Specific remodeling of splenic architecture by cytomegalovirus

Inflammation switches the chemoattractant requirements for naive lymphocyte entry into lymph nodes

Sustained lymphocyte migration from blood into lymph nodes (LNs) is important for immune responses. The CC-chemokine receptor-7 (CCR7) ligand CCL21 is required for LN entry but is downregulated during inflammation, and it has been unclear how recruitment is maintained. Here, we show that the oxysterol biosynthetic enzyme cholesterol-25-hydroxylase (Ch25h) is upregulated in LN high endothelial venules during viral infection. Lymphocytes become dependent on oxysterols, generated through a transcellular endothelial-fibroblast metabolic pathway, and the receptor EBI2 for inflamed LN entry. Additionally, Langerhans cells are an oxysterol source. Ch25h is also expressed in inflamed peripheral endothelium, and EBI2 mediates B cell recruitment in a tumor model. Finally, we demonstrate that LN CCL19 is critical in lymphocyte recruitment during inflammation. Thus, our work explains how naive precursor trafficking is sustained in responding LNs, identifies a role for oxysterols in cell recruitment into inflamed tissues, and establishes a logic for the CCR7 two-ligand system.

Immune surveillance of cytomegalovirus in tissues

Cytomegalovirus (CMV), a representative member of the Betaherpesvirinae subfamily of herpesviruses, is common in the human population, but immunocompetent individuals are generally asymptomatic when infected with this virus. However, in immunocompromised individuals and immunologically immature fetuses and newborns, CMV can cause a wide range of often long-lasting morbidities and even death. CMV is not only widespread throughout the population but it is also widespread in its hosts, infecting and establishing latency in nearly all tissues and organs. Thus, understanding the pathogenesis of and immune responses to this virus is a prerequisite for developing effective prevention and treatment strategies. Multiple arms of the immune system are engaged to contain the infection, and general concepts of immune control of CMV are now reasonably well understood. Nonetheless, in recent years, tissue-specific immune responses have emerged as an essential factor for resolving CMV infection. As tissues differ in biology and function, so do immune responses to CMV and pathological processes during infection. This review discusses state-of-the-art knowledge of the immune response to CMV infection in tissues, with particular emphasis on several well-studied and most commonly affected organs.

Unravelling the contribution of lymph node fibroblasts to vaccine responses

Vaccination is one of the most effective medical interventions, saving millions of lives and reducing the morbidity of infections across the lifespan, from infancy to older age. The generation of plasma cells and memory B cells that produce high affinity class switched antibodies is central to this protection, and these cells are the ultimate output of the germinal centre response. Optimal germinal centre responses require different immune cells to interact with one another in the right place and at the right time and this delicate cellular ballet is coordinated by a network of interconnected stromal cells. In this review we will discuss the various types of lymphoid stromal cells and how they coordinate immune cell homeostasis, the induction and maintenance of the germinal centre response, and how this is disorganised in older bodies.

Splenic stromal niches in homeostasis and immunity

The spleen is a gatekeeper of systemic immunity where immune responses against blood-borne pathogens are initiated and sustained. Non-haematopoietic stromal cells construct microanatomical niches in the spleen that make diverse contributions to physiological spleen functions and regulate the homeostasis of immune cells. Additional signals from spleen autonomic nerves also modify immune responses. Recent insight into the diversity of the splenic fibroblastic stromal cells has revised our understanding of how these cells help to orchestrate splenic responses to infection and contribute to immune responses. In this Review, we examine our current understanding of how stromal niches and neuroimmune circuits direct the immunological functions of the spleen, with a focus on T cell immunity.

Stromal structure remodeling by B lymphocytes limits T cell activation in lymph nodes of Mycobacterium tuberculosis-infected mice

An effective adaptive immune response depends on the organized architecture of secondary lymphoid organs, including the lymph nodes (LNs). While the cellular composition and microanatomy of LNs under steady state are well defined, the impact of chronic tissue inflammation on the structure and function of draining LNs is incompletely understood. Here we showed that Mycobacterium tuberculosis infection remodeled LN architecture by increasing the number and paracortical translocation of B cells. The formation of paracortical B lymphocyte and CD35+ follicular dendritic cell clusters dispersed CCL21-producing fibroblastic reticular cells and segregated pathogen-containing myeloid cells from antigen-specific CD4+ T cells. Depletion of B cells restored the chemokine and lymphoid structure and reduced bacterial burdens in LNs of the chronically infected mice. Importantly, this remodeling process impaired activation of naive CD4+ T cells in response to mycobacterial and unrelated antigens during chronic tuberculosis infection. Our studies reveal a mechanism in the regulation of LN microanatomy during inflammation and identify B cells as a critical element limiting the T cell response to persistent intracellular infection in LNs.

Inborn errors of immunity and immunodeficiencies: antibody-mediated pathology and autoimmunity as a consequence of impaired immune reactions

B cell tolerance to self-antigen is an active process that requires the temporal and spatial integration of signals of defined intensity. In common variable immune deficiency disorders (CVID), CTLA-4 deficiency, autoimmune lymphoproliferative syndrome (ALPS), or in collagen VII deficiency, genetic defects in molecules regulating development, activation, maturation and extracellular matrix composition alter the generation of B cells, resulting in immunodeficiency. Paradoxically, at the same time, the defective immune processes favor autoantibody production and immunopathology through impaired establishment of tolerance. The development of systemic autoimmunity in the framework of defective BCR signaling is relatively unusual in genetic mouse models. In sharp contrast, such reduced signaling in humans is clearly linked to pathological autoimmunity. The molecular mechanisms by which tolerance is broken in these settings are only starting to be explored resulting in novel therapeutic interventions. For instance, in CTLA-4 deficiency, homeostasis can be restored by CTLA-4 Ig treatment. Following this example, the identification of the molecular targets causing the reduced signals and their restoration is a visionary way to reestablish tolerance and develop novel therapeutic avenues for immunopathologies. This article is protected by copyright. All rights reserved.

Chemo- and mechanosensing by dendritic cells facilitate antigen surveillance in the spleen

Spleen dendritic cells (DC) are critical for initiation of adaptive immune responses against blood-borne invaders. Key to DC function is their positioning at sites of pathogen entry, and their abilities to selectively capture foreign antigens and promptly engage T cells. Focusing on conventional DC2 (cDC2), we discuss the contribution of chemoattractant receptors (EBI2 or GPR183, S1PR1, and CCR7) and integrins to cDC2 positioning and function. We give particular attention to a newly identified role in cDC2 for adhesion G-protein coupled receptor E5 (Adgre5 or CD97) and its ligand CD55, detailing how this mechanosensing system contributes to splenic cDC2 positioning and homeostasis. Additional roles of CD97 in the immune system are reviewed. The ability of cDC2 to be activated by circulating missing self-CD47 cells and to integrate multiple red blood cell (RBC)-derived inputs is discussed. Finally, we describe the process of activated cDC2 migration to engage and prime helper T cells. Throughout the review, we consider the insights into cDC function in the spleen that have emerged from imaging studies.

Expression of ACKR4 demarcates the "peri-marginal sinus," a specialized vascular compartment of the splenic red pulp

The spleen comprises defined microanatomical compartments that uniquely contribute to its diverse host defense functions. Here, we identify a vascular compartment within the red pulp of the spleen delineated by expression of the atypical chemokine receptor 4 (ACKR4) in endothelial cells. ACKR4-positive vessels form a three-dimensional sinusoidal network that connects via shunts to the marginal sinus and tightly surrounds the outer perimeter of the marginal zone. Endothelial cells lining this vascular compartment express ACKR4 as part of a distinct gene expression profile. We show that T cells enter the spleen largely through this peri-marginal sinus and initially localize extravascularly around these vessels. In the absence of ACKR4, homing of T cells into the spleen and subsequent migration into T cell areas is impaired, and organization of the marginal zone is severely affected. Our data delineate the splenic peri-marginal sinus as a compartment that supports spleen homing of T cells.

