Mechanism of lung injury in cotton rats immunized with formalin-inactivated respiratory syncytial virus

In vitro model for the assessment of human immune responses to subunit RSV vaccines

Respiratory syncytial virus (RSV) is the single most important cause of serious lower respiratory tract disease in infants and young children worldwide and a high priority for vaccine development. Despite over 50 years of research, however, no vaccine is yet available. One block to vaccine development is an incomplete understanding of the aberrant memory response to the formalin-inactivated RSV vaccine (FI-RSV) given to children in the 1960s. This vaccine caused enhanced respiratory disease (ERD) with later natural RSV infection. Concern that any non-live virus vaccine may also cause ERD has blocked development of subunit vaccines for young children. A number of animal FI-RSV studies suggest various immune mechanisms behind ERD. However, other than limited data from the original FI-RSV trial, there is no information on the human ERD-associated responses. An in vitro model with human blood specimens may shed light on the immune memory responses likely responsible for ERD. Memory T cell responses to an antigen are guided by the innate responses, particularly dendritic cells that present an antigen in conjunction with co-stimulatory molecules and cytokine signaling. Our in vitro model involves human monocyte derived dendritic cells (moDC) and allogenic T cell cultures to assess innate responses that direct T cell responses. Using this model, we evaluated human responses to live RSV, FI-RSV, and subunit RSV G vaccines (G-containing virus-like particles, G-VLP). Similar to findings in animal studies, FI-RSV induced prominent Th2/Th17-biased responses with deficient type-1 responses compared to live virus. Responses to G-VLPs were similar to live virus, i.e. biased towards a Th1 and not a Th2/Th17. Also mutating CX3C motif in G gave a more pronounced moDC responses associated with type-1 T cell responses. This in vitro model identifies human immune responses likely associated with ERD and provides another pre-clinical tool to assess the safety of RSV vaccines.

Patterns of antibody response during natural hRSV infection: insights for the development of new antibody-based therapies

INTRODUCTION

The human respiratory syncytial virus (hRSV) is the main cause of acute lower respiratory tract infection in susceptible population worldwide, such as young children and the elderly. Although hRSV is a major public health burden, there are no licensed vaccines and the only available therapy is palivizumab. During life, reinfections with hRSV are common, suggesting that the virus can impair the development of an efficient host immune response. This feature has hindered the development of efficient therapies.

AREAS COVERED

This article focuses on research about the natural development of antibodies in humans after the exposure to hRSV. The difficulties of developing anti-hRSV therapies based on monoclonal antibodies have been recently associated to the relationship between the disease outcome and the pattern of antibody response.

EXPERT OPINION

Development of monoclonal antibodies is a potentially successful approach to prevent the population from suffering severe respiratory diseases caused by hRSV infection, for which there are no available vaccines. Although the use of palivizumab is safe, its effectiveness is controversial. Recent data have prompted research to develop therapies targeting alternative viral antigens, rather than focusing only on the F protein, as well as the development of antibodies with a cell-mediated function.

Universal vaccine against respiratory syncytial virus A and B subtypes

Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract infection in infants, young children, and the elderly. Two subtypes of RSV, A and B, circulate alternately at 1-2-year intervals during epidemics. The attachment glycoprotein (G protein) of RSV is one of the major targets for immune responses. In this study, we generated a recombinant fusion protein, GcfAB, which consists of the central regions (a.a. residues 131-230) of the G proteins of both RSV A (A2 strain) and B (B1 strain) subtypes, and investigated immunogenicity, protective efficacy, and immunopathology. We immunized mice with GcfAB plus cholera toxin as a mucosal adjuvant via intranasal (IN) or sublingual (SL) routes. The IN group showed higher levels of RSV G-specific antibody responses, including serum IgG and mucosal IgA, compared with the SL group. On the contrary, more vigorous RSV G-specific CD4+ T-cell responses were elicited in the SL group than in the IN group after RSV-A but not RSV-B viral challenge. Furthermore, the SL group showed more pulmonary eosinophil recruitment and body weight loss than did the IN group after RSV-A challenge. Both IN and SL immunization with GcfAB provided potential protection against both subtypes of infections. Together, these results suggest that vaccination with GcfAB via an IN route could be a universal vaccine regimen preventing both RSV A and B infections.

Preclinical evaluation of bacterially produced RSV-G protein vaccine: Strong protection against RSV challenge in cotton rat model

In current study, we evaluated the safety and protective efficacy of recombinant unglycosylated RSV G protein ectodomain produced in E. coli (in presence and absence of oil-in-water adjuvant) in a preclinical RSV susceptible cotton rat challenge model compared to formaldehyde inactivated RSV (FI-RSV) and live RSV experimental infection. The adjuvanted G protein vaccine induced robust neutralization antibody responses comparable to those generated by live RSV infection. Importantly, adjuvanted G protein significantly reduced viral loads in both the lungs and nose at early time points following viral challenge. Antibody kinetics determined by Surface Plasmon Resonance showed that adjuvanted G generated 10-fold higher G-binding antibodies compared to non-adjvuanted G vaccine and live RSV infection, which correlated strongly with both neutralization titers and viral load titers in the nose and lungs post-viral challenge. Antibody diversity analysis revealed immunodominant antigenic sites in the N- and C-termini of the RSV-G protein, that were boosted >10-fold by adjuvant and inversely correlated with viral load titers. Enhanced lung pathology was observed only in animals vaccinated with FI-RSV, but not in animals vaccinated with unadjuvanted or adjuvanted RSV-G vaccine after viral challenge. The bacterially produced unglycosylated G protein could be developed as a protective vaccine against RSV disease.

