Infection of dendritic cells by murine cytomegalovirus induces functional paralysis

Direct antigen presentation is the canonical pathway of cytomegalovirus CD8 T-cell priming regulated by balanced immune evasion ensuring a strong antiviral response

CD8 T cells are important antiviral effectors in the adaptive immune response to cytomegaloviruses (CMV). Naïve CD8 T cells can be primed by professional antigen-presenting cells (pAPCs) alternatively by "direct antigen presentation" or "antigen cross-presentation". In the case of direct antigen presentation, viral proteins are expressed in infected pAPCs and enter the classical MHC class-I (MHC-I) pathway of antigen processing and presentation of antigenic peptides. In the alternative pathway of antigen cross-presentation, viral antigenic material derived from infected cells of principally any cell type is taken up by uninfected pAPCs and eventually also fed into the MHC class-I pathway. A fundamental difference, which can be used to distinguish between these two mechanisms, is the fact that viral immune evasion proteins that interfere with the cell surface trafficking of peptide-loaded MHC-I (pMHC-I) complexes are absent in cross-presenting uninfected pAPCs. Murine cytomegalovirus (mCMV) models designed to disrupt either of the two presentation pathways revealed that both are possible in principle and can substitute each other. Overall, however, the majority of evidence has led to current opinion favoring cross-presentation as the canonical pathway. To study priming in the normal host genetically competent in both antigen presentation pathways, we took the novel approach of enhancing or inhibiting direct antigen presentation by using recombinant viruses lacking or overexpressing a key mCMV immune evasion protein. Against any prediction, the strongest CD8 T-cell response was elicited under the condition of intermediate direct antigen presentation, as it exists for wild-type virus, whereas the extremes of enhanced or inhibited direct antigen presentation resulted in an identical and weaker response. Our findings are explained by direct antigen presentation combined with a negative feedback regulation exerted by the newly primed antiviral effector CD8 T cells. This insight sheds a completely new light on the acquisition of viral immune evasion genes during virus-host co-evolution.

Dissecting the cytomegalovirus CC chemokine: Chemokine activity and gHgLchemokine-dependent cell tropism are independent players in CMV infection

Like all herpesviruses, cytomegaloviruses (CMVs) code for many immunomodulatory proteins including chemokines. The human cytomegalovirus (HCMV) CC chemokine pUL128 has a dual role in the infection cycle. On one hand, it forms the pentameric receptor-binding complex gHgLpUL(128,130,131A), which is crucial for the broad cell tropism of HCMV. On the other hand, it is an active chemokine that attracts leukocytes and shapes their activation. All animal CMVs studied so far have functionally homologous CC chemokines. In murine cytomegalovirus (MCMV), the CC chemokine is encoded by the m131/m129 reading frames. The MCMV CC chemokine is called MCK2 and forms a trimeric gHgLMCK2 entry complex. Here, we have generated MCK2 mutant viruses either unable to form gHgLMCK2 complexes, lacking the chemokine function or lacking both functions. By using these viruses, we could demonstrate that gHgLMCK2-dependent entry and MCK2 chemokine activity are independent functions of MCK2 in vitro and in vivo. The gHgLMCK2 complex promotes the tropism for leukocytes like macrophages and dendritic cells and secures high titers in salivary glands in MCMV-infected mice independent of the chemokine activity of MCK2. In contrast, reduced early antiviral T cell responses in MCMV-infected mice are dependent on MCK2 being an active chemokine and do not require the formation of gHgLMCK2 complexes. High levels of CCL2 and IFN-γ in spleens of infected mice and MCMV virulence depend on both, the formation of gHgLMCK2 complexes and the MCK2 chemokine activity. Thus, independent and concerted functions of MCK2 serving as chemokine and part of a gHgL entry complex shape antiviral immunity and virus dissemination.

Rat cytomegalovirus efficiently replicates in dendritic cells and induces changes in their transcriptional profile

Dendritic cells (DC) play a crucial role in generating and maintaining antiviral immunity. While DC are implicated in the antiviral defense by inducing T cell responses, they can also become infected by Cytomegalovirus (CMV). CMV is not only highly species-specific but also specialized in evading immune protection, and this specialization is in part due to characteristic genes encoded by a given virus. Here, we investigated whether rat CMV can infect XCR1+ DC and if infection of DC alters expression of cell surface markers and migration behavior. We demonstrate that wild-type RCMV and a mutant virus lacking the γ-chemokine ligand xcl1 (Δvxcl1 RCMV) infect splenic rat DC ex vivo and identify viral assembly compartments. Replication-competent RCMV reduced XCR1 and MHCII surface expression. Further, gene expression of infected DC was analyzed by bulk RNA-sequencing (RNA-Seq). RCMV infection reverted a state of DC activation that was induced by DC cultivation. On the functional level, we observed impaired chemotactic activity of infected XCR1+ DC compared to mock-treated cells. We therefore speculate that as a result of RCMV infection, DC exhibit diminished XCR1 expression and are thereby blocked from the lymphocyte crosstalk.

Epidemiology and clinical impact of respiratory co-infections at diagnosis of Pneumocystis jirovecii Pneumonia

OBJECTIVES

To describe the epidemiology and clinical impact of respiratory co-infections at diagnosis of Pneumocystis jirovecii Pneumonia (PcP).

METHODS

A retrospective observational study was conducted between January 2011 and April 2019 to evaluate respiratory co-infections at diagnosis of PcP patients, in two tertiary care hospitals. Respiratory co-infection was defined by the identification of pathogens from P. jirovecii-positive samples.

RESULTS

Of the 7 882 respiratory samples tested for P. jirovecii during the 8-year study period, 328 patients with final diagnosis of PcP were included in this study. Mean age was 56.7 ± 14.9 years, 193 (58.8%) were male, 74 (22.6%) had a positive HIV serology, 125 (38.1%) had a viral co-infections, 76 (23.2%) a bacterial co-infections and 90-day mortality was 25.3%. In overall population, 90-Day mortality was independently associated with a solid tumor underlying disease (OR 11.8, 1.90-78, p=0.008), SOFA score at admission (OR 1.62, 1.34-2.05; p<0.001) and CMV respiratory co-infection (OR 3.44, 1.24-2.90; p=0.02). Among HIV-negative patients, respiratory CMV co-infection was associated with a worse prognosis, especially when treated with adjunctive corticosteroid therapy.

CONCLUSIONS

Respiratory CMV co-infection at diagnosis of PcP was independently associated with increased 90-day mortality, specifically in HIV-negative patients.

