Innate immune responses to LCMV infections: natural killer cells and cytokines

Advances in CAR-NK cell therapy for hematological malignancies

Chimeric antigen receptor T (CAR-T) cell therapy has revolutionized the treatment of hematological malignancies, demonstrably improving patient outcomes and prognosis. However, its application has introduced new challenges, such as safety concerns, off-target toxicities, and significant costs. Natural killer (NK) cells are crucial components of the innate immune system, capable of eliminating tumor cells without prior exposure to specific antigens or pre-activation. This inherent advantage complements the limitations of T cells, making CAR-NK cell therapy a promising avenue for hematological tumor immunotherapy. In recent years, preclinical and clinical studies have yielded preliminary evidence supporting the safety and efficacy of CAR-NK cell therapy in hematological malignancies, paving the way for future advancements in immunotherapy. This review aims to succinctly discuss the characteristics, significant therapeutic progress, and potential challenges associated with CAR-NK cell therapy.

Restoration of type I interferon signaling in intrahepatically primed CD8+ T cells promotes functional differentiation

Hepatitis B virus–specific (HBV-specific) CD8⁺ T cells fail to acquire effector functions after priming in the liver, but the molecular basis for the dysfunction is poorly understood. By comparing the gene expression profile of intrahepatically primed, dysfunctional HBV-specific CD8⁺ T cells with that of systemically primed, functional effector counterparts, we found that the expression of interferon-stimulated genes (ISGs) is selectively suppressed in the dysfunctional CD8⁺ T cells. The ISG suppression was associated with impaired phosphorylation of STAT1 in response to IFN-α treatment. Importantly, a strong induction of type I interferons (IFN-Is) in the liver facilitated the functional differentiation of intrahepatically primed HBV-specific CD8⁺ T cells in association with the restoration of ISGs’ expression in the T cells. These results suggest that intrahepatic priming suppresses IFN-I signaling in CD8⁺ T cells, which may contribute to the dysfunction. The data also suggest a therapeutic value of the robust induction of intrahepatic IFN-Is for the treatment of chronic HBV infection.

Functional defects of hepatitis B virus–specific CD8⁺ T cells correlate with impairment of type I interferon signaling after intrahepatic priming.

NK cell-intrinsic FcεRIγ limits CD8+ T-cell expansion and thereby turns an acute into a chronic viral infection

During viral infection, tight regulation of CD8⁺ T-cell functions determines the outcome of the disease. Recently, others and we determined that the natural killer (NK) cells kill hyperproliferative CD8⁺ T cells in the context of viral infection, but molecules that are involved in shaping the regulatory capability of NK cells remain virtually unknown. Here we used mice lacking the Fc-receptor common gamma chain (FcRγ, FcεRIγ, Fcer1g^–/– mice) to determine the role of Fc-receptor and NK-receptor signaling in the process of CD8⁺ T-cell regulation. We found that the lack of FcRγ on NK cells limits their ability to restrain virus-specific CD8⁺ T cells and that the lack of FcRγ in Fcer1g^–/– mice leads to enhanced CD8⁺ T-cell responses and rapid control of the chronic docile strain of the lymphocytic choriomeningitis virus (LCMV). Mechanistically, FcRγ stabilized the expression of NKp46 but not that of other killer cell–activating receptors on NK cells. Although FcRγ did not influence the development or activation of NK cell during LCMV infection, it specifically limited their ability to modulate CD8⁺ T-cell functions. In conclusion, we determined that FcRγ plays an important role in regulating CD8⁺ T-cell functions during chronic LCMV infection.

FcRγ is a signaling molecule for Fc receptors and NK cell killer activating receptor (KAR) complex. FcRγ is highly expressed by NK cells and involved in NK cell activity. NK cells are widely defined to regulate the expansion of T cells. Here using chronic LCMV model, we described the role of FcRγ in NK cell mediated shaping of CD8⁺ T cell response and viral control. We observed that FcRγ does not affect the early activity of NK cells which is mainly innate immune cytokines driven, but rather the specific activation due to NKp46 inadequacy. We detected that FcRγ stabilizes NKp46 protein by preventing it from proteasomal degradation. Due to lack of NKp46 expression in absence of FcRγ, we observed strong CD8⁺ T cell response and faster viral clearance during chronic LCMV infection. These data demonstrate that FcRγ is crucial for specific activity of NK cells for regulation of CD8⁺ T cell response during viral infection.

Viral Strain Determines Disease Symptoms, Pathology, and Immune Response in Neonatal Rats with Lymphocytic Choriomeningitis Virus Infection

When infection with lymphocytic choriomeningitis (LCMV) occurs during pregnancy, the virus can infect the fetus and injure the fetal brain. However, type, location, and severity of neuropathology differ among cases. One possible explanation for this diversity is that fetuses are infected with different viral strains. Using a rat model of congenital LCMV infection, we investigated how differences in LCMV strain (E350, WE2.2, and Clone 13) affect outcome. Rat pups received intracranial inoculations on postnatal day 4. E350 initially targeted glial cells, while WE2.2 and Clone 13 targeted neurons. The E350 strain induced focal destructive lesions, while the other strains induced global microencephaly. E350 attracted large numbers of CD8+ lymphocytes early in the disease course, while Clone 13 attracted CD4+ lymphocytes, and the infiltration occurred late. The E350 and WE2.2 strains induced large increases in expression of pro-inflammatory cytokines, while Clone 13 did not. The animals infected with E350 and WE2.2 became ataxic and performed poorly on the negative geotaxis assay, while the Clone 13 animals had profound growth failure. Thus, in the developing brain, different LCMV strains have different patterns of infection, neuropathology, immune responses and disease symptoms. In humans, different outcomes from congenital LCMV may reflect infection with different strains.