Protection and mechanism of action of a novel human respiratory syncytial virus vaccine candidate based on the extracellular domain of small hydrophobic protein

Infections with human respiratory syncytial virus (HRSV) occur globally in all age groups and can have devastating consequences in young infants. We demonstrate that a vaccine based on the extracellular domain (SHe) of the small hydrophobic (SH) protein of HRSV, reduced viral replication in challenged laboratory mice and in cotton rats. We show that this suppression of viral replication can be transferred by serum and depends on a functional IgG receptor compartment with a major contribution of FcγRI and FcγRIII. Using a conditional cell depletion method, we provide evidence that alveolar macrophages are involved in the protection by SHe-specific antibodies. HRSV-infected cells abundantly express SH on the cell surface and are likely the prime target of the humoral immune response elicited by SHe-based vaccination. Finally, natural infection of humans and experimental infection of mice or cotton rats does not induce a strong immune response against HRSV SHe. Using SHe as a vaccine antigen induces immune protection against HRSV by a mechanism that differs from the natural immune response and from other HRSV vaccination strategies explored to date. Hence, HRSV vaccine candidates that aim at inducing protective neutralizing antibodies or T-cell responses could be complemented with a SHe-based antigen to further improve immune protection.

The path to an RSV vaccine

Respiratory syncytial virus (RSV) is the greatest remaining unmet infant vaccine need in developed countries and an important unmet infant vaccine need worldwide. More than 40 years of effort have yet to result in a licensed RSV vaccine for humans. Key challenges to RSV vaccine development include a peak of severe disease at 2-3 months of age, problematic biochemical behavior of key vaccine antigens, a history of vaccine-mediated disease enhancement, and reliance on animal models that may not accurately reflect human disease processes. Potential paths to overcome these challenges include maternal immunization, structure-based engineering of vaccine antigens, the design of a novel platform for safe infant immunization, and the development of improved animal models for vaccine-enhanced disease.

Safety and immunogenicity of a Sf9 insect cell-derived respiratory syncytial virus fusion protein nanoparticle vaccine

OBJECTIVE

We performed a Phase 1 randomized, observer-blinded, placebo-controlled trial to evaluate the safety and immunogenicity of a recombinant respiratory syncytial virus (RSV) fusion (F) protein nanoparticle vaccine.

METHODS

Six formulations with (5, 15, 30 and 60 μg) and without (30 and 60 μg) aluminum phosphate (AdjuPhos) were administered intramuscularly on day 0 and 30 in a dose escalating fashion to healthy adults 18-49 years of age. Solicited and unsolicited events were collected through day 210. Immunogenicity measures taken at day 0, 30 and 60 included RSV A and B microneutralization, anti-F IgG, antigenic site II peptide and palivizumab competitive antibodies.

RESULTS

The vaccine was well-tolerated, with no evident dose-related toxicity or attributable SAEs. At day 60 both RSV A and B microneutralization was significantly increased in vaccinees versus placebo. Across all vaccinees there was a 7- to 19-fold increase in the anti-F IgG and a 7- to 24-fold increase in the antigenic site II binding and palivizumab competitive antibodies.

CONCLUSIONS

The RSV F nanoparticle vaccine candidate was well tolerated without dose-related increases in adverse events. Measures of immunity indicate that neutralization, anti-RSV F IgG titers and palivizumab competing antibodies were induced at levels that have been associated with decreased risk of hospitalization. NCT01290419.

Local innate and adaptive immune responses regulate inflammatory cell influx into the lungs after vaccination with formalin inactivated RSV

Inactivated respiratory syncytial virus (RSV) vaccines tend to predispose for immune mediated enhanced disease, characterized by Th2 responses and airway hypersensitivity reactions. We show in a C57BL/6 mouse model that the early innate response elicited by the challenge virus (RSV versus influenza virus) influences the outcome of the Th1/Th2 balance in the lung after intramuscular priming with inactivated vaccine. Priming of CD4(+)/IFN-γ(+) T cells by mature dendritic cells administered intravenously and/or priming of a virus specific CD8(+) T cell response ameliorated the Th2-mediated inflammatory response in the lung, suggesting that vaccination procedures are feasible that prevent vaccine induced immune pathology.

Respiratory syncytial virus (RSV) evades the human adaptive immune system by skewing the Th1/Th2 cytokine balance toward increased levels of Th2 cytokines and IgE, markers of allergy--a review