The Roseoloviruses Downregulate the Protein Tyrosine Phosphatase PTPRC (CD45)

Like all herpesviruses, the roseoloviruses (HHV6A, -6B, and -7) establish lifelong infection within their host, requiring these viruses to evade host antiviral responses. One common host-evasion strategy is the downregulation of host-encoded, surface-expressed glycoproteins. Roseoloviruses have been shown to evade the host immune response by downregulating NK-activating ligands, class I MHC, and the TCR/CD3 complex. To more globally identify glycoproteins that are differentially expressed on the surface of HHV6A-infected cells, we performed cell surface capture of N-linked glycoproteins present on the surface of T cells infected with HHV6A, and compared these to proteins present on the surface of uninfected T cells. We found that the protein tyrosine phosphatase CD45 is downregulated in T cells infected with HHV6A. We also demonstrated that CD45 is similarly downregulated in cells infected with HHV7. CD45 is essential for signaling through the T cell receptor and, as such, is necessary for developing a fully functional immune response. Interestingly, the closely related betaherpesviruses human cytomegalovirus (HCMV) and murine cytomegalovirus (MCMV) have also separately evolved unique mechanisms to target CD45. While HCMV and MCMV target CD45 signaling and trafficking, HHV6A acts to downregulate CD45 transcripts. IMPORTANCE Human herpesviruses-6 and -7 infect essentially 100% of the world's population before the age of 5 and then remain latent or persistent in their host throughout life. As such, these viruses are among the most pervasive and stealthy of all viruses. Host immune cells rely on the presence of surface-expressed proteins to identify and target virus-infected cells. Here, we investigated the changes that occur to proteins expressed on the cell surface of T cells after infection with human herpesvirus-6A. We discovered that HHV-6A infection results in a reduction of CD45 on the surface of infected T cells and impaired activation in response to T cell receptor stimulation.

Duck hepatitis A virus 1 has lymphoid tissue tropism altering the organic immune responses of mature ducks

Duck hepatitis A virus 1 (DHAV-1) is a highly prevalent pathogen within adult ducks causing acute as well as chronic hepatitis which closely emulates the progression of human hepatitis. However, the underlying mechanisms of DHAV-1 persistence and the pathogenesis of chronic liver disease are not well defined. The association between hematopoietic reservoirs of virus and persistent infection is increasingly concerning. Here, we explored the ability of lymphoid replication of DHAV-1 and the effect on immunity. We found that DHAV-1 was able to infect and replicate productively in the lymphoid organs of model ducks, persisting over 6 months. Moreover, a significant correlation of viral loads between these organs and blood was found, documenting a major contribution of lymphoid replication to DHAV-1 viraemia. Along with viral replication, the mRNA of PRRs and immune-related cytokines was up-regulated in these organs during the early phase of infection, showing tissue-dependent expression patterns but all inclining towards Th2 responses due to the consistently higher level of IL-4 than IL-2 and IFN-γ. Additionally, the expression of CCL19, CCL21, MHC-I and MHC-II, which are involved in T cell homing to the periphery and priming, was dysmodulated. Our data indicate that DHAV-1 possesses lymphoid tissue tropism, contributing to persistent infection and chronic hepatitis via altering the early endogenous transcription of immune-related genes and thereby perturbing organic immunity. These results may be useful to develop novel strategies to treat chronic viral hepatitis based on stimulation of the early innate system and regulation of T-cell trafficking.

The Innate Immune Response to Infection Induces Erythropoietin-Dependent Replenishment of the Dendritic Cell Compartment

Dendritic cells (DC) play a key role in the adaptive immune response due to their ability to present antigens and stimulate naïve T cells. Many bacteria and viruses can efficiently target DC, resulting in impairment of their immunostimulatory function or elimination. Hence, the DC compartment requires replenishment following infection to ensure continued operational readiness of the adaptive immune system. Here, we investigated the molecular and cellular mechanisms of inflammation-induced DC generation. We found that infection with viral and bacterial pathogens as well as Toll-like receptor 9 (TLR9) ligation with CpG-oligodeoxynucleotide (CpG-ODN) expanded an erythropoietin (EPO)-dependent TER119⁺CD11a⁺ cell population in the spleen that had the capacity to differentiate into TER119⁺CD11c^high and TER119^-CD11c^high cells both in vitro and in vivo. TER119⁺CD11c^high cells contributed to the conventional DC pool in the spleen and specifically increased in lymph nodes draining the site of local inflammation. Our results reveal a so far undescribed inflammatory EPO-dependent pathway of DC differentiation and establish a mechanistic link between innate immune recognition of potential immunosuppressive pathogens and the maintenance of the DC pool during and after infection.

Immune Relevant and Immune Deficient Mice: Options and Opportunities in Translational Research

In 1989 ILAR published a list and description of immunodeficient rodents used in research. Since then, advances in understanding of molecular mechanisms; recognition of genetic, epigenetic microbial, and other influences on immunity; and capabilities in manipulating genomes and microbiomes have increased options and opportunities for selecting mice and designing studies to answer important mechanistic and therapeutic questions. Despite numerous scientific breakthroughs that have benefitted from research in mice, there is debate about the relevance and predictive or translational value of research in mice. Reproducibility of results obtained from mice and other research models also is a well-publicized concern. This review summarizes resources to inform the selection and use of immune relevant mouse strains and stocks, aiming to improve the utility, validity, and reproducibility of research in mice. Immune sufficient genetic variations, immune relevant spontaneous mutations, immunodeficient and autoimmune phenotypes, and selected induced conditions are emphasized.

Spatial and temporal coordination of antiviral responses by group 1 ILCs

Group 1 innate lymphocytes consist of a phenotypically, spatially, and functionally heterogeneous population of NK cells and ILC1s that are engaged during pathogen invasion. We are only beginning to understand the context-dependent roles that different subsets of group 1 innate lymphocytes play during homeostatic perturbations. With a focus on viral infection, this review highlights the organization and regulation of spatially and temporally distinct waves of NK cell and ILC1 responses that collectively serve to achieve optimal viral control.

Subversion of Immune Response by Human Cytomegalovirus

Human cytomegalovirus (HCMV) is the most common cause of congenital infections and is an important pathogen in immunocompromised individuals. Despite a robust host immune system, HCMV able to replicate, evade host defenses, establish latency for life. A significant portion of HCMV genome dedicated to encode gene products for modulation of host immune response. Growing number of HCMV gene products are being recognized to play role in immune evasion. Information on viral immune evasion mechanisms by which HCMV persists in host will be useful in devising antiviral intervention strategies and development of new vaccines. This minireview provides a brief overview of immune evasion strategy adapted by HCMV by utilizing its gene products in modulation of host immune response.

Caspase-8-dependent control of NK- and T cell responses during cytomegalovirus infection

Caspase-8 (CASP8) impacts antiviral immunity in expected as well as unexpected ways. Mice with combined deficiency in CASP8 and RIPK3 cannot support extrinsic apoptosis or RIPK3-dependent programmed necrosis, enabling studies of CASP8 function without complications of unleashed necroptosis. These extrinsic cell death pathways are naturally targeted by murine cytomegalovirus (MCMV)-encoded cell death suppressors, showing they are key to cell-autonomous host defense. Remarkably, Casp8^-/-Ripk3^-/-, Ripk1^-/-Casp8^-/-Ripk3^-/- and Casp8^-/-Ripk3^K51A/K51A mice mount robust antiviral T cell responses to control MCMV infection. Studies in Casp8^-/-Ripk3^-/- mice show that CASP8 restrains expansion of MCMV-specific natural killer (NK) and CD8 T cells without compromising contraction or immune memory. Infected Casp8^-/-Ripk3^-/- or Casp8^-/-Ripk3^K51A/K51A mice have higher levels of virus-specific NK cells and CD8 T cells compared to matched RIPK3-deficient littermates or WT mice. CASP8, likely acting downstream of Fas death receptor, dampens proliferation of CD8 T cells during expansion. Importantly, contraction proceeds unimpaired in the absence of extrinsic death pathways owing to intact Bim-dependent (intrinsic) apoptosis. CD8 T cell memory develops in Casp8^-/-Ripk3^-/- mice, but memory inflation characteristic of MCMV infection is not sustained in the absence of CASP8 function. Despite this, Casp8^-/-Ripk3^-/- mice are immune to secondary challenge. Interferon (IFN)γ is recognized as a key cytokine for adaptive immune control of MCMV. Ifngr^-/-Casp8^-/-Ripk3^-/- mice exhibit increased lifelong persistence in salivary glands as well as lungs compared to Ifngr^-/- and Casp8^-/-Ripk3^-/- mice. Thus, mice deficient in CASP8 and RIPK3 are more dependent on IFNγ mechanisms for sustained T cell immune control of MCMV. Overall, appropriate NK- and T cell immunity to MCMV is dependent on host CASP8 function independent of RIPK3-regulated pathways.

