Vaccines against respiratory viral pathogens for use in neonates: opportunities and challenges

Mycobacterium bovis BCG Given at Birth Followed by Inactivated Respiratory Syncytial Virus Vaccine Prevents Vaccine-Enhanced Disease by Promoting Trained Macrophages and Resident Memory T Cells

Respiratory syncytial virus (RSV) infects more than 60% of infants in their first year of life. Since an experimental formalin-inactivated (FI) RSV vaccine tested in the 1960s caused enhanced respiratory disease (ERD), few attempts have been made to vaccinate infants. ERD is characterized by Th2-biased responses, lung inflammation, and poor protective immune memory. Innate immune memory displays an increased nonspecific effector function upon restimulation, a process called trained immunity, or a repressed effector function upon restimulation, a process called tolerance, which participates in host defense and inflammatory disease. Mycobacterium bovis bacillus Calmette-Guérin (BCG) given at birth can induce trained immunity as well as heterologous Th1 responses. We speculate that BCG given at birth followed by FI-RSV may alleviate ERD and enhance protection through promoting trained immunity and balanced Th immune memory. Neonatal mice were given BCG at birth and then vaccinated with FI-RSV+Al(OH)3. BCG/FI-RSV+Al(OH)3 induced trained macrophages, tissue-resident memory T cells (TRM), and specific cytotoxic T lymphocytes (CTL) in lungs and inhibited Th2 and Th17 cell immune memory, all of which contributed to inhibition of ERD and increased protection. Notably, FI-RSV+Al(OH)3 induced tolerant macrophages, while BCG/FI-RSV+Al(OH)3 prevented the innate tolerance through promoting trained macrophages. Moreover, inhibition of ERD was attributed to trained macrophages or TRM in lungs but not memory T cells in spleens. Therefore, BCG given at birth to regulate trained immunity and TRM may be a new strategy for developing safe and effective RSV killed vaccines for young infants. IMPORTANCE RSV is the leading cause of severe lower respiratory tract infection of infants. ERD, characterized by Th2-biased responses, inflammation, and poor immune memory, has been an obstacle to the development of safe and effective killed RSV vaccines. Innate immune memory participates in host defense and inflammatory disease. BCG given at birth can induce trained immunity as well as heterologous Th1 responses. Our results showed that BCG/FI-RSV+Al(OH)3 induced trained macrophages, TRM, specific CTL, and balanced Th cell immune memory, which contributed to inhibition of ERD and increased protection. Notably, FI-RSV+Al(OH)3 induced tolerant macrophages, while BCG/FI-RSV+Al(OH)3 prevented tolerance through promoting trained macrophages. Moreover, inhibition of ERD was attributed to trained macrophages or TRM in lungs but not memory T cells in spleens. BCG at birth as an adjuvant to regulate trained immunity and TRM may be a new strategy for developing safe and effective RSV killed vaccines for young infants.

Understanding Antibody Responses in Early Life: Baby Steps towards Developing an Effective Influenza Vaccine

The immune system of young infants is both quantitatively and qualitatively distinct from that of adults, with diminished responsiveness leaving these individuals vulnerable to infection. Because of this, young infants suffer increased morbidity and mortality from respiratory pathogens such as influenza viruses. The impaired generation of robust and persistent antibody responses in these individuals makes overcoming this increased vulnerability through vaccination challenging. Because of this, an effective vaccine against influenza viruses in infants under 6 months is not available. Furthermore, vaccination against influenza viruses is challenging even in adults due to the high antigenic variability across viral strains, allowing immune evasion even after induction of robust immune responses. This has led to substantial interest in understanding how specific antibody responses are formed to variable and conserved components of influenza viruses, as immune responses tend to strongly favor recognition of variable epitopes. Elicitation of broadly protective antibody in young infants, therefore, requires that both the unique characteristics of young infant immunity as well as the antibody immunodominance present among epitopes be effectively addressed. Here, we review our current understanding of the antibody response in newborns and young infants and discuss recent developments in vaccination strategies that can modulate both magnitude and epitope specificity of IAV-specific antibody.