Lymphoid stromal cells-more than just a highway to humoral immunity

The generation of high-affinity long-lived antibody responses is dependent on the differentiation of plasma cells and memory B cells, which are themselves the product of the germinal centre (GC) response. The GC forms in secondary lymphoid organs in response to antigenic stimulation and is dependent on the coordinated interactions between many types of leucocytes. These leucocytes are brought together on an interconnected network of specialized lymphoid stromal cells, which provide physical and chemical guidance to immune cells that are essential for the GC response. In this review we will highlight recent advancements in lymphoid stromal cell immunobiology and their role in regulating the GC, and discuss the contribution of lymphoid stromal cells to age-associated immunosenescence.

Chronic viral infections impinge on naive bystander CD8 T cells

INTRODUCTION

Epidemiological data suggest that persistent viral infections impair immune homeostasis and immune responsiveness. Previous studies showed that chronic virus infections negatively impact bystander T-cell differentiation and memory formation but there is limited knowledge of how chronic virus infections impinge on heterologous naive T-cell populations.

METHODS

We used adoptive transfer of naive CD8 T cells with defined nonviral specificity into hosts, which were subsequently chronically infected with lymphocytic choriomeningitis virus, followed by analyses of numeric, phenotypic, and functional changes provoked in the chronically infected host.

RESULTS

We demonstrate that chronic virus infections have a profound effect on the number and phenotype of naive bystander CD8 T cells. Moreover, primary expansion upon antigen encounter was severely compromised in chronically infected hosts. However, when naive bystander CD8 T cells were transferred from the chronically infected mice into naive hosts, they regained their expansion potential. Conversely, when chronically infected hosts were supplied with additional antigen-presenting cells (APCs), primary expansion of the naive CD8 T cells was restored to levels of the uninfected hosts.

CONCLUSIONS

Our results document numeric, phenotypic, and functional adaptation of bystander naive CD8 T cells during nonrelated chronic viral infection. Their functional impairment was only evident in the chronically infected host, indicating that T-cell extrinsic factors, in particular the quality of priming APCs, are responsible for the impaired function of naive bystander T cells in the chronically infected hosts.

Visualization of T Cell Migration in the Spleen Reveals a Network of Perivascular Pathways that Guide Entry into T Zones

Lymphocyte homeostasis and immune surveillance require that T and B cells continuously recirculate between secondary lymphoid organs. Here, we used intravital microscopy to define lymphocyte trafficking routes within the spleen, an environment of open blood circulation and shear forces unlike other lymphoid organs. Upon release from arterioles into the red pulp sinuses, T cells latched onto perivascular stromal cells in a manner that was independent of the chemokine receptor CCR7 but sensitive to Gi protein-coupled receptor inhibitors. This latching sheltered T cells from blood flow and enabled unidirectional migration to the bridging channels and then to T zones, entry into which required CCR7. Inflammatory responses modified the chemotactic cues along the perivascular homing paths, leading to rapid block of entry. Our findings reveal a role for vascular structures in lymphocyte recirculation through the spleen, indicating the existence of separate entry and exit routes and that of a checkpoint located at the gate to the T zone.

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Perivascular pathways support T cell entry into splenic T zones, but not egress from them

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Attachment to the homing paths requires activation of GPCRs other than CCR7

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CCR7 mediates one-directional migration and entry into T zones

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Inflammation leads to modification of the homing paths and to rapid block of entry

Tissue-specific transcriptomic changes associated with AmBisome® treatment of BALB/c mice with experimental visceral leishmaniasis

Background: Liposomal amphotericin B (AmBisome®) as a treatment modality for visceral leishmaniasis (VL) has had significant impact on patient care in some but not all regions where VL is endemic.  As the mode of action of AmBisome® in vivo is poorly understood, we compared the tissue-specific transcriptome in drug-treated vs untreated mice with experimental VL.    Methods:  BALB/c mice infected with L. donovani were treated with 8mg/kg AmBisome®, resulting in parasite elimination from liver and spleen over a 7-day period. At day 1 and day 7 post treatment (R x+1 and R x+7), transcriptomic profiling was performed on spleen and liver tissue from treated and untreated mice and uninfected mice.  BALB/c mice infected with M. bovis BCG (an organism resistant to amphotericin B) were analysed to distinguish between direct effects of AmBisome® and those secondary to parasite death.   Results: AmBisome® treatment lead to rapid parasitological clearance.  At R x+1, spleen and liver displayed only 46 and 88 differentially expressed (DE) genes (P<0.05; 2-fold change) respectively. In liver, significant enrichment was seen for pathways associated with TNF, fatty acids and sterol biosynthesis.  At R x+7, the number of DE genes was increased (spleen, 113; liver 400).  In spleen, these included many immune related genes known to be involved in anti-leishmanial immunity. In liver, changes in transcriptome were largely accounted for by loss of granulomas.   PCA analysis indicated that treatment only partially restored homeostasis.  Analysis of BCG-infected mice treated with AmBisome® revealed a pattern of immune modulation mainly targeting macrophage function.   Conclusions: Our data indicate that the tissue response to AmBisome® treatment varies between target organs and that full restoration of homeostasis is not achieved at parasitological cure.  The pathways required to restore homeostasis deserve fuller attention, to understand mechanisms associated with treatment failure and relapse and to promote more rapid restoration of immune competence.

STING induces early IFN-β in the liver and constrains myeloid cell-mediated dissemination of murine cytomegalovirus

Cytomegalovirus is a DNA-encoded β-herpesvirus that induces STING-dependent type 1 interferon responses in macrophages and uses myeloid cells as a vehicle for dissemination. Here we report that STING knockout mice are as resistant to murine cytomegalovirus (MCMV) infection as wild-type controls, whereas mice with a combined Toll-like receptor/RIG-I-like receptor/STING signaling deficiency do not mount type 1 interferon responses and succumb to the infection. Although STING alone is dispensable for survival, early IFN-β induction in Kupffer cells is STING-dependent and controls early hepatic virus propagation. Infection experiments with an inducible reporter MCMV show that STING constrains MCMV replication in myeloid cells and limits viral dissemination via these cells. By contrast, restriction of viral dissemination from hepatocytes to other organs is independent of STING. Thus, during MCMV infection STING is involved in early IFN-β induction in Kupffer cells and the restriction of viral dissemination via myeloid cells, whereas it is dispensable for survival.

Chikungunya virus impairs draining lymph node function by inhibiting HEV-mediated lymphocyte recruitment

Chikungunya virus (CHIKV) causes acute and chronic rheumatologic disease. Pathogenic CHIKV strains persist in joints of immunocompetent mice, while the attenuated CHIKV strain 181/25 is cleared by adaptive immunity. We analyzed the draining lymph node (dLN) to define events in lymphoid tissue that may contribute to CHIKV persistence or clearance. Acute 181/25 infection resulted in dLN enlargement and germinal center (GC) formation, while the dLN of mice infected with pathogenic CHIKV became highly disorganized and depleted of lymphocytes. Using CHIKV strains encoding ovalbumin-specific TCR epitopes, we found that lymphocyte depletion was not due to impaired lymphocyte proliferation. Instead, the accumulation of naive lymphocytes transferred from the vasculature to the dLN was reduced, which was associated with fewer high endothelial venule cells and decreased CCL21 production. Following NP-OVA immunization, NP-specific GC B cells in the dLN were decreased during pathogenic, but not attenuated, CHIKV infection. Our data suggest that pathogenic, persistent strains of CHIKV disable the development of adaptive immune responses within the dLN.

