I think the paper was misusing the phrase. It seems like they were trying to emphasize the fact that the PRR's don't require somatic recombination like t cell receptors.

So...yes and no... in immunology germline is often used to describe origin of the immune population. So, germline immune cells are those that do not arise out of the bone marrow, and they usually are tissues resident cells, like alveolar macrophages, langerhan cells, etc.

So the PRR technically is germline encoded since they don't change, and it is passed down...which is why almost everyone respond to the same pathogen motifs.

I think there is a distinction between embryonic or yolk Sac derived immune cells and germline. For instance monocytes express prr's that don't involve somatic mutation but monocytes are not yolk Sac derived.

Yes, @Daniel McKim, I didn't mean to imply that only embryonic derived immune cells are the only ones that express PRRs. They are two separate topics. As for germline immune cells... I still consider the embryonic tissue resident APCs as germline, as most, if I'm not mistaken, are pretty homogeneous in response, unlike the bone marrow derived APCs. Although you are correct that they are more correctly referred to as embryonic derived immune cells.