Murine cytomegalovirus degrades MHC class II to colonize the salivary glands

Cytomegaloviruses (CMVs) persistently and systemically infect the myeloid cells of immunocompetent hosts. Persistence implies immune evasion, and CMVs evade CD8⁺ T cells by inhibiting MHC class I-restricted antigen presentation. Myeloid cells can also interact with CD4⁺ T cells via MHC class II (MHC II). Human CMV (HCMV) attacks the MHC II presentation pathway in vitro, but what role this evasion might play in host colonization is unknown. We show that Murine CMV (MCMV) down-regulates MHC II via M78, a multi-membrane spanning viral protein that captured MHC II from the cell surface and was necessary although not sufficient for its degradation in low pH endosomes. M78-deficient MCMV down-regulated MHC I but not MHC II. After intranasal inoculation, it showed a severe defect in salivary gland colonization that was associated with increased MHC II expression on infected cells, and was significantly rescued by CD4⁺ T cell loss. Therefore MCMV requires CD4⁺ T cell evasion by M78 to colonize the salivary glands, its main site of long-term shedding.

Human cytomegalovirus is the commonest infectious cause of harm to unborn children. Vaccines have not stopped it establishing chronic, systemic infections. Murine cytomegalovirus (MCMV) provides an accessible model to understand why. We show that MCMV evades CD4⁺ T cells via its M78 protein, and that this helps infection to spread despite the immune response. Thus while CD4⁺ T cells are important for host defence, viral evasion limits their capacity to act alone in controlling infection.

Safety and Efficacy of Antiviral Therapy for Prevention of Cytomegalovirus Reactivation in Immunocompetent Critically Ill Patients: A Randomized Clinical Trial

IMPORTANCE

Latent cytomegalovirus (CMV) infection is present in more than half the adult population, and a viral reactivation (ie, when the virus becomes measurable in body fluids such as blood) can occur in up to one-third of these individuals during episodes of critical illness.

OBJECTIVE

To determine whether antiviral therapy is safe and effective for preventing CMV reactivation in a general population of critically ill patients.

DESIGN, SETTING, AND PARTICIPANTS

A single-center, open-label, randomized, controlled clinical trial recruited 124 CMV-seropositive patients undergoing mechanical ventilation for at least 24 hours in the intensive care unit between January 1, 2012, and January 31, 2014. The mean baseline Acute Physiology and Chronic Health Evaluation II score of all patients was 17.6.

INTERVENTIONS

Patients were randomized to receive anti-CMV prophylaxis with valacyclovir hydrochloride (n = 34) or low-dose valganciclovir hydrochloride (n = 46) for up to 28 days to suppress viral reactivation, or to a control group with no intervention (n = 44).

MAIN OUTCOMES AND MEASURES

Time to first CMV reactivation in blood within the 28-day follow-up period following initiation of the study drug.

RESULTS

Among the 124 patients in the study (46 women and 78 men; mean [SD] age, 56.9 [16.9] years), viral reactivation in the blood occurred in 12 patients in the control group, compared with 1 patient in the valganciclovir group and 2 patients in the valacyclovir group (combined treatment groups vs control: hazard ratio, 0.14; 95% CI 0.04-0.50). Although this trial was not powered to assess clinical end points, the valacyclovir arm was halted prematurely because of higher mortality; 14 of 34 patients (41.2%) had died by 28 days, compared with 5 of 37 (13.5%) patients in the control arm at the point of the decision to halt this arm. Other safety end points showed similar outcomes between groups.

CONCLUSIONS AND RELEVANCE

Antiviral prophylaxis with valacyclovir or low-dose valganciclovir suppresses CMV reactivation in patients with critical illness. However, given the higher mortality, a large-scale trial would be needed to determine the clinical efficacy and safety of CMV suppression.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01503918.

The Mouse Cytomegalovirus Gene m42 Targets Surface Expression of the Protein Tyrosine Phosphatase CD45 in Infected Macrophages

The receptor-like protein tyrosine phosphatase CD45 is expressed on the surface of cells of hematopoietic origin and has a pivotal role for the function of these cells in the immune response. Here we report that following infection of macrophages with mouse cytomegalovirus (MCMV) the cell surface expression of CD45 is drastically diminished. Screening of a set of MCMV deletion mutants allowed us to identify the viral gene m42 of being responsible for CD45 down-modulation. Moreover, expression of m42 independent of viral infection upon retroviral transduction of the RAW264.7 macrophage cell line led to comparable regulation of CD45 expression. In immunocompetent mice infected with an m42 deletion mutant lower viral titers were observed in all tissues examined when compared to wildtype MCMV, indicating an important role of m42 for viral replication in vivo. The m42 gene product was identified as an 18 kDa protein expressed with early kinetics and is predicted to be a tail-anchored membrane protein. Tracking of surface-resident CD45 molecules revealed that m42 induces internalization and degradation of CD45. The observation that the amounts of the E3 ubiquitin ligases Itch and Nedd4 were diminished in cells expressing m42 and that disruption of a PY motif in the N-terminal part of m42 resulted in loss of function, suggest that m42 acts as an activator or adaptor for these Nedd4-like ubiquitin ligases, which mark CD45 for lysosomal degradation. In conclusion, the down-modulation of CD45 expression in MCMV-infected myeloid cells represents a novel pathway of virus-host interaction.

Human cytomegalovirus (HCMV) is a tenacious pathogen, which can be life-threatening for immunocompromised patients and immunologically immature newborns. The pathogenicity of HCMV is owed to a plethora of immunomodulatory functions that interfere with host defense mechanisms. Such viral functions can teach us about viral pathogenesis mechanisms, and also about the functioning of immune cells. In this study we report that the mouse cytomegalovirus (MCMV)–a close relative of HCMV–influences surface expression of the cellular protein CD45 on macrophages and we identified the viral gene m42 mediating this effect. CD45 has long been known to be essential for the functioning of lymphocytes, however, its role in macrophages is less well understood. Growth analysis of a viral mutant indicated that the m42 gene confers a replication advantage to MCMV in vivo. We found that the m42 protein induces internalization of CD45 from the plasma membrane and degradation in lysosomes—most likely triggered by interaction of m42 with a ubiquitin ligase. In our study we detected a new element in the complex interaction of cytomegaloviruses with host cells, and further investigation into this mechanism may provide us with new insights into the functions of CD45 in myeloid cells.

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.