STAT2-Dependent Immune Responses Ensure Host Survival despite the Presence of a Potent Viral Antagonist

A pathogen encounter induces interferons, which signal via Janus kinases and STAT transcription factors to establish an antiviral state. However, the host and pathogens are situated in a continuous arms race which shapes host evolution toward optimized immune responses and the pathogens toward enhanced immune-evasive properties. Mouse cytomegalovirus (MCMV) counteracts interferon responses by pM27-mediated degradation of STAT2, which directly affects the signaling of type I as well as type III interferons. Using MCMV mutants lacking M27 and mice lacking STAT2, we studied the opposing relationship between antiviral activities and viral antagonism in a natural host-pathogen pair in vitro and in vivo In contrast to wild-type (wt) MCMV, ΔM27 mutant MCMV was efficiently cleared from all organs within a few days in BALB/c, C57BL/6, and 129 mice, highlighting the general importance of STAT2 antagonism for MCMV replication. Despite this effective and relevant STAT2 antagonism, wt and STAT2-deficient mice exhibited fundamentally different susceptibilities to MCMV infections. MCMV replication was increased in all assessed organs (e.g., liver, spleen, lungs, and salivary glands) of STAT2-deficient mice, resulting in mortality during the first week after infection. Taken together, the results of our study reveal the importance of cytomegaloviral interferon antagonism for viral replication as well as a pivotal role of the remaining STAT2 activity for host survival. This mutual influence establishes a stable evolutionary standoff situation with fatal consequences when the equilibrium is disturbed.IMPORTANCE The host limits viral replication by the use of interferons (IFNs), which signal via STAT proteins. Several viruses evolved antagonists targeting STATs to antagonize IFNs (e.g., cytomegaloviruses, Zika virus, dengue virus, and several paramyxoviruses). We analyzed infections caused by MCMV expressing or lacking the STAT2 antagonist pM27 in STAT2-deficient and control mice to evaluate its importance for the host and the virus in vitro and in vivo The inability to counteract STAT2 directly translates into exaggerated IFN susceptibility in vitro and pronounced attenuation in vivo Thus, the antiviral activity mediated by IFNs via STAT2-dependent signaling drove the development of a potent MCMV-encoded STAT2 antagonist which became indispensable for efficient virus replication and spread to organs required for dissemination. Despite this clear impact of viral STAT2 antagonism, the host critically required the remaining STAT2 activity to prevent overt disease and mortality upon MCMV infection. Our findings highlight a remarkably delicate balance between host and virus.

Origin and specialization of splenic macrophages

Macrophage heterogeneity in the spleen has been long documented, with four subsets populating the different splenic compartments. The diverse environments on the splenic compartments determine their varied phenotype and functions. In the white pulp, highly phagocytic macrophages contribute to the generation of the immune response. The marginal zone contains two populations of macrophages, which also contribute to the immune response. Their strategic position in the bloodstream and their unique phenotype confer them a crucial role in the defense against blood borne pathogens, placing them at the crossroad between innate and adaptive immune responses. Macrophages in the red pulp are classically linked to homeostatic and metabolic functions in erythrocyte phagocytosis and iron recycling. We review here recent advances demonstrating the importance of macrophage ontogeny and organ development in determining the phenotype, transcriptional profile and, ultimately, the functions of the populations of splenic macrophages.

A TNF-p100 pathway subverts noncanonical NF-κB signaling in inflamed secondary lymphoid organs

Lymphotoxin-beta receptor (LTβR) present on stromal cells engages the noncanonical NF-κB pathway to mediate RelB-dependent expressions of homeostatic chemokines, which direct steady-state ingress of naïve lymphocytes to secondary lymphoid organs (SLOs). In this pathway, NIK promotes partial proteolysis of p100 into p52 that induces nuclear translocation of the RelB NF-κB heterodimers. Microbial infections often deplete homeostatic chemokines; it is thought that infection-inflicted destruction of stromal cells results in the downregulation of these chemokines. Whether inflammation per se also regulates these processes remains unclear. We show that TNF accumulated upon non-infectious immunization of mice similarly downregulates the expressions of these chemokines and consequently diminishes the ingress of naïve lymphocytes in inflamed SLOs. Mechanistically, TNF inactivated NIK in LTβR-stimulated cells and induced the synthesis of Nfkb2 mRNA encoding p100; these together potently accumulated unprocessed p100, which attenuated the RelB activity as inhibitory IκBδ. Finally, a lack of p100 alleviated these TNF-mediated inhibitions in inflamed SLOs of immunized Nfkb2-/- mice. In sum, we reveal that an inhibitory TNF-p100 pathway modulates the adaptive compartment during immune responses.

Conventional Dendritic Cells Confer Protection against Mouse Cytomegalovirus Infection via TLR9 and MyD88 Signaling

Cytomegalovirus (CMV) is an opportunistic virus severely infecting immunocompromised individuals. In mice, endosomal Toll-like receptor 9 (TLR9) and downstream myeloid differentiation factor 88 (MyD88) are central to activating innate immune responses against mouse CMV (MCMV). In this respect, the cell-specific contribution of these pathways in initiating anti-MCMV immunity remains unclear. Using transgenic mice, we demonstrate that TLR9/MyD88 signaling selectively in CD11c⁺ dendritic cells (DCs) strongly enhances MCMV clearance by boosting natural killer (NK) cell CD69 expression and IFN-γ production. In addition, we show that in the absence of plasmacytoid DCs (pDCs), conventional DCs (cDCs) promote robust NK cell effector function and MCMV clearance in a TLR9/MyD88-dependent manner. Simultaneously, cDC-derived IL-15 regulates NK cell degranulation by TLR9/MyD88-independent mechanisms. Overall, we compartmentalize the cellular contribution of TLR9 and MyD88 signaling in individual DC subsets and evaluate the mechanism by which cDCs control MCMV immunity.

Chronic Brucella Infection Induces Selective and Persistent Interferon Gamma-Dependent Alterations of Marginal Zone Macrophages in the Spleen

The spleen is known as an important filter for blood-borne pathogens that are trapped by specialized macrophages in the marginal zone (MZ): the CD209+ MZ macrophages (MZMs) and the CD169+ marginal metallophilic macrophages (MMMs). Acute systemic infection strongly impacts MZ populations and the location of T and B lymphocytes. This phenomenon has been linked to reduced chemokine secretion by stromal cells. Brucella spp. are the causative agent of brucellosis, a widespread zoonotic disease. Here, we used Brucella melitensis infection as a model to investigate the impact of chronic stealth infection on splenic MZ macrophage populations. During the late phase of Brucella infection, we observed a loss of both MZMs and MMMs, with a durable disappearance of MZMs, leading to a reduction of the ability of the spleen to take up soluble antigens, beads, and unrelated bacteria. This effect appears to be selective as every other lymphoid and myeloid population analyzed increased during infection, which was also observed following Brucella abortus and Brucella suis infection. Comparison of wild-type and deficient mice suggested that MZ macrophage population loss is dependent on interferon gamma (IFN-γ) receptor but independent of T cells or tumor necrosis factor alpha receptor 1 (TNF-αR1) signaling pathways and is not correlated to an alteration of CCL19, CCL21, and CXCL13 chemokine mRNA expression. Our results suggest that MZ macrophage populations are particularly sensitive to persistent low-level IFN-γ-mediated inflammation and that Brucella infection could reduce the ability of the spleen to perform certain MZM- and MMM-dependent tasks, such as antigen delivery to lymphocytes and control of systemic infection.

In-bag enzymatic splenic digestion: a novel alternative to manual morcellation?

