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Ralph Steinman is perhaps best known as a codiscoverer of dendritic cells (DCs) and as a founding father of the research area that these cells have spawned. For his discovery, Steinman was recently awarded the 2007 Albert Lasker Award for Basic Medical Research. Yet the man behind the research holds his praise for the many other scientists—in the U.S. and abroad—who have further advanced the therapeutic promise of DCs.

Dengue virus is the causative agent of dengue fever and the more-severe dengue hemorrhagic fever (DHF). Human studies suggest that the increased risk of DHF during secondary infection is due to immunopathology partially mediated by cross-reactive memory T cells from the primary infection. To model T cell responses to sequential infections, we immunized mice with different sequences of dengue virus serotypes and measured the frequency of peptide-specific T cells after infection. The acute response after heterologous secondary infections was enhanced compared with the acute or memory response after primary infection. Also, the hierarchy of epitope-specific responses was influenced by the specific sequence of infection. Adoptive-transfer experiments showed that memory T cells responded preferentially to the secondary infection. These findings demonstrate that cross-reactive T cells from a primary infection alter the immune response during a heterologous secondary infection.

In 1993, David Nemazee and Martin Weigert independently showed that autoreactive B cells could proofread, alter, and reexpress modified receptors to become nonautoreactive. This process, called “receptor editing,” has since gained prominence as the main mechanism of B cell tolerance.

In 1978, Jonathan Sprent and Robert Korngold proved that graft-versus-host disease (GVHD) is caused by donor T cells that attack the host's non-MHC antigens. T cell depletion of donor grafts has since become a staple of transplantation strategies to combat leukemia and other inherited blood disorders.

Reactivation of serotype cross-reactive CD8+ memory T lymphocytes is thought to contribute to the immunopathogenesis of dengue disease during secondary infection by a heterologous serotype. Using cytokine flow cytometry, we have defined four novel HLA-A*02-restricted dengue viral epitopes recognized by up to 1.5% of circulating CD8+ T cells in four donors after primary vaccination. All four donors had the highest cytokine response to the epitope NS4b 2353. We also studied the effect of sequence differences in heterologous dengue serotypes on dengue-reactive CD8+ memory T cell cytokine and proliferative responses. The D3 variant of a different NS4b epitope 2423 and the D2 variant of the NS4a epitope 2148 induced the largest cytokine response, compared with their respective heterologous sequences in all donors regardless of the primary vaccination serotype. Stimulation with variant peptides also altered the relative frequencies of the various subsets of cells that expressed IFN-γ, TNF-α, MIP-1β, and combinations of these cytokines. These results indicate that the prior infection history of the individual as well as the serotypes of the primary and heterologous secondary viruses influence the nature of the secondary response. These differences in the effector functions of serotype cross-reactive memory T cells induced by heterologous variant epitopes, which are both quantitative and qualitative, may contribute to the clinical outcome of secondary dengue infection.

Text and Interview by Hema Bashyam By figuring out how mycobacteria survive within the very cells that try to kill them, Ehrt hopes to find a way to combat chronic tuberculosis infections. For most bacteria, getting scooped up by macrophages means a rapid death at the hands of a lethal cocktail of

A cell contact protein found in the heart does more than provide structural support, according to Lisewski et al. (page 2369). It also helps maintain a steady heartbeat. Figure 1 The loss of CAR (green) expression leads to cardiac arrhythmia because connexins (red) are no longer held in place at heart AV nodes. In developing or injured heart muscle, a cell–cell adhesion protein known as the Coxsackievirus–adenovirus receptor (CAR) helps growing muscle fibers stick to each other and settle into place. The relatively low levels of CAR in adult hearts suggest that they've outgrown the need for it. But the authors now find that this bit of CAR keeps ion channels called connexins in place, thus aiding the transmission of electrical connections between heart cells. Decreasing CAR expression in the heart, the group found, did not disrupt the structural stability of the organ but triggered an erratic heartbeat. Electrical signals within each heart chamber were normal, but they were delayed in passing from atria to ventricles. Electrical signals pass between the chambers through a cell cluster called the atrial-ventricular (AV) node, via ion channels made of connexins 45. The AV node cells of CAR-deficient mice had fewer of these connexins, which drifted throughout the cell membrane instead of clustering at gap junctions, where they belong. Although it's possible that CAR-driven signals induce connexin gene expression, the recent discovery that the two proteins bind to each other suggests instead that CAR might pin down connexins in adjacent gap junctions, thereby preventing their mislocalization and subsequent degradation. The findings might explain the association between cardiac arrhythmia and CAR loss seen in patients who develop autoantibodies against CAR or who become infected with Coxsackievirus. The group is now trying to confirm that this arrhythmia is due to a connexin disturbance at the AV node. HB