Harnessing Cellular Immunity for Vaccination against Respiratory Viruses

Severe respiratory viral infections, such as influenza, metapneumovirus (HMPV), respiratory syncytial virus (RSV), rhinovirus (RV), and coronaviruses, including severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), cause significant mortality and morbidity worldwide. These viruses have been identified as important causative agents of acute respiratory disease in infants, the elderly, and immunocompromised individuals. Clinical signs of infection range from mild upper respiratory illness to more serious lower respiratory illness, including bronchiolitis and pneumonia. Additionally, these illnesses can have long-lasting impact on patient health well beyond resolution of the viral infection. Aside from influenza, there are currently no licensed vaccines against these viruses. However, several research groups have tested various vaccine candidates, including those that utilize attenuated virus, virus-like particles (VLPs), protein subunits, and nanoparticles, as well as recent RNA vaccines, with several of these approaches showing promise. Historically, vaccine candidates have advanced, dependent upon the ability to activate the humoral immune response, specifically leading to strong B cell responses and neutralizing antibody production. More recently, it has been recognized that the cellular immune response is also critical in proper resolution of viral infection and protection against detrimental immunopathology associated with severe disease and therefore, must also be considered when analyzing the efficacy and safety of vaccine candidates. These candidates would ideally result in robust CD4+ and CD8+ T cell responses as well as high-affinity neutralizing antibody. This review will aim to summarize established and new approaches that are being examined to harness the cellular immune response during respiratory viral vaccination.

Harnessing Cellular Immunity for Vaccination against Respiratory Viruses

Severe respiratory viral infections, such as influenza, metapneumovirus (HMPV), respiratory syncytial virus (RSV), rhinovirus (RV), and coronaviruses, including severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), cause significant mortality and morbidity worldwide. These viruses have been identified as important causative agents of acute respiratory disease in infants, the elderly, and immunocompromised individuals. Clinical signs of infection range from mild upper respiratory illness to more serious lower respiratory illness, including bronchiolitis and pneumonia. Additionally, these illnesses can have long-lasting impact on patient health well beyond resolution of the viral infection. Aside from influenza, there are currently no licensed vaccines against these viruses. However, several research groups have tested various vaccine candidates, including those that utilize attenuated virus, virus-like particles (VLPs), protein subunits, and nanoparticles, as well as recent RNA vaccines, with several of these approaches showing promise. Historically, vaccine candidates have advanced, dependent upon the ability to activate the humoral immune response, specifically leading to strong B cell responses and neutralizing antibody production. More recently, it has been recognized that the cellular immune response is also critical in proper resolution of viral infection and protection against detrimental immunopathology associated with severe disease and therefore, must also be considered when analyzing the efficacy and safety of vaccine candidates. These candidates would ideally result in robust CD4+ and CD8+ T cell responses as well as high-affinity neutralizing antibody. This review will aim to summarize established and new approaches that are being examined to harness the cellular immune response during respiratory viral vaccination.

Neonatal and infant immunity for tuberculosis vaccine development: importance of age-matched animal models

Neonatal and infant immunity differs from that of adults in both the innate and adaptive arms, which are critical contributors to immune-mediated clearance of infection and memory responses elicited during vaccination. The tuberculosis (TB) research community has openly admitted to a vacuum of knowledge about neonatal and infant immune responses to Mycobacterium tuberculosis (Mtb) infection, especially in the functional and phenotypic attributes of memory T cell responses elicited by the only available vaccine for TB, the Bacillus Calmette-Guérin (BCG) vaccine. Although BCG vaccination has variable efficacy in preventing pulmonary TB during adolescence and adulthood, 80% of endemic TB countries still administer BCG at birth because it has a good safety profile and protects children from severe forms of TB. As such, new vaccines must work in conjunction with BCG at birth and, thus, it is essential to understand how BCG shapes the immune system during the first months of life. However, many aspects of the neonatal and infant immune response elicited by vaccination with BCG remain unknown, as only a handful of studies have followed BCG responses in infants. Furthermore, most animal models currently used to study TB vaccine candidates rely on adult-aged animals. This presents unique challenges when transitioning to human trials in neonates or infants. In this Review, we focus on vaccine development in the field of TB and compare the relative utility of animal models used thus far to study neonatal and infant immunity. We encourage the development of neonatal animal models for TB, especially the use of pigs.