BACKGROUND

Contained in-bag spleen morcellation is a conventional extraction technique for safe spleen removal during laparoscopic splenectomy. Existing data for the use of in-bag enzymatic splenic digestion as an alternative to morcellation are lacking. This proof-of-concept study sought to evaluate the effectiveness of single and combinatorial enzyme digestion of murine spleens.

MATERIALS AND METHODS

Murine spleens were digested with collagenase alone or with combinations of commercially available enzymes (collagenase, elastase, hyaluronidase, neutral protease) to determine their degradation effect. The primary end point was the percentage of mass reduction at 15 and 30 min.

RESULTS

For collagenase alone (n = 15), the mean reduction in mass was 14 ± 10% (range: 2%-31%) at 15 min and 30 ± 25% (range: 7%-100%) at 30 min. Using combinatorial dissolution with collagenase, hyaluronidase, and elastase (n = 8), the mean reduction in mass was 27 ± 16% (range: 6%-42%) at 15 min and 48 ± 27% (range: 3%-100%) at 30 min. Injecting the enzyme solution into whole spleens (n = 9) yielded a mean reduction in mass of 22 ± 13% (range: 9%-42%) at 15 min and 55 ± 31% (range: 9%-100%) at 30 min; mean reduction was 9 ± 13% (range: 0%-39%) at 15 min and 23 ± 13% (range: 3%-53%) with no injection (n = 12).

CONCLUSIONS

We provide the first demonstration of successful enzymatic murine spleen digestion as an alternative method for in-bag spleen removal during laparoscopic splenectomy. However, the significant cost and quantities of commercial enzyme required for clinical application dampens the enthusiasm for this novel approach.

Impaired B cell function during viral infections due to PTEN-mediated inhibition of the PI3K pathway

Getahun et al. show that the inositol phosphatase PTEN plays a role in the inhibition of B cell functions observed during acute viral infections.

Transient suppression of B cell function often accompanies acute viral infection. However, the molecular signaling circuitry that enforces this hyporesponsiveness is undefined. In this study, experiments identify up-regulation of the inositol phosphatase PTEN (phosphatase and tensin homolog) as primarily responsible for defects in B lymphocyte migration and antibody responses that accompany acute viral infection. B cells from mice acutely infected with gammaherpesvirus 68 are defective in BCR- and CXCR4-mediated activation of the PI3K pathway, and this, we show, is associated with increased PTEN expression. This viral infection-induced PTEN overexpression appears responsible for the suppression of antibody responses observed in infected mice because PTEN deficiency or expression of a constitutively active PI3K rescued function of B cells in infected mice. Conversely, induced overexpression of PTEN in B cells in uninfected mice led to suppression of antibody responses. Finally, we demonstrate that PTEN up-regulation is a common mechanism by which infection induces suppression of antibody responses. Collectively, these findings identify a novel role for PTEN during infection and identify regulation of the PI3K pathway, a mechanism previously shown to silence autoreactive B cells, as a key physiological target to control antibody responses.

Blocking Virus Replication during Acute Murine Cytomegalovirus Infection Paradoxically Prolongs Antigen Presentation and Increases the CD8+ T Cell Response by Preventing Type I IFN-Dependent Depletion of Dendritic Cells

Increasing amounts of pathogen replication usually lead to a proportionate increase in size and effector differentiation of the CD8⁺ T cell response, which is attributed to increased Ag and inflammation. Using a murine CMV that is highly sensitive to the antiviral drug famciclovir to modulate virus replication, we found that increased virus replication drove increased effector CD8⁺ T cell differentiation, as expected. Paradoxically, however, increased virus replication dramatically decreased the size of the CD8⁺ T cell response to two immunodominant epitopes. The decreased response was due to type I IFN-dependent depletion of conventional dendritic cells and could be reproduced by specific depletion of dendritic cells from day 2 postinfection or by sterile induction of type I IFN. Increased virus replication and type I IFN specifically inhibited the response to two immunodominant epitopes that are known to be dependent on Ag cross-presented by DCs, but they did not inhibit the response to "inflationary" epitopes whose responses can be sustained by infected nonhematopoietic cells. Our results show that type I IFN can suppress CD8⁺ T cell responses to cross-presented Ag by depleting cross-presenting conventional dendritic cells.

Lymphotoxin in physiology of lymphoid tissues - Implication for antiviral defense

Lymphotoxin (LT) is a member of the tumor necrosis factor (TNF) superfamily of cytokines which serves multiple functions, including the control of lymphoid organ development and maintenance, as well as regulation of inflammation and autoimmunity. Although the role of LT in organogenesis and maintenance of lymphoid organs is well established, the contribution of LT pathway to homeostasis of lymphoid organs during the immune response to pathogens is less understood. In this review, we highlight recent advances on the role of LT pathway in antiviral immune responses. We discuss the role of LT signaling in lymphoid organ integrity, type I IFN production and regulation of protection and immunopathology during viral infections. We further discuss the potential of therapeutic targeting LT pathway for controlling immunopathology and antiviral protection.

Disruption of Splenic Lymphoid Tissue and Plasmacytosis in Canine Visceral Leishmaniasis: Changes in Homing and Survival of Plasma Cells

Visceral leishmaniasis (VL) is a disease caused by Leishmania infantum, which is transmitted by phlebotomine sandflies. Dogs are the main urban reservoir of this parasite and the disease presents similar characteristics in both humans and dogs. In this paper, we investigated the potential pathways involved in plasma cell replacement of normal cell populations in the spleen, with respect to disease severity in dogs from an endemic area for visceral leishmaniasis. To this end, canine spleen samples were grouped into three categories: TYPE1SC- (non-infected dogs or without active infection with organized white pulp), TYPE1SC+ (infected dogs with organized white pulp) or TYPE3SC+ (infected animals with disorganized white pulp). We analyzed the distribution of different plasma cell isotypes (IgA, IgG and IgM) in the spleen. The expression of cytokines and chemokines involved in plasma cell homing and survival were assessed by real time RT-PCR. Polyclonal B cell activation and hypergammaglobulinemia were also evaluated. The proportion of animals with moderate or intense plasmacytosis was higher in the TYPE3SC+ group than in the other groups (Fisher test, P<0.05). This was mainly due to a higher density of IgG+ plasma cells in the red pulp of this group. The albumin/globulin ratio was lower in the TYPE3SC+ animals than in the TYPE1SC- or TYPE1SC+ animals, which evidences VL-associated dysproteinemia. Interestingly, TYPE3SC+ animals showed increased expression of the BAFF and APRIL cytokines, as well as chemokine CXCL12. Aberrant expression of BAFF, APRIL and CXCL12, together with amplified extrafollicular B cell activation, lead to plasma cell homing and the extended survival of these cells in the splenic red pulp compartment. These changes in the distribution of immunocompetent cells in the spleen may contribute to the progression of VL, and impair the spleen's ability to protect against blood borne pathogens.

Disorganization of the splenic microanatomy in ageing mice

The precise mechanisms responsible for immunosenescence still remain to be determined, however, considering the evidence that disruption of the organization of primary and secondary lymphoid organs results in immunodeficiency, we propose that this could be involved in the decline of immune responses with age. Therefore, we investigated the integrity of the splenic microarchitecture in mice of increasing age and its reorganization following immune challenge in young and old mice. Several differences in the anatomy of the spleen with age in both the immune and stromal cells were observed. There is an age-related increase in the overall size of the white pulp, which occurs primarily within the T-cell zone and is mirrored by the enlargement of the T-cell stromal area, concurrent to the distinct boundary between T cells and B cells becoming less defined in older mice. In conjunction, there appears to be a loss of marginal zone macrophages, which is accompanied by an accumulation of fibroblasts in the spleens from older animals. Furthermore, whereas the reorganization of the white pulp is resolved after several days following antigenic challenge in young animals, it remains perturbed in older subjects. All these age-related changes within the spleen could potentially contribute to the age-dependent deficiencies in functional immunity.