DCs can either magnify or mute immunity by sampling antigens throughout body and presenting them to lymphocytes in combination with secondary signals that either activate or tolerize the responding cell. This quality, which makes DCs a handy weapon against both cancer and autoimmune disease, spurred Caetano Reis e Sousa to dissect the pathways that control different DC functions. Figure 1 Caetano Reis e Sousa Reis e Sousa first encountered DCs as a graduate student in Jonathan Austyn's laboratory at Oxford University. At the time, these cells were thought to lack the ability to phagocytose antigens—a troubling deficiency in cells known as the initiators of immune responses to microbes. Using DCs found in the skin, Reis e Sousa showed that the inability to pick up antigen was limited to mature cells; freshly isolated ("immature") DCs could engulf particulate antigens with ease (1). Determined that his DC expertise become more than skin-deep, Reis e Sousa accepted a post-doctoral position with Ron Germain at the NIH, where he studied antigen processing and cross-presentation by DCs and other cells (2). While at the NIH, he also collaborated with Alan Sher to investigate how microbe-activated DCs induce a T helper (Th)-1 cell response (3). Since starting his own laboratory at the London Research Institute in 1998, Reis e Sousa has defined several pathways of DC activation that involve Toll-like receptors (TLRs) and non-TLRs such as RIG-I and C-type lectins (4–7). His group recently discovered a marker for a DC subset adept at cross-presentation, which might help bring DCs further into the realm of translational medicine (8).

Early in an immune response, B cells latch on to antigens embedded in the surface membranes of follicular dendritic cells and macrophages. Engagement with antigen spurs a flurry of activity at the point of contact on the B cell surface—a phenomenon that Facundo Batista is documenting in molecular detail. Figure 1 Facundo Batista During his post-doctoral research, Batista discovered that B cell receptors (BCRs) must exceed an affinity threshold to trigger B cell activation (1). He also found that B cells form an organized molecular structure, known as an immunological synapse, when they encounter membrane-tethered antigen. BCRs accumulate antigens at the synapse, then extract and internalize them for subsequent presentation to T cells (2). After Batista set up his own laboratory at the London Research Institute, he began to follow these events in real time. His studies revealed that antigen-bound B cells reorganize their actin cytoskeleton to first spread across the surface of the antigen presenting cell and then to contract their membranes, concentrating the antigen at the synapse within a ring of adhesion molecules (3). High affinity antigens induce more spreading than low affinity antigens, leading to increased antigen capture. Using a mathematical model based on this data, Batista's group hypothesized that deactivating the spreading mechanism would prevent cells from distinguishing between high and low affinity antigens—a prediction confirmed by their later experiments with spreading-defective B cells. The group has since identified the signaling molecules and pathways that enhance B cell adhesion at the synapse and contribute to activation (4–6). They have now begun to use multi-photon microscopy techniques to image B cell behavior in real time in living tissue.

Antibody diversity is generated during B cell development by the rearrangement of variable (V), diversity (D), and joining (J) segments of immunoglobulin (Ig) genes. Both alleles of each Ig locus can initiate rearrangement, but only one allele successfully rearranges. As soon as this occurs, the other allele is prevented from following suit. This mechanism, known as allelic exclusion, guarantees that each B cell produces antibodies specific for just one antigen. In her laboratory at New York University (NYU), Jane Skok studies how exclusion is established. Figure 1 Jane Skok Skok's initial foray into research involved dissecting the genetics of the complement cascade with Ellen Solomon at Cancer Research UK (1). But immediately after earning her Ph.D., Skok left science to care for her ill child. When she returned nearly a decade later, she had to start from scratch. After getting up to speed with a Master's in immunology, she joined Amanda Fisher's lab, where she gravitated toward B cells and the question of allelic exclusion. Skok has since helped define some of the steps that make exclusion possible. She discovered that the excluded Ig allele gets sent to repressive DNA domains near the centromere, whereas the productive allele remains within accessible domains (2–4). She later focused on one of the more curious aspects of allelic exclusion. During rearrangement, the Ig locus contracts, allowing V, D, and J segments to get close to each other and join up. Skok found that the successful rearrangement of one allele triggers locus relaxation, thereby stopping rearrangement of the other allele (5). She is now hunting for the proteins and pathways that direct the contortion and movement of Ig loci.

Text and Interview by Hema Bashyam Combining his medical training with his love of computational biology, Mootha is compiling an exhaustive list of mitochondrial genes and networks. Mitochondria, the energy reactors of the cell, are estimated to contain approximately 1,500 proteins, but the

The most elusive period in Plasmodium's life cycle spans from the moment a human feels the sting of an infected mosquito to the time the victim starts to shiver—a sign that the parasite has infected the red blood cells. During this interval, the bugs are few in number, and they dash to the liver at high speeds. Figure 1 Robert Menard At the Pasteur Institute in Paris, Robert Menard spies on the parasites' journey in real-time to figure out how they move and gain access to their various cellular targets. How pathogens sneak into cells is a question that has interested Menard since the start of his research career. As a Ph.D. student, he identified the proteins that Shigella, the bacterium that causes dysentery, uses to penetrate the gut epithelium (1, 2). Later, as a postdoctoral fellow, Menard discovered that Plasmodium uses a protein called circumsporozoite to both develop inside the mosquito midgut and to grab on to mammalian liver cells (3). After starting his own lab in 2001, Menard developed techniques to track Plasmodium during its odyssey within infected mosquitoes and mice. His discoveries have challenged many presumptions about Plasmodium's early behavior within the host. Menard's movies show that injected parasites don't just glide to the liver as previously thought. Some make a detour to lymph nodes—one of the training centers of the immune system (4), while others remain at the bite site (5). Those that make it to the liver shield themselves from macrophages and dendritic cells by clustering within vacuole-like structures called merosomes that eventually bud out into the blood (6). Menard is now combining genetic tools with his movie-making skills to identify host and parasite genes that direct the parasites' early itinerary within the host.