The PD-1/PD-L1 Axis and Virus Infections: A Delicate Balance

Programmed cell death protein (PD-1) and its ligands play a fundamental role in the evasion of tumor cells from antitumor immunity. Less well appreciated is the fact that the PD-1/PD-L1 axis also regulates antiviral immune responses and is therefore modulated by a number of viruses. Upregulation of PD-1 and its ligands PD-L1 and PD-L2 is observed during acute virus infection and after infection with persistent viruses including important human pathogens such as human immunodeficiency virus (HIV), hepatitis C virus (HCV), and hepatitis B virus (HBV). Experimental evidence suggests that insufficient signaling through the PD-1 pathway promotes immunopathology during acute infection by exaggerating primary T cell responses. If chronic infection is established, however, high levels of PD-1 expression can have unfavorable immunological consequences. Exhaustion and suppression of antiviral immune responses can result in viral immune evasion. The role of the PD-1/PD-L1 axis during viral infections is further complicated by evidence that PD-L1 also mediates inflammatory effects in the acute phase of an immune response. In this review, we discuss the intricate interplay between viruses and the PD-1/PD-L1 axis.

Complexities of Type I Interferon Biology: Lessons from LCMV

Over the past decades, infection of mice with lymphocytic choriomeningitis virus (LCMV) has provided an invaluable insight into our understanding of immune responses to viruses. In particular, this model has clarified the central roles that type I interferons play in initiating and regulating host responses. The use of different strains of LCMV and routes of infection has allowed us to understand how type I interferons are critical in controlling virus replication and fostering effective antiviral immunity, but also how they promote virus persistence and functional exhaustion of the immune response. Accordingly, these discoveries have formed the foundation for the development of novel treatments for acute and chronic viral infections and even extend into the management of malignant tumors. Here we review the fundamental insights into type I interferon biology gained using LCMV as a model and how the diversity of LCMV strains, dose, and route of administration have been used to dissect the molecular mechanisms underpinning acute versus persistent infection. We also identify gaps in the knowledge regarding LCMV regulation of antiviral immunity. Due to its unique properties, LCMV will continue to remain a vital part of the immunologists' toolbox.

The lipid-sensor TREM2 aggravates disease in a model of LCMV-induced hepatitis

Lipid metabolism is increasingly being appreciated to affect immunoregulation, inflammation and pathology. In this study we found that mice infected with lymphocytic choriomeningitis virus (LCMV) exhibit global perturbations of circulating serum lipids. Mice lacking the lipid-sensing surface receptor triggering receptor expressed on myeloid cells 2 (Trem2 ^-/-) were protected from LCMV-induced hepatitis and showed improved virus control despite comparable virus-specific T cell responses. Non-hematopoietic expression of TREM2 was found to be responsible for aggravated hepatitis, indicating a novel role for TREM2 in the non-myeloid compartment. These results suggest a link between virus-perturbed lipids and TREM2 that modulates liver pathogenesis upon viral infection. Targeted interventions of this immunoregulatory axis may ameliorate tissue pathology in hepatitis.

Inhibition of Innate Immune Responses Is Key to Pathogenesis by Arenaviruses

Mammalian arenaviruses are zoonotic viruses that cause asymptomatic, persistent infections in their rodent hosts but can lead to severe and lethal hemorrhagic fever with bleeding and multiorgan failure in human patients. Lassa virus (LASV), for example, is endemic in several West African countries, where it is responsible for an estimated 500,000 infections and 5,000 deaths annually. There are currently no FDA-licensed therapeutics or vaccines available to combat arenavirus infection. A hallmark of arenavirus infection (e.g., LASV) is general immunosuppression that contributes to high viremia. Here, we discuss the early host immune responses to arenavirus infection and the recently discovered molecular mechanisms that enable pathogenic viruses to suppress host immune recognition and to contribute to the high degree of virulence. We also directly compare the innate immune evasion mechanisms between arenaviruses and other hemorrhagic fever-causing viruses, such as Ebola, Marburg, Dengue, and hantaviruses. A better understanding of the immunosuppression and immune evasion strategies of these deadly viruses may guide the development of novel preventative and therapeutic options.

