BCG Vaccination Enhances the Immunogenicity of Subsequent Influenza Vaccination in Healthy Volunteers: A Randomized, Placebo-Controlled Pilot Study

The preventable efficacy of β-glucan against leptospirosis

Leptospirosis, caused by pathogenic Leptospira species, has emerged as an important neglected zoonotic disease. Few studies have reported the preventable effects of immunoregulators, except for antibiotics, against leptospirosis. Generally, immunostimulatory agents are considered effective for enhancing innate immune responses. Many studies have found that beta-glucan (β-glucan) could be a potent and valuable immunostimulant for improving immune responses and controlling diseases. In this study, we investigated the preventable role of β-glucan against Leptospira infection in hamsters. First, β-glucan was administered 24 h prior to, during and after infection. The results showed that β-glucan increased the survival rate to 100%, alleviated tissue injury, and decreased leptospire loads in target organs. Additionally, we found using quantitative real-time PCR that application of β-glucan significantly enhanced the expression of Toll-like receptor (TLR) 2, interleukin (IL)-1β and iNOS at 2 dpi (days post infection) and reduced the increase of TLR2, IL-1β and iNOS induced by Leptospira at 5 dpi. Furthermore, to induce memory immunity, β-glucan was administered 5 days prior to infection. β-Glucan also significantly increased the survival rates and ameliorated pathological damage to organs. Moreover, we demonstrated that β-glucan-trained macrophages exhibited elevated expression of proinflammatory cytokines (IL-1β and IL-6) in vitro, indicating that β-glucan induces an enhanced inflammatory response against Leptospira infection. These results indicate that administration of β-glucan and other immunostimulants could be potential valuable options for the control of Leptospira infection.

Leptospirosis, an important emerging neglected zoonotic disease, is caused by Leptospira and affects humans as well as animals. Due to the emergence of bacterial resistance to antibiotics, the development of alternatives to antibiotics has become an inevitable requirement in this new situation. Immunoregulators act as biological response regulators that do not induce toxicity, side effects, or resistance and can enhance, regulate, and restore nonspecific immunity to a host's immune response. β-Glucan, an immunostimulant, increased the survival rate, alleviated tissue injury, and decreased the abundance of leptospires in target organs. β-Glucan enhanced the inflammatory response, which was associated with enhanced prevention hamsters from Leptospira infection. Our findings also demonstrated that β-glucan-induced trained immunity protected against Leptospira infection. These results contributed to an explanation for the preventable mechanism against Leptospira infection and revealed that β-glucan and even other immunostimulants could be potent and valuable agents for controlling Leptospira infection.

Impact of H1N1 Influenza Vaccination on Child Morbidity in Guinea-Bissau

BACKGROUND

In addition to vaccines' specific effects, vaccines may have non-specific effects (NSEs) altering the susceptibility to unrelated infections. Non-live vaccines have been associated with negative NSEs. In 2010, a campaign with the non-live H1N1-influenza vaccine targeted children 6-59 months in Guinea-Bissau.

METHODS

Bandim Health Project runs a health and demographic surveillance system site in Guinea-Bissau. Using a Cox proportional hazards model, we compared all-cause consultation rates after vs. before the campaign, stratified by participation status.

RESULTS

Among 10 290 children eligible for the campaign, 60% had participated, 18% had not and for 22% no information was obtained. After the H1N1 campaign, the consultation rates tended to decline less for participants [HR = 0.80 (95% confidence interval, CI: 0.75; 0.85)] than for non-participants [HR = 0.68 (95% CI: 0.58; 0.79)], p = 0.06 for same effect.

CONCLUSION

The decline in the vaccinated group may have been smaller than the decline in the non-vaccinated group consistent with H1N1-vaccine increasing susceptibility to unrelated infections.

The influence of neonatal Bacille Calmette-Guérin (BCG) immunisation on heterologous vaccine responses in infants

INTRODUCTION

Bacillus Calmette-Guérin vaccine (BCG), one of the most widely used vaccines, does not only provide protection against tuberculosis and other mycobacterial infections, but also has non-specific (heterologous) immunomodulatory effects. In participants in a randomised trial, we investigated the effect of neonatal BCG immunisation on antibody responses to routine infant vaccines given in the first year of life.