The human genome encodes snippets of RNAs ranging from 21–23 nucleotides in length, called microRNAs (miRNAs). Rather than code for proteins, miRNAs instead sabotage the translation of mRNAs. miRNAs are first generated as longer, hairpin-like precursors in the nucleus. In the cytoplasm, a collection of enzymes strips down the hairpins into their tiny and unwound final structure. miRNA-bound mRNAs are either sequestered from translation or destroyed outright—processes known as RNA interference (RNAi). Figure 1 Michael McManus Michael McManus has been a fan of small RNAs and their manipulative power his entire career. As a graduate student, he studied how small RNAs in trypanosomes guide RNA editing, whereby incomplete mitochondrial mRNAs are filled in with uridine residues (1, 2). During his postdoctoral stay in Nobel Laureate Phillip Sharp's laboratory at MIT, McManus's work led to one of the first reports of RNAi activity in mammalian cells (3). He showed that T cell genes could be suppressed by a class of small double-stranded RNA called short interfering RNAs (siRNA). His next discovery—that synthetic miRNA hairpin structures could be stably introduced into cells via DNA vectors—gave the field a reliable tool for studying RNAi (4). Now heading his own lab at the University of California, San Francisco, McManus is exploring the role of miRNAs during development and disease. His team has developed a mouse model in which the gene for the miRNA-processing enzyme Dicer can be selectively switched off (5) and a technique to visualize miRNA expression patterns in situ (6). They are now creating RNAi libraries that can be used to analyze gene function in cells and in animal models. McManus is also involved in collaborations to understand how miRNA failures lead to diseases such as diabetes. He recently began a project funded by the Keck Foundation to create 100 different miRNA mouse knockouts.

Text and Interview by Hema Bashyam As a student, Anthony De Tomaso was fascinated by many aspects of biology, from molecular biology to marine biology and development to immunology. Now as head of the same lab that he joined as a postdoctoral fellow, De Tomaso has marshaled his diverse scientific

[Graphic][1] Cells exposed to high glucose (HG) levels overexpress NF-κB due to histone modifications of the NF-κB promoter by the enzyme Set7. Short-lived sugar highs might give a quick energy boost, but according to El-Osta et al. (page [2409][2]), they also leave a lasting bad

On page 2125, Quezada et al. find that big tumors might evade T cell destruction by shutting down adhesion molecules. Figure 1 Boosting adhesion molecule expression in tumor blood vessels (red) allows more tumor-killing T cells (green) to gain access. Tumor-fighting T cells work better when suppressive regulatory T (T reg) cells are not lurking. But treatments that either deplete T reg cells or boost anti-tumor T cell activity aren't enough to destroy tumors, as Quezada et al. now show. As seen in other tumor models, skin tumors in mice were destroyed when the group depleted T reg cells before tumor implantation. But if they depleted T reg cells after tumors had already developed, the treatment failed, even if the animals were given T cell–boosting drugs. Plenty of potential tumor-killing T cells were available, but they did not seem to infiltrate the tumors. Small tumors, at least, were sensitive to T cells. When T reg cell depletion was combined with T cell–boosting drugs, small, newly implanted tumors were destroyed. But well-established tumors resisted the effects of this regimen. Established tumors, the group found, had fewer adhesion molecules on surrounding blood vessels. To determine whether these adhesives might help T cells invade bigger tumors, the group induced adhesion molecules in mice using irradiation. Zapped mice that were then injected with anti-tumor T cells along with activity-enhancing drugs fought off tumors. But irradiated mice that were also injected with T reg cells put up a weak fight. How tumors dial down adhesion molecules as they grow and whether T reg cells coerce them into doing so remain to be seen. The findings might explain why treatments that deplete T reg cells and enhance killer T cell activity haven't fared well in clinical trials. Perhaps combining them with irradiation might make them more effective.

Unlike some religious leaders, suppressive T cells need more than just charisma to spread messages of tolerance: the cells rely on a cell surface cytokine complex to convert others, according to Andersson et al. (page 1975). Figure 1 Naive-turned-suppressor cells dampen colitis (left). Regulatory T (T reg) cells suppress other T cells in part via the cytokine TGF-β, which can convert naive CD4+ T cells into suppressor cells. Andersson et al. now find that naive CD4+ T cells turn into suppressor cells only when they bump into T reg cells that have an inactive TGF-β complex on their surface. Proteolysis of the inactive complex, which liberates soluble protein, was required for conversion. Inhibiting the protease or swamping the cells with inactive complex prevented cleavage and thus conversion. The converted cells could suppress gut inflammation when transferred into mice suffering from colitis. Other immune cells also secrete soluble TGF-β, but they do not seem to force conversion—at least in this system. Previous experiments showed that naive T cells cultured with T reg cell–depleted spleen cells, which secrete TGFβ, fail to convert. Non-T reg cells don't express the inactive surface complex, so perhaps its cleavage is needed to trigger other conversion-supporting signals. The latent TGF-β complex was present only on T reg cells that had been stimulated via their T cell receptors. This specificity might allow T reg cells to limit conversion by delivering the TGF-β signal only to T cells bound to the same antigen-presenting cell and thus on the verge of being activated. “We wouldn't want T reg cells to run amok and convert all naive T cells,” says lead author Ethan Shevach. How often T reg cells rely on this mechanism and its impact on the induction of tolerance in vivo are unknown.