Influenza-infected newborn and adult monkeys exhibit a strong primary antibody response to hemagglutinin stem

The specificity of antibodies (Abs) generated against influenza A virus (IAV) infection can significantly alter protection and viral clearance. At present, the impact of age upon this process is relatively unexplored. Here, we evaluated the Ab response in newborn and adult African green monkeys following infection with IAV using a strain that enables us to determine the immunodominance (ID) hierarchy of the Ab response to hemagglutinin (HA), the principal target of protective Abs. This revealed altered ID patterns in the early IgM anti-HA response in newborns versus adults that converged over time. While the IgG ID profiles for HA in newborn and adult monkeys were similar, this was not the case for IgA. Importantly, HA stem-specific Abs were generated robustly and similarly in newborns and adults in terms of quality and quantity. Together, these results demonstrate that newborns and adults can differ in the Ab ID pattern established following infection and that the ID pattern can vary across isotypes. In addition, newborns have the ability to generate potent HA stem-specific Ab responses. Our findings further the understanding of the newborn response to IAV antigens and inform the development of improved vaccines for this at-risk population.

Adjuvanting an inactivated influenza vaccine with conjugated R848 improves the level of antibody present at 6months in a nonhuman primate neonate model

Generation of a potent antibody response that can be sustained over time is highly challenging in young infants. Our previous studies using a nursery-reared nonhuman primate model identified R848 conjugated to inactivated influenza virus as a highly immunogenic vaccine for neonates. Here we determined the effectiveness of this vaccine in mother-reared infants as well as its ability to promote improved responses at 6months compared to vaccination in the absence of R848. In agreement with our nursery study, R848 conjugated to influenza virus induced a higher antibody response in neonates compared to the non-adjuvanted vaccine. Further, the increase in the response relative to that induced by the non-adjuvanted vaccine was maintained at 6months suggesting the increased antibody secreting cells that resulted from inclusion of conjugated R848 production were capable of surviving long term. There was no significant difference in quality of antibody (i.e. neutralization or affinity), suggesting the beneficial effect of conjugated R848 during vaccination of neonates with inactivated influenza virus is likely manifest during the early generation of antibody secreting cells.

Influenza and respiratory syncytial virus in infants study (IRIS) of hospitalized and non-ill infants aged <1 year in four countries: study design and methods

Background

This multi-country prospective study of infants aged <1 year aims to assess the frequency of influenza virus and respiratory syncytial virus (RSV) infections associated with hospitalizations, to describe clinical features and antibody response to infection, and to examine predictors of very severe disease requiring intensive care.

Methods/Design

We are enrolling a hospital-based cohort and a sample of non-ill infants in four countries (Albania, Jordan, Nicaragua, and the Philippines) using a common protocol. We are currently starting year 2 of a 2- to 3-year study and will enroll approximately 3,000 infants hospitalized for any acute illness (respiratory or non-respiratory) during periods of local influenza and/or RSV circulation. After informed consent and within 24 h of admission, we collect blood and respiratory specimens and conduct an interview to assess socio-demographic characteristics, medical history, and symptoms of acute illness (onset ≤10 days). Vital signs, interventions, and medications are documented daily through medical record abstraction. A follow-up health assessment and collection of convalescent blood occurs 3-5 weeks after enrollment. Influenza and RSV infection is confirmed by singleplex real time reverse transcriptase polymerase chain reaction (rRT-PCR) assays. Serologic conversion will be assessed comparing acute and convalescent sera using hemagglutination inhibition assay for influenza antibodies and enzyme-linked immunosorbent assay (ELISA) for RSV. Concurrent with hospital-based enrollment, respiratory specimens are also being collected (and tested by rRT-PCR) from approximately 1,400 non-ill infants aged <1 year during routine medical or preventive care.

Discussion

The Influenza and RSV in Infants Study (IRIS) promises to expand our knowledge of the frequency, clinical features, and antibody profiles of serious influenza and RSV disease among infants aged <1 year, quantify the proportion of infections that may be missed by traditional surveillance, and inform decisions about the potential value of existing and new vaccines and other prevention and treatment strategies.

