Measles virus-specific CD4 T-cell activity does not correlate with protection against lung infection or viral clearance

Nonsteroidal anti-inflammatory drugs restore immune function to respiratory syncytial virus in geriatric cotton rats (Sigmodon hispidus)

Respiratory syncytial virus (RSV) infection is not only a childhood disease, but also a serious health risk for the elderly. We investigated in cotton rats how age affected viral clearance, immune responses, and whether pharmacological intervention was beneficial. Our results demonstrated that in geriatric animals, virus grew to similar titers, but with delayed clearance, compared to adult animals. After primary infection with RSV, geriatric animals were susceptible to secondary infection and results indicated a defective humoral immune response. Depletion of cytotoxic T lymphocytes (CTL) during primary infection delayed clearance, indicating the necessary role of CTL. Pharmacological intervention through nonsteroidal anti-inflammatory ibuprofen resulted in faster viral clearance and complete protection after immunization. In addition, the CTL response in the presence of ibuprofen seemed to be restored. It appears that in geriatric animals, immune functions are not as effective as in adult animals and that anti-inflammatory therapy may restore effective immune function.

Cooperation of Oligodeoxynucleotides and Synthetic Molecules as Enhanced Immune Modulators

Unmethylated cytosine–guanine dinucleotide (CpG) motifs are potent stimulators of the host immune response. Cellular recognition of CpG motifs occurs via Toll-like receptor 9 (TLR9), which normally activates immune responses to pathogen-associated molecular patterns (PAMPs) indicative of infection. Oligodeoxynucleotides (ODNs) containing unmethylated CpGs mimic the immunostimulatory activity of viral/microbial DNA. Synthetic ODNs harboring CpG motifs resembling those identified in viral/microbial DNA trigger an identical response, such that these immunomodulatory ODNs have therapeutic potential. CpG DNA has been investigated as an agent for the management of malignancy, asthma, allergy, and contagious diseases, and as an adjuvant in immunotherapy. In this review, we discuss the potential synergy between synthetic ODNs and other synthetic molecules and their immunomodulatory effects. We also summarize the different synthetic molecules that function as immune modulators and outline the phenomenon of TLR-mediated immune responses. We previously reported a novel synthetic ODN that acts synergistically with other synthetic molecules (including CpG ODNs, the synthetic triacylated lipopeptide Pam₃CSK₄, lipopolysaccharide, and zymosan) that could serve as an immune therapy. Additionally, several clinical trials have evaluated the use of CpG ODNs with other immune factors such as granulocyte-macrophage colony-stimulating factor, cytokines, and both endosomal and cell-surface TLR ligands as adjuvants for the augmentation of vaccine activity. Furthermore, we discuss the structural recognition of ODNs by TLRs and the mechanism of functional modulation of TLRs in the context of the potential application of ODNs as wide-spectrum therapeutic agents.

Diversity of aging of the immune system classified in the cotton rat (Sigmodon hispidus) model of human infectious diseases

Susceptibility and declined resistance to human pathogens like respiratory syncytial virus (RSV) at old age is well represented in the cotton rat (Sigmodon hispidus). Despite providing a preferred model of human infectious diseases, little is known about aging of its adaptive immune system. We aimed to define aging-related changes of the immune system of this species. Concomitantly, we asked whether the rate of immunological alterations may be stratified by physiological aberrations encountered during aging. With increasing age, cotton rats showed reduced frequencies of T cells, impaired induction of antibodies to RSV, higher incidence of aberrations of organs and signs of lipemia. Moreover, old animals expressed high biological heterogeneity, but the age-related reduction of T cell frequency was only observed in those specimens that displayed aberrant organs. Thus, cotton rats show age-related alterations of lymphocytes that can be classified by links with health status.