STAT2 Knockout Syrian Hamsters Support Enhanced Replication and Pathogenicity of Human Adenovirus, Revealing an Important Role of Type I Interferon Response in Viral Control

Human adenoviruses have been studied extensively in cell culture and have been a model for studies in molecular, cellular, and medical biology. However, much less is known about adenovirus replication and pathogenesis in vivo in a permissive host because of the lack of an adequate animal model. Presently, the most frequently used permissive immunocompetent animal model for human adenovirus infection is the Syrian hamster. Species C human adenoviruses replicate in these animals and cause pathology that is similar to that seen with humans. Here, we report findings with a new Syrian hamster strain in which the STAT2 gene was functionally knocked out by site-specific gene targeting. Adenovirus-infected STAT2 knockout hamsters demonstrated an accentuated pathology compared to the wild-type control animals, and the virus load in the organs of STAT2 knockout animals was 100- to 1000-fold higher than that in wild-type hamsters. Notably, the adaptive immune response to adenovirus is not adversely affected in STAT2 knockout hamsters, and surviving hamsters cleared the infection by 7 to 10 days post challenge. We show that the Type I interferon pathway is disrupted in these hamsters, revealing the critical role of interferon-stimulated genes in controlling adenovirus infection. This is the first study to report findings with a genetically modified Syrian hamster infected with a virus. Further, this is the first study to show that the Type I interferon pathway plays a role in inhibiting human adenovirus replication in a permissive animal model. Besides providing an insight into adenovirus infection in humans, our results are also interesting from the perspective of the animal model: STAT2 knockout Syrian hamster may also be an important animal model for studying other viral infections, including Ebola-, hanta-, and dengue viruses, where Type I interferon-mediated innate immunity prevents wild type hamsters from being effectively infected to be used as animal models.

Changing partners at the dance: Variations in STAT concentrations for shaping cytokine function and immune responses to viral infections

Differential use of cellular and molecular components shapes immune responses, but understanding of how these are regulated to promote defense and health during infections is still incomplete. Examples include signaling from members of the Janus activated kinase-signal transducer and activator of transcription (JAK-STAT) cytokine family. Following receptor stimulation, individual JAK-STAT cytokines have preferences for particular key STAT molecules to lead to specific cellular responses. Certain of these cytokines, however, can conditionally activate alternative STATs as well as elicit pleiotropic and paradoxical effects. Studies examining basal and infection conditions are revealing intrinsic and induced cellular differences in various intracellular STAT concentrations to control the biological consequences of cytokine exposure. The system can be likened to changing partners at a dance based on competition and relative availability, and sets a framework for understanding the particular conditions promoting subset biological functions of cytokines as needed during evolving immune responses to infections.

Intrahepatic innate lymphoid cells secrete IL-17A and IL-17F that are crucial for T cell priming in viral infection

Intrahepatic cell-derived, early IL-17 is important for activating APCs in viral infection; however, the source and regulation of this IL-17 surge in the liver microenvironment are not well defined. In this article, we present evidence for a significant expansion of IL-17A/F-producing cells in mouse liver within 24 h of adenovirus infection. In addition to γδ T cells, a subset of IL-17A/F(+) cells expressed no myeloid or lymphoid lineage markers. Instead, they expressed high levels of stem cell markers, IL-7R and RORγt, consistent with the newly described innate lymphoid cells (ILCs). Based on their unique surface markers and cytokine profiles, these cells were confirmed as group 3 ILCs. In addition to adenovirus infection, group 3 ILCs were also found in mouse liver within 24 h of lymphocytic choriomeningitis virus infection. They contributed significantly to the establishment of the early cytokine milieu in virus-infected liver. Functional studies with mice deficient of IL-17R, IL-17A, and IL-17F further revealed that IL-17 signaling was critical for priming T cell responses in viral hepatitis. IL-17A repressed IL-17F secretion in vitro and in vivo; IL-17F(+) intrahepatic cells expanded more vigorously in IL-17A knockout animals, permitting efficient Ag presentation and T cell function. However, IL-17F neither inhibited IL-17A in vitro nor regulated its secretion in vivo. Together, this study has demonstrated the importance of a unique intrahepatic subpopulation and subsequent IL-17A/F regulation at initial stages of viral infection in the liver. These results have important implications for anticytokine biologic therapy and vaccine development.

Role of natural killer and Gamma-delta T cells in West Nile virus infection

Natural Killer (NK) cells and Gamma-delta T cells are both innate lymphocytes that respond rapidly and non-specifically to viral infection and other pathogens. They are also known to form a unique link between innate and adaptive immunity. Although they have similar immune features and effector functions, accumulating evidence in mice and humans suggest these two cell types have distinct roles in the control of infection by West Nile virus (WNV), a re-emerging pathogen that has caused fatal encephalitis in North America over the past decade. This review will discuss recent studies on these two cell types in protective immunity and viral pathogenesis during WNV infection.

Immune responses and Lassa virus infection

Lassa fever is a hemorrhagic fever endemic to West Africa and caused by Lassa virus, an Old World arenavirus. It may be fatal, but most patients recover from acute disease and some experience asymptomatic infection. The immune mechanisms associated with these different outcomes have not yet been fully elucidated, but considerable progress has recently been made, through the use of in vitro human models and nonhuman primates, the only relevant animal model that mimics the pathophysiology and immune responses induced in patients. We discuss here the roles of the various components of the innate and adaptive immune systems in Lassa virus infection and in the control of viral replication and pathogenesis.

Arenavirus evasion of host anti-viral responses

The innate response to infection by an Old World arenavirus is initiated and mediated by extracellular and intracellular receptors, and effector molecules. In response, the invading virus has evolved to inhibit these responses and create the best environment possible for replication and spread. Here, we will discuss both the host's response to infection with data from human infection and lessons learned from animal models, as well as the multitude of ways the virus combats the resulting immune response. Finally, we will highlight recent work identifying TLR2 as an innate sensor for arenaviruses and how the TLR2-dependent response differs depending on the pathogenicity of the strain.

