CpG and LPS can interfere negatively with prion clearance in macrophage and microglial cells

Cells of the innate immune system play important roles in the progression of prion disease after peripheral infection. It has been found in vivo and in vitro that the expression of the cellular prion protein (PrPc) is up-regulated on stimulation of immune cells, also indicating the functional importance of PrPc in the immune system. The aim of our study was to investigate the impact of cytosine-phosphate-guanosine- and lipopolysaccharide-induced PrPc up-regulation on the uptake and processing of the pathological prion protein (PrPSc) in phagocytic innate immune cells. For this purpose, we challenged the macrophage cell line J774, the microglial cell line BV-2 and primary bone marrow-derived macrophages in a resting or stimulated state with various prion strains, and monitored the uptake and clearance of PrPSc. Interestingly, stimulation led either to a transient increase in the level of PrPSc relative to unstimulated cells or to a decelerated degradation of PrPSc. These features were dependent on cell type and prion strain. Our data indicate that the stimulation of innate immune cells may be able to support transient prion propagation, possibly explained by an increased PrPc cell surface expression in stimulated cells. We suggest that stimulation of innate immune cells can lead to an imbalance between the propagation and degradation of PrPSc.