Prevention of measles virus infection by intranasal delivery of fusion inhibitor peptides

Measles virus (MV) infection is undergoing resurgence and remains one of the leading causes of death among young children worldwide despite the availability of an effective measles vaccine. MV infects its target cells by coordinated action of the MV H and the fusion (F) envelope glycoprotein; upon receptor engagement by H, the prefusion F undergoes a structural transition, extending and inserting into the target cell membrane and then refolding into a postfusion structure that fuses the viral and cell membranes. By interfering with this structural transition of F, peptides derived from the heptad-repeat (HR) regions of F can potently inhibit MV infection at the entry stage. We show here that specific features of H's interaction with its receptors modulate the susceptibility of MV F to peptide fusion inhibitors. A higher concentration of inhibitory peptides is required to inhibit F-mediated fusion when H is engaged to its nectin-4 receptor than when H is engaged to its CD150 receptor. Peptide inhibition of F may be subverted by continued engagement of receptor by H, a finding that highlights the ongoing role of H-receptor interaction after F has been activated and that helps guide the design of more potent inhibitory peptides. Intranasal administration of these peptides results in peptide accumulation in the airway epithelium with minimal systemic levels of peptide and efficiently prevents MV infection in vivo in animal models. The results suggest an antiviral strategy for prophylaxis in vulnerable and/or immunocompromised hosts.

IMPORTANCE

Measles virus (MV) infection causes an acute illness that may be associated with infection of the central nervous system (CNS) and severe neurological disease. No specific treatment is available. We have shown that parenterally delivered fusion-inhibitory peptides protect mice from lethal CNS MV disease. Here we show, using established small-animal models of MV infection, that fusion-inhibitory peptides delivered intranasally provide effective prophylaxis against MV infection. Since the fusion inhibitors are stable at room temperature, this intranasal strategy is feasible even outside health care settings, could be used to protect individuals and communities in case of MV outbreaks, and could complement global efforts to control measles.

Maternal antibodies: clinical significance, mechanism of interference with immune responses, and possible vaccination strategies

Neonates have an immature immune system, which cannot adequately protect against infectious diseases. Early in life, immune protection is accomplished by maternal antibodies transferred from mother to offspring. However, decaying maternal antibodies inhibit vaccination as is exemplified by the inhibition of seroconversion after measles vaccination. This phenomenon has been described in both human and veterinary medicine and is independent of the type of vaccine being used. This review will discuss the use of animal models for vaccine research. I will review clinical solutions for inhibition of vaccination by maternal antibodies, and the testing and development of potentially effective vaccines. These are based on new mechanistic insight about the inhibitory mechanism of maternal antibodies. Maternal antibodies inhibit the generation of antibodies whereas the T cell response is usually unaffected. B cell inhibition is mediated through a cross-link between B cell receptor (BCR) with the Fcγ-receptor IIB by a vaccine-antibody complex. In animal experiments, this inhibition can be partially overcome by injection of a vaccine-specific monoclonal IgM antibody. IgM stimulates the B cell directly through cross-linking the BCR via complement protein C3d and antigen to the complement receptor 2 (CR2) signaling complex. In addition, it was shown that interferon alpha binds to the CD21 chain of CR2 as well as the interferon receptor and that this dual receptor usage drives B cell responses in the presence of maternal antibodies. In lieu of immunizing the infant, the concept of maternal immunization as a strategy to protect neonates has been proposed. This approach would still not solve the question of how to immunize in the presence of maternal antibodies but would defer the time of infection to an age where infection might not have such a detrimental outcome as in neonates. I will review successful examples and potential challenges of implementing this concept.

Synergistic induction of interferon α through TLR-3 and TLR-9 agonists stimulates immune responses against measles virus in neonatal cotton rats

Immunization of neonates is problematic because of the immaturity of their immune system and the presence of maternal antibodies, both of which affect B cell responses. We tested the effects of co-administration of measles vaccine with a combination of TLR-3 (pI:C) and TLR-9 (ODN2216, optimized for human TLR-9) agonists on the ability to induce an effective immune response in neonatal cotton rats. TLR-9 expression in cotton rat lymphocytes was at the same low level as in human lymphocytes, which is in contrast to mice that express higher levels. TLR-3 expression levels were comparable between cotton rats, mice, and humans. A combination of TLR-3 and TLR-9 agonists synergistically induced high levels of type I interferon in neonatal spleen cells and higher levels of IL-10 as compared to adult spleen cells. Previously, it was shown that type I interferon stimulates B cell generation and antibody secretion in vitro and in vivo, and that IL-10 has immunomodulatory effects. The simultaneous induction of both type I interferon and IL-10 indicated that this immunization regimen could be both effective and safe. Neonatal cotton rats did not generate neutralizing antibodies after measles vaccination in the first week of life (although a T cell response was detectable). However, co-administration of the TLR-3 and TLR-9 agonist combination with measles vaccine in neonatal cotton rats induced neutralizing antibody responses comparable to those after adult immunization. This immunization regimen was also effective in neonatal cotton rats in the presence of natural maternal antibodies, although antibody titers were lower than those after immunization in the absence of maternal antibodies.