Plasmacytoid dendritic cells are productively infected and activated through TLR-7 early after arenavirus infection

The antiviral response is largely mediated by dendritic cells (DCs), including conventional (c) DCs that function as antigen-presenting cells, and plasmacytoid (p) DCs that produce type I interferons, making them an attractive target for viruses. We find that the Old World arenaviruses lymphocytic choriomeningitis virus clone 13 (LCMV Cl13) and Lassa virus bind pDCs to a greater extent than cDCs. Consistently, LCMV Cl13 targets pDCs early after in vivo infection of its natural murine host and establishes a productive and robust replication cycle. pDCs coproduce type I interferons and proinflammatory cytokines, with the former being induced in both infected and uninfected pDCs, demonstrating a dissociation from intrinsic virus replication. TLR7 globally mediates pDC responses, limits pDC viral load, and promotes rapid innate and adaptive immune cell activation. These early events likely help dictate the outcome of infections with arenaviruses and other DC-replicating viruses and shed light on potential therapeutic targets.

Natural killer cell activation enhances immune pathology and promotes chronic infection by limiting CD8+ T-cell immunity

Infections with HIV, hepatitis B virus, and hepatitis C virus can turn into chronic infections, which currently affect more than 500 million patients worldwide. It is generally thought that virus-mediated T-cell exhaustion limits T-cell function, thus promoting chronic disease. Here we demonstrate that natural killer (NK) cells have a negative impact on the development of T-cell immunity by using the murine lymphocytic choriomeningitis virus. NK cell-deficient (Nfil3(-/-), E4BP4(-/-)) mice exhibited a higher virus-specific T-cell response. In addition, NK cell depletion caused enhanced T-cell immunity in WT mice, which led to rapid virus control and prevented chronic infection in lymphocytic choriomeningitis virus clone 13- and reduced viral load in DOCILE-infected animals. Further experiments showed that NKG2D triggered regulatory NK cell functions, which were mediated by perforin, and limited T-cell responses. Therefore, we identified an important role of regulatory NK cells in limiting T-cell immunity during virus infection.

Inhibition of the type I interferon antiviral response during arenavirus infection

Arenaviruses merit interest both as tractable experimental model systems to study acute and persistent viral infections, and as clinically-important human pathogens. Several arenaviruses cause hemorrhagic fever (HF) disease in humans. In addition, evidence indicates that the globally-distributed prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a human pathogen of clinical significance in congenital infections, and also poses a great danger to immunosuppressed individuals. Arenavirus persistence and pathogenesis are facilitated by their ability to overcome the host innate immune response. Mammalian hosts have developed both membrane toll-like receptors (TLR) and cytoplasmic pattern recognition receptors (PRRs) that recognize specific pathogen-associated molecular patterns (PAMPs), resulting in activation of the transcription factors IRF3 or IRF7, or both, which together with NF-κB and ATF-2/c-JUN induce production of type I interferon (IFN-I). IFN-I plays a key role in host anti-microbial defense by mediating direct antiviral effects via up-regulation of IFN-I stimulated genes (ISGs), activating dendritic cells (DCs) and natural killer (NK) cells, and promoting the induction of adaptive responses. Accordingly, viruses have developed a plethora of strategies to disrupt the IFN-I mediated antiviral defenses of the host, and the viral gene products responsible for these disruptions are often major virulence determinants. IRF3- and IRF7-dependent induction of host innate immune responses is frequently targeted by viruses. Thus, the arenavirus nucleoprotein (NP) was shown to inhibit the IFN-I response by interfering with the activation of IRF3. This NP anti-IFN activity, together with alterations in the number and function of DCs observed in mice chronically infected with LCMV, likely play an important role in LCMV persistence in its murine host. In this review we will discuss current knowledge about the cellular and molecular mechanisms by which arenaviruses can subvert the host innate immune response and their implications for understanding HF arenaviral disease as well as arenavirus persistence in their natural hosts.

Role of PKR and Type I IFNs in viral control during primary and secondary infection

Type I interferons (IFNs) are known to mediate viral control, and also promote survival and expansion of virus-specific CD8+ T cells. However, it is unclear whether signaling cascades involved in eliciting these diverse cellular effects are also distinct. One of the best-characterized anti-viral signaling mechanisms of Type I IFNs is mediated by the IFN-inducible dsRNA activated protein kinase, PKR. Here, we have investigated the role of PKR and Type I IFNs in regulating viral clearance and CD8+ T cell response during primary and secondary viral infections. Our studies demonstrate differential requirement for PKR, in viral control versus elicitation of CD8+ T cell responses during primary infection of mice with lymphocytic choriomeningitis virus (LCMV). PKR-deficient mice mounted potent CD8+ T cell responses, but failed to effectively control LCMV. The compromised LCMV control in the absence of PKR was multifactorial, and linked to less effective CD8+ T cell-mediated viral suppression, enhanced viral replication in cells, and lower steady state expression levels of IFN-responsive genes. Moreover, we show that despite normal expansion of memory CD8+ T cells and differentiation into effectors during a secondary response, effective clearance of LCMV but not vaccinia virus required PKR activity in infected cells. In the absence of Type I IFN signaling, secondary effector CD8+ T cells were ineffective in controlling both LCMV and vaccinia virus replication in vivo. These findings provide insight into cellular pathways of Type I IFN actions, and highlight the under-appreciated importance of innate immune mechanisms of viral control during secondary infections, despite the accelerated responses of memory CD8+ T cells. Additionally, the results presented here have furthered our understanding of the immune correlates of anti-viral protective immunity, which have implications in the rational design of vaccines.