An amino acid might help fight chronic hepatitis, according to a study by Das et al. (page 2111). Figure 1 The arginine-destroying enzyme arginase is elevated in the livers of patients with chronic hepatitis B virus infections. The amino acid arginine is indispensible for the host's T cell response against tumors or chronic infection. Without arginine, a T cell receptor (TCR) component cannot be produced efficiently, thus dampening TCR signaling. As a result, the cells do not proliferate normally or produce the survival cytokine IL-2, although they still make inflammatory cytokines such as IFNγ and TNF. Das et al. now find that CD8+ T cells in the livers of patients with chronic hepatitis B infections show signs of arginine starvation. Despite proliferating poorly, the cells' production of IFNγ and TNF might explain the liver damage seen in these patients. The amino acid deficiency stemmed from the activity of the arginine-destroying enzyme arginase, which was elevated in inflamed livers. The arginase source is still unknown but seems to be confined to the liver, as T cells elsewhere in the patients were less affected. Culturing patients' liver-derived CD8+ T cells with arginine corrected their defects. Whether arginine supplements can help cure hepatitis B remains to be seen.

[Graphic][1] Different HIV strains in one individual that recombine (red/blue overlap) at immune-targeted sites escape T cell attack. On page [1789][2], Streeck et al. reveal that a second HIV strain is encouraged by the host's immune pressures to recombine with an earlier strain.

Fat cooperation keeps sleeping sickness at bay, say Molina-Portela et al. (page 1721). The authors find that all three components of a human high density cholesterol complex are needed to kill disease-causing trypanosomes.

On page 1659, Xia et al. find that a protein that counters cancer also stops wound-healing fibroblasts from overdoing it.

[Graphic][1] IgM+ B cells (brown) develop in the mouse gut even when the spleen and gut lymphoid tissues are missing. On page [1343][2], Shimomura et al. identify a group of B cells that rebels against convention: unlike other B cells, this group matures in the gut and might

Lipids heading toward lysosomes that carry a mutated ion channel are spared from destruction, according to Meidel et al. (page 1477). The accumulating undigested fat eventually dooms the cell. Figure 1 Lysosomes (arrowhead) in TRP-ML1–deficient cells internalize endosomal cargo (spots) but fail to degrade it. Cells get rid of excess fat, which comes from worn-out organelles or engulfed pathogens by, for instance, shuttling them in endosomes to degradative lysosomes. The two organelles are thought to fuse when the lysosomal ion channel TRP-ML1 lets in calcium. Individuals with mutated TRP-ML1 suffer from a fatal disease called mucolipidosis type IV, which is characterized by fat accumulation in tissues. The failure of endosomes to fuse with lysosomes in these patients' cells, as seen in previous imaging studies, was thought to cause the fat pile up and trigger disease. But because TRP-ML1 also lets through protons, which lowers the pH and thus activates lysosomal enzymes, Meidel et al. wondered whether the fat backlog is caused by a degradation defect rather than a fusion failure. By silencing TRP-ML1 in normal cells, the group now shows that the channel is not necessary for fusion. In its absence, however, lysosomes became overacidified and thus failed to process their lipid load. The pH defect suggests that the channel leaks protons out of the lysosome to prevent the interior from becoming too acidic. Lipid-clogged lysosomes might send feedback signals that prevent them from taking on new cargo.

[Graphic][1] The naive RORγt+ T cell population (green) includes Foxp3-expressing regulatory T cells (arrowheads). On page [1381][2], Lochner et al. expose the promiscuity of a transcription factor that was previously thought to be exclusive to inflammatory T helper (Th) 17 cells

During severe bacterial infections, the transcription factor NF-κB is both an asset and a liability. On page 1303, Ye et al. show that its activation in endothelial cells single-handedly perpetuates the inflammation that drives septic shock. NF-κB is required in endothelial cells for new blood vessels to grow. And in immune cells, its activation is essential for defense against bacteria and other infectious agents. Blocking NF-κB in mice, for example, turns a normally manageable infection deadly. But more selectively inactivating the protein has proven beneficial in other systems, including certain cancer models. Following this lead, Ye et al. disabled NF-κB activation only in endothelial cells. The resulting mice survived a normally lethal bacterial infection and avoided septic shock. Being free of active NF-κB allowed the endothelial cells lining blood vessels to maintain their cell–cell junctions and minimize their stickiness, thus limiting the fluid leakage and massive immune cell infiltration into tissues that triggers shock. Thus, NF-κB–driven activation of endothelial cells, not immune cells, seems to be the driving force behind the escalating inflammation that causes sepsis.