MCMV exploits the spleen as a transfer hub for systemic dissemination upon oronasal inoculation

Murine cytomegalovirus (MCMV) infection in mice is a commonly used animal model for studying human cytomegalovirus (HCMV) infections. In our previous studies, a mouse model based on an oronasal MCMV infection was set up for mimicking a natural infection, and the spleen was hypothesized to regulate viremia and virus dissemination to distal organs such as submandibular glands. Here, the role of the spleen during an MCMV infection was investigated by the comparison of intact and splenectomized Balb/c mice. Both highly passaged MCMV Smith and low passaged MCMV HaNa1 were used. Various samples were collected at 7, 14, and 21 days post inoculation (dpi) for analyses by virus isolation/titration, co-cultivation and qPCR. The results showed that for both virus strains, 1) cell-associated virus in PBMC (determined by co-cultivation) was detected in intact mice but not in splenectomized mice; 2) the mean viral DNA load in PBMC of splenectomized mice was 4.4-(HaNa1)/2.7-(Smith) fold lower at the peak viremia (7dpi) in contrast to that of intact mice; and 3) infectious virus in the submandibular glands was detected later in splenectomized mice (14dpi) than in intact mice (7dpi). Moreover, the average virus titers in submandibular glands of splenectomized mice were 10-(HaNa1)/7.9-(Smith) fold lower at 14dpi and 1.7-(HaNa1)/2.1-(Smith) fold lower at 21dpi compared with that of intact mice. Upon inoculation with MCMV Smith, infectious virus was found in the kidneys and liver of intact mice, but not in splenectomized mice. Taken together, all these data clearly demonstrate that virus dissemination to distant organs is reduced in splenectomized mice, further confirming the importance of the spleen as a viremia booming site for a natural MCMV infection.

Infection with Salmonella enterica Serovar Typhimurium Leads to Increased Proportions of F4/80+ Red Pulp Macrophages and Decreased Proportions of B and T Lymphocytes in the Spleen

Infection of mice with Salmonella enterica serovar Typhimurium (Salmonella) causes systemic inflammatory disease and enlargement of the spleen (splenomegaly). Splenomegaly has been attributed to a general increase in the numbers of phagocytes, lymphocytes, as well as to the expansion of immature CD71+Ter119+ reticulocytes. The spleen is important for recycling senescent red blood cells (RBCs) and for the capture and eradication of blood-borne pathogens. Conservation of splenic tissue architecture, comprised of the white pulp (WP), marginal zone (MZ), and red pulp (RP) is essential for initiation of adaptive immune responses to captured pathogens. Using flow cytometry and four color immunofluorescence microscopy (IFM), we show that Salmonella-induced splenomegaly is characterized by drastic alterations of the splenic tissue architecture and cell population proportions, as well as in situ cell distributions. A major cause of splenomegaly appears to be the significant increase in immature RBC precursors and F4/80+ macrophages that are important for recycling of heme-associated iron. In contrast, the proportions of B220+, CD4+ and CD8+ lymphocytes, as well as MZ MOMA+ macrophages decrease significantly as infection progresses. Spleen tissue sections show visible tears and significantly altered tissue architecture with F4/80+ macrophages and RBCs expanding beyond the RP and taking over most of the spleen tissue. Additionally, F4/80+ macrophages actively phagocytose not only RBCs, but also lymphocytes, indicating that they may contribute to declining lymphocyte proportions during Salmonella infection. Understanding how these alterations of spleen microarchitecture impact the generation of adaptive immune responses to Salmonella has implications for understanding Salmonella pathogenesis and for the design of more effective Salmonella-based vaccines.

Cytomegalovirus immune evasion of myeloid lineage cells

Cytomegalovirus (CMV) evades the immune system in many different ways, allowing the virus to grow and its progeny to spread in the face of an adverse environment. Mounting evidence about the antiviral role of myeloid immune cells has prompted the research of CMV immune evasion mechanisms targeting these cells. Several cells of the myeloid lineage, such as monocytes, dendritic cells and macrophages, play a role in viral control, but are also permissive for CMV and are naturally infected by it. Therefore, CMV evasion of myeloid cells involves mechanisms that qualitatively differ from the evasion of non-CMV-permissive immune cells of the lymphoid lineage. The evasion of myeloid cells includes effects in cis, where the virus modulates the immune signaling pathways within the infected myeloid cell, and those in trans, where the virus affects somatic cells targeted by cytokines released from myeloid cells. This review presents an overview of CMV strategies to modulate and evade the antiviral activity of myeloid cells in cis and in trans.

The neurotrophic receptor Ntrk2 directs lymphoid tissue neovascularization during Leishmania donovani infection

The neurotrophic tyrosine kinase receptor type 2 (Ntrk2, also known as TrkB) and its ligands brain derived neurotrophic factor (Bdnf), neurotrophin-4 (NT-4/5), and neurotrophin-3 (NT-3) are known primarily for their multiple effects on neuronal differentiation and survival. Here, we provide evidence that Ntrk2 plays a role in the pathologic remodeling of the spleen that accompanies chronic infection. We show that in Leishmania donovani-infected mice, Ntrk2 is aberrantly expressed on splenic endothelial cells and that new maturing blood vessels within the white pulp are intimately associated with F4/80(hi)CD11b(lo)CD11c(+) macrophages that express Bdnf and NT-4/5 and have pro-angiogenic potential in vitro. Furthermore, administration of the small molecule Ntrk2 antagonist ANA-12 to infected mice significantly inhibited white pulp neovascularization but had no effect on red pulp vascular remodeling. We believe this to be the first evidence of the Ntrk2/neurotrophin pathway driving pathogen-induced vascular remodeling in lymphoid tissue. These studies highlight the therapeutic potential of modulating this pathway to inhibit pathological angiogenesis.

Tumor necrosis factor superfamily in innate immunity and inflammation

The tumor necrosis factor superfamily (TNFSF) and its corresponding receptor superfamily (TNFRSF) form communication pathways required for developmental, homeostatic, and stimulus-responsive processes in vivo. Although this receptor-ligand system operates between many different cell types and organ systems, many of these proteins play specific roles in immune system function. The TNFSF and TNFRSF proteins lymphotoxins, LIGHT (homologous to lymphotoxins, exhibits inducible expression, and competes with HSV glycoprotein D for herpes virus entry mediator [HVEM], a receptor expressed by T lymphocytes), lymphotoxin-β receptor (LT-βR), and HVEM are used by embryonic and adult innate lymphocytes to promote the development and homeostasis of lymphoid organs. Lymphotoxin-expressing innate-acting B cells construct microenvironments in lymphoid organs that restrict pathogen spread and initiate interferon defenses. Recent results illustrate how the communication networks formed among these cytokines and the coreceptors B and T lymphocyte attenuator (BTLA) and CD160 both inhibit and activate innate lymphoid cells (ILCs), innate γδ T cells, and natural killer (NK) cells. Understanding the role of TNFSF/TNFRSF and interacting proteins in innate cells will likely reveal avenues for future therapeutics for human disease.

