Following an infection, CD8+ T cells are activated and undergo a characteristic kinetic sequence of rapid expansion, subsequent contraction and formation of memory cells1–3. The pool of naïve T cell clones is diverse and contains cells bearing T cell antigen receptors (TCR) that differ in their affinity for the same antigen4,5. How these differences in affinity impact the function and the response kinetics of individual T cell clones was previously unknown. Here we show that during the in vivo response to microbial infection, even very weak TCR-ligand interactions are sufficient to activate naïve T cells, induce rapid initial proliferation and generate effector and memory cells. The strength of the TCR-ligand interaction critically impacts when expansion stops, when the cells exit lymphoid organs and when contraction begins, i.e. strongly stimulated T cells contract and exit lymphoid organs later than do weakly stimulated cells. Our data challenges the prevailing view that strong TCR ligation is a prerequisite for CD8+ T cell activation. Instead, very weak interactions are sufficient for activation, but strong TCR ligation is required to sustain T cell expansion. We propose that in response to microbial challenge, T cell clones with a broad range of avidities for foreign ligands are initially recruited, and that the pool of T cells subsequently matures in affinity due to the more prolonged expansion of high affinity T cell clones.

The immaturity of the CNS at birth greatly affects injury after stroke but the contribution of the blood-brain barrier (BBB) to the differential response to stroke in adults and neonates is poorly understood. We asked if the structure and function of the BBB is disrupted differently in neonatal and adult rats by transient middle cerebral artery occlusion. In adult rats, albumin leakage into injured regions was markedly increased during 2–24 h reperfusion but leakage remained low in the neonates. Functional assays employing intravascular tracers in the neonates showed that BBB permeability to both large (70-kDa dextran) and small (3-kDa dextran, Gd-DTPA) tracers remained largely undisturbed 24h after reperfusion. The profoundly different functional integrity of the BBB was associated with the largely nonoverlapping patterns of regulated genes in endothelial cells purified from injured and uninjured adult and neonatal brain at 24h (endothelial transcriptome, 31,042 total probe sets). Within significantly regulated 1,266 probe sets in injured adults and 361 probe sets in neonates, changes in the gene expression of the basal lamina components, adhesion molecules, the tight junction protein occludin, and MMP-9 were among the key differences. The protein expression of collagen-IV, laminin, claudin-5, occludin and ZO-1 was also better preserved in neonatal rats. Neutrophil infiltration remained low in acutely injured neonates but neutralization of CINC-1 in the systemic circulation enhanced neutrophil infiltration, BBB permeability and injury. The markedly more integrant BBB in neonatal brain than in adult brain after acute stroke may have major implications for the treatment of neonatal stroke.

Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system manifested with varying clinical course, pathology, and inflammatory patterns. There are multiple animal models that reflect different aspects of this heterogeneity. Collectively, these models reveal a balance between pathogenic and regulatory CD4+ T cells, CD8+ T cells and B cells that influences the incidence, timing, and severity of central nervous system autoimmunity. In this review we discuss experimental autoimmune encephalomyelitis (EAE) models that have been used to study the pathogenic and regulatory roles of these immune cells, models that recapitulate different aspects of the disease seen in patients with MS, and questions remaining for future studies.

Macrophages can be both beneficial and detrimental after CNS injury. We previously showed rapid accumulation of macrophages in injured immature brain acutely after ischemia-reperfusion. To determine whether these macrophages are microglia or invading monocytes, we subjected post-natal day 7 (P7) rats to transient 3 h middle cerebral artery (MCA) occlusion and used flow cytometry at 24 and 48 h postreperfusion to distinguish invading monocytes (CD45high/ CD11b+) from microglia (CD45low/medium/CD11b+). Inflammatory cytokines and chemokines were determined in plasma, injured and contralateral tissue 1-24 h post-reperfusion using ELISA-based cytokine multiplex assays. At 24 h, the number of CD45+/CD11b+ cells increased 3-fold in injured compared to uninjured brain tissue and CD45 expression shifted from low to medium with less than 10% of the population expressing CD45high. MCA occlusion induced rapid and transient asynchronous increases in the pro-inflammatory cytokine IL-b and chemokines cytokine-induced neutrophil chemoattractant protein 1 (CINC-1) and monocyte-chemoattractant protein 1 (MCP-1), first in systemic circulation and then in injured brain. Double immunofluorescence with celltype specific markers showed that multiple cell types in the injured brain produce MCP-1. Our findings show that despite profound increases in MCP-1 in injured regions, monocyte infiltration is low and the majority of macrophages in acutely injured regions are microglia. Keywords: cytokines, flow cytometry, inflammation, macrophage, microglia, neonatal stroke. Inflammation is a significant contributing factor to neurodegenerative diseases. Depending on injury setting, the relative involvement of systemic and local inflammation, and communication between the two compartments can vary (Carson and Sutcliffe 1999;Baggiolini 2001;Perry 2004). It is well known that stroke in adult triggers a robust inflammatory reaction, largely involving an influx of peripheral leukocytes into the brain parenchyma (for reviews, see Feuerstein et al. 1997;Dirnagl et al. 1999;Han and Yenari 2003) and disruption of the blood-brain barrier (BBB) (Gidday et al. 2005). In the adult, neutrophils are typically the first leukocyte type to infiltrate (Barone et al. 1991;Garcia et al. 1994;Matsuo et al. 1995) and are followed by macrophages and lymphocytes (Barone et al. 1991;Garcia et al. 1994). The damaging role of infiltrating neutrophils in ischemia was shown by studies in which neutrophil depletion (Garcia et al. 1994) or administration of anti-adhesion molecules (Kishimoto and Rothlein 1994;Fassbender et al. 1999) reversed the reduced local tissue perfusion, BBB disruption, and the release of free radicals, proteinases, and other cytotoxins seen after stroke. The key role of macrophages in cerebral ischemic injury has also been firmly established (Feuerstein et al. 1997;Dirnagl et al. 1999;Han and Yenari 2003). Macrophage populations in the injured brain, however, are diverse (Carson et al. 1998;Dalmau et al. 2003) Abbreviations used: BBB, blood-bra...