Infection of infants in their first year of life, children and elderly people with the respiratory syncytial virus (RSV) endangers the life of the patient. An attempt to develop a formalin-inactivated RSV (FI-RSV) vaccine during the 1960s resulted in an aggravated infection in immunized children, leading to hospitalization, while infection of non-immunized children produced much milder symptoms. The reason for this remained an enigma, one which was gradually solved over the last decade by many researchers who studied the molecular biology of RSV infection of respiratory ciliary cells. Clinical studies of RSV-infected patients indicated increased levels of Th2 cytokines and IgE in the patients' sera, suggesting that an allergy-like condition developed during infection. The biomarkers of allergy caused by endogenous or environmental allergens include a marked increase of the Th2 cytokine IL-4 and IgE non-neutralizing antibodies to the allergen. The way allergens trigger allergy was deciphered recently, and will be discussed later. Studies of RSV infection led to the suggestion that RSV patients suffer from allergy prior to RSV infection, a concept that was later abandoned. Studies on HIV-1 [Y. Becker, Virus Genes 28, 319-331 (2005)] research led me to the hypothesis that since HIV-1 infection induces a marked increase of IL-4 and IgE in serum, an allergy-like condition, the AIDS stage is the result of an allergen motif that is embedded in the shed viral gp120 molecules. It is hypothesized that the viral-soluble G glycoprotein (sG) contains a T cell superantigen (Tsag) that is capable of binding to the V(H)3 domain of IgE/FcepsilonRI(+) hematopoietic cells, basophils, mast cells and monocytes, similar to the case of allergens, and that this aggregation causes these innate system cells to degranulate and release large amounts of Th2 cytokines (IL-4, IL-5, IL-10, IL-13) into the blood. The way these Th2 cytokines skew the Th1/Th2 balance toward Th2 > Th1 will be discussed. The aim of the present review is to base RSV pathogenicity on the numerous very good analyses of the virus genes and to suggest a therapeutic approach to treatment that is directed at preventing the inhibitory effects of Th2 cytokines on the adaptive immune system of the patients, instead of inhibiting RSV replication by antivirals. The review of the molecular research on the role of the viral fusion (F) and attachment (G) glycoproteins of RSV provided information on their role in the virus infection: early in infection the F glycoprotein induces Th1 cells to release the Th1 cytokines IL-2, IL-12 and IFN-gamma to activate precursors CTLs (pCTLs) to become anti-RSV CTLs. The G and sG glycoproteins attach to FKNR1(+) ciliary respiratory epithelial cells as well as directly to eosinophils to the lungs. The sG T cell antigen can also induce the release of large amounts of Th2 cytokines from CD4(+) T cells and from FCepsilonRI(+) mast cells, basophils and monocytes. By comparison to HIV-1 gp120 it is possible to show that in the G and sG proteins the T cell antigen resembles the CD4(+) T cell superantigen (=allergen) domain of HIV-1 gp120 which aggregates with IgE/FCepsilonRI(+) hematopoietic cells. The increased IL-4 level in the serum inhibits the adaptive immune response: IL-4Ralpha(+) Th1 cells stop Th1 cytokine synthesis and IL-4Ralpha(+) B cells stop the synthesis of antiviral IgG and IgA and switch to IgE synthesis. In addition, the hematopoietic cells release histamine and prostaglandin which induce wheezing. The gradual increase of sG molecules creates a gradient of fractalkine (FKN) which directs IL-5-activated eosinophils to the lungs of the patient.

Carriers for the delivery of a vaccine against respiratory syncytial virus

Respiratory syncytial virus (RSV) is a major cause of bronchiolitis and pneumonia in young children and the elderly. Despite its clinical importance, there is no licensed vaccine available at present. Vaccine development has been hampered by observations of increased pathology after RSV infection in infants vaccinated with formalin-inactivated RSV; incomplete immunity following natural infection; and the need to be effective during the neonatal period when levels of maternal antibody are high. Four categories of RSV vaccine carriers--live-attenuated RSVs, recombinant vectors expressing the protective antigens of RSV, DNA vaccines and subunit vaccines--have been evaluated in animal models and/or clinical trials. So far, studies with live-attenuated virus vaccines highlight the need to improve immunogenicity whilst maintaining a suitable level of attenuation. Studies with recombinant vectors, DNA and subunit vaccines illustrate the pivotal nature of the vaccine carrier in determining the balance between immune-mediated protection against infection and the induction of immune-mediated pulmonary pathology.

Vbeta14(+) T cells mediate the vaccine-enhanced disease induced by immunization with respiratory syncytial virus (RSV) G glycoprotein but not with formalin-inactivated RSV

Mice immunized with respiratory syncytial virus (RSV) G glycoprotein or with formalin-inactivated RSV (FI-RSV) exhibit severe disease following RSV challenge. This results in type 2 cytokine production and pulmonary eosinophilia, both hallmarks of vaccine-enhanced disease. RSV G-induced T-cell responses were shown to be restricted to CD4(+) T cells expressing Vbeta14 in the T-cell receptor (TCR), and the deletion of these T cells resulted in less severe disease. We therefore examined the role of Vbeta14(+) T cells in FI-RSV-induced disease. BALB/c mice were immunized with vaccinia virus expressing secreted RSV G (vvGs) or with FI-RSV. At the time of challenge with live RSV, mice were injected with antibody to the Vbeta14 component of the TCR. vvGs-immunized mice treated with anti-Vbeta14 had reduced cytokine levels in the lung. Eosinophil recruitment to the lung was also significantly reduced. In contrast, depletion of Vbeta14(+) T cells in FI-RSV-immunized mice had little impact on cytokine production or pulmonary eosinophilia. An analysis of TCR Vbeta chain usage confirmed a bias toward Vbeta14 expression on CD4(+) T cells from vvGs-immunized mice, whereas the CD4(+) T cells in FI-RSV-immunized mice expressed a diverse array of Vbeta chains. These data show that although FI-RSV and vvGs induce responses resulting in similar immunopathology, the T-cell repertoire mediating the response is different for each immunogen and suggest that the immune responses elicited by RSV G are not the basis for FI-RSV vaccine-enhanced disease.

