-coded tags identified seven proteins which co-eluted equimolarly with PrPC and may represent component of a multiprotein complex. Selected PrPC interactors were validated using independent methods. Several of these proteins appear to exert functions in axomyelinic maintenance.

Dysfunctional variants of the innate immune cell surface receptor TREM2 (triggering receptor expressed on myeloid cells-2) were identified as major genetic risk factors for Alzheimer's disease and other neurodegenerative conditions. Here we assessed a possible involvement of TREM2 in prion disease. We report that TREM2 expression by microglia is significantly up-regulated upon prion infection. However, depletion of TREM2 did not affect disease incubation time and survival after intracerebral prion infection. Interestingly, markers of microglial activation were attenuated in prion-infected TREM2(-/-) mice, suggesting an involvement of TREM2 in prion-induced microglial activation. Further phenotype profiling of microglia revealed that TREM2 deficiency did not change microglial phenotypes. We conclude that TREM2 is involved in prion-induced microglial activation but does not noticeably modulate the pathogenesis of experimental prion infections. | P a g e AbstractDysfunctional variants of the innate immune cell surface receptor TREM2 (triggering receptor expressed on myeloid cells-2) were identified as major genetic risk factors for Alzheimer's disease and other neurodegenerative conditions. Here we assessed a possible involvement of TREM2 in prion disease. We report that TREM2 expression by microglia is significantly upregulated upon prion infection. However, depletion of TREM2 did not affect disease incubation time and survival after intracerebral prion infection. Interestingly, markers of microglial activation were attenuated in prion-infected TREM2 -/-mice, suggesting an involvement of TREM2 in prion-induced microglial activation. Further phenotype profiling of microglia revealed that TREM2 deficiency did not change microglial phenotypes. We conclude that TREM2 is involved in prion-induced microglial activation, but does not noticeably modulate the pathogenesis of experimental prion infections.

Earlier studies have shown that Kaposi sarcomas contain cells infected with human herpesvirus (HHV) 6B, and in current studies we report that both AIDSassociated and classic-sporadic Kaposi sarcoma contain HHV-7 genome sequences detectable by PCR. To determine the distribution of HHV-7-infected cells relative to those infected with HHV-6, sections from paraffin-embedded tissues were allowed to react with antibodies to HHV-7 virion tegument phosphoprotein pp85 and to HHV-6B protein p101. The antibodies are specific for HHV-7 and HHV-6B, respectively, and they retained reactivity for antigens contained in formalin-fixed, paraffin-embedded tissue samples. We report that (i) HHV-7 pp85 was present in 9 of 32 AIDS-associated Kaposi sarcomas, and in 1 of 7 classical-sporadic HIV-negative Kaposi sarcomas; (ii) HHV-7 pp85 was detected primarily in cells bearing the CD68 marker characteristic of the monocyte͞ macrophage lineage present in or surrounding the Kaposi sarcoma lesions; and (iii) in a number of Kaposi sarcoma specimens, tumor-associated CD68 ؉ monocytes͞macro-phages expressed simultaneously antigens from both HHV-7 and HHV-6B, and therefore appeared to be doubly infected with the two viruses. CD68 ؉ monocytes͞macrophages infected with HHV-7 were readily detectable in Kaposi sarcoma, but virtually absent from other normal or pathological tissues that harbor macrophages. Because all of the available data indicate that HHV-7 infects CD4 ؉ T lymphocytes, these results suggest that the environment of the Kaposi sarcoma (i) attracts circulating peripheral lymphocytes and monocytes, triggers the replication of latent viruses, and thereby increases the local concentration of viruses, (ii) renders CD68 ؉ monocytes͞macrophages susceptible to infection with HHV-7, and (iii) the combination of both events enables double infections of cells with both HHV-6B and HHV-7.

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