Disruption of the blood-brain barrier (BBB) underlies the development of experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis. Environmental factors, such as Bordetella pertussis, are thought to sensitize central endothelium to biogenic amines like histamine, thereby leading to increased BBB permeability. B. pertussis-induced histamine sensitization (Bphs) T he blood-brain barrier (BBB) involves endothelial cells that line the blood vessels of the central nervous system (CNS) and the tight junction protein complexes between these endothelial cells. The BBB thereby acts as a physical and metabolic barrier by separating the vasculature from the parenchyma of the CNS (1). Breakdown of the BBB is associated with the onset and pathogenesis of multiple sclerosis (MS), a degenerative, demyelinating, inflammatory disease of the CNS (2). In MS and its autoimmune model, experimental autoimmune encephalomyelitis (EAE), activated T cells cross the BBB into the perivascular space of the CNS, causing damage to neurons (2). Surveillance of the CNS by T cells does occur (3), but the paucity of leukocytes found in the CNS under noninflammatory conditions suggests that extravasation of cells across the BBB is tightly regulated. Therefore, identifying factors that modulate BBB permeability may be of therapeutical value in treating inflammatory demyelinating diseases of the CNS.Bordetella pertussis-induced hypersensitivity to histamine (Bphs/ Bphs) is a genetically controlled intermediate phenotype associated with susceptibility to EAE (4). Bphs is a state wherein mice are rendered highly susceptible to histamine after a preceding injection of B. pertussis toxin (PTX) (5). Bphs-susceptible strains of mice die within 30 min after histamine challenge, presumably attributable to hypotensive and hypovolemic shock. The cells targeted during Bphs are not known. Histamine has also been implicated in the pathophysiology of MS and EAE. Increased tissue levels of histamine correlate with the onset of EAE (6-8). Mast cells, the major source of histamine (9), are present in MS lesions (10-12), and evidence of mast cell activation is found in the cerebrospinal fluid (CSF) of patients who have MS (13). In mice, mast cells (14) and their activation (15) are required for early EAE onset and maximal disease severity.The effects of histamine are mediated by four surface histamine receptors: H 1 R, H 2 R, H 3 R, and H 4 R (16). H 1 R protein and mRNA are highly expressed in MS lesions (17), and H 1 antihistamines reduce EAE in mice and rats (17,18). In patients with MS, the use of sedating H 1 antihistamines is correlated with decreased disease incidence and amelioration of symptoms (19,20). Our laboratory has shown that susceptibility to Bphs and EAE requires expression of Hrh1, the gene encoding H 1 R (21).Expression of H 1 R on T cells is required for their full encephalitogenic potential (22), but the contribution of H 1 R signaling in other cell types to the pathogenesis of EAE has not been formally addressed. In this study, w...