METHODS

Antibodies against antigens in the diphtheria, tetanus, pertussis, polio, Haemophilus influenzae type b (Hib), and the 13-valent pneumococcal conjugate vaccines were measured in 91 (45 BCG-vaccinated, 46 BCG-naïve) infants one month after, and in 310 (169 BCG-vaccinated, 141 BCG-naïve) infants seven months after immunisation at 6 weeks, 4 and 6 months of age. In addition, antibodies against meningococcus C, Hib, measles, mumps, and rubella were measured in 147 (78 BCG-vaccinated, 69 BCG-naïve) infants one month after immunisation at 12 months of age. The seroprotection rates for each vaccine and the geometric mean concentrations (GMC) of antibodies were compared in BCG-vaccinated and BCG-naïve infants.

RESULTS

At 7 months of age, seroprotection rates were high in both BCG-vaccinated and BCG-naïve infants. At 13 months of age, seroprotection rates were lower than at 7 months of age, particularly for pertussis and a number of pneumococcal antigens, with generally higher rates for the latter in BCG-vaccinated infants. Although not statistically significant, antibody responses in BCG-vaccinated infants were consistently higher against diphtheria, tetanus, and pneumococcal antigens at both 7 and 13 months of age, and against measles and mumps at 13 months of age, but were lower against Hib one month after immunisation at both 7 and 13 months of age.

CONCLUSION

The immunomodulatory effect of BCG on antibody responses to heterologous vaccines adds to the evidence that BCG immunisation at birth has broad heterologous effects on the infant immune system.

Non-specific effects of BCG vaccine on viral infections

BACKGROUND

Some strains of Bacillus Calmette-Guérin (BCG) vaccine not only confer protection against disseminated forms of tuberculosis, but also reduce all-cause mortality by the induction of protection against infections with non-related pathogens.

OBJECTIVES

We review evidence for non-specific protection induced by BCG vaccination against viral infections, discuss possible mechanisms of action, and summarize implications for vaccination policies and vaccine discovery.

SOURCES

Relevant studies retrieved from PubMed and clinicaltrials.gov.

CONTENT

Numerous epidemiological, clinical and immunological studies demonstrate that BCG vaccination impacts the immune response to subsequent infections, resulting in reduced morbidity and mortality. Important lines of evidence indicating that BCG protects against viral pathogens comes from experimental studies in mice showing that BCG offers protection against various DNA and RNA viruses, including herpes and influenza viruses. Recently, the effect of BCG on an experimental viral infection in humans has been demonstrated. These effects are thought to be mediated via the induction of innate immune memory and heterologous lymphocyte activation, resulting in enhanced cytokine production, macrophage activity, T-cell responses and antibody titres.

IMPLICATIONS

The discovery of innate immune memory has greatly improved our understanding of the mechanisms underlying the non-specific effects induced by BCG vaccination. However, a full understanding of the molecular mechanisms that underlie this phenomenon is still evolving. By identifying the factors that impact the non-specific effects of BCG, we will take an important step towards novel therapeutic options and vaccination strategies, which might lead to a reduction in severe morbidity and mortality associated with viral infections.

The impact of vaccines on heterologous adaptive immunity

BACKGROUND

Vaccines induce antigen-specific memory in adaptive immune cells that enables long-lived protection against the target pathogen. In addition to this, several vaccines have beneficial effects greater than protection against their target pathogen. These non-specific effects are proposed to be the result of vaccine-induced immunomodulation. In the case of bacille Calmette-Guérin (BCG) vaccine, this involves induction of innate immune memory, termed 'trained immunity', in monocytes and natural killer cells.

OBJECTIVES

This review discusses current evidence for vaccine-induced immunomodulation of adaptive immune cells and heterologous adaptive immune responses.

CONTENT

The three vaccines that have been associated with changes in all-cause infant mortality: BCG, diphtheria-tetanus-pertussis (DTP) and measles-containing vaccines (MCV) alter T-cell and B-cell immunity. The majority of studies that investigated non-specific effects of these vaccines on the adaptive immune system report changes in numbers or proportions of adaptive immune cell populations. However, there is also evidence for effects of these vaccines on adaptive immune cell function and responses to heterologous stimuli. There is some evidence that, in addition to BCG, DTP and MCV, other vaccines (that have not been associated with changes in all-cause mortality) may alter adaptive immune responses to unrelated stimuli.

IMPLICATIONS

This review concludes that vaccines alter adaptive immune cell populations and heterologous immune responses. The non-specific effects differ between various vaccines and their effects on heterologous adaptive immune responses may also involve bystander activation, cross-reactivity and other as yet undefined mechanisms. This has major implications for future vaccine design and vaccination scheduling.