Sinusoidal immunity: macrophages at the lymphohematopoietic interface

Macrophages are widely distributed throughout the body, performing vital homeostatic and defense functions after local and systemic perturbation within tissues. In concert with closely related dendritic cells and other myeloid and lymphoid cells, which mediate the innate and adaptive immune response, macrophages determine the outcome of the inflammatory and repair processes that accompany sterile and infectious injury and microbial invasion. This article will describe and compare the role of specialized macrophage populations at two critical interfaces between the resident host lymphohematopoietic system and circulating blood and lymph, the carriers of cells, humoral components, microorganisms, and their products. Sinusoidal macrophages in the marginal zone of the spleen and subcapsular sinus and medulla of secondary lymph nodes contribute to the innate and adaptive responses of the host in health and disease. Although historically recognized as major constituents of the reticuloendothelial system, it has only recently become apparent that these specialized macrophages in close proximity to B and T lymphocytes play an indispensable role in recognition and responses to exogenous and endogenous ligands, thus shaping the nature and quality of immunity and inflammation. We review current understanding of these macrophages and identify gaps in our knowledge for further investigation.

Stromal infrastructure of the lymph node and coordination of immunity

The initiation of adaptive immune responses depends upon the careful maneuvering of lymphocytes and antigen into and within strategically placed lymph nodes (LNs). Non-hematopoietic stromal cells form the cellular infrastructure that directs this process. Once regarded as merely structural features of lymphoid tissues, these cells are now appreciated as essential regulators of immune cell trafficking, fluid flow, and LN homeostasis. Recent advances in the identification and in vivo targeting of specific stromal populations have resulted in striking new insights to the function of stromal cells and reveal a level of complexity previously unrealized. We discuss here recent discoveries that highlight the pivotal role that stromal cells play in orchestrating immune cell homeostasis and adaptive immunity.

Animal models in dengue

Validation of a mouse model of dengue virus (DENV) infection relies on verification of viremia and productive replication in mouse tissues following infection. Here, we describe a quantitative assay for determining viral RNA levels in mouse serum and tissues. For the purpose of confirming DENV replication, we outline a fluorescence immunohistochemistry (FIHC) protocol for staining a nonstructural protein of DENV.

Deciphering the role of DC subsets in MCMV infection to better understand immune protection against viral infections

Infection of mice with murine cytomegalovirus (MCMV) recapitulates many physiopathological characteristics of human CMV infection and enables studying the interactions between a virus and its natural host. Dendritic cells (DC) are mononuclear phagocytes linking innate and adaptive immunity which are both necessary for MCMV control. DC are critical for the induction of cellular immunity because they are uniquely efficient for the activation of naïve T cells during their first encounter with a pathogen. DC are equipped with a variety of innate immune recognition receptors (I2R2) allowing them to detect pathogens or infections and to engulf molecules, microorganisms or cellular debris. The combinatorial engagement of I2R2 during infections controls DC maturation and shapes their response in terms of cytokine production, activation of natural killer (NK) cells and functional polarization of T cells. Several DC subsets exist which express different arrays of I2R2 and are specialized in distinct functions. The study of MCMV infection helped deciphering the physiological roles of DC subsets and their molecular regulation. It allowed the identification and first in vivo studies of mouse plasmacytoid DC which produce high level of interferons-α/β early after infection. Despite its ability to infect DC and dampen their functions, MCMV induces very robust, efficient and long-lasting CD8 T cell responses. Their priming may rely on the unique ability of uninfected XCR1⁺ DC to cross-present engulfed viral antigens and thus to counter MCMV interference with antigen presentation. A balance appears to have been reached during co-evolution, allowing controlled replication of the virus for horizontal spread without pathological consequences for the immunocompetent host. We will discuss the role of the interplay between the virus and DC in setting this balance, and how advancing this knowledge further could help develop better vaccines against other intracellular infectious agents.

Severe clinical presentation of visceral leishmaniasis in naturally infected dogs with disruption of the splenic white pulp

In this work, we investigated the association between the disruption of splenic lymphoid tissue and the severity of visceral leishmaniasis in dogs. Clinical and laboratory data from 206 dogs were reviewed. Spleen sections collected during the euthanasia of these animals were analyzed, and the splenic lymphoid tissue samples were classified as well organized (spleen type 1), slightly disorganized (spleen type 2), or moderately to extensively disorganized (spleen type 3). Of 199 dogs with evidence of Leishmania infection, 54 (27%) had spleen type 1, 99 (50%) had spleen type 2, and 46 (23%) had spleen type 3. The number of clinical signs associated with visceral leishmaniasis was significantly higher in the animals with evidence of Leishmania infection and spleen type 2 or 3 than in the animals with spleen type 1. Alopecia, anemia, dehydration, dermatitis, lymphadenopathy, and onychogryphosis were all more frequent among animals with evidence of Leishmania infection and spleen type 3 than among the dogs with evidence of Leishmania infection and spleen type 1. The association between the severity of canine visceral leishmaniasis and the disorganization of the splenic lymphoid tissue was even more evident in the group of animals with positive spleen culture. Conjunctivitis and ulceration were also more common in the animals with spleen type 3 than in the animals with spleen type 1. The serum levels (median, interquartile range) of albumin (1.8, 1.4–2.3 g/dL) and creatinine (0.7, 0.4–0.8 mg/dL) were significantly lower and the serum levels of aspartate aminotransferase were significantly higher (57, 39–95 U) in animals with spleen type 3 than in animals with spleen type 1 (2.8, 2.4–3.4 g/dL; 0.9, 0.7–1.2 mg/dL and 23, 20–32 U, respectively). Our data confirm the hypothesis that disruption of the splenic lymphoid tissue is associated with a more severe clinical presentation of canine visceral leishmaniasis.

Networking at the level of host immunity: immune cell interactions during persistent viral infections

Persistent viral infections are the result of a series of connected events that culminate in diminished immunity and the inability to eliminate infection. By building our understanding of how distinct components of the immune system function both individually and collectively in productive versus abortive responses, new potential therapeutic targets can be developed to overcome immune dysfunction and thus fight persistent infections. Using lymphocytic choriomeningitis virus (LCMV) as a model of a persistent virus infection and drawing parallels to persistent human viral infections such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV), we describe the cellular relationships and interactions that determine the outcome of initial infection and highlight immune targets for therapeutic intervention to prevent or treat persistent infections. Ultimately, these findings will further our understanding of the immunologic basis of persistent viral infection and likely lead to strategies to treat human viral infections.

Trapping of naive lymphocytes triggers rapid growth and remodeling of the fibroblast network in reactive murine lymph nodes

Adaptive immunity is initiated in T-cell zones of secondary lymphoid organs. These zones are organized in a rigid 3D network of fibroblastic reticular cells (FRCs) that are a rich cytokine source. In response to lymph-borne antigens, draining lymph nodes (LNs) expand several folds in size, but the fate and role of the FRC network during immune response is not fully understood. Here we show that T-cell responses are accompanied by the rapid activation and growth of FRCs, leading to an expanded but similarly organized network of T-zone FRCs that maintains its vital function for lymphocyte trafficking and survival. In addition, new FRC-rich environments were observed in the expanded medullary cords. FRCs are activated within hours after the onset of inflammation in the periphery. Surprisingly, FRC expansion depends mainly on trapping of naïve lymphocytes that is induced by both migratory and resident dendritic cells. Inflammatory signals are not required as homeostatic T-cell proliferation was sufficient to trigger FRC expansion. Activated lymphocytes are also dispensable for this process, but can enhance the later growth phase. Thus, this study documents the surprising plasticity as well as the complex regulation of FRC networks allowing the rapid LN hyperplasia that is critical for mounting efficient adaptive immunity.

Natural killer cells are required for extramedullary hematopoiesis following murine cytomegalovirus infection

The immune response against a variety of pathogens can lead to activation of blood formation at ectopic sites, a process termed extramedullary hematopoiesis (EMH). The underlying mechanisms of EMH have been enigmatic. Investigating splenic EMH in mice infected with murine cytomegalovirus (MCMV), we find that, while cells of the adaptive immune system were dispensable for EMH, natural killer (NK) cells were essential. EMH required recognition of infected cells via activating NK cell receptors Ly49H or NKG2D, and correspondingly, viral interference with NK cell recognition abolished EMH. Surprisingly, development of EMH was not induced by NK cell-derived cytokines but was dependent on perforin-mediated cytotoxicity in order to control virus spread. Spreading virus reduced the numbers of F4/80(+) macrophages that were crucial for inflammatory EMH. Hence, whereas MCMV suppresses inflammation-induced EMH, NK cells confine virus spread, thereby protecting extramedullary hematopoietic niches and facilitating EMH.

