Microglial cells in the central nervous system play important roles in neurodevelopment and resistance to
infection, yet microglia can become neurotoxic under some conditions. An early event during
prion infection is the activation of microglia and astrocytes in the brain prior to damage or death of neurons. Previous
prion disease studies using two different strategies to manipulate signaling through the microglial receptor CSF-1R reported contrary effects on survival from
prion disease. However, in these studies, reductions of microglial numbers and function were variable, thus confounding interpretation of the results. In the present work, we used oral treatment with a potent inhibitor of CSF-1R,
PLX5622, to eliminate 78 to 90% of microglia from cortex early during the course of
prion infection. Oral drug treatment early after
infection with the RML
scrapie strain significantly accelerated vacuolation,
astrogliosis, and deposition of disease-associated
prion protein. Furthermore, drug-treated mice had advanced clinical disease requiring
euthanasia 31 days earlier than untreated control mice. Similarly,
PLX5622 treatment during the preclinical phase at 80 days postinfection with RML
scrapie also accelerated disease and resulted in
euthanasia of mice 33 days earlier than infected controls.
PLX5622 also accelerated clinical disease after
infection with
scrapie strains ME7 and 22L. Thus, microglia are critical in host defense during
prion disease. The early accumulation of PrPSc in the absence of microglia suggested that microglia may function by clearing PrPSc, resulting in longer survival.IMPORTANCE Microglia contribute to many aspects of health and disease. When activated, microglia can be beneficial by repairing damage in the central nervous system (CNS) or they can turn harmful by becoming neurotoxic. In
prion and prionlike diseases, the involvement of microglia in disease is unclear. Previous studies suggest that microglia can either speed up or slow down disease. In this study, we infected mice with
prions and depleted microglia from the brains of mice using
PLX5622, an effective CSF-1R
tyrosine kinase inhibitor. Microglia were markedly reduced in brains, and
prion disease was accelerated, so that mice needed to be euthanized 20 to 33 days earlier than infected control mice due to advanced clinical disease. Similar results occurred when mice were treated with
PLX5622 at 80 days after
infection, which was just prior to the start of clinical signs. Thus, microglia are important for removing
prions, and the disease is faster when microglia are depleted.