The cotton rat (Sigmodon hispidus) as an animal model for respiratory tract infections with human pathogens

Respiratory viral infection is a great human health concern, resulting in disease, death and economic losses. Cotton rats (Sigmodon hispidus) have been particularly useful in the study of the pathogenesis of human respiratory virus infections, including the development and testing of antiviral compounds and vaccines. In this article, the authors outline the advantages of the cotton rat compared with the mouse as a model for infection with measles virus, respiratory syncytial virus, influenza virus, human parainfluenza virus and human metapneumovirus. From the literature and their own experience, the authors summarize guidelines for handling, maintaining and breeding cotton rats. In addition, they offer technical tips for carrying out infection experiments and provide information about the large array of immunological assays and reagents available for the study of immune responses (macrophages, dendritic cells, T cells, B cells, antibodies, chemokines and cytokines) in cotton rats.

Synergistic induction of interferon α through TLR-3 and TLR-9 agonists identifies CD21 as interferon α receptor for the B cell response

Maternal antibodies inhibit seroconversion and the generation of measles virus (MeV)-specific antibodies (both neutralizing and non-neutralizing antibodies) after vaccination whereas T cell responses are usually unaffected. The lack of seroconversion leaves individuals susceptible to vaccine-preventable infections. Inhibition of antibody secretion is due to the inhibition of B cells through a cross-link of the B cell receptor with the inhibitory FcγIIB receptor (CD32) by maternal antibody/vaccine complexes. Here, we demonstrate that a combination of TLR-3 and TLR-9 agonists induces synergistically higher levels of type I interferon in vitro and in vivo than either agonist alone. The synergistic action of TLR-3 and TLR-9 agonists is based on a feedback loop through the interferon receptor. Finally, we have identified CD21 as a potential receptor for interferon α on B cells which contributes to interferon α-mediated activation of B cells in the presence of maternal antibodies. The combination leads to complete restoration of B cell and antibody responses after immunization in the presence of inhibitory MeV-specific IgG. The strong stimulatory action of type I interferon is due to the fact that type I interferon uses not only the interferon receptor but also CD21 as a functional receptor for B cell activation.

Maternal antibodies provide protection against infection with pathogens early in life but also interfere with vaccination. This interference is caused by a vaccine/maternal antibody complex which links the B cell receptor to the inhibitory CD32 molecule. Here, we show that this cross-link results in impaired B cell activation and proliferation which is correlated with diminished antibody responses. We also found that induction of large amounts of type I interferon restores the neutralizing antibody response in the presence of maternal antibodies. The best induction of type I interferon was accomplished by a combination of known activators of interferon secretion (a combination of TLR-3 and TLR-9 agonists). The strong stimulation by interferon is due to the previously unappreciated role of CD21 as functional receptor for interferon alpha. Our findings demonstrate that the dual receptor usage of type I interferon receptor and CD21 is crucial for B cell activation in the presence of maternal antibodies. This study suggests that measles vaccine, and potentially other vaccines, may induce optimal antibody responses when they are reconstituted with TLR-3 and TLR-9 agonists and thus these agonists may have great potential for clinical use.