A chemokine-like viral protein enhances alpha interferon production by plasmacytoid dendritic cells but delays CD8+ T cell activation and impairs viral clearance

Murine cytomegalovirus encodes numerous proteins that act on a variety of pathways to modulate the innate and adaptive immune responses. Here, we demonstrate that a chemokine-like protein encoded by murine cytomegalovirus activates the early innate immune response and delays adaptive immunity, thereby impairing viral clearance. The protein, m131/129 (also known as MCK-2), is not required to establish infection in the spleen; however, a mutant virus lacking m131/129 was cleared more rapidly from this organ. In the absence of m131/129 expression, there was enhanced activation of dendritic cells (DC), and virus-specific CD8(+) T cells were recruited into the immune response earlier. Viral mutants lacking m131/129 elicited weaker production of alpha interferon (IFN-α) at 40 h postinfection, indicating that this protein exerts its effects during early rounds of viral replication in the spleen. Furthermore, while wild-type and mutant viruses activated plasmacytoid dendritic cells (pDC) equally at this time, as measured by the upregulation of costimulatory molecules, the presence of m131/129 stimulated more pDC to secrete IFN-α, accounting for the stronger IFN-α response than from the wild-type virus. These data provide evidence for a novel immunomodulatory function of a viral chemokine and expose the multifunctionality of immune evasion proteins. In addition, these results broaden our understanding of the interplay between innate and adaptive immunity.

Effect of IFN-α on KC and LIX expression: role of STAT1 and its effect on neutrophil recruitment to the spleen after lipopolysaccharide stimulation

The spleen is a crucial lymphoid organ. It is involved in the recruitment of various immunocytes to their correct locations using specific chemokines, but little is known concerning the role of type-I interferon (IFN) in the regulation of chemokines. In this study, we first used protein microarrays to assess the expression of keratinocyte-derived chemokine (KC) and lipopolysaccharide-induced CXC chemokine (LIX) in murine spleens. Both expressions were smoothly enhanced by IFN-α pretreatment after LPS injection. Then, we focused on the IFN-α regulation of KC, LIX, and their target cells, neutrophils, using an IFN-α neutralizing antibody and fludarabine (specific signal transducers and activators of transcription 1 - STAT1 inhibitor). Next, LPS was found to attenuate the production of KC and LIX in spleen. Even the elevated production of chemokines caused by exogenous IFN-α was found to be attenuated by fludarabine pretreatment. We later determined that the marginal zone and red pulp are the main sites of KC and LIX production. Last, we determined that the number of neutrophils was slightly increased by IFN-α treatment and diminished by IFN-α neutralization or fludarabine treatment. Further, the elevated neutrophils due to exogenous IFN-α were partially reversed by fludarabine pretreatment. In this way, these results indicate that IFN-α facilitates KC and LIX expression in mouse spleens after an LPS challenge. This effect was found to be mainly dependent upon the activation of STAT1, it may be involved in the recruitment of neutrophils to the spleen for the clearance of pathogens.

MCMV-mediated inhibition of the pro-apoptotic Bak protein is required for optimal in vivo replication

Successful replication and transmission of large DNA viruses such as the cytomegaloviruses (CMV) family of viruses depends on the ability to interfere with multiple aspects of the host immune response. Apoptosis functions as a host innate defence mechanism against viral infection, and the capacity to interfere with this process is essential for the replication of many viruses. The Bcl-2 family of proteins are the principle regulators of apoptosis, with two pro-apoptotic members, Bax and Bak, essential for apoptosis to proceed. The m38.5 protein encoded by murine CMV (MCMV) has been identified as Bax-specific inhibitor of apoptosis. Recently, m41.1, a protein product encoded by the m41 open reading frame (ORF) of MCMV, has been shown to inhibit Bak activity in vitro. Here we show that m41.1 is critical for optimal MCMV replication in vivo. Growth of a m41.1 mutant was attenuated in multiple organs, a defect that was not apparent in Bak^−/− mice. Thus, m41.1 promotes MCMV replication by inhibiting Bak-dependent apoptosis during in vivo infection. The results show that Bax and Bak mediate non-redundant functions during MCMV infection and that the virus produces distinct inhibitors for each protein to counter the activity of these proteins.

The cytomegaloviruses (CMV) are a family of viruses that establish a latent infection that lasts for the life of the host, with the virus able to reactivate when the host is immunosuppressed. We have used murine CMV (MCMV) as a model to understand how CMV interferes with the anti-viral immune response. Apoptosis, or programmed cell death, is one of the defence mechanisms used by multicellular organisms to impair viral infection. In order for viral replication to proceed, many viruses have evolved mechanisms to prevent the apoptosis of infected host cells. Under most circumstances the activation of Bax, or the closely related protein Bak, is required for apoptosis to proceed. The m41.1 protein was recently identified as a candidate Bak inhibitor during in vitro infection. We have generated a mutant virus which is unable to produce the m41.1 protein and found that growth of this virus was attenuated in wild-type mice. Importantly, growth of the mutant virus was equivalent to that of the wild-type virus in mice lacking the Bak protein. These studies establish that m41.1 is an inhibitor of Bak and that the capacity to prevent apoptosis triggered by Bak is required for efficient replication of MCMV in vivo.

Downregulation of key early events in the mobilization of antigen-bearing dendritic cells by leukocyte immunoglobulin-like Receptor B4 in a mouse model of allergic pulmonary inflammation

Leukocyte Immunoglobulin-like Receptor B4 (LILRB4) null mice have an exacerbated T helper cell type 2 (Th2) immune response and pulmonary inflammation compared with Lilrb4(+/+) animals when sensitized intranasally with ovalbumin (OVA) and low-dose lipopolysaccharide (LPS) followed by challenge with OVA. Moreover, OVA-challenged Lilrb4(-/-) mice exhibit greater migration of antigen (Ag)-bearing dendritic cells (DCs) to lymph nodes and accumulation of interleukin 4- and interleukin 5-producing lymph node lymphocytes. The main objective of this study was to determine how the absence of LILRB4 leads to a greater number of DCs in the lymph nodes of Ag-challenged mice and increased lung Th2 inflammation. Mice were sensitized intranasally with PBS alone or containing OVA and LPS; additional cohorts were subsequently challenged with OVA. Expression of chemokine (C-C motif) ligand 21 (CCL21) in the lung was assessed immunohistologically. OVA ingestion and expression of LILRB4 and chemokine (C-C motif) receptor 7 (CCR7) were quantified by flow cytometry. Inhalation of OVA and LPS induced upregulation of LILRB4 selectively on lung Ag-bearing DCs. After sensitization and challenge, the lung lymphatic vessels of Lilrb4(-/-) mice expressed more CCL21, a chemokine that directs the migration of DCs from peripheral tissue to draining lymph nodes, compared with Lilrb4(+/+) mice. In addition, lung DCs of challenged Lilrb4(-/-) mice expressed more CCR7, the CCL21 receptor. The lungs of challenged Lilrb4(-/-) mice also contained significantly greater numbers of CD4+ cells expressing interleukin-4 or interleukin-5, consistent with the greater number of Ag-bearing DCs and Th2 cells in lymph nodes and the attendant exacerbated Th2 lung pathology. Our data establish a new mechanism by which LILRB4 can downregulate the development of pathologic allergic inflammation: reduced upregulation of key molecules needed for DC migration leading to decreases in Th2 cells in lymph nodes and their target tissue.