Circulating natural killer and gammadelta T cells decrease soon after infection of rhesus macaques with lymphocytic choriomeningitis virus

Rhesus macaques infected with the WE strain of lymphocytic choriomeningitis virus (LCMV-WE) serve as a model for human infection with Lassa fever virus. To identify the earliest events of acute infection, rhesus macaques were monitored immediately after lethal infection for changes in peripheral blood mononuclear cells (PBMCs). Changes in CD3, CD4, CD8 and CD20 subsets did not vary outside the normal fluctuations of these blood cell populations; however, natural killer (NK) and gammadelta T cells increased slightly on day 1 and then decreased significantly after two days. The NK subsets responsible for the decrease were primarily CD3-CD8+ or CD3-CD16+ and not the NKT (primarily CD3+CD56+) subset. Macaques infected with a non-virulent arenavirus, LCMV-Armstrong, showed a similar drop in circulating NK and gammadelta T cells, indicating that this is not a pathogenic event. V(3)9 T cells, representing the majority of circulating gammadelta T cells in rhesus macaques, displayed significant apoptosis when incubated with LCMV in cell culture; however, the low amount of cell death for virus-co-cultured NK cells was insufficient to account for the observed disappearance of this subset. Our observations in primates are similar to those seen in LCMV-infected mice, where decreased circulating NK cells were attributed to margination and cell death. Thus, the disappearance of these cells during acute hemorrhagic fever in rhesus macaques may be a cytokine-induced lymphopenia common to many virus infections.

Polymorphisms of KIRs gene and HLA-C alleles in patients with ankylosing spondylitis: possible association with susceptibility to the disease

INTRODUCTION

An emerging body of evidence is accumulating to suggest that killer cell immunoglobulin-like receptors (KIRs) and human leukocyte antigen (HLA) class I ligands contribute to the pathogenesis of diverse kinds of autoimmune diseases. However, the functional effects of their polymorphism remain largely unknown to date. Thus, the present study was undertaken to determine the association of the polymorphisms KIRs gene and HLA-C alleles with the susceptibility to ankylosing spondylitis (AS) by means of polymerase chain reaction/sequence-specific primers for genotyping KIRs from genomic DNA of 119 patients with AS together with 128 healthy donors as a control group.

RESULTS AND DISCUSSION

We found that the frequencies of KIR3DS1 and KIR2DL5 were statistically significantly higher in the patient group than those in the control group (P = 0.016 and P = 0.003, respectively). Meanwhile, the percentage of patients, who were carrying two or more of the activating KIRs, was higher than that of control group. With respect to HLA-C alleles, individuals with AS showed an increased frequency of HLA-Cw02. If HLA-C was divided into group 1 or group 2 based on whether there was an asparagine or lysine present at position 80 of the alpha-chain, HLA-C group 2 was more common in subjects with AS compared to control subjects. The genotype 2DS1+/HLA-C lys(80)+ was more common in subjects with AS. Moreover, the CD69 expression, a NK activation marker, remarkably increased in patient with AS.

CONCLUSION

In conclusions, this study suggests that KIR3DS1 may severe as AS susceptive genes to trigger continuous injury of arthrosis. The imbalance of activating and inhibitory KIR as well as HLA-C group 1 and group 2 may be the key factor, which influences the pathogenesis of AS. Moreover, KIR2DS1 might associate with the susceptibility of AS by influencing NK cell activity once group 2 HLA-C ligands are present.

Congenital viral infections of the brain: lessons learned from lymphocytic choriomeningitis virus in the neonatal rat

The fetal brain is highly vulnerable to teratogens, including many infectious agents. As a consequence of prenatal infection, many children suffer severe and permanent brain injury and dysfunction. Because most animal models of congenital brain infection do not strongly mirror human disease, the models are highly limited in their abilities to shed light on the pathogenesis of these diseases. The animal model for congenital lymphocytic choriomeningitis virus (LCMV) infection, however, does not suffer from this limitation. LCMV is a well-known human pathogen. When the infection occurs during pregnancy, the virus can infect the fetus, and the developing brain is particularly vulnerable. Children with congenital LCMV infection often have substantial neurological deficits. The neonatal rat inoculated with LCMV is a superb model system of human congenital LCMV infection. Virtually all of the neuropathologic changes observed in humans congenitally infected with LCMV, including microencephaly, encephalomalacia, chorioretinitis, porencephalic cysts, neuronal migration disturbances, periventricular infection, and cerebellar hypoplasia, are reproduced in the rat model. Within the developing rat brain, LCMV selectively targets mitotically active neuronal precursors. Thus, the targets of infection and sites of pathology depend on host age at the time of infection. The rat model has further shown that the pathogenic changes induced by LCMV infection are both virus-mediated and immune-mediated. Furthermore, different brain regions simultaneously infected with LCMV can undergo widely different pathologic changes, reflecting different brain region-virus-immune system interactions. Because the neonatal rat inoculated with LCMV so faithfully reproduces the diverse neuropathology observed in the human counterpart, the rat model system is a highly valuable tool for the study of congenital LCMV infection and of all prenatal brain infections In addition, because LCMV induces delayed-onset neuronal loss after the virus has been cleared, the neonatal rat infected with LCMV may be an excellent model system to study neurodegenerative or psychiatric diseases whose etiologies are hypothesized to be virus-induced, such as autism, schizophrenia, and temporal lobe epilepsy.