Immune markers and correlates of protection for vaccine induced immune responses

Vaccines have been a major innovation in the history of mankind and still have the potential to address the challenges posed by chronic intracellular infections including tuberculosis, HIV and malaria which are leading causes of high morbidity and mortality across the world. Markers of an appropriate humoral response currently remain the best validated correlates of protective immunity after vaccination. Despite advancements in the field of immunology over the past few decades currently there are, however, no sufficiently validated immune correlates of vaccine induced protection against chronic infections in neither human nor veterinary medicine. Technological and conceptual advancements within cell-mediated immunology have led to a number of new immunological read-outs with the potential to emerge as correlates of vaccine induced protection. For T(H)1 type responses, antigen-specific production of interferon-gamma (IFN-γ) has been promoted as a quantitative marker of protective cell-mediated immune responses over the past couple of decades. More recently, however, evidence from several infections has pointed towards the quality of the immune response, measured through increased levels of antigen-specific polyfunctional T cells capable of producing a triad of relevant cytokines, as a better correlate of sustained protective immunity against this type of infections. Also the possibilities to measure antigen-specific cytotoxic T cells (CTL) during infection or in response to vaccination, through recombinant major histocompatibility complex (MHC) class I tetramers loaded with relevant peptides, has opened a new vista to include CTL responses in the evaluation of protective immune responses. Here, we review different immune markers and new candidates for correlates of a protective vaccine induced immune response against chronic infections and how successful they have been in defining the protective immunity in human and veterinary medicine.

Measles virus, immune control, and persistence

Measles remains one of the most important causes of child morbidity and mortality worldwide with the greatest burden in the youngest children. Most acute measles deaths are owing to secondary infections that result from a poorly understood measles-induced suppression of immune responses. Young children are also vulnerable to late development of subacute sclerosing panencephalitis, a progressive, uniformly fatal neurologic disease caused by persistent measles virus (MeV) infection. During acute infection, the rash marks the appearance of the adaptive immune response and CD8(+) T cell-mediated clearance of infectious virus. However, after clearance of infectious virus, MeV RNA persists and can be detected in blood, respiratory secretions, urine, and lymphoid tissue for many weeks to months. This prolonged period of virus clearance may help to explain measles immunosuppression and the development of lifelong immunity to re-infection, as well as occasional infection of the nervous system. Once MeV infects neurons, the virus can spread trans-synaptically and the envelope proteins needed to form infectious virus are unnecessary, accumulate mutations, and can establish persistent infection. Identification of the immune mechanisms required for the clearance of MeV RNA from multiple sites will enlighten our understanding of the development of disease owing to persistent infection.

Insights into the regulatory mechanism controlling the inhibition of vaccine-induced seroconversion by maternal antibodies

The inhibition of vaccination by maternal antibodies is a widely observed phenomenon in human and veterinary medicine. Maternal antibodies are known to suppress the B-cell response. This is similar to antibody feedback mechanism studies where passively transferred antibody inhibits the B-cell response against particulate antigens because of epitope masking. In the absence of experimental data addressing the mechanism underlying inhibition by maternal antibodies, it has been suggested that epitope masking explains the inhibition by maternal antibodies, too. Here we report that in the cotton rat model of measles virus (MV) vaccination passively transferred MV-specific immunoglobulin G inhibit B-cell responses through cross-linking of the B-cell receptor with FcγRIIB. The extent of inhibition increases with the number of antibodies engaging FcγRIIB and depends on the Fc region of antibody and its isotype. This inhibition can be partially overcome by injection of MV-specific monoclonal IgM antibody. IgM stimulates the B-cell directly through cross-linking the B-cell receptor via complement protein 3d and antigen to the complement receptor 2 signaling complex. These data demonstrate that maternal antibodies inhibit B-cell responses by interaction with the inhibitory/regulatory FcγRIIB receptor and not through epitope masking.

Induction of type I interferon secretion through recombinant Newcastle disease virus expressing measles virus hemagglutinin stimulates antibody secretion in the presence of maternal antibodies