Respiratory syncytial virus (RSV) G glycoprotein is not necessary for vaccine-enhanced disease induced by immunization with formalin-inactivated RSV

Following respiratory syncytial virus (RSV) challenge, mice immunized with RSV G or with formalin-inactivated RSV (FI-RSV) exhibit severe disease associated with type 2 cytokine production and pulmonary eosinophilia. This has led to the proposal that the presence of RSV G is the factor in FI-RSV that induces disease-enhancing T-cell responses. Therefore, we evaluated the role of RSV G and its immunodominant region in the induction of aberrant immune responses during FI-RSV immunization. BALB/c mice were immunized with FI preparations of wild-type (wt) RSV or recombinant RSV (rRSV) containing deletions of (i) the entire G gene, (ii) the region of the G gene encoding amino acids 187 to 197 of the immunodominant region, or (iii) the entire SH gene. After challenge, illness, RSV titers, cytokine levels, and pulmonary eosinophilia were measured. Peak RSV titers postchallenge were significantly greater in mice immunized with FI preparations of the deletion viruses than in those immunized with FI-rRSV wt, suggesting that the absence of G or SH in FI-RSV reduced its protective efficacy. Deletion of G or its epitope did not reduce illness, cytokine production, or eosinophilia relative to that in mice immunized with FI-rRSV wt. While cytokine levels and eosinophilia were similar, illness was reduced in mice immunized with SH-deleted FI-RSV. These data suggest that G-specific immune responses may be important for vaccine-induced protection and are not solely the basis for FI-RSV vaccine-enhanced illness. These data suggest that the method of RSV antigen delivery, rather than the protein composition, influences the phenotype of the induced immune responses and that RSV G should not necessarily be excluded from potential vaccine strategies.

Experimental Chlamydia pneumoniae infection in NIH/S mice: effect of reinoculation with chlamydial or cell preparation on culture, PCR and histological findings of lung tissue

The cellular components present in chlamydial preparations may contribute to the course of the experimental infection. NIH/S mice were inoculated and reinoculated intranasally with Chlamydia pneumoniae or a cellular preparation. The mock inoculation induced only mild histological changes in the lungs, which possibly induced partial protection against subsequent C. pneumoniae infection and, when given as reinoculation, possibly reactivated the culture-negative infection as culture-positive. In addition, serum antibodies against mouse heat shock protein 60 (Hsp60) were found in a few mice. In conclusion, the main immunopathogenic factors in a C. pneumoniae mouse model are chlamydial components. However, a cellular preparation may participate in an inflammatory reaction. Autoimmunity against Hsp60 may also play a role in the pathogenesis of C. pneumoniae infection.

Vaccine-enhanced respiratory syncytial virus disease in cotton rats following immunization with Lot 100 or a newly prepared reference vaccine

A formalin-inactivated respiratory syncytial virus vaccine was used to immunize infants in the mid-1960s; when these children later were naturally infected by the virus they developed markedly accentuated disease, and two died. For the present work, a new batch of vaccine was prepared using the original formula. Administration of either the old or new vaccines resulted in enhanced lesions in immunized cotton rats subsequently challenged with live virus, although administration of the vaccine reduced virus replication by 90%. Animals primed with formalin-inactivated virus and challenged developed markedly accentuated lesions of the same type as in animals undergoing primary or secondary infection. In addition, the animals with the vaccine-enhanced disease developed alveolitis and interstitial pneumonitis, which appear to be specific markers for the vaccine enhancement. The newly prepared vaccine appears suitable as a reference standard for studying the mechanism of vaccine-enhanced disease caused by this virus. Additionally, we reviewed the lesions in the lungs of the two humans who died with the vaccine-enhanced disease in 1967, and found that they were similar to, but more severe than those seen in the cotton rats.

Replication-competent or attenuated, nonpropagating vesicular stomatitis viruses expressing respiratory syncytial virus (RSV) antigens protect mice against RSV challenge

Foreign glycoproteins expressed in recombinant vesicular stomatitis virus (VSV) can elicit specific and protective immunity in the mouse model. We have previously demonstrated the expression of respiratory syncytial virus (RSV) G (attachment) and F (fusion) glycoprotein genes in recombinant VSV. In this study, we demonstrate the expression of RSV F and G glycoproteins in attenuated, nonpropagating VSVs which lack the VSV G gene (VSVDeltaG) and the incorporation of these RSV proteins into recombinant virions. We also show that intranasal vaccination of mice with nondefective VSV recombinants expressing RSV G (VSV-RSV G) or RSV F (VSV-RSV F) elicited RSV-specific antibodies in serum (by enzyme-linked immunosorbent assay [ELISA]) as well as neutralizing antibodies to RSV and afford complete protection against RSV challenge. In contrast, VSVDeltaG-RSV F induced detectable serum antibodies to RSV by ELISA, but no detectable neutralizing antibodies, yet it still protected from RSV challenge. VSVDeltaG-RSV G failed to induce any detectable serum (by ELISA) or neutralizing antibodies and failed to protect from RSV challenge. The attenuated, nonpropagating VSVDeltaG-RSV F is a particularly attractive candidate for a live attenuated recombinant RSV vaccine.

