gms | German Medical Science

Objective: Mild or moderate traumatic brain injury (TBI) often includes the disruption of the cranial nerve fibers; among these most frequently the olfactory is affected. But those pre-existing pathways are a traveling route for various immunological cells, for example t-cells and macrophages as we show in our study presented here.

After the establishment of a protocol which allowed the cross-sectioning of an entire head-neck preparation while preserving the signal of green fluorescent protein (GFP), we show that CD4+ t cells are able to leave the CNS by using pre-existing pathways. We monitored the temporal and spatial passage of cells along the filiae olfactoriae, through the cribroid plate, their appearance in the nasal mucosa, and finally, their accumulation in one specific cervical lymph node.

Methods: Six- to eight-week-old mice expressing GFP under the control of the beta-actin promoter were used for the isolation of GFP-expressing cells. Adult wild-type C57BL/6 mice were used as recipients. GFP expressing cells were injected at the lesion site immediately after entorhinal cortex lesion with a 5μl Hamilton pipette. Animals were sacrificed at 12h, 24h and 36h post injection of the cells. Whole heads and necks were dissected from the torso and prepared for histological analyses.

Results: From 12-48 hpi, cells were found in close association to the olfactory nerves penetrating the cribroid plate and within the nasal mucosa, where the cells were found under the epithelial layer. In keeping with the finding that only some deep cervical lymph nodes of individual animals were populated by GFP cells at 24 hpi, there was a clear accumulation in distinct nodes, while neighboring structures were mainly devoid of the injected cells. The number of cells subsequently increased in deep cervical nodes leading to dense cellular accumulations at 48 hpi. Using histological analyses we could identify T cells and macrophages among the cells found in the cervical lymph nodes.

Conclusions: As a proof of principle, these data demonstrate that cells are capable of leaving the brain and the subarachnoid space (SAS) via the cribroid plate along olfactory nerves and the nasal mucosa to eventually reach cervical lymph nodes. The specific targeting suggests that the cells, once inside the nasal mucosa, gain access to lymphatic vessels draining to certain lymph nodes. The consequences of the disruption of those routes by TBI need to be studied.