Type I Interferon Released by Myeloid Dendritic Cells Reversibly Impairs Cytomegalovirus Replication by Inhibiting Immediate Early Gene Expression

Cytomegalovirus (CMV) is a ubiquitous beta-herpesvirus whose reactivation from latency is a major cause of morbidity and mortality in immunocompromised hosts. Mouse CMV (MCMV) is a well-established model virus to study virus-host interactions. We showed in this study that the CD8-independent antiviral function of myeloid dendritic cells (mDC) is biologically relevant for the inhibition of MCMV replication in vivo and in vitro. In vivo ablation of CD11c(+) DC resulted in higher viral titers and increased susceptibility to MCMV infection in the first 3 days postinfection. We developed in vitro coculture systems in which we cocultivated MCMV-infected endothelial cells or fibroblasts with T cell subsets and/or dendritic cells. While CD8 T cells failed to control MCMV replication, bone marrow-derived mDC reduced viral titers by a factor of up to 10,000. Contact of mDC with the infected endothelial cells was crucial for their antiviral activity. Soluble factors secreted by the mDC blocked MCMV replication at the level of immediate early (IE) gene expression, yet the viral lytic cycle reinitiated once the mDC were removed from the cells. On the other hand, the mDC did not impair MCMV replication in cells deficient for the interferon (IFN) alpha/beta receptor (IFNAR), arguing that type I interferons were critical for viral control by mDC. In light of our recent observation that type I IFN is sufficient for the induction of latency immediately upon infection, our results imply that IFN secreted by mDC may play an important role in the establishment of CMV latency.

IMPORTANCE

Numerous studies have focused on the infection of DC with cytomegaloviruses and on the establishment of latency within them. However, almost all of these studies have relied on the infection of DC monocultures in vitro, whereas DC are just one among many cell types present in an infection site in vivo. To mimic this aspect of the in vivo situation, we cocultured DC with infected endothelial cells or fibroblasts. Our data suggest that direct contact with virus-infected endothelial cells activates CD11c(+) DC, which leads to reversible suppression of MCMV replication at the level of IE gene expression by a mechanism that depends on type I IFN. The effect matches the formal definition of viral latency. Therefore, our data argue that the interplay of dendritic cells and infected neighboring cells might play an important role in the establishment of viral latency.

Acute GVHD results in a severe DC defect that prevents T-cell priming and leads to fulminant cytomegalovirus disease in mice

Viral infection is a common, life-threatening complication after allogeneic bone marrow transplantation (BMT), particularly in the presence of graft-versus-host disease (GVHD). Using cytomegalovirus (CMV) as the prototypic pathogen, we have delineated the mechanisms responsible for the inability to mount protective antiviral responses in this setting. Although CMV infection was self-limiting after syngeneic BMT, in the presence of GVHD after allogeneic BMT, CMV induced a striking cytopathy resulting in universal mortality in conjunction with a fulminant necrotizing hepatitis. Critically, GVHD induced a profound dendritic cell (DC) defect that led to a failure in the generation of CMV-specific CD8(+) T-cell responses. This was accompanied by a defect in antiviral CD8(+) T cells. In combination, these defects dramatically limited antiviral T-cell responses. The transfer of virus-specific cells circumvented the DC defects and provided protective immunity, despite concurrent GVHD. These data demonstrate the importance of avoiding GVHD when reconstructing antiviral immunity after BMT, and highlight the mechanisms by which the adoptive transfer of virus-specific T cells overcome the endogenous defects in priming invoked by GVHD.

Breaking the co-operation between bystander T-cells and natural killer cells prevents the development of immunosuppression after traumatic skeletal muscle injury in mice

Nosocomial infections represent serious complications after traumatic or surgical injuries in intensive care units. The pathogenesis of the underlying immunosuppression is only incompletely understood. In the present study, we investigated whether injury interferes with the function of the adaptive immune system in particular with the differentiation of antigen-specific T helper (Th)-cell responses in vivo. We used a mouse model for traumatic gastrocnemius muscle injury. Ovalbumin (OVA), which served as a foreign model antigen, was injected into the hind footpads for determination of the differentiation of OVA-specific Th-cells in the draining popliteal lymph node (pLN). The release of interferon (IFN)-γ from OVA-specific Th-cells was impaired within 24 h after injury and this impairment persisted for at least 7 days. In contrast, the proliferation of OVA-specific Th-cells remained unaffected. Injury did not modulate the function of antigen-presenting cells (APCs) in the pLN. Adoptive transfer of total T-cells from pLNs of injured mice inhibited IFN-γ production by OVA-specific Th-cells in naive mice. Suppressed Th1 priming did not occur in lymphocyte-deficient mice after injury but was restored by administration of T-cells before injury. Moreover, the suppression of Th1 differentiation required the presence of natural killer (NK) cells that were recruited to the pLN after injury; this recruitment was dependent on lymphocytes, toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88). In summary, upon traumatic skeletal muscle injury T-cells and NK cells together prevent the development of protective Th1 immunity. Breaking this co-operation might be a novel approach to reduce the risk of infectious complications after injury.

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.

Sphingosine analog AAL-R promotes activation of LCMV-infected dendritic cells

Sphingosine analogs display diverse immunoregulatory activities with curative potential in autoimmune diseases and viral infections. Recently, the sphingosine analog AAL-R was shown to increase DC activation upon TLR7 stimulation. Here, we investigated the effect of AAL-R on activation of dendritic cells (DCs) infected by lymphocytic choriomeningitis virus (LCMV). Concomitant treatment of LCMV-infected DCs with AAL-R enhanced DC maturation and DC ability to stimulate and expand antiviral CD8(+) T cells. Importantly, AAL-R's stimulatory activity was abrogated in type I interferon (IFN) receptor-deficient DCs following LCMV infection. In support of this observation, AAL-R increased type I IFN production from DCs infected with LCMV. Taken together, the sphingosine analog could directly act on DCs to promote defensive host DC responses to the viral invasion via type I IFN signaling.

Interferon-α production by plasmacytoid dendritic cells is dispensable for an effective anti-cytomegalovirus response in adaptor protein-3-deficient mice

Adaptor protein-3 (AP-3) is a heterotetrameric complex, which regulates vesicular trafficking. Mutations of the β3A subunit cause the Hermansky-Pudlak syndrome type 2 (HPS-2), a rare genetic disease characterized by albinism, platelet defects, and recurrent infections. Likewise, pearl mice, which lack functional AP-3, show several HPS-2 defects. The AP-3 absence results in defective toll-like receptor trafficking and signaling in dendritic cells (DC), but its effect on the efficiency of the in vivo antiviral response is unclear. We evaluated the impact of AP-3 deficiency on the distribution of DC subsets, interferon (IFN) production, and the susceptibility to murine cytomegalovirus (MCMV) infection. Pearl mice showed a distribution and frequency of conventional (cDC) and plasmacytoid DC (pDC) similar to that of wild-type mice both before and after MCMV infection. Moreover, pearl mice controlled MCMV infection even at high virus doses and showed a normal production of IFN-α. Since pDC, but not cDC, from pearl mice showed an impaired IFN-α and tumor necrosis factor-α production in response to prototypic DNA (MCMV and Herpes Simplex virus) or RNA (Vesicular Stomatitis virus) viruses in vitro, it is likely that MCMV infection can be controlled in vivo independently of an efficient production of IFN-α by pDC, and that the AP-3 complex has a minimal impact on protective antiviral responses.

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.