Increased replication of respiratory syncytial virus (RSV) in pulmonary infiltrates is associated with enhanced histopathological disease in bonnet monkeys (Macaca radiata) pre-immunized with a formalin-inactivated RSV vaccine

The pathology of respiratory syncytial virus (RSV) disease in bonnet monkeys parallels findings with human RSV disease. RSV-infected animals pre-immunized with a formalin-inactivated (FI) RSV vaccine develop inflammation in peribronchiolar, perivascular, interstitial and intra-alveolar sites with lung inflammation scores significantly higher than animals with a primary RSV infection and those pre-immunized with an FI-Vero cell control vaccine (P=0.05). Animals previously infected and re-exposed to RSV had significantly lower alveolar, interstitial and total lung inflammation scores than in primary infection (P=0.05). Immunization with two intra-muscular doses of 0.5 ml of the FI-RSV vaccine administered 21 days apart resulted in little serum-neutralizing and ELISA antibody, low levels of secretory IgA and a low lymphocyte proliferative response that was significantly lower than the response observed in animals that were previously infected with live RSV. Higher RSV virus titres were detected in the lungs and lung lavage fluid of monkeys immunized with the FI-RSV vaccine than in those with a primary infection (P=0.001). RSV was detected by in situ hybridization in pulmonary inflammatory infiltrates, where the single most abundant infiltrating cellular species was macrophages, so it may be these cells that support the enhanced virus replication that contributes to the enhanced pulmonary pathology of FI-RSV immunization.

Construction and characterization of recombinant vaccinia viruses co-expressing a respiratory syncytial virus protein and a cytokine

Recombinant vaccinia viruses are well-characterized tools that can be used to define novel approaches to vaccine formulation and delivery. While vector co-expression of immune mediators has enormous potential for optimizing the composition of vaccine-induced immune responses, the impact on antigen expression and vector antigenicity must also be considered. Co-expression of IL-4 increased vaccinia virus vector titres, while IFN-gamma co-expression reduced vaccinia virus replication in BALB/c mice and in C57BL/6 mice infected with some recombinant viruses. Protection against respiratory syncytial virus (RSV) challenge was similar in mice immunized with vaccinia virus expressing RSV G glycoprotein and IFN-gamma, even though the replication efficiency of the vector was diminished. These data demonstrate the ability of vector-expressed cytokine to influence the virulence of the vector and to direct the development of selected immune responses. This suggests that the co-expression of cytokines and other immunomodulators has the potential to improve the safety of vaccine vectors while improving the immunogenicity of vaccine antigens.

Both immunisation with a formalin-inactivated respiratory syncytial virus (RSV) vaccine and a mock antigen vaccine induce severe lung pathology and a Th2 cytokine profile in RSV-challenged mice

Respiratory syncytial virus (RSV) is the most important cause of bronchiolitis and pneumonia in infants and young children. Immunopathology may play a role in RSV-induced disease and a severe RSV infection may also be associated with an increased risk of developing asthma. Vaccination with formalin-inactivated RSV (FI-RSV) prior to infection resulted both in human and in the mouse model in extensive lung pathology. In the mouse model, it has been shown that this aggravation of disease was associated with a shift in the balance between Th1 and Th2 cytokines towards a Th2-type response. The aim of the present study was to characterise the immunological and inflammatory responses in BALB/c mice upon RSV infection with or without prior vaccination with aluminium-adjuvanted FI-RSV or control antigens (FI-Mock). As previously reported by others, we also observed that a primary RSV infection in BALB/c mice resulted in a predominant Th1-type cytokine response, which was associated with slight bronchiolitis and alveolitis. FI-RSV vaccination prior to RSV challenge prevented virus replication and was associated with an aggravation of pulmonary histopathology and a shift towards a Th2-type response. Vaccination with FI-Mock did not prevent RSV replication in the lung but resulted in an even more pronounced Th2 response after infection while these mice were not sensitised to specific viral antigens. Thus, viral replication in a Th2 responding animal (induced by aluminium-adjuvanted mock vaccine) appears to boost the Th2 response upon RSV infection.

Respiratory syncytial virus infects the Bonnet monkey, Macaca radiata

The Bonnet monkey model of respiratory syncytial virus (RSV) infection may be a useful nonhuman primate model for studying RSV disease in humans because Bonnet monkeys can predictably be infected to obtain an orderly sequence of morphologic, cytologic, virologic, serologic, and inflammatory changes related to time of infection. Young feral Bonnet monkeys, Macaca radiata, were infected endotracheally with 10(6) plaque-forming units (pfu) of the Long strain of RSV. RSV was recovered from the animals' lungs at necropsy on days 3, 5, and 7 with the highest viral titer obtained on day 3 (1.1 and 5.2 x 10(3) pfu/g of tissue in the upper and lower lobes, respectively). RSV antigen and F protein mRNA were detected 3-5 days after infection in alveolar macrophages and in the epithelium of bronchi, terminal bronchioles, and alveoli. Histologic analysis of RSV-infected lungs at necropsy revealed progressive bronchiolar mucosal and submucosal inflammation, periarterial mononuclear interstitial inflammation, and focal alveolitis, with a maximal response at 7 days after infection. Cell counts in bronchoalveolar lavage (BAL) increased with time with neutrophils and macrophages predominating on day 3 (6.47 and 5.85 x 10(5)/mm3, respectively) and lymphocytes predominating on day 9 (4.18 x 10(5)/mm3). Serum-neutralizing antibody appeared on day 5 and IgG antibody to RSV was detected on day 9. This sequence of morphologic, cytologic, virologic, serologic, and inflammatory change following RSV infection creates a useful model in the study of experimentally induced RSV disease with a potential for testing future vaccine-induced alterations in RSV disease response.