Phenotypic and functional changes of cytotoxic CD56pos natural T cells determine outcome of acute hepatitis C virus infection

Innate CD56(pos) natural killer (NK) and natural T (NT) cells comprise important hepatic antiviral effector lymphocytes whose activity is fine-tuned through surface NK receptors (NKRs). Dysregulation of NKRs in patients with long-standing hepatitis C virus (HCV) infection has been shown, but little is known regarding NKRs in acute infection. Treatment-naïve patients with acute HCV (n = 22), including 10 with spontaneous recovery, were prospectively studied. CD56(pos) NT levels were reduced early in acute HCV infection and did not fluctuate over time. In resolving HCV infection, NT cells with a more activated phenotype (lower CD158A and higher natural cytotoxicity receptor expression) at baseline predated spontaneous recovery. Moreover, NKG2A expression on CD56(+) NT cells correlated directly with circulating HCV RNA levels. Deficient interleukin-13 (IL-13) production by NT cells and reduced IL-2-activated killing (LAK) at baseline were associated with the ultimate development of persistence. These results indicate a previously unappreciated role for NT cells in acute HCV infection and identify a potential target for pharmacologic manipulation.

Stage of primary infection with lymphocytic choriomeningitis virus determines predisposition or resistance of mice to secondary bacterial infections

We investigated the effect of a primary non-lethal infection with lymphocytic choriomeningitis virus (LCMV) on the course and outcome of a secondary infection with the Gram-negative Salmonella enterica serovar Typhimurium or the Gram-positive Listeria monocytogenes in mice. We found that at each stage of the viral infection the susceptibility of mice to bacterial super-infections changes dramatically and depends also on whether the secondary infection is a Gram-positive or Gram-negative one. The study shows that the outcome of the secondary infection is determined by a delicate balance between the overproduction of and the hypersensitivity to inflammatory cytokines (TNF-alpha and IFN-gamma), as well as by the changes in blood leukocytes occurring in mice in the course of viral infection.

A-to-G hypermutation in the genome of lymphocytic choriomeningitis virus

The interferon-inducible adenosine deaminase that acts on double-stranded RNA (ADAR1-L) has been proposed to be one of the antiviral effector proteins within the complex innate immune response. Here, the potential role of ADAR1-L in the innate immune response to lymphocytic choriomeningitis virus (LCMV), a widely used virus model, was studied. Infection with LCMV clearly upregulated ADAR1-L expression and activity. The editing activity of ADAR1-L on an RNA substrate was not inhibited by LCMV replication. Accordingly, an adenosine-to-guanosine (A-to-G) and uracil-to-cytidine (U-to-C) hypermutation pattern was found in the LCMV genomic RNA in infected cell lines and in mice. In addition, two hypermutated clones with a high level of A-to-G or U-to-C mutations within a short stretch of the viral genome were isolated. Analysis of the functionality of viral glycoprotein revealed that A-to-G- and U-to-C-mutated LCMV genomes coded for nonfunctional glycoprotein at a surprisingly high frequency. Approximately half the GP clones with an amino acid mutation lacked functionality. These results suggest that ADAR1-L-induced mutations in the viral RNA lead to a loss of viral protein function and reduced viral infectivity. This study therefore provides strong support for the contribution of ADAR1-L to the innate antiviral immune response.

Complementary activation of peripheral natural killer cell immunity in nasopharyngeal carcinoma

NK cells and alphabeta- and gammadelta-CTL play important roles in cellular immunity against tumors. We previously demonstrated that NPC patients have a quantitative and qualitative deficit in gammadelta-CTL and EBV-specific alphabeta-CTL when compared to normal subjects and NPC long-term survivors. In this study we report further observations of a complementary activation of peripheral NK cells in NPC patients. The NK cells in these patients, compared to those of healthy subjects and NPC survivors, were preferentially activated in response to the stimulation of myeloma cell line XG-7 and expanded in the presence of exogenous IL-2. The production of IFN-gamma was lowest in the patient group, whereas IL-12, IL-15 and TNF-alpha were produced in higher levels in patients than in the donors and survivors. The cytolytic effect of the NK cells against NPC cells in the patient group was also higher than that of the donors and survivors. Furthermore, the patients at later stages of NPC had lower gammadelta-CTL activity but higher NK cytotoxicity towards NPC targets, with higher production of IL-12, IL-15 and TNF-alpha but lower production of IFN-gamma than in patients at earlier stages. This might be part of a triggered compensatory re-activation of the innate immunity, believed to be mediated through various cytokines and chemokines when adaptive T cell immunity is breached. Together, these data suggest complementary roles of innate and adaptive immune response in tumor immunity where NK cells, gammadelta- and alphabeta-CTL compensate for the deficits of one another at different stages of tumor invasion.