Controlling cytomegalovirus: helping the immune system take the lead

Cytomegalovirus, of the Herpesviridae family, has evolved alongside humans for thousands of years with an intricate balance of latency, immune evasion, and transmission. While upwards of 70% of humans have evidence of CMV infection, the majority of healthy people show little to no clinical symptoms of primary infection and CMV disease is rarely observed during persistent infection in immunocompetent hosts. Despite the fact that the majority of infected individuals are asymptomatic, immunologically, CMV hijacks the immune system by infecting and remaining latent in antigen-presenting cells that occasionally reactivate subclinically and present antigen to T cells, eventually causing the inflation of CMV-specific T cells until they can compromise up to 10% of the entire T cell repertoire. Because of this impact on the immune system, as well as its importance in fields such as stem cell and organ transplant, the relationship between CMV and the immune response has been studied in depth. Here we provide a review of many of these studies and insights into how CMV-specific T cells are currently being used therapeutically.

Distinct requirements for activation of NKT and NK cells during viral infection

NK cells are key regulators of innate defense against mouse CMV (MCMV). Like NK cells, NKT cells also produce high levels of IFN-γ rapidly after MCMV infection. However, whether similar mechanisms govern activation of these two cell types, as well as the significance of NKT cells for host resistance, remain unknown. In this article, we show that, although both NKT and NK cells are activated via cytokines, their particular cytokine requirements differ significantly in vitro and in vivo. IL-12 is required for NKT cell activation in vitro but is not sufficient, whereas NK cells have the capacity to be activated more promiscuously in response to individual cytokines from innate cells. In line with these results, GM-CSF-derived dendritic cells activated only NK cells upon MCMV infection, consistent with their virtual lack of IL-12 production, whereas Flt3 ligand-derived dendritic cells produced IL-12 and activated both NK and NKT cells. In vivo, NKT cell activation was abolished in IL-12(-/-) mice infected with MCMV, whereas NK cells were still activated. In turn, splenic NK cell activation was more IL-18 dependent. The differential requirements for IL-12 and IL-18 correlated with the levels of cytokine receptor expression by NK and NKT cells. Finally, mice lacking NKT cells showed reduced control of MCMV, and depleting NK cells further enhanced viral replication. Taken together, our results show that NKT and NK cells have differing requirements for cytokine-mediated activation, and both can contribute nonredundantly to MCMV defense, revealing that these two innate lymphocyte subsets function together to fine-tune antiviral responses.

NKp46 regulates allergic responses

Natural killer (NK) cells are cytotoxic cells that are able to rapidly kill viruses, tumor cells, parasites, bacteria, and even cells considered "self". The activity of NK cells is controlled by a fine balance of inhibitory and activating signals mediated by a complex set of different receptors. However, the function of NK cells is not restricted only to the killing of target cells, NK cells also possess other properties such as the secretion of proangiogenic factors during pregnancy. Here, we demonstrate another unique NK-cell activity, namely the regulation of T-cell mediated allergic responses, which is dependent on the NK-cell specific receptor NKp46 (Ncr1 in mice). Using mice in which the Ncr1 gene has been replaced with a green fluorescent protein, we demonstrate reduced delayed-type hypersensitivity and airway hypersensitivity. Interestingly, we show that this reduction in airway hypersensitivity is due to differences in the stimulation of T cells resulting in an altered cytokine profile.

Cytomegalovirus expresses the chemokine homologue vXCL1 capable of attracting XCR1+ CD4- dendritic cells

Cytomegaloviruses (CMV) have developed various strategies to escape the immune system of the host. One strategy involves the expression of virus-encoded chemokines to modulate the host chemokine network. We have identified in the English isolate of rat CMV (murid herpesvirus 8 [MuHV8]) an open reading frame encoding a protein homologous to the chemokine XCL1, the only known C chemokine. Viral XCL1 (vXCL1), a glycosylated protein of 96 amino acids, can be detected 13 h postinfection in the supernatant of MuHV8-infected rat embryo fibroblasts. vXCL1 exclusively binds to CD4(-) rat dendritic cells (DC), a subset of DC that express the corresponding chemokine receptor XCR1. Like endogenous rat XCL1, vXCL1 selectively chemoattracts XCR1(+) CD4(-) DC. Since XCR1(+) DC in mice and humans have been shown to excel in antigen cross-presentation and thus in the induction of cytotoxic CD8(+) T lymphocytes, the virus has apparently hijacked this gene to subvert cytotoxic immune responses. The biology of vXCL1 offers an interesting opportunity to study the role of XCL1 and XCR1(+) DC in the cross-presentation of viral antigens.

Priming of T follicular helper cells by dendritic cells

T follicular helper cells (Tfh) are required to generate long-lived antibody responses, which confer long-term protection to pathogens following vaccination or infection. Despite significant advances in the field, however, little is known about the early steps that drive Tfh cell differentiation. In this review, we will discuss the mechanisms by which dendritic cells promote the initial commitment of activated CD4(+) T cells to the Tfh cell differentiation pathway.

Superior induction and maintenance of protective CD8 T cells in mice infected with mouse cytomegalovirus vector expressing RAE-1γ

Due to a unique pattern of CD8 T-cell response induced by cytomegaloviruses (CMVs), live attenuated CMVs are attractive candidates for vaccine vectors for a number of clinically relevant infections and tumors. NKG2D is one of the most important activating NK cell receptors that plays a role in costimulation of CD8 T cells. Here we demonstrate that the expression of CD8 T-cell epitope of Listeria monocytogenes by a recombinant mouse CMV (MCMV) expressing the NKG2D ligand retinoic acid early-inducible protein 1-gamma (RAE-1γ) dramatically enhanced the effectiveness and longevity of epitope-specific CD8 T-cell response and conferred protection against a subsequent challenge infection with Listeria monocytogenes. Unexpectedly, the attenuated growth in vivo of the CMV vector expressing RAE-1γ and its capacity to enhance specific CD8 T-cell response were preserved even in mice lacking NKG2D, implying additional immune function for RAE-1γ beyond engagement of NKG2D. Thus, vectors expressing RAE-1γ represent a promising approach in the development of CD8 T-cell-based vaccines.

Impaired direct priming of CD8 T cells by donor-derived cytomegalovirus following kidney transplantation

Cytomegalovirus (CMV) infection increases the risk of complications after renal transplantation, but the mechanisms controlling donor-derived infection are not adequately characterized. Here, we assessed the risk of clinically significant CMV disease in donor-seropositive, recipient-seropositive (D+R+) renal transplantation and examined recipients' CMV antigen-specific cellular immune responses primed directly by donor cells. In a retrospective cohort of 569 patients administered standardized basiliximab-tacrolimus-mycophenolate-corticosteroid immunosuppressive therapy, CMV disease rates increased in D+R+ serostatus pairings compared with D-R+ pairings (hazard ratio [HR], 2.61; 95% confidence interval [CI], 1.36 to 5.01; P=0.004) and associated with increased donor-recipient HLA mismatch in the D+R+ group (HR [per class 1 mismatch], 1.43; 95% CI, 1.12 to 1.82]; P=0.02). D+R+ and D+R- transplants in which the donor and recipient differentially expressed at least one HLA class I allele were followed prospectively from the time of transplantation. During the first year after transplantation, four of eight seropositive recipients and one of three seronegative recipients displayed peripheral blood CD8+ T cell responses to CMV presented by recipient-specific HLA. Notably, no recipients mounted responses to CMV presented by donor-specific HLA, despite the detection of CMV antigen expression in all seropositive donor organs examined (n=10), suggesting that the allograft of Class I HLA-mismatched seropositive donors is inaccessible to CD8+ T cell responses. Finally, pretransplant assays of anti-CMV cellular immunity predicted post-transplant CMV replication less accurately in D+R+ pairings than in D-R+ pairings, possibly reflecting in vitro assay specificity for recipient, rather than donor, HLA. These findings are relevant to the clinical management and immunologic understanding of donor-transmitted viral infection.