Some tumors avoid getting killed by turning macrophages into immune-suppressive cells. Hagemann et al. (page 1261) now find that these impotent cells can be reverted into weapons of tumor destruction by simply suppressing a kinase. Figure 1 Ovarian tumors (blue) are destroyed in mice injected with macrophages in which IKKβ signals are inhibited (bottom). Macrophages can destroy tumor cells by producing inflammatory molecules. But macrophages within tumors often secrete harmless antiinflammatory cytokines and proteins that promote tumor growth. Tumor cells induce this transformation, but the signals that drive the conversion were unknown. Hagemann et al. now find that tumor macrophages in mice are disarmed by signals that activate NF-κB—a transcription factor that normally drives inflammation. As in inflammation, tumor macrophage NF-κB was turned on by IκB kinase (IKK) β. In tumors, however, IKKβ also suppressed STAT1—a transcription factor that turns on tumor-fighting genes. The basis for this difference is unclear. Perhaps the tumor contains unique cues that instruct the IKK pathway to shut off STAT1. STAT1 suppression in macrophages depended solely on the cytokine receptor IL-1R and its downstream adaptor, MyD88, suggesting that tumors might protect themselves by secreting the IL-1R ligand, IL-1β. Macrophages from tumors or from healthy animals became in vivo tumor killers when engineered to express dominant-negative IKKβ. These reprogrammed macrophages produced high levels of IL-12, which recruited tumor-fighting NK cells. The group is now investigating whether infusing similarly reprogramed macrophages into cancer patients will help reverse tumor growth.

A protein previously hailed as a tumor suppressor has a sinister agenda, according to Rolny et al. (page 1155). Although this false savior, semaphorin 3B (SEMA3B), tells tumor cells to stop growing, it then sends them on their way to grow elsewhere. Figure 1 Infiltration of macrophages (red, left) into tumors is blocked when tumors are treated with anti–IL-8 antibody (right). Semaphorins were initially identified as extracellular navigation signals for growing axons. Some of them are now known to enhance tumor progression by increasing the growth of blood vessels that feed the tumor. SEMA3B, however, is thought to be a tumor suppressor because it also inhibits tumor cell proliferation. Rolny et al. now find that this suppression comes at a potentially fatal price. Most tumor suppressors are either absent or expressed at low levels in cancerous tissues. But the group found high levels of SEMA3B in many cancer cell lines and in metastatic tumors from patients. Although SEMA3B inhibited the proliferation of these tumor cells in vitro, the poor prognosis of these patients suggested that SEMA3B signaling might have detrimental effects in vivo. To test this hypothesis, the authors engineered human tumor cells in which SEMA3B expression was controlled by a drug-inducible promoter and injected these cells into the skin of immune cell-depleted mice. Drug-treated mice had smaller skin tumors but developed secondary lung tumors, suggesting that SEMA3B induced metastasis. The authors found that SEMA3B inhibited growth and simultaneously triggered metastasis by activating the signaling kinase p38. p38 then activated a cell cycle inhibitor and induced tumor cells to secrete IL-8, a cytokine known to induce leukocyte chemotaxis. The IL-8–secreting tumors were full of infiltrating macrophages, which are thought to spur metastasis by producing soluble factors such as VEGF. Blocking the release of IL-8 in response to SEMA3B, the group found, blocked the metastasis of these tumors.

[Graphic][1] Old mice that fight off a second strain of the flu (left column) have more NP epitope-specific T cells than do mice that stay infected (right column). An aged immune system has experienced CD8 + T cells to combat old enemies but is missing some of the fresh CD8+ T cells

Giving infecting bacteria a little freedom early on can be good for the host, according to a new study by Rotta et al. (page 657). Confining the bugs too quickly, they find, can tie up the immune cells that need to spread the word. Figure 1 Granulomas (blue) do not form when mice lack the collagen-building protein SPARC (bottom). Certain bacteria, such as Mycobacterium tuberculosis and Salmonella, are quickly surrounded by macrophages, dendritic cells, and other immune cells that seal off the invaders from surrounding tissue. The convergence of these cells creates immune cages—or granulomas—that contain several extracellular matrix proteins, including SPARC, which enhances collagen assembly. Rotta et al. now find that SPARC-induced collagen prevents Salmonella-laden dendritic cells from slipping out of granulomas to the nearest lymph node to alert T cells. In mice infected through the skin with an attenuated strain of Salmonella, bacteria were hemmed in by a SPARC-enhanced granuloma within a day. After 9 days, however, the bacteria breached the fortifications. In the resulting absence of bacterium-killing T cells, the bugs rapidly spread out into other organs, and the animals soon died. SPARC-deficient mice, on the other hand, never formed granulomas when they were infected with this crippled strain of Salmonella. Immune cells found the infection site but failed to coalesce because SPARC-formed collagen was absent, thereby allowing dendritic cells to reach lymph nodes. The mice thus fought off this initial infection and survived a later challenge with a virulent strain of Salmonella. The immune system's knee-jerk reaction in trapping bacteria right away might be initially protective for widely infecting bugs that invade via the airways or the blood rather than the skin. The current findings suggest that antibacterial vaccines injected through the skin might be more effective if they are coinjected with a SPARC-blocking molecule.

[Graphic][1] Mice injected with glutamate receptor–blocking drugs (left) form fewer clots (spots). A chemical that boosts our ability to learn and remember also boosts blood clotting, say Morrell et al. (page [575][2]). The learning chemical, glutamate, is released at neuronal

[Graphic][1] Inflammation-related weight loss caused by symbiotic bacteria in A20-deficient mice (middle) is rescued by loss of MyD88 (right). On page [451][2], Turer et al. find that loss of a single protein turns symbiotic bacteria into killers. Helpful bacteria in the gut