Natural killer-dendritic cell cross-talk in cancer immunotherapy

Natural killer (NK) cells and dendritic cells (DCs), two important components of the immune system, can exchange bidirectional activating signals in a positive feedback. Myeloid DCs, the cell type specialised in the presentation of antigen and initiation of antigen-specific immune responses, have recently been documented to be involved in supporting innate immunity, promoting the production of cytokines and cytotoxicity of NK cells, and enhancing their tumouricidal activity. Natural interferon-producing cells/plasmacytoid DCs (IPCs/PDCs) play an additional role in NK cell activation. Reciprocally, NK cells, traditionally considered to be major innate effector cells, have also recently been shown to play immunoregulatory 'helper' functions, being able to activate DCs and to enhance their ability to produce pro-inflammatory cytokines, and to stimulate T helper (Th) 1 and cytotoxic T lymphocyte (CTL) responses of tumour-specific CD4+ and CD8+ T cells. Activated NK cells induce the maturation of myeloid DCs into stable type-1 polarised DCs (DC1), characterised by up to a 100-fold enhanced ability to produce IL-12p70 in response to subsequent interaction with Th cells. In addition, the ability of NK cells to kill tumour cells may facilitate the generation of tumour-related antigenic material, further accelerating the induction of tumour-specific immunity. DC1, induced by NK cells or by NK cell-related soluble factors, are stable, resistant to tumour-related suppressive factors, and demonstrate a strongly enhanced ability to induce Th1 and CTL responses in human in vitro and mouse in vivo models. Compared with the standard mature DCs that are used in clinical trials at present, human NK cell-induced DC1s act as superior inducers of anticancer CTL responses during in vitro sensitisation. This provides a strong rationale for the combined use of NK cells and DCs in the immunotherapy of patients with cancer and patients with chronic infections that are resistant to standard forms of treatment. Stage I/II clinical trials that are being implemented at present should allow evaluation of the immunological and clinical efficacy of combined NK-DC therapy of melanoma and other cancers.

Shared alterations in NK cell frequency, phenotype, and function in chronic human immunodeficiency virus and hepatitis C virus infections

Human immunodeficiency virus (HIV) and hepatitis C virus (HCV) cause clinically important persistent infections. The effects of virus persistence on innate immunity, including NK cell responses, and the underlying mechanisms are not fully understood. We examined the frequency, phenotype, and function of peripheral blood CD3- CD56+ NK subsets in HIV+ and HCV+ patients and identified significantly reduced numbers of total NK cells and a striking shift in NK subsets, with a marked decrease in the CD56(dim) cell fraction compared to CD56(bright) cells, in both infections. This shift influenced the phenotype and functional capacity (gamma interferon production, killing) of the total NK pool. In addition, abnormalities in the functional capacity of the CD56(dim) NK subset were observed in HIV+ patients. The shared NK alterations were found to be associated with a significant reduction in serum levels of the innate cytokine interleukin 15 (IL-15). In vitro stimulation with IL-15 rescued NK cells of HIV+ and HCV+ patients from apoptosis and enhanced proliferation and functional activity. We hypothesize that the reduced levels of IL-15 present in the serum during HIV and HCV infections might impact NK cell homeostasis, contributing to the common alterations of the NK pool observed in these unrelated infections.

Type I interferons act directly on CD8 T cells to allow clonal expansion and memory formation in response to viral infection

T cell expansion and memory formation are generally more effective when elicited by live organisms than by inactivated vaccines. Elucidation of the underlying mechanisms is important for vaccination and therapeutic strategies. We show that the massive expansion of antigen-specific CD8 T cells that occurs in response to viral infection is critically dependent on the direct action of type I interferons (IFN-Is) on CD8 T cells. By examining the response to infection with lymphocytic choriomeningitis virus using IFN-I receptor–deficient (IFN-IR⁰) and –sufficient CD8 T cells adoptively transferred into normal IFN-IR wild-type hosts, we show that the lack of direct CD8 T cell contact with IFN-I causes >99% reduction in their capacity to expand and generate memory cells. The diminished expansion of IFN-IR⁰ CD8 T cells was not caused by a defect in proliferation but by poor survival during the antigen-driven proliferation phase. Thus, IFN-IR signaling in CD8 T cells is critical for the generation of effector and memory cells in response to viral infection.

Natural killer cell receptors: regulating innate immune responses to hematologic malignancy

Critical to innate immunity, the natural killer (NK) cell performs its function of immunosurveillance through its recognition of altered or missing self on damaged, infected, or transformed malignant cells. NK cell receptors responsible for detection of human leukocyte antigen (HLA) class I and class I-like proteins on potential target cells transmit inhibitory and activating signals that integrate to determine NK cell function. Advances in the fields of NK cell receptor biology and immunogenetics have enhanced our understanding of NK cell target recognition and may now guide studies to determine NK cell effects in the clinical setting. Analysis of NK cell receptor-ligand relationships, such as the inhibitory killer immunoglobulin-like receptors (KIRs) and their HLA class I ligands, has revealed the potential for NK cell-mediated benefit in allogeneic hematopoietic stem cell transplantation for hematologic malignancies.