Enhanced pulmonary pathology associated with the use of formalin-inactivated respiratory syncytial virus vaccine in cotton rats is not a unique viral phenomenon

The specificity of viral antigens in the formalin-inactivated, alum-precipitated respiratory syncytial virus (FI-RSV) vaccine in augmenting the pulmonary inflammatory response was evaluated. Cotton rats were immunized with a FI-RSV vaccine derived from Vero cells, a monkey cell line, or HEp-2 cells, a human cell line. The FI-RSV/Vero and the FI-RSV/HEp-2 vaccines were prepared similarly to the original Lot-100 FI-RSV vaccine that was associated with enhanced disease in the mid-1960s field trials. Each vaccine was administered intramuscularly at various doses and intervals. At 1, 4 or 7 weeks after the last vaccine dose, cotton rats were challenged with 10(6) plaque-forming units of live RSV grown in HEp-2 cells. For controls, FI-parainfluenza, FI-HEp-2 and alum vaccines, and live RSV primary infection were used. For measuring virus replication and histopathology, lungs were harvested at 4 and 8 days postchallenge. A dose-response relationship to vaccine dose was observed for ELISA, neutralizing and antifusion antibodies. All animals given three doses or two of the higher doses of FI-RSV/Vero vaccine developed significant neutralizing antibody, were protected against pulmonary virus replication and had similar low levels of histopathology compared with live RSV and controls. Two immunizations of the lowest dose of FI-RSV/Vero vaccine did not induce neutralizing antibody, did not provide protection of the lung against RSV and did not enhance the pulmonary cellular response. However, FI-RSV/HEp-2 vaccine was associated with significant enhanced pulmonary histopathology despite inducing high titres of neutralizing antibody and protecting the lungs against RSV infection.(ABSTRACT TRUNCATED AT 250 WORDS)

A lamb model for human respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the most important cause of bronchiolitis and pneumonia in young children. The development of an animal model of RSV disease serves to better understanding the pathophysiology of airway disease from RSV infection in infants and children. Groups of six lambs were inoculated intratracheally (IT) or intranasally (IN) with a human strain of RSV (H-RSV). For controls 8 lambs received IT virus-free cell lysate. Tachypnea and fever were observed significantly more often following IT than following IN inoculation of H-RSV or IT placebo (for tachypnea: 20 of 69 days, 5 of 63 days, and 3 of 89 days, respectively, P < 0.001; for fever: 6 of 69 days, 0 of 63 days, and 1 of 89 days, respectively, P < 0.02). Nasal fluid production was significantly more frequent in both IT (14 of 69 days) and IN (15 of 63 days) groups than in the placebo group (2 of 87 days, P < 0.001). Postvaccination geometric mean titers (GMT, arithmetic transformation of log 2) of RSV-specific neutralizing antibody were significantly increased in the IT H-RSV group compared with postplacebo GMTs at 1 week (72 vs. 6.7, P < 0.03). By the second week postinoculation both H-RSV-infected groups had comparable levels of RSV-specific neutralizing antibody titers and had significantly greater GMTs for the second through to the fourth week than the placebo group (144, 128, and 4.8, respectively P < 0.0008). Bacterial isolates of the upper airway were comparable among the three groups. Histopathology at day 28 postinoculation was unremarkable for the three study groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Parainfluenza virus type 3 (PIV3)-specific and non-virus-specific delayed type hypersensitivity responses in cotton rats given different PIV3 antigen preparations

Virus-specific, T-lymphocyte-mediated delayed type hypersensitivity (DTH) responses were studied in cotton rats using replicating (wild-type parainfluenza virus type 3 (PIV3) and recombinant vaccinia virus expressing PIV3 haemagglutinin-neuraminidase (HN) or fusion (F) glycoproteins), and non-replicating (detergent-solubilized, affinity chromatography purified HN and F glycoproteins or inactivated whole PIV3) virus preparations. Significant virus-specific DTH responses were observed in all test groups 1-2 weeks after a single antigen inoculation or 5 days after two inoculations given 3 weeks apart. Peak swelling of ear pinnas in these animals generally occurred 24 h after elicitation and was marked by a cellular infiltration consisting of mono- and polymorphonuclear leucocytes. A considerable non-virus-specific inflammatory response, presumably due to tissue culture or media components in the priming antigen preparations, was observed 3 weeks after priming. No significant differences in DTH responses were observed in cotton rats primed with any of the PIV3 preparations. The possible roles and significance of both the virus-specific and non-virus-specific DTH responses in paramyxovirus-induced disease in humans and cotton rats are discussed.

