Central nervous system recruitment of effector memory CD8+ T lymphocytes during neuroinflammation is dependent on α4 integrin.

Clonally expanded CD8(+) T lymphocytes are present in multiple sclerosis lesions, as well as in the cerebrospinal fluid of patients with multiple sclerosis. In experimental autoimmune encephalomyelitis, CD8(+) T lymphocytes are found in spinal cord and brainstem lesions. However, the exact phenotype of central nervous system-infiltrating CD8(+) T lymphocytes and the mechanism by which these cells cross the blood-brain barrier remain largely unknown. Using cerebrospinal fluid from patients with multiple sclerosis, spinal cord from experimental autoimmune encephalomyelitis and coronavirus-induced encephalitis, we demonstrate that central nervous system-infiltrating CD8(+) T lymphocytes are mostly of the effector memory phenotype (CD62L(-) CCR7(-) granzymeB(hi)). We further show that purified human effector memory CD8(+) T lymphocytes transmigrate more readily across blood-brain barrier-endothelial cells than non-effector memory CD8(+) T lymphocytes, and that blood-brain barrier endothelium promotes the selective recruitment of effector memory CD8(+) T lymphocytes. Furthermore, we provide evidence for the recruitment of interferon-γ- and interleukin-17-secreting CD8(+) T lymphocytes by human and mouse blood-brain barrier endothelium. Finally, we show that in vitro migration of CD8(+) T lymphocytes across blood-brain barrier-endothelial cells is dependent on α4 integrin, but independent of intercellular adhesion molecule-1/leucocyte function-associated antigen-1, activated leucocyte cell adhesion molecule/CD6 and the chemokine monocyte chemotactic protein-1/CCL2. We also demonstrate that in vivo neutralization of very late antigen-4 restricts central nervous system infiltration of CD8(+) T lymphocytes in active immunization and adoptive transfer experimental autoimmune encephalomyelitis, and in coronavirus-induced encephalitis. Our study thus demonstrates an active role of the blood-brain barrier in the recruitment of effector memory CD8(+) T lymphocytes to the CNS compartment and defines α4 integrin as a major contributor of CD8(+) T lymphocyte entry into the brain.