Fig. 9

CLN5⁺ leukocyte subtypes in blood and CNS during EAE. Shown is a schematic of how acquisition of CLN-5 by leukocytes is envisioned to take place early in disease at the intact blood–brain barrier (BBB). Leukocytes in the blood are depicted focally acquiring CLN-5 from nascent CLN-5⁺ extracellular vesicles (EVs) released by brain microvascular endothelial cells (BMEC) of the BBB, though some endogenous gene expression of CLN-5 has been detected in leukocytes in normal and disease states [17, 19]. And, while populations of CLN-5⁺ cells are observed in all the leukocyte subtypes shown, T cells, specifically, exhibit a high correlation between CLN-5 abundance and activation state. Since activation is required for efficient T cell entry into the non-inflamed CNS [48,49,50], T cells crossing the intact BBB are depicted with a halo representing their high level of activation. Moreover, a higher proportion of leukocytes with attached CLN-5⁺ EVs is shown in the perivascular space compared to the blood, as current results reveal a higher percentage of CLN-5⁺ leukocytes in the CNS than in the circulation – possibly suggesting a preference for CLN-5⁺ leukocytes that are the most highly activated to extravasate. The inset depicts a T cell undergoing paracellular diapedesis by exploiting a hypothetical zipper mechanism whereby leukocyte-associated CLN-5⁺ EVs temporarily form bridges with CLN-5 on the BMEC surface