IL-6 Production by TLR-Activated APC Broadly Enhances Aged Cognate CD4 Helper and B Cell Antibody Responses In Vivo

Naive CD4 T cell responses, especially their ability to help B cell responses, become compromised with aging. We find that using APC pretreated ex vivo with TLR agonists, polyinosinic-polycytidylic acid and CpG, to prime naive CD4 T cells in vivo, restores their ability to expand and become germinal center T follicular helpers and enhances B cell IgG Ab production. Enhanced helper responses are dependent on IL-6 production by the activated APC. Aged naive CD4 T cells respond suboptimally to IL-6 compared with young cells, such that higher doses are required to induce comparable signaling. Preactivating APC overcomes this deficiency. Responses of young CD4 T cells are also enhanced by preactivating APC with similar effects but with only partial IL-6 dependency. Strikingly, introducing just the activated APC into aged mice significantly enhances otherwise compromised Ab production to inactivated influenza vaccine. These findings reveal a central role for the production of IL-6 by APC during initial cognate interactions in the generation of effective CD4 T cell help, which becomes greater with age. Without APC activation, aging CD4 T cell responses shift toward IL-6-independent Th1 and CD4 cytotoxic Th cell responses. Thus, strategies that specifically activate and provide Ag to APC could potentially enhance Ab-mediated protection in vaccine responses.

Adjuvanting an inactivated influenza vaccine with flagellin improves the function and quantity of the long-term antibody response in a nonhuman primate neonate model

Young infants are at significantly increased risk of developing severe disease following infection with influenza virus. At present there is no approved vaccine for individuals below the age of six months given previous studies showing a failure of these individuals to efficiently seroconvert. Given the major impact of influenza on infant health, it is critical that we develop vaccines that will be safe and effective in this population. Using a nonhuman primate (NHP) model, we have evaluated the ability of an inactivated influenza virus vaccine adjuvanted with flagellin to result in long term immune responses in neonates. To evaluate this critical attribute, neonate NHP were vaccinated and boosted with inactivated influenza virus in combination with either flagellin or a mutant inactive flagellin control. Our studies show that inclusion of flagellin resulted in a significant increase (5-fold, p=0.04) in influenza virus-specific IgG antibody at 6months post-vaccination. In addition, the antibody present at this late time was of higher affinity (2.4-fold, p=0.02). Finally a greater percentage of infants had detectable neutralizing antibody. These results support the use of flagellin in neonates as an adjuvant that promotes long-lived, high affinity antibody responses.

Determinants of early life immune responses to RSV infection

Respiratory syncytial virus causes significant morbidity and mortality in both developed and developing countries, and a vaccine that adequately protects from severe disease remains an important unmet need. RSV disease has an inordinate impact on the very young, and the physical and immunological immaturity of early life complicates vaccine design. Defining and targeting the functional capacities of early life immune responses and controlling responses during primary antigen exposure with selected vaccine delivery approaches will be important for protecting infants by active immunization. Alternatively, vaccination of older children and pregnant mothers may ameliorate disease burden indirectly until infants reach about six months of age, when they can generate more effective anti-RSV immune responses.

Age at Mycobacterium bovis BCG Priming Has Limited Impact on Anti-Tuberculosis Immunity Boosted by Respiratory Mucosal AdHu5Ag85A Immunization in a Murine Model

Tuberculosis (TB) remains a global pandemic despite the use of Bacillus Calmette-Guérin (BCG) vaccine, partly because BCG fails to effectively control adult pulmonary TB. The introduction of novel boost vaccines such as the human Adenovirus 5-vectored AdHu5Ag85A could improve and prolong the protective immunity of BCG immunization. Age at which BCG immunization is implemented varies greatly worldwide, and research is ongoing to discover the optimal stage during childhood to administer the vaccine, as well as when to boost the immune response with potential novel vaccines. Using a murine model of subcutaneous BCG immunization followed by intranasal AdHu5Ag85A boosting, we investigated the impact of age at BCG immunization on protective efficacy of BCG prime and AdHu5Ag85A boost immunization-mediated protection. Our results showed that age at parenteral BCG priming has limited impact on the efficacy of BCG prime-AdHu5Ag85A respiratory mucosal boost immunization-enhanced protection. However, when BCG immunization was delayed until the maturity of the immune system, longer sustained memory T cells were generated and resulted in enhanced boosting effect on T cells of AdHu5Ag85A respiratory mucosal immunization. Our findings hold implications for the design of new TB immunization protocols for humans.