Fatty foods plug up arteries in more ways than one, according to Mullick et al., on page 373. They also trigger the expression of an innate immune receptor that makes the arteries more fat friendly. Figure 1 A high fat diet increases the expression of Toll-like receptor 2 (green) on endothelial cells within four weeks (left to right). Recent data suggest that the immune receptor in question, Toll-like receptor (TLR) 2, which recognizes bacterial lipids, might also enhance atherosclerosis—a disease that can lead to restricted blood flow due to the accumulation of fat in blood vessels. Mice that are plaque prone because they cannot properly dispose of fat stay plaque free if TLR2 is knocked out. Its loss from immune cells alone, however, does not protect these mice, suggesting that TLR2 on a nonimmune cell favors plaque growth. Mullick et al. now find that TLR2 is expressed on the endothelial cells that line the curves of blood vessels. These cells are plaque prone by nature; the uneven blood flow here stimulates them to express more adhesion molecules and chemokines. They thus attract both circulating fat and macrophages that eat the fat and then harden into plaques. A high-fat diet thus increases plaque formation in part by giving macrophages more to chew on. A high-fat diet also increased endothelial TLR2 levels in the atherosclerosis-prone mice, the authors found. Components of dietary fat may bind to TLR2 and thereby increase expression of the receptor on endothelial cells. These mice accumulated larger fat deposits and more macrophages in vessel curves than did their TLR2-deficient counterparts. The team has yet to determine how TLR2 expands plaques. Perhaps, like uneven blood flow, it increases endothelial cell adhesion molecule or chemokine levels.

Mending wounds quickly while keeping them scar-free is not an impossible task. Mori et al. (page 43) find that shutting down a protein called osteopontin does the trick.

[Graphic][1] HCMV induces infected cells to produce leukotrienes (pink) that attract more inflammatory cells (dark blue). A dormant virus that is awoken by inflammation enhances this potentially dangerous immune response to remain active, say Qiu et al. (page [19][2]). The human

On page 117, Giefing et al. report the discovery of two new vaccine candidates that might protect humans against all 90 versions of a deadly bacterium. This bacterium, Pneumococcus, invades the lungs, blood, and eventually brain to cause pneumonia, sepsis, and meningitis, respectively. Surface proteins from this bug are currently being used as vaccines to induce the body to produce protective antibodies. But these vaccines, which comprise proteins from only a few types of Pneumococcus, are not effective against other variants, due to extreme sequence diversity in the surface proteins. Giefing et al. now identify two bacterial antigens that are nearly identical among all pneumococcal strains and that induce potent antibodies in vivo. The authors hunted for conserved bacterial antigens that would be exposed on the pathogen during disease. Antibodies isolated from exposed but healthy humans and from those recovering from infection were used to identify pneumococcal proteins that were targeted by the protective antibody response. Both of the newly identified antigens induced antibodies that protected immunized mice against several other pneumococcal variants. The antigens might be potent targets because they come from a protein that is essential for bacterial growth and survival and thus unlikely to mutate. The team is currently testing their vaccines in clinical trials.

On page [565][1], Haxhinasto et al. pinpoint the signaling roadblock that stops CD4+ T cells from turning into regulatory T (T reg) cells. Some T reg cells develop directly from thymic progenitor cells, but others seem to be converts that were once CD4+ T cells. These potentially inflammatory CD4+

For more than 70 years, alum has been used as a trusty aide to improve antibody responses to vaccines. But how this aluminum-containing compound boosts the response to vaccines has been a mystery. On page 869, Kool et al. expose its mysterious mechanism; they find that alum causes cells to produce a stimulator of dendritic cells (DCs). Figure 1 Mice injected with antigens mixed with alum produce uric acid that then attracts monocytes. DCs were once the favored hypothetical link between alum and B cells, as they activate CD4+ T cells, which can then enhance B cell activation and antibody production. But DCs were later dismissed when it was found that they were not stimulated by alum in vitro. These in vitro results are now shown to be a red herring by Kool et al., who find that alum activates DCs in vivo by provoking the secretion of uric acid—a molecule that is triggered by tissue and cell trauma. The injection of alum, the group found, induced an influx of neutrophils and inflammatory cytokines and chemokines—a combination that was previously seen in response to the injection of uric acid into mice. In mice injected with antigens mixed with alum, uric acid levels increased within hours. The uric acid might be released by the cells' lining the body's cavities that turn necrotic after contacting the alum. The absence of these uric acid sources in in vitro assays might have led to the previous misleading results. In response to the uric acid, inflammatory monocytes flocked to the injection site, took up the antigens, and broke them down into T cell–stimulating epitopes. The monocytes then migrated to lymph nodes, where they matured into DCs and activated CD4+ T cells. Without alum, the antigens were not taken up at the injection site. Still, they eventually reached lymph nodes via the flowing lymph. The resident node DCs, however, did not process the alum-free antigens efficiently or express T cell costimulating receptors. The resulting subdued immunity was similar to that seen in mice that were depleted of inflammatory monocytes or those injected with enzymes that degrade uric acid.

On page 1917, Nishikii et al. produce a plethora of platelets by picking perfect stem cell progenitors and preventing shearing of the platelets' activating receptors. Figure 1 MMP inhibitors help maximize the numbers and function of cultured platelets (green) expressing integrin receptors (red). Platelets for therapeutic use are currently filtered from donated blood, which increases the risk of infections and other side effects in patients who need frequent transfusions. Scientists have been trying to generate platelets from embryonic stem cell (ESC) lines instead, but their efforts have so far been stymied by two problems. First, ESCs cultured with stromal cells produce a tiny platelet population that is quickly drowned out by other cell lineages. Nishikii et al. resolved this issue by using ESCs that had already differentiated into platelet-committed hematopoietic stem cells, which express the clot-promoting αIIbβ3 receptor. The second and more worrying problem is that the ESC-derived platelets don't aggregate properly. This defect was previously seen in vivo in long-lived platelets whose matrix-binding receptors had been sheared off by matrix metalloproteinases (MMPs). The group found that this also happened in vitro, and platelets cultured with MMP inhibitors formed clots in vitro and enhanced tissue repair in injured mice. This approach awaits testing in humans.

