The contribution of local factors to the activation of immune cells infiltrating the CNS of patients with multiple sclerosis (MS) remains to be defined. The cytokine IL-15 is pivotal in the maintenance and activation of CD8 T lymphocytes, a prominent lymphocyte population found in MS lesions. We investigated whether astrocytes are a functional source of IL-15 sufficient to enhance CD8 T lymphocyte responses and whether they provide IL-15 in the inflamed CNS of patients with MS. We observed that human astrocytes in primary cultures increased surface IL-15 levels upon activation with combinations of proinflammatory cytokines. Expanded human myelin autoreactive CD8 T lymphocytes cultured with such activated astrocytes displayed elevated lytic enzyme content, NKG2D expression, and Ag-specific cytotoxicity. These functional enhancements were abrogated by anti–IL-15–blocking Abs. Immunohistochemical analysis of brain tissue sections obtained from patients with MS demonstrated colocalization for IL-15 and the astrocyte marker glial fibrillary acidic protein within white matter lesions. The majority of astrocytes (80–90%) present in demyelinating MS lesions expressed IL-15, whereas few astrocytes in normal control brain sections had detectable IL-15. IL-15 could be detected in the majority of Iba-1–expressing microglia in the control sections, albeit in lower numbers when compared with microglia/macrophages in MS lesions. Furthermore, infiltrating CD8 T lymphocytes in MS lesions were in close proximity to IL-15–expressing cells. Astrocyte production of IL-15 resulting in the activation of CD8 T lymphocytes ascribes a role for these cells as contributors to the exacerbation of tissue damage during MS pathogenesis.

Activated leukocyte cell adhesion molecule (ALCAM) is a cell adhesion molecule found on blood-brain barrier endothelial cells (BBB-ECs) that was previously shown to be involved in leukocyte transmigration across the endothelium. In the present study, we found that ALCAM knockout (KO) mice developed a more severe myelin oligodendrocyte glycoprotein (MOG) 35-55 -induced experimental autoimmune encephalomyelitis (EAE). The exacerbated disease was associated with a significant increase in the number of CNS-infiltrating proinflammatory leukocytes compared with WT controls. Passive EAE transfer experiments suggested that the pathophysiology observed in active EAE was linked to the absence of ALCAM on BBB-ECs. In addition, phenotypic characterization of unimmunized ALCAM KO mice revealed a reduced expression of BBB junctional proteins. Further in vivo, in vitro, and molecular analysis confirmed that ALCAM is associated with tight junction molecule assembly at the BBB, explaining the increased permeability of CNS blood vessels in ALCAM KO animals. Collectively, our data point to a biologically important function of ALCAM in maintaining BBB integrity. multiple sclerosis | ALCAM | blood-brain barrier | EAE | tight junctions

Central nervous system (CNS) cells locally modulate immune responses using numerous molecules that are not fully elucidated. Engagement of programmed death-1 (PD-1), expressed on activated T cells, by its ligands (PD-L1 or PD-L2) suppresses T-cell responses. Enhanced CNS PD-1 and PD-L1 expression has been documented in inflammatory murine models; however, human CNS data are still incomplete. We determined that human primary cultures of astrocytes, microglia, oligodendrocytes, or neurons expressed low or undetectable PD-L1 under basal conditions, but inflammatory cytokines significantly induced such expression, especially on astrocytes and microglia. Blocking PD-L1 expression in astrocytes using specific siRNA led to significantly increased CD8 T-cell responses (proliferation, cytokines, lytic enzyme). Thus, our results establish that inflamed human glial cells can express sufficient and functional PD-L1 to inhibit CD8 T cell responses. Extensive immunohistochemical analysis of postmortem brain tissues demonstrated a significantly greater PD-L1 expression in multiple sclerosis (MS) lesions compared with control tissues, which colocalized with astrocyte or microglia/macrophage cell markers. However, more than half of infiltrating CD8 T lymphocytes in MS lesions did not express PD-1, the cognate receptor. Thus, our results demonstrate that inflamed human CNS cells such as in MS lesions express significantly elevated PD-L1, providing a means to reduce CD8 T cell responses, but most of these infiltrating immune cells are devoid of PD-1 and thus insensitive to PD-L1/L2. Strategies aimed at inducing PD-1 on deleterious activated human CD8 T cells that are devoid of this receptor could provide therapeutic benefits since PD-L1 is already increased in the target organ.