Sex bias in susceptibility to MCMV infection: implication of TLR9

Toll-like receptor (TLR)-dependent pathways control the activation of various immune cells and the production of cytokines and chemokines that are important in innate immune control of viruses, including mouse cytomegalovirus (MCMV). Here we report that upon MCMV infection wild-type and TLR7^−/− male mice were more resistant than their female counterparts, while TLR9^−/− male and female mice showed similar susceptibility. Interestingly, 36 h upon MCMV infection TLR9 mRNA expression was higher in male than in female mouse spleens. MCMV infection led to stronger reduction of marginal zone (MZ) B cells, and higher infiltration of plasmacytoid dendritic cells and neutrophils in wild-type male than female mice, while no such sex differences were observed in TLR9^−/− mice. In accordance, the serum levels of KC and MIP-2, major neutrophil chemoattractants, were higher in wild-type, but not in TLR9^−/−, male versus female mice. Wild-type MCMV-infected female mice showed more severe liver inflammation, necrosis and steatosis compared to infected male mice. Our data demonstrate sex differences in susceptibility to MCMV infection, accompanied by a lower activation of the innate immune system in female mice, and can be attributed, at least in a certain degree, to the lower expression of TLR9 in female than male mice.

Immune response and immunopathology during toxoplasmosis

Toxoplasma gondii is a protozoan parasite of medical and veterinary significance that is able to infect any warm-blooded vertebrate host. In addition to its importance to public health, several inherent features of the biology of T. gondii have made it an important model organism to study host-pathogen interactions. One factor is the genetic tractability of the parasite, which allows studies on the microbial factors that affect virulence and allows the development of tools that facilitate immune studies. Additionally, mice are natural hosts for T. gondii, and the availability of numerous reagents to study the murine immune system makes this an ideal experimental system to understand the functions of cytokines and effector mechanisms involved in immunity to intracellular microorganisms. In this article, we will review current knowledge of the innate and adaptive immune responses required for resistance to toxoplasmosis, the events that lead to the development of immunopathology, and the natural regulatory mechanisms that limit excessive inflammation during this infection.

Retention of anergy and inhibition of antibody responses during acute γ herpesvirus 68 infection

The majority of the human population becomes infected early in life by the gammaherpesvirus EBV. Some findings suggest that there is an association between EBV infection and the appearance of pathogenic Abs found in lupus. Gammaherpesvirus 68 infection of adult mice (an EBV model) was shown to induce polyclonal B cell activation and hypergammaglobulinemia, as well as increased production of autoantibodies. In this study, we explored the possibility that this breach of tolerance reflects loss of B cell anergy. Our findings show that, although anergic B cells transiently acquire an activated phenotype early during infection, they do not become responsive to autoantigen, as measured by the ability to mobilize Ca2+ following AgR cross-linking or mount Ab responses following immunization. Indeed, naive B cells also acquire an activated phenotype during acute infection but are unable to mount Ab responses to either T cell-dependent or T cell-independent Ags. In acutely infected animals, Ag stimulation leads to upregulation of costimulatory molecules and relocalization of Ag-specific B cells to the B-T cell border; however, these cells do not proliferate or differentiate into Ab-secreting cells. Adoptive-transfer experiments show that the suppressed state is reversible and is dictated by the environment in the infected host. Finally, B cells in infected mice deficient of CD4+ T cells are not suppressed, suggesting a role for CD4+ T cells in enforcing unresponsiveness. Thus, rather than promoting loss of tolerance, gammaherpesvirus 68 infection induces an immunosuppressed state, reminiscent of compensatory anti-inflammatory response syndrome.

Infection with Toxoplasma gondii alters lymphotoxin expression associated with changes in splenic architecture

B cell responses are required for resistance to Toxoplasma gondii; however, the events that lead to production of class-switched antibodies during T. gondii infection have not been defined. Indeed, mice challenged with the parasite exhibited an expansion of T follicular helper cells and germinal center B cells in the spleen. Unexpectedly, this was not associated with germinal center formation and was instead accompanied by profound changes in splenic organization. This phenomenon was transient and was correlated with a decrease in expression of effector proteins that contribute to splenic organization, including lymphotoxins α and β. The importance of lymphotoxin was confirmed, as the use of a lymphotoxin β receptor agonist results in partial restoration of splenic structure. Splenectomized mice were used to test the splenic contribution to the antibody response during T. gondii infection. Analysis of splenectomized mice revealed delayed kinetics in the production of parasite-specific antibody, but the mice did eventually develop normal levels of parasite-specific antibody. Together, these studies provide a better understanding of how infection with T. gondii impacts the customized structures required for the optimal humoral responses to the parasite and the role of lymphotoxin in these events.

Low CXCL13 expression, splenic lymphoid tissue atrophy and germinal center disruption in severe canine visceral leishmaniasis

Visceral leishmaniasis is associated with atrophy and histological disorganization of splenic compartments. In this paper, we compared organized and disorganized splenic lymphoid tissue from dogs naturally infected with Leishmania infantum assessing the size of the white pulp compartments, the distribution of T, B and S100⁺ dendritic cells, using immunohistochemistry and morphometry and the expression of CCR7 and the cytokines, CXCL13, lymphotoxin (LT)-α, LT-β, CCL19, CCL21, TNF-α, IL-10, IFN-γ and TGF-β, using by real time RT-PCR. The lymphoid follicles and marginal zones were smaller (3.2 and 1.9 times, respectively; Mann-Whitney, P<0.02) in animals with disorganized splenic tissue in comparison to those with organized splenic lymphoid tissue. In spleens with disorganized lymphoid tissue, the numbers of T cells and S100⁺ dendritic cells were decreased in the follicles, and the numbers of B cells were reduced in both the follicles and marginal zones. CXCL13 mRNA expression was lower in animals with disorganized lymphoid tissue (0.5±0.4) compared to those with organized lymphoid tissue (2.7±2.9, both relative to 18S expression, P = 0.01). These changes in the spleen were associated with higher frequency of severe disease (7/12) in the animals with disorganized than in animals with organized (2/13, Chi-square, P = 0.01) splenic lymphoid tissue. The data presented herein suggest that natural infection with Leishmania infantum is associated with the impairment of follicular dendritic cells, CXCL13 expression, B cell migration and germinal center formation and associates these changes with severe clinical forms of visceral leishmaniasis. Furthermore the fact that this work uses dogs naturally infected with Leishmania infantum emphasizes the relevance of the data presented herein for the knowledge on the canine and human visceral leishmaniasis.

Mechanism of T cell exhaustion in a chronic environment

T cell exhaustion develops under conditions of antigen-persistence caused by infection with various chronic pathogens, such as human immunodeficiency virus (HIV) and mycobacterium tuberculosis (TB), or by the development of cancer. T cell exhaustion is characterized by stepwise and progressive loss of T cell function, which is probably the main reason for the failed immunological control of chronic pathogens and cancers. Recent observations have detailed some of the intrinsic and extrinsic factors that influence the severity of T cell exhaustion. Duration and magnitude of antigenic activation of T cells might be associated with up-regulation of inhibitory receptors, which is a major intrinsic factor of T cell exhaustion. Extrinsic factors might include the production of suppressive cytokines, T cell priming by either non-professional antigen-presenting cells (APCs) or tolerogenic dendritic cells (DCs), and alteration of regulatory T (Treg) cells. Further investigation of the cellular and molecular processes behind the development of T cell exhaustion can reveal therapeutic targets and strategies for the treatment of chronic infections and cancers. Here, we report the properties and the mechanisms of T cell exhaustion in a chronic environment.