Neutralizing antibodies have been thought to be required for protection against acutely cytopathic viruses, such as the neurotropic vesicular stomatitis virus (VSV). Utilizing mice that possess B cells but lack antibodies, we show here that survival upon subcutaneous (s.c.) VSV challenge was independent of neutralizing antibody production or cell-mediated adaptive immunity. However, B cells were absolutely required to provide lymphotoxin (LT) α1β2, which maintained a protective subcapsular sinus (SCS) macrophage phenotype within virus draining lymph nodes (LNs). Macrophages within the SCS of B cell-deficient LNs, or of mice that lack LTα1β2 selectively in B cells, displayed an aberrant phenotype, failed to replicate VSV, and therefore did not produce type I interferons, which were required to prevent fatal VSV invasion of intranodal nerves. Thus, although B cells are essential for survival during VSV infection, their contribution involves the provision of innate differentiation and maintenance signals to macrophages, rather than adaptive immune mechanisms.

A defining feature of vertebrate immunity is the acquisition of immunological memory, which confers enhanced protection against pathogens by mechanisms that are incompletely understood. Here, we compared responses by virus-specific naive T cells (T(N)) and central memory T cells (T(CM)) to viral antigen challenge in lymph nodes (LNs). In steady-state LNs, both T cell subsets localized in the deep T cell area and interacted similarly with antigen-presenting dendritic cells. However, upon entry of lymph-borne virus, only T(CM) relocalized rapidly and efficiently toward the outermost LN regions in the medullary, interfollicular, and subcapsular areas where viral infection was initially confined. This rapid peripheralization was coordinated by a cascade of cytokines and chemokines, particularly ligands for T(CM)-expressed CXCR3. Consequently, in vivo recall responses to viral infection by CXCR3-deficient T(CM) were markedly compromised, indicating that early antigen detection afforded by intranodal chemokine guidance of T(CM) is essential for efficient antiviral memory.

Cover by Megan Perdue (oil on canvas)

ERM (ezrin, radixin moesin) proteins in lymphocytes link cortical actin to plasma membrane, which is regulated in part by ERM protein phosphorylation. To assess whether phosphorylation of ERM proteins regulates lymphocyte migration and membrane tension, we generated transgenic mice whose T-lymphocytes express low levels of ezrin phosphomimetic protein (T567E). In these mice, T-cell number in lymph nodes was reduced by 27%. Lymphocyte migration rate in vitro and in vivo in lymph nodes decreased by 18% to 47%. Lymphocyte membrane tension increased by 71%. Investigations of other possible underlying mechanisms revealed impaired chemokine-induced shape change/lamellipod extension and increased integrin-mediated adhesion. Notably, lymphocyte homing to lymph nodes was decreased by 30%. Unlike most described homing defects, there was not impaired rolling or sticking to lymph node vascular endothelium but rather decreased migration across that endothelium. Moreover, decreased numbers of transgenic T cells in efferent lymph suggested defective egress. These studies confirm the critical role of ERM dephosphorylation in regulating lymphocyte migration and transmigration. Of particular note, they identify phospho-ERM as the first described regulator of lymphocyte membrane tension, whose increase probably contributes to the multiple defects observed in the ezrin T567E transgenic mice.

Differentiation of naive CD4(+) T cells into T helper (Th) cells is a defining event in adaptive immunity. The cytokines and transcription factors that control Th cell differentiation are understood, but it is not known how this process is orchestrated within lymph nodes (LNs). Here we have shown that the CXCR3 chemokine receptor was required for optimal generation of interferon-γ (IFN-γ)-secreting Th1 cells in vivo. By using a CXCR3 ligand reporter mouse, we found that stromal cells predominately expressed the chemokine ligand CXCL9 whereas hematopoietic cells expressed CXCL10 in LNs. Dendritic cell (DC)-derived CXCL10 facilitated T cell-DC interactions in LNs during T cell priming while both chemokines guided intranodal positioning of CD4(+) T cells to interfollicular and medullary zones. Thus, different chemokines acting on the same receptor can function locally to facilitate DC-T cell interactions and globally to influence intranodal positioning, and both functions contribute to Th1 cell differentiation.

When refrigerated platelets are rewarmed, they secrete active sialidases, including the lysosomal sialidase Neu1, and express surface Neu3 that remove sialic acid from platelet von Willebrand factor receptor (VWFR), specifically the GPIbα subunit. The recovery and circulation of refrigerated platelets is greatly improved by storage in the presence of inhibitors of sialidases. Desialylated VWFR is also a target for metalloproteinases (MPs), because GPIbα and GPV are cleaved from the surface of refrigerated platelets. Receptor shedding is inhibited by the MP inhibitor GM6001 and does not occur in Adam17(ΔZn/ΔZn) platelets expressing inactive ADAM17. Critically, desialylation in the absence of MP-mediated receptor shedding is sufficient to cause the rapid clearance of platelets from circulation. Desialylation of platelet VWFR therefore triggers platelet clearance and primes GPIbα and GPV for MP-dependent cleavage.

Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-related death. The biologic processes contributing to TRALI are poorly understood. All blood products can cause TRALI, and no specific treatment is available. A "2-event model" has been proposed as the trigger. The first event may include surgery, trauma, or infection; the second involves the transfusion of antileukocyte antibodies or bioactive lipids within the blood product. Together, these events induce neutrophil activation in the lungs, causing endothelial damage and capillary leakage. Neutrophils, in response to pathogens or under stress, can release their chromatin coated with granule contents, thus forming neutrophil extracellular traps (NETs). Although protective against infection, these NETs are injurious to tissue. Here we show that NET biomarkers are present in TRALI patients' blood and that NETs are produced in vitro by primed human neutrophils when challenged with anti-HNA-3a antibodies previously implicated in TRALI. NETs are found in alveoli of mice experiencing antibody-mediated TRALI. DNase 1 inhalation prevents their alveolar accumulation and improves arterial oxygen saturation even when administered 90 minutes after TRALI onset. We suggest that NETs form in the lungs during TRALI, contribute to the disease process, and thus could be targeted to prevent or treat TRALI.

B cells require MHC class II (MHC II)-restricted cognate help and CD40 engagement by CD4(+) T follicular helper (T(FH)) cells to form germinal centers and long-lasting Ab responses. Invariant NKT (iNKT) cells are innate-like lymphocytes that jumpstart the adaptive immune response when activated by the CD1d-restricted lipid α-galactosylceramide (αGalCer). We previously observed that immunization of mice lacking CD4(+) T cells (MHC II(-/-)) elicits specific IgG responses only when protein Ags are mixed with αGalCer. In this study, we investigated the mechanisms underpinning this observation. We find that induction of Ag-specific Ab responses in MHC II(-/-) mice upon immunization with protein Ags mixed with αGalCer requires CD1d expression and CD40 engagement on B cells, suggesting that iNKT cells provide CD1d-restricted cognate help for B cells. Remarkably, splenic iNKT cells from immunized MHC II(-/-) mice display a typical CXCR5(hi)programmed death-1(hi)ICOS(hi)Bcl-6(hi) T(FH) phenotype and induce germinal centers. The specific IgG response induced in MHC II(-/-) mice has shorter duration than that developing in CD4-competent animals, suggesting that iNKT(FH) cells preferentially induce transient rather than long-lived Ab responses. Together, these results suggest that iNKT cells can be co-opted into the follicular helper function, yet iNKT(FH) and CD4(+) T(FH) cells display distinct helper features, consistent with the notion that these two cell subsets play nonredundant functions throughout immune responses.

After host entry through mucosal surfaces, human immunodeficiency virus-1 (HIV-1) disseminates to lymphoid tissues to establish a generalized infection of the immune system. The mechanisms by which this virus spreads among permissive target cells locally during the early stages of transmission and systemically during subsequent dissemination are not known. In vitro studies suggest that the formation of virological synapses during stable contacts between infected and uninfected T cells greatly increases the efficiency of viral transfer. It is unclear, however, whether T-cell contacts are sufficiently stable in vivo to allow for functional synapse formation under the conditions of perpetual cell motility in epithelial and lymphoid tissues. Here, using multiphoton intravital microscopy, we examine the dynamic behaviour of HIV-infected T cells in the lymph nodes of humanized mice. We find that most productively infected T cells migrate robustly, resulting in their even distribution throughout the lymph node cortex. A subset of infected cells formed multinucleated syncytia through HIV envelope-dependent cell fusion. Both uncoordinated motility of syncytia and adhesion to CD4(+) lymph node cells led to the formation of long membrane tethers, increasing cell lengths to up to ten times that of migrating uninfected T cells. Blocking the egress of migratory T cells from the lymph nodes into efferent lymph vessels, and thus interrupting T-cell recirculation, limited HIV dissemination and strongly reduced plasma viraemia. Thus, we have found that HIV-infected T cells are motile, form syncytia and establish tethering interactions that may facilitate cell-to-cell transmission through virological synapses. Migration of T cells in lymph nodes therefore spreads infection locally, whereas their recirculation through tissues is important for efficient systemic viral spread, suggesting new molecular targets to antagonize HIV infection.

Millions of platelets are produced each hour by bone marrow (BM) megakaryocytes (MKs). MKs extend transendothelial proplatelet (PP) extensions into BM sinusoids and shed new platelets into the blood. The mechanisms that control platelet generation remain incompletely understood. Using conditional mutants and intravital multiphoton microscopy, we show here that the lipid mediator sphingosine 1-phosphate (S1P) serves as a critical directional cue guiding the elongation of megakaryocytic PP extensions from the interstitium into BM sinusoids and triggering the subsequent shedding of PPs into the blood. Correspondingly, mice lacking the S1P receptor S1pr1 develop severe thrombocytopenia caused by both formation of aberrant extravascular PPs and defective intravascular PP shedding. In contrast, activation of S1pr1 signaling leads to the prompt release of new platelets into the circulating blood. Collectively, our findings uncover a novel function of the S1P-S1pr1 axis as master regulator of efficient thrombopoiesis and might raise new therapeutic options for patients with thrombocytopenia.