Bovine respiratory syncytial virus-specific immune responses in cattle following immunization with modified-live and inactivated vaccines. Analysis of the specificity and activity of serum antibodies

Cattle were immunized with vaccines containing modified-live or inactivated bovine respiratory syncytial virus (BRSV) and serum antibody responses were analyzed. Compared with preinculation values, at Day 14 after two biweekly immunizations with modified-live or inactivated vaccines there were significant increases in BRSV-specific titers in the sera of cattle that received both types of vaccines, as determined by a whole cell ELISA. Using a blocking ELISA and radioimmune precipitation it was determined that there was recognition of the fusion (F) protein by antibodies from cattle that received both types of BRSV antigens: however, virus neutralization assays revealed that only cattle that received modified live virus, either in monovalent or polyvalent vaccines, developed neutralizing antibodies to BRSV after two immunizations. These results indicate that inactivation of BRSV can lead to a dissociation between serological recognition of the F protein and virus neutralization in vaccinated cattle.

Definition and application of a histopathological scoring scheme for an animal model of acute Mycoplasma pneumoniae pulmonary infection

A histopathological scoring system was developed to assess the pathology of acute Mycoplasma pneumoniae pulmonary infection in a hamster model. A final score per animal (ranging 0-26) is obtained by averaging scores from each lung which have been accumulated by the addition of subscores from the assessments of quantity and quality of peribronchiolar and peribronchial infiltrates, luminal exudates, perivascular infiltrates, and parenchymal pneumonia. The scoring scheme was then applied to test the ability of a heat-killed inoculum to induce pulmonary pathology and to the trial of a 43 kDa protein-associated antigen as a vaccine immunogen. A heat-killed inoculum delivered by both intratracheal and intranasal routes did not induce pulmonary pathology compared to a live inoculum (respective mean scores 0.1, 6.7; P less than 0.01). Animals prevaccinated with the 43 kDa antigen developed an accentuated pathological response after live challenge compared to those unvaccinated (respective mean scores 16.8, 5.8; P = 0.00007). Hypersensitization to growth medium components may, however, have contributed to the accentuated disease since the lungs of vaccinated animals challenged with culture-negative media also were affected (mean score 5.4). Reproducibility of the scoring system was measured by duplicate reading of histology slides which were randomized to the observer upon the second reading (r = 0.93; P = 0.000009). The scoring system has the ability to differentiate disease severity in small groups of animals.

Mucosal T cell distribution during infection with respiratory syncytial virus

Groups of 12-week-old Balb/c mice were inoculated intranasally with respiratory syncytial virus (RSV) and sacrificed at regular intervals after infection. T lymphocyte subset distribution was determined in lung tissue, bronchoalveolar lavage (BAL), peripheral blood, and spleen by means of flow cytometry employing monoclonal antibodies against the T cell membrane antigens Thy1.2 (pan-T), Ly2 (CD8), and L3T4 (CD4). Thy1.2+ cells increased in the lung from 35.4% of total lymphocytes before infection to 47.6% on day 7 after infection. This increase was largely accounted for by an increase in Ly2+ cells, which manifested a rise from 7.8% preinfection to 19.8% on day 7. The level of L3T4+ cells remained constant (27.9% preinfection vs. 25.2% on day 7). The L3T4+/Ly2+ ratio in the lungs reached a nadir 7 days post infection (1.5 vs. 3.5 before infection). The total cell count in BAL increased more than tenfold during the first week after infection. At the same time Thy1.2+ cells in the BAL increased from 41.1% of total lymphocytes on day 1 to 85.3% on day 7. Ly2+ influx was the most important (5.8% on day 1 vs. 41.1% on day 7). L3T4+ cell levels increased from 17.2% on day 1 to 40.1% on day 7. RSV-specific lymphocyte transformation was observed in BAL and blood but not in the lung tissue and spleen on day 7 postinfection. The disappearance of infectious virus in the lung correlated directly to the peak appearance of Ly2+ T cells in the lung tissue and BAL.

Comparison of the ability of formalin-inactivated respiratory syncytial virus, immunopurified F, G and N proteins and cell lysate to enhance pulmonary changes in Balb/c mice

Formalin-inactivated respiratory syncytial virus (FI-RSV), a lysate of HEp-2 cells and proteins F, G and N, immunopurified from infected cell cultures, were compared for their ability to prevent infection and to enhance changes in lung cytology associated with RSV challenge. Mice were immunized at three weekly intervals with serial dilutions of the preparations and challenged by the nasal route 1 week after the last injection; their lungs were analysed 4 days later. The concentration of the immunogens was adjusted to test at least a range of 2 to 500 ng of proteins per injection. The dose of FI-RSV used for immunization influenced both the protection against infection and the potentiation of lung histopathology. There was a strong correlation between the lesion scores and the proportion of larger cells recovered in bronchoalveolar lavage fluid. We therefore used cytological changes as an index of lung alterations in further experiments. Glycoproteins F and G but not protein N were protective against challenge infection. Potentiation was observed in mice immunized with minute amounts (2 ng per injection) of F, G or N. HEp-2 cell lysate also caused potentiation but this required greater than 125 ng of proteins per injection.

