Follicular dendritic cell dedifferentiation reduces scrapie susceptibility following inoculation via the skin

Abstract

Transmissible spongiform encephalopathies (TSEs) are a group of subacute infectious neurodegenerative diseases that are characterized by the accumulation in affected tissues of PrP(Sc), an abnormal isoform of the host prion protein (PrPc). Following peripheral exposure, TSE infectivity and PrP(Sc) usually accumulate in lymphoid tissues prior to neuroinvasion. Studies in mice have shown that exposure through scarified skin is an effective means of TSE transmission. Following inoculation via the skin, a functional immune system is critical for the transmission of TSEs to the brain, but until now, it has not been known which components of the immune system are required for efficient neuroinvasion. Temporary dedifferentiation of follicular dendritic cells (FDCs) by treatment with an inhibitor of the lymphotoxin-beta receptor signalling pathway (LTbetaR-Ig) 3 days before or 14 days after inoculation via the skin, blocked the early accumulation of PrP(Sc) and TSE infectivity within the draining lymph node. Furthermore, in the temporary absence of FDCs before inoculation, disease susceptibility was reduced and survival time significantly extended. Treatment with LTbetaR-Ig 14 days after TSE inoculation also significantly extended the disease incubation period. However, treatment 42 days after inoculation did not affect disease susceptibility or survival time, suggesting that the infection may have already have spread to the nervous system. Together these data show that FDCs are essential for the accumulation of PrP(Sc) and infectivity within lymphoid tissues and subsequent neuroinvasion following TSE exposure via the skin.

Figure 1

Effect of treatment with lymphotoxin β-receptor immunoglobulin (LTβR-Ig) on follicular dendritic cell (FDC) status in the inguinal lymph nodes of uninfected mice. Tissues were taken 3 days after injection with polyclonal human immunoglobulin G (hu-Ig) (control) or LTβR-Ig, and adjacent frozen sections were stained with the FDC-specific monoclonal antiserum FDC-M2 (top row; red) and 8C12 antiserum to detect CD35 (bottom row; red). Expression of FDC-M2 and CD35 were undetectable in inguinal lymph nodes after treatment with LTβR-Ig. All sections were counterstained with haematoxylin (blue). Original magnification × 200.

Figure 2

Effect of treatment with lymphotoxin β-receptor immunoglobulin (LTβR-Ig) on the early accumulation of PrP^Sc in the draining inguinal lymph node and in the spleen. Mice were given a single intraperitoneal (i.p.) injection of LTβR-Ig or polyclonal human immunoglobulin G (hu-Ig) (control) 42 days after inoculation with scrapie via skin scarification on the right thigh. Tissues from two mice from each group were collected 3 days after treatment, and PrP^Sc accumulations were determined by paraffin-embedded tissue (PET) immunoblotting. Abundant PrP^Sc accumulations were detected in the lymphoid follicles of polyclonal human immunoglobulin G (hu-Ig)-treated animals (a and c, dark staining, arrowheads). In contrast, PrP^Sc accumulations were undetectable in tissues from LTβR-Ig-treated mice (b and d). Terminally scrapie affected brain tissue (e) and uninfected normal brain tissue (f) were included as controls to confirm the specificity of PrP^Sc detection.

Figure 3

Immunohistological analysis of brain tissue from mice treated with polyclonal human immunoglobulin G (hu-Ig) (control) or with lymphotoxin β-receptor immunoglobulin (LTβR-Ig) 3 days before scrapie inoculation by skin scarification. Large disease-specific PrP accumulations (upper row; brown), and reactive astrocytes expressing high levels of glial fibrillary acid protein (GFAP) (middle row; red) and spongiform pathology (haematoxylin & eosin, lower row) were detected in the hippocampi of all mice showing clinical signs of scrapie. In contrast, in the brains of LTβR-Ig-treated mice that remained free of the clinical signs of disease, no evidence of PrP accumulation, reactive astrocytes or spongiform pathology was detected 480 days after inoculation. All sections were counterstained with haematoxylin (blue). Original magnification × 200. dpi, days postinoculation on which the tissues were analysed; pos., mice that developed clinical signs of scrapie; neg., mice that were free of the clinical signs of scrapie.