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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.

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.