VIPhyb, an antagonist of vasoactive intestinal peptide receptor, enhances cellular antiviral immunity in murine cytomegalovirus infected mice

Vasoactive intestinal peptide (VIP) is a neuropeptide hormone that suppresses Th1-mediated cellular immunity. We previously reported that VIP-knockout (VIP-KO) mice have enhanced cellular immune responses and increased survival following murine cytomegalovirus (mCMV) infection in C57BL/6 mice. In this study, we tested whether treatment with a VIP receptor antagonistic peptide protects C57BL/6 and BALB/c mice from mCMV-infection. One week of daily subcutaneous injections of VIPhyb was non-toxic and did not alter frequencies of immune cell subsets in non-infected mice. VIPhyb administration to mCMV-infected C57BL/6 and BALB/c mice markedly enhanced survival, viral clearance, and reduced liver and lung pathology compared with saline-treated controls. The numbers of effector/memory CD8+ T-cells and mature NK cells were increased in VIPhyb-treated mice compared with PBS-treated groups. Pharmacological blockade of VIP-receptor binding or genetic blockade of VIP-signaling prevented the up-regulation of PD-L1 and PD-1 expression on DC and activated CD8⁺ T-cells, respectively, in mCMV-infected mice, and enhanced CD80, CD86, and MHC-II expression on conventional and plasmacytoid DC. VIPhyb-treatment increased type-I IFN synthesis, numbers of IFN-γ- and TNF-α-expressing NK cells and T-cells, and the numbers of mCMV-M45 epitope-peptide-MHC-I tetramer CD8⁺ T-cells following mCMV infection. VIP-treatment lowered the percentage of Treg cells in spleens compared with PBS-treated WT mice following mCMV infection, while significantly decreasing levels of serum VEGF induced by mCMV-infection. The mice in all treated groups exhibited similar levels of anti-mCMV antibody titers. Short-term administration of a VIP-receptor antagonist represents a novel approach to enhance innate and adaptive cellular immunity in a murine model of CMV infection.

CpG pretreatment enhances antiviral T-cell immunity against cytomegalovirus

Major histocompatibility complex class I-restricted T-cell immunity is essential to control infection with cytomegalovirus (CMV), a clinically important virus that causes significant disease in immunocompromised individuals. Cross-presentation is considered the primary mode of antigen presentation to generate protective antiviral CD8⁺ T-cell immunity. Herpesviruses, including CMV, encode numerous proteins that interfere with direct antigen presentation, leading to the paradigm that T-cell immunity to these pathogens necessitates cross-presentation. However, the antigen presentation requirements needed to generate a protective T-cell response to CMV remain unknown. Here, we show that a fully functional antiviral CD8⁺ T-cell response can be generated in a system where cross-presentation is shut down by pretreatment with CpG. Notably, in this setting, CD8⁺ T cells demonstrate accelerated control of infection, and organ pathology is limited. These data indicate that protective antiviral T-cell immunity to CMV is generated by direct presentation and can be enhanced by pretreatment with CpG.

The evolutionary arms race between NK cells and viruses: who gets the short end of the stick?

NK cells are innate lymphocytes that play a key role in the control of various viral infections. Recent studies indicate that NK cells may acquire some features of adaptive immune cells, including the formation of long-lived memory cells. A large and growing body of data indicates that NK cells regulate the adaptive immune response as well. The function and the activation status of NK cells are tightly regulated by signals induced by a broad range of inhibitory and activating cell surface receptors and cytokines released by other immune cells. Here, we review the function of mouse NK-cell receptors involved in virus control and in the regulation of the adaptive immune response. In addition, we discuss viral strategies used to evade NK-cell-mediated control during infection. Finally, the role of several activating Ly49 receptors specific for mouse cytomegalovirus (MCMV), as well as some controversial issues in the field, will be discussed.

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.

Priming of CD8+ T cells against cytomegalovirus-encoded antigens is dominated by cross-presentation

CMV can infect dendritic cells (DCs), and direct Ag presentation could, therefore, lead to the priming of CMV-specific CD8(+) T cells. However, CMV-encoded immune evasins severely impair Ag presentation in the MHC class I pathway; thus, it is widely assumed that cross-presentation drives the priming of antiviral T cells. We assessed the contribution of direct versus cross priming in mouse CMV (MCMV) infection using recombinant viruses. DCs infected with an MCMV strain encoding the gB498 epitope from HSV-1 were unable to stimulate in vitro naive gB498-specific CD8(+) T cells from TCR transgenic mice. Infection of C57BL/6 mice with this recombinant virus led, however, to the generation of abundant numbers of gB498-specific T cells in vivo. Of the DC subsets isolated from infected mice, only CD8α(+) DCs were able to stimulate naive T cells, suggesting that this DC subset cross-presents MCMV-encoded Ag in vivo. Upon infection of mice with MCMV mutants encoding Ag that can either be well or hardly cross-presented, mainly CD8(+) T cells specific for cross-presented epitopes were generated. Moreover, even in the absence of immune evasion genes interfering with MHC class I-mediated Ag presentation, priming of T cells to Ag that can only be presented directly was not observed. We conclude that the host uses mainly DCs capable of cross-presentation to induce the CMV-specific CD8(+) T cell response during primary, acute infection and discuss the implications for the development of a CMV vaccine.

MHC class I cross-presentation by dendritic cells counteracts viral immune evasion

DCs very potently activate CD8(+) T cells specific for viral peptides bound to MHC class I molecules. However, many viruses have evolved immune evasion mechanisms, which inactivate infected DCs and might reduce priming of T cells. Then MHC class I cross-presentation of exogenous viral Ag by non-infected DCs may become crucial to assure CD8(+) T cell responses. Although many vital functions of infected DCs are inhibited in vitro by many different viruses, the contributions of cross-presentation to T cell immunity when confronted with viral immune inactivation in vivo has not been demonstrated up to now, and remains controversial. Here we show that priming of Herpes Simplex Virus (HSV)-, but not murine cytomegalovirus (mCMV)-specific CD8(+) T cells was severely reduced in mice with a DC-specific cross-presentation deficiency. In contrast, while CD8(+) T cell responses to mutant HSV, which lacks crucial inhibitory genes, also depended on CD8α(+) DCs, they were independent of cross-presentation. Therefore HSV-specific CTL-responses entirely depend on the CD8α(+) DC subset, which present via direct or cross-presentation mechanisms depending on the immune evasion equipment of virus. Our data establish the contribution of cross-presentation to counteract viral immune evasion mechanisms in some, but not all viruses.

Downregulation of antigen-presenting cells in tonsil and lymph nodes of porcine reproductive and respiratory syndrome virus-infected pigs

Porcine reproductive and respiratory syndrome virus (PRRSV) can persist in different organs of infected pigs, which suggests a failure in the immune response. Antigen-presenting cells (APCs) play a pivotal role in the induction of effective T- and B-cell responses. In this study, we investigated the changes in the different APC subpopulations and T- and B-cell counts in the tonsil, retropharyngeal and mediastinal lymph nodes of pigs experimentally infected with a European PRRSV field isolate. Our results demonstrated that the expression of S100, SWC3, HLA-DR molecule and CD3 was diminished in the studied organs throughout the study, observing a significant negative correlation between viral antigen and HLA-DR expression in both retropharyngeal and mediastinal lymph nodes. In contrast, λ-light chains showed an increase during the study. Taking all into account, after PRRSV infection, no enhancement in the number of APCs and T cells was observed, suggesting an impairment of the immune function which may allow the persistence of PRRSV into the organism.

