Features of follicular dendritic cells in ovine pharyngeal tonsil: An in vivo and in vitro study in the context of scrapie pathogenesis

Although the alimentary tract has been suggested as the most likely portal of entry in
natural scrapie, a growing amount of data indicates that the respiratory system and more
specifically the pharyngeal tonsils serve as a natural portal of entry for scrapie.
This study describes for the first time the broad cell populations in the lymphoid compartment
of pharyngeal tonsils and more specifically inside the lymphoid follicles where
the scrapie agent accumulates during the period of latency.
Follicular dendritic cells (FDCs), stromal cells located in the light zone of the germinal
centre of lymphoid follicles, seem to be the principal causal factor in the accumulation of
the infectious agent in transmissible spongiform encephalopathy (TSE) diseases. Knowing
that efficient lymphoreticular prion propagation requires PrPc expression, we analysed
the expression of PrPc with the mouse monoclonal antibody Pri 909 both in situ and on
FDC-cluster-enriched cell suspensions.
In situ, a positive staining was observed in the germinal centre of pharyngeal lymph
follicles. The germinal centre labelling was due to the presence of a follicular dendritic
network as revealed after immunogold staining of isolated FDC clusters. Our results suggest
that the pharyngeal lymphoreticular system and more specifically PrPc expressing follicular
dendritic cells could serve as a prion “reservoir” during the latency phase, thus playing a
key role during the scrapie lymphoinvasion.