Measles virus (MV) vaccine effectively protects seronegative individuals against infection. However, inhibition of vaccine-induced seroconversion by maternal antibodies after vaccination remains a problem, as it leaves infants susceptible to MV infection. In cotton rats, passive transfer of MV-specific IgG mimics maternal antibodies and inhibits vaccine-induced seroconversion. Here, we report that immunization in the presence of passively transferred IgG inhibits the secretion of neutralizing antibodies but not the generation of MV-specific B cells. This finding suggested that MV-specific B cells require an additional stimulus to mature into antibody-secreting plasma cells. In order to provide such a stimulus, we generated a recombinant Newcastle disease virus (NDV) expressing the MV hemagglutinin (NDV-H). In contrast to MV, NDV-H induced high levels of type I interferon in plasmacytoid dendritic cells and in lung tissue. In cotton rats immunized with NDV-H, neutralizing antibodies were also generated in the presence of passively transferred antibodies. In the latter case, however, the level and kinetics of antibody generation were reduced. In vitro, alpha interferon stimulated the activation of MV-specific B cells from MV-immune spleen cells. NDV infection (which induces alpha interferon) had the same effect, and stimulation could be abrogated by antibodies neutralizing alpha interferon, but not interleukin 6 (IL-6). In vivo, coapplication of UV-inactivated MV with NDV led to increased MV-specific antibody production in the presence and absence of passively transferred antibodies. These data indicate that MV-specific B cells are being generated after immunization in the presence of maternal antibodies and that the provision of alpha interferon as an additional signal leads to antibody secretion.

Correlates of protection induced by vaccination

This paper attempts to summarize current knowledge about immune responses to vaccines that correlate with protection. Although the immune system is redundant, almost all current vaccines work through antibodies in serum or on mucosa that block infection or bacteremia/viremia and thus provide a correlate of protection. The functional characteristics of antibodies, as well as quantity, are important. Antibody may be highly correlated with protection or synergistic with other functions. Immune memory is a critical correlate: effector memory for short-incubation diseases and central memory for long-incubation diseases. Cellular immunity acts to kill or suppress intracellular pathogens and may also synergize with antibody. For some vaccines, we have no true correlates, but only useful surrogates, for an unknown protective response.

Specific CD8(+) T-lymphocytes control dissemination of measles virus

Measles continues to be an important cause of childhood mortality in developing countries. Measles virus (MV) is lymphotropic and infects high percentages of B- and T-lymphocytes in lymphoid tissues. Cellular immunity is considered crucial for viral clearance; however, MV-specific T-lymphocytes generated during primary infection also constitute a potential target for MV infection. We therefore aimed to identify T-lymphocyte subsets that can clear MV infection without becoming infected. To this end, we infected human EBV transformed B-lymphoblastic cell lines (B-LCL) with a recombinant MV strain expressing enhanced GFP, and co-cultured these with non-infected B-LCL resulting in rapid viral spread. MV-specific CD8(+) T-cell clones efficiently suppressed MV dissemination in autologous and HLA-matched, but not in HLA-mismatched B-LCL. In contrast, CD4(+) T-cell clones could not control MV dissemination but became a target for MV infection themselves. Furthermore, PBMC collected 6-9 months after acute measles and stimulated with autologous MV-infected B-LCL also efficiently suppressed MV dissemination; this was mediated by the fraction containing CD8(+) T-lymphocytes. In conclusion, we have developed a powerful tool to study cellular immunity against measles, and demonstrate that control of MV dissemination is mediated by virus-specific CD8(+) rather than by CD4(+) T-lymphocytes.

Cytokine imbalance after measles virus infection has no correlation with immune suppression

Measles virus infection leads to immune suppression. A potential mechanism is the reduction of interleukin 12 (IL-12) secretion during acute measles, resulting in a TH2 response. Studies in humans have reported conflicting results, detecting either a TH2 or a TH1 response. We have investigated the correlation between a TH2 response and immune suppression in specific-pathogen-free inbred cotton rats which were infected with measles vaccine and wild-type viruses. After infection of bone marrow-derived macrophages with wild-type virus, IL-12 secretion was reduced in contrast to the level for vaccine virus infection. In bronchoalveolar lavage cells, IL-12 secretion was suppressed after infection with both wild-type and vaccine virus on days 2, 4, and 6 and was detectable on days 8 and 10. After stimulation of mediastinal lymph node and spleen cells with UV-inactivated measles virus at various time points after infection, gamma interferon but no IL-4 was found. After stimulation with phorbol myristate acetate-ionomycin, high gamma interferon and low IL-4 levels were detected. To investigate whether the secretion of IL-4 contributes to immune suppression, a recombinant vaccine virus was created which secretes cotton rat IL-4. After infection with this recombinant virus, IL-4 secretion was enhanced. However, neither inhibition of concanavalin A-stimulated spleen cells nor keyhole limpet hemocyanin-specific proliferation of spleen cells was altered after infection with the recombinant virus in comparison to the levels with the parental virus. Our data indicate that measles virus infection leads to a decrease in IL-12 secretion and an increase in IL-4 secretion, but this does not seem to correlate with immune suppression.

