Biology of the blood–nerve barrier and its alteration in immune mediated neuropathies

The blood–nerve barrier (BNB) is a dynamic and competent interface between the endoneurial microenvironment and the surrounding extracellular space or blood. It is localised at the innermost layer of the multilayered ensheathing perineurium and endoneurial microvessels, and is the key structure that controls the internal milieu of the peripheral nerve parenchyma. Since the endoneurial BNB is the point of entry for pathogenic T cells and various soluble factors, including cytokines, chemokines and immunoglobulins, understanding this structure is important to prevent and treat human immune mediated neuropathies such as Guillain–Barré syndrome, chronic inflammatory demyelinating polyneuropathy, POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein and skin changes) syndrome and a subset of diabetic neuropathy. However, compared with the blood–brain barrier, only limited knowledge has been accumulated regarding the function, cell biology and clinical significance of the BNB. This review describes the basic structure and functions of the endoneurial BNB, provides an update of the biology of the cells comprising the BNB, and highlights the pathology and pathomechanisms of BNB breakdown in immune mediated neuropathies. The human immortalised cell lines of BNB origin established in our laboratory will facilitate the future development of BNB research. Potential therapeutic strategies for immune mediated neuropathies manipulating the BNB are also discussed.