A study by Didierlaurent et al. (page 323) explains why fighting the flu makes us vulnerable to bacterial infections in the following months. A clampdown of innate immune receptors in the lung might leave the door open to opportunistic bacteria. The team previously showed that mice that had just recovered from the flu had a weak inflammatory response that made them more susceptible to other pathogens over the next six months. The group now finds that post-flu inflammation in these mice is tempered by a dampened innate immune system. The innate system usually responds to ligands that activate Toll-like receptors (TLRs). But in mice recovering from the flu, these ligands did not activate TLRs on lung macrophages. The cells therefore did not produce the chemokines necessary to attract neutrophils—the main instigators of lung inflammation. Although the blunted TLR response might protect the lungs against inflammation-induced damage, it made them vulnerable to bacterial infection. After beating influenza, the mice were killed by pneumonia-causing bacteria that are normally held at bay in the airways. Influenza and other respiratory viruses are not known to encode TLR-suppressing proteins. This suppression may thus represent the lung's attempts to protect itself from damaging levels of inflammation. The authors suspect that the lungs might suppress TLR expression in response to any insult that causes a massive inflammatory response, including pathogens, injury, or environmental allergens. Their theory is bolstered by the fact that another respiratory pathogen, respiratory syncytial virus, also suppressed macrophage TLR signaling.

Neutrophils need just enough copies of a gene for an antibody-binding receptor, say Willcocks et al. (page 1573). Whereas too few copies can contribute to lupus, too many can predispose to vasculitis. Figure 1 Neutrophils with fewer copies of FCGR3B bind poorly to IgG complexes. Cells can have as many as four copies of this gene, FCGR3B, which encodes the FcγRIIIb receptor, but some lupus patients have fewer than two. Willcocks et al. now find that this scarcity leads to fewer receptors on neutrophils, which thus fail to bind and destroy inflammation-causing antibody clusters. The paucity of receptors did not impair other neutrophil functions such as the production of oxygen radicals. Some healthy individuals also had fewer copies of the gene and thus similarly defective neutrophils. But these individuals do not develop lupus, most likely because they lack other genetic defects required to cause disease. The compensatory influence of other genes might also explain why low copy number of FCGR3B is not a risk factor for lupus among all populations. Having more FcγRIIIb receptors, however, doesn't guarantee good health. The group found that a high copy number of FCGR3B was associated with vasculitis—a disease in which neutrophils release damaging oxygen radicals in response to cross-linking of FcγRIIIb receptors on blood vessel walls.

On page 1201, Huang et al. find that an antigen-presenting protein suffers from an identity crisis: it looks like MHC class I but behaves like MHC class II. This molecule, MR1, is recognized by a subset of gut-roving T cells. When MR1 carries the right antigen, these T cells suppress gut inflammation, but the identity of that antigen is a mystery. Because MR1 is structurally similar to MHC class I molecules, which usually pick up proteasome-processed peptides in the ER, MR1 was thought to do the same. This notion is now disproved by Huang et al., who show that MR1-expressing cells treated with proteasome inhibitors or lacking ER peptide–loading chaperones can still activate these T cells. Instead, they found, MR1 picked up its antigens in endosomes, where MHC class II molecules get theirs. MR1 bound to a chaperone called the invariant chain, which ferries MHC class II molecules from the ER into endosomes. There, it bound another chaperone, called DM, which loads them up with peptides. MR1-expressing cells that lacked the invariant chain were unable to activate gut T cells, whereas high levels of this protein increased their activation. The immune-suppressing ability of MR1-activated T cells suggests that they might prevent the gut from attacking its helpful resident flora. The authors therefore speculate that the MR1 ligand might be derived from one or more of these residents.

A factor that promotes blood vessel growth keeps two types of brain cells alive, say Maharaj et al. (page 491). Their findings might explain why anticancer drugs that block this factor cause neurological side effects. Figure 1 Mice that express high levels of VEGF-blocking molecules develop brain lesions (arrows). The drug target in question—vascular endothelial growth factor (VEGF)—binds to receptors on blood vessel endothelial cells. A localized increase in VEGF, which is produced by underlying epithelial cells and other neighboring cells, promotes vessel growth in developing organs and injured tissues. Lower levels of VEGF might also be necessary for the survival of endothelial cells, which become apoptotic when cultured in the absence of VEGF. VEGF-blocking drugs that limit tumor growth by inhibiting the tumors' blood supply can cause seizures and brain swelling in some cancer patients. To find out whether these symptoms stem from the destruction of brain blood vessels, Maharaj et al. examined the brains of mice that had been engineered to express high levels of proteins that block VEGF and TGFβ—a cytokine that stimulates VEGF production. Besides damage to brain endothelia, the mice developed brain lesions due to a breakdown of the barrier that prevents cerebral spinal fluid from seeping into the brain. The specialized type of epithelial cells that make up this barrier expressed VEGF receptors and became apoptotic when VEGF was systemically blocked. Why brain complications only occur in a subset of cancer patients on anti-VEGF therapy is unknown.