Nitric oxide production and nitric oxide synthase type 2 expression by cotton rat (Sigmodon hispidus) macrophages reflect the same pattern as human macrophages

Our knowledge of the antibacterial role of nitric oxide (NO) during infection is based on studies of murine macrophages, which secrete large amounts of NO. In contrast, human macrophages produce very little NO and its relevance as an antibacterial mediator during infection of humans is uncertain. We have defined bone marrow-derived macrophages from cotton rats (Sigmodon hispidus). These macrophages display phenotypical and functional characteristics similar to other rodent and human macrophages. The most interesting finding was the low level of NO production which is in contrast to findings for murine macrophages, but consistent with those of humans. In spite of these low levels, inhibition of NO production led to a decrease in killing of bacteria. Cotton rats are highly susceptible to a variety of human pathogens and therefore offer a rodent model of infectious diseases with similar characteristics to humans in terms of NO production.

Current animal models: cotton rat animal model

The cotton rat (Sigmodon hispidus) model has proven to be a suitable small animal model for measles virus pathogenesis to fill the niche between tissue culture and studies in macaques. Similar to mice, inbred cotton rats are available in a microbiologically defined quality with an ever-increasing arsenal of reagents and methods available for the study of infectious diseases. Cotton rats replicate measles virus in the respiratory tract and (depending on virus strain) in lymphoid organs. They can be infected with vaccine, wild-type, and recombinant measles viruses and have been used to study viruses with genetic modifications. Other areas of study include efficacy testing of antivirals and vaccines. The cotton rat also has been an informative animal model to investigate measles virus-induced immune suppression and suppression of vaccination by maternal antibodies. In addition, the cotton rat promises to be a useful model for the study of polymicrobial disease (interaction between measles virus and secondary pathogens).

Sindbis virus-based measles DNA vaccines protect cotton rats against respiratory measles: relevance of antibodies, mucosal and systemic antibody-secreting cells, memory B cells, and Th1-type cytokines as correlates of immunity

Measles remains an important cause of pediatric morbidity and mortality in developing countries, especially among infants who are too young to receive the current licensed live attenuated measles vaccine. We developed two Sindbis virus DNA vaccines encoding the measles virus hemagglutinin (pMSIN-H) and fusion proteins (pMSINH-FdU) and examined their immunogenicities and protective efficacies when administered alone or followed by the live measles virus vaccine in cotton rats. Neutralizing antibodies, mucosal and systemic antibody-secreting cells, memory B cells, and gamma interferon-secreting T cells developed after priming and increased after boosting. pMSIN-H priming conferred 100% protection against pulmonary measles, whereas pMSINH-FdU protected only in conjunction with the live measles virus vaccine boost.

Analysis of antibody response by temperature-sensitive measles vaccine strain in the cotton rat model

Measles virus (MeV) vaccine strain, AIK-C, is temperature sensitive (ts), which is thought to be associated with attenuation of virus pathogenicity. In this study, replication and antibody response were examined in cotton rats using viruses carrying different forms of the P gene, which is responsible for the ts phenotype of strain AIK-C and its parental Edmonston strain. When cotton rats were inoculated intranasally, ts viruses neither replicated in lungs, nor reproducibly generated an antibody response. When inoculated intramusculary (i.m.), however, ts strains raised an antibody titer in all animals. This response was not observed when ultraviolet-inactivated virus was used. ts virus, inoculated i.m., was recovered from cotton rat drainage lymph nodes. These results suggest that ts virus, inoculated i.m., could replicate in the cotton rat, presumably at the superficial lymph node, and induce an antibody response. Therefore, cotton rats can serve as a small-animal model for investigating immune responses to safer ts vaccine, as well as recombinant vaccine using AIK-C as a vector for protection against other infectious agents.