Response of aged mice to primary virus infections

Aging is associated with an increased morbidity to virus infections as well as a delay in clearance of symptoms after infection. Studies of sublethal virus infections of aged mice closely mirror the human situation: there is a delay in clearance of virus. The delay in virus clearance is accompanied by a delay and a decrease in T-cell response, particularly of CD8(+) T cells. Intrinsic alterations of T cells of aged mice contribute to this decrease in virus-specific T-cell response; however, evidence suggests that environmental or innate components of the aged host also influence this age-associated decline in clearance of virus. While the changes in the adaptive immune response have been carefully described, the early events in the generation of the T-cell response after virus infection have received limited attention. Importantly, age-associated changes in the innate response to virus infection, particularly production of and response to interferon (IFN)-alpha/beta, cytotoxicity and IFN-gamma production by natural killer cells, interleukin-12 induction, and depletion of non-specific T cells early during virus infection need further evaluation.

MyD88 is critical for the development of innate and adaptive immunity during acute lymphocytic choriomeningitis virus infection

We investigated the roles of Toll-like receptor 2 (TLR2) and myeloid differentiation factor 88 (MyD88) in the course of a lymphocytic choriomeningitis virus (LCMV) infection and revealed the following: (i) studies of transfected cells and murine peritoneal macrophages demonstrated that TLR2 and MyD88 are essential for the initial pro-inflammatory cytokine response (human IL-8, mouse IL-6) to LCMV; (ii) TLR2 knockout (KO) mice and MyD88 KO mice challenged with LCMV produced less IL-6 and monocyte chemotactic protein-1 in the serum than wild-type mice; (iii) in contrast to inflammatory cytokines, the production of type 1 IFN (IFN-alpha) in response to LCMV was MyD88 independent; (iv) MyD88 plays an essential role in antiviral CD8(+) T cell responses, CD8(+) T cells in MyD88 KO mice were defective in their expression of intracellular antiviral cytokines; and (v) the failure of MyD88 KO mice to activate CD8(+) T cells was accompanied by persistent viral infection in MyD88 KO mice. We demonstrate that TLR-mediated responses are important in the innate immune response to LCMV and that MyD88 is essential for the control of the LCMV infection and the maturation/activation of virus-specific CD8(+) T cells.

Deciphering the involvement of innate immune factors in the development of the host response to PRRSV vaccination

The natural response of pigs to porcine reproductive and respiratory syndrome virus (PRRSV) infections and vaccinations needs to be altered so that better protection is afforded against both homologous and heterologous challenges by this pathogen. To address this problem, real-time gene expression assays were coupled with cytokine Elispot and protein analyses to assess the nature of the anti-PRRSV response of pigs immunized with modified live virus (MLV) vaccine. Although T helper 1 (Th1) immunity was elicited in all vaccinated animals, as evidenced by the genesis of PRRSV-specific interferon-gamma secreting cells (IFNG SC), the overall extent of the memory response was variable and generally weak. Peripheral blood mononuclear cells (PBMC) isolated from these pigs responded to PRRSV exposure with a limited increase in their expression of the Th1 immune markers, IFNG, tumor necrosis factor-alpha and interleukin-15 (IL15), and a reduction in the quantity of mRNAs encoding the innate and inflammatory proteins, IL1B, IL8 and IFNA. Efforts to enhance Th1 immunity, by utilizing an expression plasmid encoding porcine IFNA (pINA) as an adjuvant, resulted in a temporary increase in the frequency of PRRSV-specific IFNG SC but only minor changes overall in the expression of Th1 associated cytokine or innate immune marker mRNA by virus-stimulated PBMC. Administration of pINA, however, did correlate with decreased IL1B secretion by cultured, unstimulated PBMC but had no effect on their ability to release IFNG. Thus, while exogenous addition of IFNA during PRRSV vaccination has an impact on the development of a Th1 immune response, other alterations will be required for substantial boosting of virus-specific protection.

Natural killer cells, miscarriage, and infertility

Enthusiasm for new treatments aimed at natural killer cells in women with reproductive failure is unfortunately not backed up by the science

The natural interferon-alpha producing cells in systemic lupus erythematosus

Prolonged exposure of the immune system to type I interferons (IFN-alpha/beta/omega) in patients receiving IFN-alpha therapy frequently results in development of autoantibodies and autoimmune disease. This is attributed to the many immunostimulatory effects of these cytokines. Patients with the autoimmune disease systemic lupus erythematosus (SLE) have an ongoing IFN-alpha production. Recent studies of SLE demonstrated the presence of endogenous IFN-alpha inducers, acting specifically on natural IFN-alpha producing cells (NIPC), often termed plasmacytoid dendritic cells (PDC). These IFN-alpha inducers were potent, present at the blood level, and characterized as immune complexes that contained DNA and IgG as essential components. They were considered a likely reason for the activated IFN-alpha production in SLE, which, in turn, might be an important etiopathogenic factor. Here, we briefly review the biology of the type I IFN system, with emphasis on inducers, producing cells (especially NIPC/PDC), IFN-alpha actions, and target immune cells, which might be relevant in SLE. Based on such information and results from studies in SLE patients, we propose a hypothesis that explains how NIPC/PDC become activated and play a pivotal etiopathogenic role in SLE and perhaps also other autoimmune diseases. This hypothesis furthermore indicates new therapeutic targets.