Comparison of class and subclass antibody response to live and UV-inactivated RSV administered intranasally in mice

To determine the effect of viral dose and replication on the subclass antibody response to RSV, mice were immunized intranasally with different doses of live RSV (10(4)-10(6) pfu) and compared to mice given an immunizing regimen of UV-inactivated RSV. Mice given the 10(6) pfu dose of live RSV and mice given the 40 micrograms dose of UV-inactivated RSV had comparable class specific antibody responses to whole RSV in serum and respiratory secretions. Serum from these two groups of mice were then compared for IgG subclass response to whole RSV. A predominance of IgG2a subclass antibody was found for both immunizing regimens, and no significant differences in subclass proportions were noted between regimens. These two regimens were then compared for serum total IgG response to RSV surface glycoproteins F and G. The serum IgG response to these glycoproteins was lower after immunization with UV-inactivated RSV than after live-RSV immunization (F: P = 0.03; G: P less than 0.05), even though the serum IgG response of the two groups to whole RSV was comparable. The IgG subclass response to surface glycoproteins was evaluated for live RSV immunization. The proportions of subclass antibody responses to glycoprotein F were comparable to the subclass response proportions to whole RSV and were not characteristic of a T-dependent response pattern. The subclass profile for glycoprotein G was not comparable to that of whole RSV but was suggestive of a T-independent response pattern.

Similar subclass antibody responses after intranasal immunization with UV-inactivated RSV mixed with cholera toxin or live RSV

To determine the effect of cholera toxin as a mucosal adjuvant on the class and subclass antibody response to RSV, mice were immunized intranasally with different doses of live RSV or UV-inactivated RSV mixed with cholera toxin. A single 10(6) pfu dose of live RSV and a single 50 micrograms dose of UV-inactivated RSV mixed with cholera toxin produced comparable serum IgG and respiratory secretion IgG and IgA antibody titers. Subclass antibody titers to whole RSV were also comparable between these two immunizing regimens. A predominance of IgG2a subclass to whole RSV was found for both regimens. The quantity of serum total IgG antibody to glycoprotein F or glycoprotein G did not differ between these regimens. The serum IgG subclass antibody response to both glycoprotein F and G was also not significantly different between regimens. Cholera toxin as a mucosal adjuvant can stimulate class and subclass antibody responses to UV-inactivated RSV that are similar in quantity and distribution to those after live RSV infection.

Comparison of lung histopathology and bronchoalveolar lavage cytology in mice and cotton rats infected with respiratory syncytial virus

Lung histology as well as cell number and size distribution in bronchoalveolar lavage (BAL) were compared in Balb/c mice and in cotton rats, of various immune status regarding the respiratory syncytial virus (RSV), when subjected to challenge with RSV. In mock-immunized animals, RSV infection typically caused microscopic inflammatory lesions of the lungs and the presence of inflammatory cells in the BAL. Immunization with a formalin inactivated vaccine prior to challenge increased the severity of the lung lesions and the number of cells recovered in the BAL. We observed that cotton rats are more convenient to study primary RSV infections, inasmuch naive subjects show more pronounced lesions than do naive mice. Conversely, the changes in lung histopathology and in BAL cytology associated with exposure to formalin inactivated vaccine prior to challenge were more apparent in mice, making them a more suitable model for potentiation studies. Moreover, mice showed less individual fluctuations than cotton rats. The reading of the lung sections could be made less tedious by use of computer image analysis, which results paralleled those of conventional examination. In mice, cytological analysis of the BAL could be used in place of lung histology for some potentiation studies since the number of cells recovered in BAL reflected the intensity of the lung lesions and size distribution profiles typical of potentiation were recognized.

Rapid recovery in mice after combined nasal/oral immunization with killed respiratory syncytial virus

Based on the concept of a common mucosal immune system, the murine gastrointestinal tract was inoculated (oral) with three doses (5, 20, and 40 micrograms) of UV-inactivated respiratory syncytial virus (RSV) in order to elicit a virus-specific immune response in the respiratory tract. Only the 40 micrograms dose induced significant (P less than 0.01) anti-RSV-IgG rises in serum and lung wash compared to controls. To improve the immune response, mice were immunized intranasally under light anesthesia with the same 40 micrograms dosage regimen of killed RSV so that each dose passed through the nose and was swallowed. This combined nasal/oral immunization stimulated anti-RSV-IgG in serum, lung wash and nasal wash (P less than 0.001) and anti-RSV-IgA in lung and nasal wash (P less than 0.001) that were comparable to levels after infection with live RSV. Three days after challenge with live RSV, mice given combined nasal/oral immunization showed suppressed nasal virus shedding (P = 0.025). Nasal virus shedding correlated inversely with concentrations of anti-RSV-Ig in nasal secretions but did not correlate with concentrations in serum.

Towards vaccine optimisation

New molecular technologies have accelerated the search for sub-unit candidate vaccines. However, once identified the use of a candidate antigen must be optimised to reap the maximum benefit from the eventual vaccine. This optimisation should take into account both the needs of the target population, and the various ways of potentiating the protective immune response induced. One must be sure that the final product will be used. Hence, vaccine optimisation should strive toward meeting the needs of a specific epidemiological problem within the economic constraints of a given situation. This may be possible using novel delivery systems designed to limit the number of doses needed, improve the stability or facilitate the delivery of a particular vaccine. In meeting the needs of a target population in a field situation, one must also keep in mind certain safety factors that go beyond the usual regulatory constraints. The immune response to vaccine candidates can be potentiated in many ways. The ability to preferentially induce specific protective effector mechanisms: i.e., antibody isotypes, T-cell subsets, and T-cell sub-subsets, is becoming a reality. Carrier molecules designed to avoid the problems of epitope suppression and competition, and perhaps an eventual "carrier jam," are being developed. Adjuvants and immunostimulants may also help, but the critical issue here remains their acceptability for use in man. Finally novel strategies for the induction of the immune response may also potentiate the immune response in the optimisation of vaccines.