Impact of innate immune modifications in prion disease pathogenesis

Abstract

Prion diseases are a group of infectious, chronic, fatal, and progressive neurodegenerative diseases with no treatments or cures to date. During disease, the central nervous system (CNS) presents extensive accumulation of misfolded prions, reactive gliosis, neuronal spongiform vacuolation and dementia. The mononuclear phagocytes (MNPs) comprise a fundamental component of the innate immune system and are considered to mediate the sequestration and destruction of infectious prions. Therefore, the aims of this thesis were to interrogate prion disease pathogenesis and susceptibility after reducing the abundance of MNPs, restricting their pro-inflammatory phenotype, and modifying their activation through innate immune stimulation.

Data in this thesis show that peripheral stimulation of mice with an intraperitoneal (IP) injection of bacterial lipopolysaccharide (LPS) 3 hours prior to a subsequent IP injection with a limiting dose of prions significantly enhanced prion disease susceptibility. Further analysis showed that within 3 hours of LPS treatment the number of CD11b+MHCIIlowCD102high large peritoneal macrophages and CD11b+MHCIIhighCD102low small peritoneal macrophages were significantly reduced compared to mice treated with saline. This revealed that peripheral LPS stimulation had induced the disappearance of macrophages in the peritoneal cavity. Together these data suggest that the increased susceptibility to IP prion infection in the LPS- treated mice was due to the reduced abundance of MNPs in the peritoneal cavity which increased the ability of the prions to infect the CNS.

In the brain, a single IP injection with LPS (1x LPS) had been shown to induce microglial innate immune training, whereas four consecutive doses (4x LPS) every 24 hours induced microglial innate immune tolerance. The innate immune tolerised phenotype in the microglia had been previously demonstrated to reduce the amyloidosis in an Alzheimer’s like mouse model, and this response was observed 6 months after LPS stimulation. To test whether innate immune tolerance in microglia might similarly impede the development of CNS prion disease, groups of mice received an intracerebral (IC) injection of prions then 35 days later stimulated with 4x LPS by the IP route. Data from this study showed that neither microglial innate immune training (1x LPS) or tolerance (4x LPS) significantly affected the clinical presentation of prion disease nor the accumulation of misfolded prions in the CNS. This suggests that LPS mediated innate immune tolerance in the microglia does not influence prion mediated neuropathology in mice, despite their beneficial effects previously seen in an Alzheimer’s like mouse model.

MNPs require constant signalling through their Colony Stimulating Factor 1 Receptor (CSF1R) for their survival and maintenance. Previous data had shown that a transgenic mouse containing a Csf1r tyrosine kinase dead mutation (Csf1rE631K/+) had a significantly reduced number of microglia in the brain. In addition, bone marrow cells collected from these mice were unresponsive to stimulation with the cytokine CSF1 which suggests an impairment in CSF1R signalling. To test whether an impairment in CSF1R signalling restricts microglial expansion in CNS prion disease or modifies disease pathogenesis, a group of Csf1rE631K/+ mice received an IC injection with prions and the effects on CNS prion disease determined. Whilst the prion infected Csf1rE631K/+ mice presented a significantly lower abundance of microglia compared to those found in prion infected wildtype mice, a significant increase in the number of microglia was observed in both Csf1rE631K/+ mice and wildtype mice during CNS prion disease. However, it was found that the rate of microglial expansion across disease was not significantly affected in the Csf1rE631K/+ mice with an impairment in CSF1R signalling. Likewise, there were no significant changes in the development or severity of prion mediated neuropathology. This suggests that although an impairment in CSF1R signalling in the Csf1rE631K/+ mice restricted the overall density of the microglia in the brain, this did not impact on their rate of expansion, nor CNS prion disease pathogenesis. These data infer that microglial expansion does not likely require CSF1R signalling in prion disease.

Together data from this thesis suggest a protective role of macrophages in the peritoneal cavity by constraining peripheral prion infection. However, modification of the microglial phenotype using innate immune tolerance or reducing their abundance using the Csf1rE631K/+ mutation provided limited effects on the development and pathogenesis of CNS prion disease. Further elucidation into the factors that drive MNP activation and interrogating their functional roles in the development and susceptibility to prion disease may uncover novel interventions to ameliorating CNS pathogenesis.