While previous studies have described CD25 expression on mature dendritic cells (mDCs) and their production of IL-2, it remains unclear how these molecules participate in the activation of T cells. In search of the mechanisms by which daclizumab, a humanized monoclonal antibody against CD25, inhibits brain inflammation in multiple sclerosis (MS), we observed that while the drug has limited effect on polyclonal T cell activation, it potently inhibits activation of antigen (Ag)-specific T cells by mDCs. We demonstrate that in an Ag-specific manner, mDCs (and Ag-experienced T cells) secrete IL-2 to the mDC-T cell interface and mDCs “lend” their CD25 to primed T cells in trans, in order to facilitate early high affinity IL-2 signaling, which is critical for subsequent T cell expansion and development of Ag-specific effectors. Our data reveal a novel mechanism for the IL-2 receptor system in DC-mediated activation of T cells.

Cellular cholesterol levels alter the processing of the amyloid precursor protein (APP) to produce A␤. Activation of liver X receptors (LXRs), one cellular mechanism to regulate cholesterol homeostasis, has been found to alter A␤ levels in vitro and in vivo. To identify genes regulated by LXR, we treated human neuroblastoma cells with an LXR agonist (TO-901317) and examined gene expression by microarray. As expected, TO-901317 upregulated several cholesterol metabolism genes, but it also decreased expression of a metalloprotease inhibitor, TIMP-3. We confirmed this finding using real-time PCR and by measuring TIMP-3 protein in glia, SY5Y cells, and COS7 cells. TIMP-3 is a member of a family of metalloproteinase inhibitors and blocks A disintegrin and metalloproteinase-10 (ADAM-10) and ADAM-17, two APP ␣-secretases. We found that TIMP-3 inhibited ␣-secretase cleavage of APP and an apolipoprotein E (apoE) receptor, ApoER2. TIMP-3 decreased surface levels of ADAM-10, APP, and ApoER2. These changes were accompanied by increased APP ␤-Cterminal fragment and A␤ production. These data suggest that TIMP-3 preferentially routes APP and ApoER2 away from the cell surface and ␣-secretase cleavage and encourages endocytosis and ␤-secretase cleavage. In vivo, TO-901317 decreased brain TIMP-3 levels. TIMP-3 protein levels were increased in human Alzheimer's disease (AD) brain and in APP transgenic mice, suggesting that increased levels of TIMP-3 in AD may contribute to higher levels of A␤.

Closed-head concussive injury is one of the most common causes of traumatic brain injury (TBI). Isolated concussions frequently produce acute neurological impairments, and individuals typically recover spontaneously within a short time frame. In contrast, brain injuries resulting from multiple concussions can result in cumulative damage and elevated risk of developing chronic brain pathologies. Increased attention has focused on identification of diagnostic markers that can prognostically serve as indices of brain health after injury, revealing the temporal profile of vulnerability to a second insult. Such markers may demarcate adequate recovery periods before concussed patients can return to required activities. We developed a noninvasive closed-head impact model that captures the hallmark symptoms of concussion in the absence of gross tissue damage. Animals were subjected to single or repeated concussive impact and examined using a battery of neurological, vestibular, sensorimotor, and molecular metrics. A single concussion induced transient, but marked, acute neurological impairment, gait alterations, neuronal death, and increased glial fibrillary acidic protein (GFAP) expression in brain tissue. As expected, repeated concussions exacerbated sensorimotor dysfunction, prolonged gait abnormalities, induced neuroinflammation, and upregulated GFAP and tau. These animals also exhibited chronic functional neurological impairments with sustained astrogliosis and white matter thinning. Acute changes in molecular signatures correlated with behavioral impairments, whereas increased times to regaining consciousness and balance impairments were associated with higher GFAP and neuroinflammation. Overall, behavioral consequences of either single or repeated concussive impact injuries appeared to resolve more quickly than the underlying molecular, metabolic, and neuropathological abnormalities. This observation, which is supported by similar studies in other mTBI models, underscores the critical need to develop more objective prognostic measures for guiding return-to-play decisions.

In traumatic brain injury (TBI), cellular loss from initial impact as well as secondary neurodegeneration leads to increased cholesterol and lipid debris at the site of injury. Cholesterol accumulation in the periphery can trigger inflammatory mechanisms while cholesterol clearance may be anti-inflammatory. Here we investigated whether TBI altered the regulation of cholesterol 24S-hydroxylase (Cyp46), an enzyme that converts cholesterol to the more hydrophilic 24S-hydroxycholesterol. We examined by Western blot and immunohistochemistry changes in Cyp46 expression following fluid percussion injury. Under normal conditions, most Cyp46 was present in neurons, with very little measurable in glia. Cyp46 levels were significantly increased at 7 days post-injury, and cell type specific analysis at 3 days post-injury showed a significant increase in levels of Cyp46 (84%) in microglia. Since 24-hydroxycholesterol induces activation of genes through the liver X receptor (LXR), we examined protein levels of ATP-binding cassette transporter A1 and apolipoprotein E, two LXR regulated cholesterol homeostasis proteins. Apolipoprotein E and ATP-binding cassette transporter A1 were increased at 7 days post-injury, indicating that increased LXR activity coincided with increased Cyp46 levels. We found that activation of primary rat microglia by LPS in vitro caused increased Cyp46 levels. These data suggest that increased microglial Cyp46 activity is part of a system for removal of damaged cell membranes post-injury, by conversion of cholesterol to 24-hydroxycholesterol and by activation of LXR-regulated gene transcription.