Interleukin-2 production by dendritic cells and its immuno-regulatory functions

Dendritic cells (DCs) are uniquely potent antigen presenting cells that acquire microbial products and prime adaptive immune responses against pathogens. Furthermore, DCs also play a key role in induction and maintenance of tolerance. Although numerous studies have assessed the diverse functions of DCs, many unanswered questions remain regarding the molecular mechanisms that DCs use to achieve immunoregulation. While not widely regarded as a significant provider of T-cell growth factors, DCs have previously been identified as a potential source of IL-2 cytokine. Recent research indicates that microbes are the most effective stimuli to trigger IL-2 production in DCs by activating the calcineurin/NFAT signaling pathway. Herein we describe recent insights into the production and function of IL-2 cytokine and IL-2 receptor in DCs early after stimulation through pattern recognition receptors. These findings clarify how DCs fine-tune effector and regulatory responses by modulating IL-2 production in both tolerance and immunity.

Altered dendritic cell-natural killer interaction in Kenyan sex workers resistant to HIV-1 infection

BACKGROUND

Natural killer (NK) cells are members of the innate immune system that play an important role in the defense against viral infection. They are also involved in the regulation of adaptive immune responses through cytokine secretion and the interaction with antigen-presenting cells. However, their role in HIV infection is only partially understood.

OBJECTIVE

Here we studied the phenotype and function of NK cells of highly HIV-exposed but seronegative (HESN) uninfected commercial sex workers from Kenya who can be epidemiologically defined as relatively resistant to HIV infection.

DESIGN

The purpose of this study was to gain insight into the role of NK cells in mediating resistance to HIV-1. This information can be used to better understand protection from infection which can be used for informing future design of effective prophylactics and therapeutics for HIV.

METHODS

Whole blood samples were collected from study participants and isolated NK cells and dendritic cells were used in assays for phenotyping and cell function.

RESULTS

Activated NK cells from resistant women killed autologous immature dendritic cells more efficiently and also secreted more interferon (IFN)-γ than those of uninfected, susceptible women. Interestingly, NK cells from HIV-resistant women were significantly more effective in inducing secretion of IL-12 in immature dendritic cells.

CONCLUSIONS

These data suggest that an altered NK cell-dendritic cell interaction plays an important role in the protection from infection with HIV-1.

Recognition and prevention of tumor metastasis by the NK receptor NKp46/NCR1

NK cells employ a variety of activating receptors to kill virally infected and tumor cells. Prominent among these receptors are the natural cytotoxicity receptors (NCRs) (NKp30, NKp44, and NKp46), of which only NKp46 has a mouse ortholog (NCR1). The tumor ligand(s) of NKp46/NCR1 is still unknown, but it was shown that the human NKp46 and the mouse NCR1 are involved in tumor eradication both in vitro and in vivo. Whether any of the NK activating receptors is involved in the prevention of tumor metastasis is unknown. To address this question, we studied the activity of the NK cell receptor NKp46/NCR1 in two spontaneous metastasis models, the B16F10.9 melanoma (B16) and the Lewis lung carcinoma (D122) in the NCR1 knockout mouse that was generated by our group, in various in vitro and in vivo assays. We demonstrated that all B16 and D122 tumors, including those generated in vivo, express an unknown ligand(s) for NKp46/NCR1. We have characterized the properties of the NKp46/NCR1 ligand(s) and demonstrated that NKp46/NCR1 is directly involved in the killing of B16 and D122 cells. Importantly, we showed in vivo that NKp46/NCR1 plays an important role in controlling B16 and D122 metastasis. Thus, to our knowledge, in this study we provide the first evidence for the direct involvement of a specific NK killer receptor in preventing tumor metastasis.

Manipulation of NKG2D ligands by cytomegaloviruses: impact on innate and adaptive immune response

NKG2D is a potent activating receptor expressed on NK cells, NKT cells, γδ T cells, and CD8 T cells. NKG2D recognizes cell surface molecules structurally related to MHC class I proteins induced by infection or other type of cellular stress. The engagement of NKG2D leads to NK cell cytotoxicity and cytokine secretion or to a co-stimulation of CD8 T cells. Both human and mouse cytomegalovirus (CMV) have evolved numerous mechanisms to evade NKG2D-mediated immune response. This review describes the mechanisms used by CMV to inhibit NKG2D ligand expression and the recent advances in exploiting the NKG2D recognition pathway for mounting efficient and long-lasting immune response.

Murid herpesvirus-4 exploits dendritic cells to infect B cells

Dendritic cells (DCs) play a central role in initiating immune responses. Some persistent viruses infect DCs and can disrupt their functions in vitro. However, these viruses remain strongly immunogenic in vivo. Thus what role DC infection plays in the pathogenesis of persistent infections is unclear. Here we show that a persistent, B cell-tropic gamma-herpesvirus, Murid Herpesvirus-4 (MuHV-4), infects DCs early after host entry, before it establishes a substantial infection of B cells. DC-specific virus marking by cre-lox recombination revealed that a significant fraction of the virus latent in B cells had passed through a DC, and a virus attenuated for replication in DCs was impaired in B cell colonization. In vitro MuHV-4 dramatically altered the DC cytoskeleton, suggesting that it manipulates DC migration and shape in order to spread. MuHV-4 therefore uses DCs to colonize B cells.

We detect invading viruses with dendritic cells and eliminate them with lymphocytes. A key interaction is lymphocyte activation by dendritic cells presenting viral antigens. Not all viruses can be eliminated, and some that persist deliberately colonize lymphocytes and dendritic cells, such that parasitism and host defence co-exist within the same sites. Once established, these infections are very hard to eliminate. Therefore to vaccinate against them we must determine how infection first occurs. Here we show that a gamma-herpesvirus relation of the Kaposi's Sarcoma-associated Herpesvirus and Epstein-Barr virus - B cell-tropic human pathogens that cause cancers - uses dendritic cells to reach and infect B lymphocytes. Dendritic cells were infected before B cells; viruses marked genetically in dendritic cells were recovered from B cells; and a virus unable to replicate in dendritic cells infected B cells poorly. Thus dendritic cells not only present viral antigens to lymphocytes, but can be exploited by evasive viruses to infect lymphocytes. Therefore targeting dendritic cell infection could be an effective means of vaccine-primed host defence.

