Francisella tularensis is a gram-negative facultative bacterium that causes the disease tularemia, even upon exposure to low numbers of bacteria. One critical characteristic of Francisella is its ability to dampen or subvert the host immune response. In order to help understand the mechanisms by which this occurs, we performed Affymetrix microarray analysis on transcripts from blood monocytes infected with the virulent Type A Schu S4 strain. Results showed that expression of several host response genes were reduced such as those associated with interferon signaling, Toll-like receptor signaling, autophagy and phagocytosis. When compared to microarrays from monocytes infected with the less virulent F. tularensis subsp. novicida, we found qualitative differences and also a general pattern of quantitatively reduced pro-inflammatory signaling pathway genes in the Schu S4 strain. Notably, the PI3K/Akt1 pathway appeared specifically down-regulated following Schu S4 infection and a concomitantly lower cytokine response was observed. This study identifies several new factors potentially important in host cell subversion by the virulent Type A F. tularensis that may serve as novel targets for drug discovery.

Infection with the obligate intracellular protozoan Leishmania is thought to be initiated by direct parasitization of macrophages, but the early events following transmission to the skin by vector sand flies have been difficult to examine directly. Using dynamic intravital microscopy and flow cytometry, we observed a rapid and sustained neutrophilic infiltrate at localized sand fly bite sites. Invading neutrophils efficiently captured Leishmania major (L.m.) parasites early after sand fly transmission or needle inoculation, but phagocytosed L.m. remained viable and infected neutrophils efficiently initiated infection. Furthermore, neutrophil depletion reduced, rather than enhanced, the ability of parasites to establish productive infections. Thus, L.m. appears to have evolved to both evade and exploit the innate host response to sand fly bite in order to establish and promote disease.

Biting insects transmit numerous viral, bacterial, and parasitic infections of human and veterinary importance. However, the initial events that occur as pathogens are introduced by these vectors at sites of local feeding (wounds) are poorly understood. On page 970 in this issue, Peters et al. (1) report that early events in vector-mediated injury influence the outcome of infection with the sand–fly–transmitted parasite Leishmania major. Wounds are the points of entry for multiple pathogens, and neutrophils are the first of a wave of inflammatory cells that migrate into these sites. These highly phagocytic cells have been regarded as foot soldiers, armed with toxic oxygen radicals, lytic enzymes, and cationic proteins to destroy the microorganisms they ingest. Indeed, their vital role in efficiently mounting an immune response to pathogens is emphasized by the susceptibility of neutropenic patients to bacterial infections (2). However, in current models, neutrophils are short-lived and undergo programmed cell death (apoptosis). Their corpses, when phagocytosed by macrophages (part of the wound-healing response), have an anti-inflammatory effect. In a twist on this model, van Zandbergen et al. (3) showed that neutrophils internalize L. major and, as these infected cells die, they are engulfed by other immune cells--macrophages. Thus, this allows silent entry of the parasites into their host immune cell of preference.

One hundred years ago the birth of immunology was made official by the Nobel Prize award to Elie Metchnikoff and Paul Ehrlich. Metchnikoff discovered phagocytosis by macrophages and microphages as a critical host-defense mechanism and thus is considered the father of cellular innate immunity. Ehrlich described the side-chain theory of antibody formation and the mechanisms of how antibodies neutralize toxins and induce bacterial lysis with the help of complement and thus is considered one of the fathers of humoral adaptive immunity. Despite many discordant discussions in the initial phase after these discoveries, innate and adaptive responses are now known to be complementary partners in producing robust immunity.

This report presents themes highlighted during the eclectic and stimulating Metchnikoff's Legacy in 2008 meeting hosted at the Institut Pasteur in April 2008 in honor of the 100th anniversary of the 1908 Nobel Prize.

The vaccinia virus acts like a Trojan Horse to enter its host cells: it envelops itself in the membrane of a dying cell, and is then taken up by healthy cells.

Efficient phagocytosis of apoptotic cells is crucial for tissue homeostasis and the immune response1, 2. Rab5 is known as a key regulator of the early endocytic pathway3 and we have recently shown that Rab5 is also implicated in apoptotic cell engulfment4; however, the precise spatio-temporal dynamics of Rab5 activity remain unknown. Here, using a newly developed fluorescence resonance energy transfer biosensor, we describe a change in Rab5 activity during the engulfment of apoptotic thymocytes. Rab5 activity on phagosome membranes began to increase on disassembly of the actin coat encapsulating phagosomes. Rab5 activation was either continuous or repetitive for up to 10 min, but it ended before the collapse of engulfed apoptotic cells. Expression of a dominant-negative mutant of Rab5 delayed this collapse of apoptotic thymocytes, showing a role for Rab5 in phagosome maturation. Disruption of microtubules with nocodazole inhibited Rab5 activation on the phagosome membrane without perturbing the engulfment of apoptotic cells. Furthermore, we found that Gapex-5 is the guanine nucleotide exchange factor essential for Rab5 activation during the engulfment of apoptotic cells. Gapex-5 was bound to a microtubule-tip-associating protein, EB1, whose depletion inhibited Rab5 activation during phagocytosis. We therefore propose a mechanistic model in which the recruitment of Gapex-5 to phagosomes through the microtubule network induces the transient Rab5 activation.

A poxvirus tricks cells into drinking it up, say Jason Mercer and Ari Helenius (Institute of Biochemistry, Zurich). Vaccinia, studied here, is a prototypical poxvirus, whose members also include the human smallpox virus. In studying how vaccinia enters cells, the authors observed that mature virus particles bound to filopodia and surfed toward the cell. After arriving at the cell body, the virus particles induced the entire cell surface to erupt into blebs, which, when retracting, engulfed the virus. To the authors’ amazement, a single virus particle was sufficient to induce this dramatic behavior.

The vaccinia virus had a dirty secret. Researchers in Switzerland have revealed that it dupes cells into taking it in by mimicking the detritus that many types of cell would normally mop up. The work is important because vaccinia typifies pox viruses such as smallpox. Jason Mercer and Ari Helenius of ETH Zurich watched fluorescently labelled virus particles trigger the membranes of their target cells to develop a spherical bulge called a 'bleb'. Blebbing proved crucial for infection, and a fat molecule called phosphatidylserine in the viral membrane proved crucial for virus-induced blebbing. Because the process normally deals with 'rubbish' in healthy cells, it flies under the radar of the immune system and thus enables pox viruses to evade detection as they spread between cells.