Try as it might to pull itself forward, an immune cell will go nowhere unless a myosin also pushes it from behind, report Morin et al. (page 195).

[Graphic][1] A citrulline-containing IL-8 isoform fails to recruit neutrophils. On page [2085][2], Proost et al. find that altering a single arginine turns an inflammatory chemokine docile. The chemokine IL-8, which activates and recruits neutrophils, is secreted as a mixture of

On page 2019, Deruaz et al. find that ticks might prefer to take on immune cells one at a time rather than brave the whole lot at once. Figure 1 Ticks secrete several different evasins, each of which blocks a different chemokine. Some worms and viruses fend off immune cells by neutralizing the chemokines that attract them. These species make one chemokine-blocking protein that covers all the bases. But ticks don't use this one-for-all strategy, the authors now find. Instead, they made at least three single-chemokine blockers, which probably prevent host immune cells from swarming into the bite site and clogging up the ticks' food pipeline. These anti-chemokines, called evasins, each had a unique shape that might contribute to their selectivity. Ticks might have evolved this selectivity to counter the host's step-wise immune response to the Lyme disease parasite they carry. In parasite-infected hosts, neutrophils usually arrive first to the bite site, followed by eosinophils and monocytes, and finally other immune cells. Tick saliva has different anti-chemokine activity at different times during feeding, suggesting that having several evasins might somehow better counter this staggered cell influx. Viruses, on the other hand, do not have the luxury of combating one cell type at a time and might therefore depend on a single multi-purpose protein.

On page 1077, Machado et al. reveal a new way in which aspirin reins in inflammation—it triggers the destruction of proinflammatory signaling proteins. Figure 1 The destruction (top) of TRAF2 (green) via SOCS2 (red) is blocked in mice treated with proteasome inhibitors (bottom). Aspirin's power was initially attributed to its inhibition of proinflammatory lipids called prostaglandins—a discovery that won the Nobel Prize in 1982. Later, aspirin was also shown to beef up levels of lipids called lipoxins, which help resolve inflammation by blocking NF-κB activation and the recruitment of inflammatory cells. Lipoxins were recently found to activate SOCS2, a protein that blocks signals from growth hormone receptors by targeting downstream signaling proteins to the proteasome. To determine whether SOCS2 also blocks inflammatory signals, Machado et al. fished for its binding partners among molecules that transmit innate immune receptor signals. They now find that SOCS2 binds TRAF2 and TRAF6—adaptor proteins that are required for cytokine production by activated dendritic cells (DCs)—and seems to target them to the proteasome. Treating mice with aspirin decreased DC levels of cytokines and TRAF2 and TRAF6—effects that were mitigated by proteasome inhibitors. The same effects were not found in DCs from SOCS2-deficient mice. Mice treated with a proteasome inhibitor after parasitic infection beat the bug but died from inflammatory damage. The deadly inflammation probably resulted from prolonged signaling via TRAF2 and TRAF6, as inflammation in SOCS2-deficient mice was severe even without the inhibitor. Infected mice that were left untreated resolved the inflammation and survived the infection. Because proteasome inhibitors also block the degradation of IκB—the molecule that holds NF-κB in check—they are being developed as alternatives to aspirin, which can cause stomach problems and other side effects. But the current findings suggest that this strategy might be counterproductive.

[Graphic][1] Overproliferating T cells infiltrate the liver and trigger lupus-like autoimmunity in older Egr-2–deficient mice. Self-tolerance is well known to be enforced in part by an external force of regulatory T cells. Now, Zhu et al. (page [2295][2]) show that T cells also

Refilling an empty niche with proliferating CD8+ T cells can cause autoimmune disease, say Tajima et al. (page 1019). Figure 1 Autoimmune colitis (top) caused by homeostatically proliferating CD8+ T cells is resolved in mice injected with anti–IL-6 antibody (bottom). Infusion of naive T cells into an environment where there are few native T cells—such as in patients undergoing cancer therapy—spurs two kinds of proliferation. T cells activated by non-self-peptide/MHC complexes proliferate rapidly, whereas those activated by self-peptide/MHC in the presence of cytokines such as IL-7 and IL-15 proliferate slowly. CD4+ T cells that proliferate by these methods have previously been shown to drive the inflammation seen in several autoimmune diseases. Tajima et al. now find a new variation in homeostatic proliferation of CD8+ T cells that also induces autoimmunity. In gut lymph nodes of T cell–deficient mice, injected CD8+ T cells proliferated rapidly in response to IL-6. This inflammatory cytokine was found at high levels in these lymph nodes, perhaps due to their proximity to the bacterium-laden gut. The mice developed a thicker gut epithelium and lost weight—signs of an autoimmune disease called colitis. T cell–injected mice treated with an IL-6–blocking antibody or with bacterium-depleting drugs, however, remained healthy. The proliferating T cells secreted several inflammatory cytokines, including IL-17. The IL-17 alone seemed to be responsible for the colitis, as mice injected with T cells that were unable to produce IL-17 remained healthy. If these IL-17–producing CD8+ T cells are found in patients suffering from diseases such as colitis, the anti–IL-6 antibody might be a good therapeutic option.