Absence of vasoactive intestinal peptide expression in hematopoietic cells enhances Th1 polarization and antiviral immunity in mice

Vasoactive intestinal peptide (VIP) induces regulatory dendritic cells (DC) in vitro that inhibit cellular immune responses. We tested the role of physiological levels of VIP on immune responses to murine CMV (mCMV) using VIP-knockout (VIP-KO) mice and radiation chimeras engrafted with syngenic VIP-KO hematopoietic cells. VIP-KO mice had less weight loss and better survival following mCMV infection compared with wild-type (WT) littermates. mCMV-infected VIP-KO mice had lower viral loads, faster clearance of virus, with increased numbers of IFN-γ(+) NK and NKT cells, and enhanced cytolytic activity of NK cells. Adaptive antiviral cellular immunity was increased in mCMV-infected VIP-KO mice compared with WT mice, with more Th1/Tc1-polarized T cells, fewer IL-10(+) T cells, and more mCMV-M45 epitope peptide MHC class I tetramer(+) CD8(+) T cells (tetramer(+) CD8 T cells). mCMV-immune VIP-KO mice had enhanced ability to clear mCMV peptide-pulsed target cells in vivo. Enhanced antiviral immunity was also seen in WT transplant recipients engrafted with VIP-KO hematopoietic cells, indicating that VIP synthesized by neuronal cells did not suppress immune responses. Following mCMV infection there was a marked upregulation of MHC-II and CD80 costimulatory molecule expression on DC from VIP-KO mice compared with DC from WT mice, whereas programmed death-1 and programmed death ligand-1 expression were upregulated in activated CD8(+) T cells and DC, respectively, in WT mice, but not in VIP-KO mice. Because the absence of VIP in immune cells increased innate and adaptive antiviral immunity by altering costimulatory and coinhibitory pathways, selective targeting of VIP signaling represents an attractive therapeutic target to enhance antiviral immunity.

Suppression of TLR9 immunostimulatory motifs in the genome of a gammaherpesvirus

Multiple receptors within the innate immune system have evolved to recognize nucleic acids as signatures of viral infection. It is believed that this specificity is essential for viral detection, as viruses often lack other invariant features that can serve as suitable targets for innate receptors. One such innate receptor, TLR9, has been implicated in the detection of many dsDNA viruses. In this study, we investigate the detection of murine gammaherpesvirus 68 (MHV68) by TLR9. We find that the genomic DNA of the murine CMV, a very potent inducer of innate responses. Genome-wide analysis of the number of stimulatory versus nonstimulatory CpG motifs present in the genome of each virus reveals that the MHV68 genome contains only a fraction of the number of immunostimulatory motifs present in murine CMV. Notably, MHV68 appears to have selectively suppressed the number of stimulatory motifs through cytosine to thymine conversion. These data suggest that certain viruses may have evolved and modified their genomic content to avoid recognition by nucleic acid-sensing receptors of the innate immune system.

Antigen-presenting cells of haematopoietic origin prime cytomegalovirus-specific CD8 T-cells but are not sufficient for driving memory inflation during viral latency

Expansion of the CD8 T-cell memory pool, also known as 'memory inflation', for certain but not all viral epitopes in latently infected host tissues is a special feature of the immune response to cytomegalovirus. The L(d)-presented murine cytomegalovirus (mCMV) immediate-early (IE) 1 peptide is the prototype of an epitope that is associated with memory inflation. Based on the detection of IE1 transcripts in latently infected lungs it was previously proposed that episodes of viral gene expression and antigenic activity due to desilencing of a limited number of viral genes may drive epitope-specific memory inflation. This would imply direct antigen presentation through latently infected host tissue cells rather than cell death-associated cross-presentation of viral antigens derived from productively infected cells through uninfected, professional antigen-presenting cells (profAPCs). To address the role of bone marrow-derived profAPCs in CD8 T-cell priming and memory to mCMV, we have used here a combined sex-mismatched and MHC class-I mismatched dual-marker bone marrow chimera model in which presentation of the IE1 epitope is restricted to donor-derived sry(+)L(d+) cells of haematopoietic differentiation lineages. Successful CD8 T-cell priming specific for the L(d)- and D(d)-presented inflationary epitopes IE1 and m164, respectively, but selective failure in IE1 epitope-specific memory inflation in these chimeras indicates different modes of antigen presentation involved in CD8 T-cell priming and memory inflation. These data suggest that memory inflation during mCMV latency requires expression of the epitope-presenting MHC class-I molecule by latently infected non-haematopoietic host tissue cells and thus predicts a role for direct antigen presentation in memory inflation.

The role of cell types in cytomegalovirus infection in vivo

Human cytomegalovirus (HCMV) is the major viral cause of morbidity in immune compromised patients and of pre- and perinatal pathology in newborns. The clinical manifestations are highly variable and the principles which govern these differences cannot be understood without detailed knowledge on tissue specific aspects of HCMV infection. For decades the role of individual cell types during cytomegalovirus infection and disease has been discussed. The pathogenesis of mouse cytomegalovirus (MCMV) mirrors the human infection in many aspects. Although only MCMV infection is studied extensively at the level of organs, the relative contribution of specific cell types to viral pathogenesis in vivo has remained enigmatic. Here we discuss new approaches based on the cre/loxP-system to label nascent virus progeny or to lift a replication block. The salient aspect of this technique is the change of viral genome properties selectively in cells that express cre during infection in vivo. This allowed us to study the role of endothelial cells and hepatocytes for virus dissemination and will permit detailed studies on innate and adaptive immune responses to CMV.

Virus-induced transient immune suppression and the inhibition of T cell proliferation by type I interferon

Vaccine-induced memory is necessary for protective immunity to pathogens, but many viruses induce a state of transient immune suppression that might contribute to the inability of a vaccine to elicit immunity. We evaluated here the fate of bystander T cells activated by third party cognate antigens during acute viral infections in vivo, using distinct models to track and specifically activate HY and P14 transgenic bystander CD8 T cells in vivo during acute arenavirus infections of mice. Viral infections acted as stimulatory adjuvants when bystander T cells were exposed to an inflammatory milieu and cognate antigens at the beginning of infections, but bystander CD8 T cell proliferation in response to cognate antigen was inhibited 3 to 9 days after virus infection. Reduced proliferation was not dependent on Fas-FasL- or tumor necrosis factor (TNF)-induced activation-induced cell death or on deficiencies of antigen presentation. Instead, reduced proliferation was associated with a delayed onset of division that was an intrinsic defect of T cells. Inhibition of proliferation could be simulated by exposure of T cells to the Toll-like receptor agonist and type I interferon (IFN) inducer poly(I · C). T cells lacking IFN-α/β receptors resisted both the suppressive effects of preexposure to poly(I · C) and the stimulatory effects of type I IFN, indicating that the timing of exposure to IFN can have negative or positive effects on T cell proliferation. Inhibition of T cell receptor-stimulated bystander CD8 T cell proliferation during acute viral infections may reflect the reduced ability of vaccines to elicit protective immunity when administered during an acute illness.

Self MHC class I-licensed NK cells enhance adaptive CD8 T-cell viral immunity

MHC class I (MHC I) is essential to NK- and T-cell effector and surveillance functions. However, it is unknown whether MHC I polymorphism influences adaptive immunity through NK cells. Previously, we found that MHC I D(k), a cognate ligand for the Ly49G2 inhibitory receptor, was essential to NK control of murine (M)CMV infection. Here we assessed the significance of NK inhibitory receptor recognition of MCMV on CD8 T cells in genetically defined MHC I D(k) disparate mice. We observed that D(k)-licensed Ly49G2⁺ NK cells stabilized and then enhanced conventional dendritic cells (cDCs) recovery after infection. Furthermore, licensed NK support of cDC recovery was essential to enhance the tempo, magnitude, and effector activity of virus-specific CD8 T cells. Minimal cDC and CD8 T-cell number differences after low-dose MCMV in D(k) disparate animals further implied that licensed NK recognition of MCMV imparted qualitative cDC changes to enhance CD8 T-cell priming.