Outcomes of controlled human malaria infection after BCG vaccination

Recent evidence suggests that certain vaccines, including Bacillus-Calmette Guérin (BCG), can induce changes in the innate immune system with non-specific memory characteristics, termed ‘trained immunity’. Here we present the results of a randomised, controlled phase 1 clinical trial in 20 healthy male and female volunteers to evaluate the induction of immunity and protective efficacy of the anti-tuberculosis BCG vaccine against a controlled human malaria infection. After malaria challenge infection, BCG vaccinated volunteers present with earlier and more severe clinical adverse events, and have significantly earlier expression of NK cell activation markers and a trend towards earlier phenotypic monocyte activation. Furthermore, parasitemia in BCG vaccinated volunteers is inversely correlated with increased phenotypic NK cell and monocyte activation. The combined data demonstrate that BCG vaccination alters the clinical and immunological response to malaria, and form an impetus to further explore its potential in strategies for clinical malaria vaccine development.

Immune activation induces long-term alterations of setpoints, impacting responses to subsequent unrelated stimuli. Here the authors show that volunteers vaccinated with BCG respond to controlled human malaria infection with increased clinical symptoms and an inverse correlation between immune activation markers and parasitemia.

Heterologous effects of infant BCG vaccination: potential mechanisms of immunity

The current antituberculosis vaccine, BCG, was derived in the 1920s, yet the mechanisms of BCG-induced protective immunity and the variability of protective efficacy among populations are still not fully understood. BCG challenges the concept of vaccine specificity, as there is evidence that BCG may protect immunized infants from pathogens other than Mycobacterium tuberculosis – resulting in heterologous or nonspecific protection. This review summarizes the up-to-date evidence for this phenomenon, potential immunological mechanisms and implications for improved childhood vaccine design. BCG induces functional changes in infant innate and adaptive immune compartments, encouraging their collaboration in the first year of life. Understanding biological mechanisms beyond heterologous BCG effects is crucial to improve infant protection from infectious diseases.

Non-specific effects of vaccines: Current evidence and potential implications

Besides protection against specific microorganisms, vaccines can induce heterologous or non-specific effects (NSE). Epidemiological data suggest that vaccination with live-attenuated vaccines such as Bacillus Calmette-Guérin (BCG), measles vaccine, and oral polio vaccine results in increased overall childhood survival, and several of these observations have been confirmed in randomized trials. Immunological mechanisms mediating NSE include heterologous lymphocyte effects and induction of innate immune memory (trained immunity). Trained immunity induces long-term functional upregulation of innate immune cells through epigenetic and metabolic reprogramming. An overview of the epidemiological evidence of non-specific effects of vaccines and the latest insights regarding the biological mechanisms behind this phenomenon is presented, and future research priorities and potential implications are discussed.

Bacillus Calmette-Guérin-Induced Trained Immunity Is Not Protective for Experimental Influenza A/Anhui/1/2013 (H7N9) Infection in Mice

Avian influenza A of the subtype H7N9 has been responsible for almost 1,600 confirmed human infections and more than 600 deaths since its first outbreak in 2013. Although sustained human-to-human transmission has not been reported yet, further adaptations to humans in the viral genome could potentially lead to an influenza pandemic, which may have severe consequences due to the absence of pre-existent immunity to this strain at population level. Currently there is no influenza A (H7N9) vaccine available. Therefore, in case of a pandemic outbreak, alternative preventive approaches are needed, ideally even independent of the type of influenza virus outbreak. Bacillus Calmette-Guérin (BCG) is known to induce strong heterologous immunological effects, and it has been shown that BCG protects against non-related infection challenges in several mouse models. BCG immunization of mice as well as human induces trained innate immune responses, resulting in increased cytokine responses upon subsequent ex vivo peripheral blood mononuclear cell restimulation. We investigated whether BCG (Statens Serum Institut-Denmark)-induced trained immunity may protect against a lethal avian influenza A/Anhui/1/2013 (H7N9) challenge. Here, we show that isolated splenocytes as well as peritoneal macrophages of BCG-immunized BALB/c mice displayed a trained immunity phenotype resulting in increased innate cytokine responses upon ex vivo restimulation. However, after H7N9 infection, no significant differences were found between the BCG immunized and the vehicle control group at the level of survival, weight loss, pulmonary influenza A nucleoprotein staining, or histopathology. In conclusion, BCG-induced trained immunity did not result in protection in an oseltamivir-sensitive influenza A/Anhui/1/2013 (H7N9) challenge mouse model.

Evidence of Increase in Mortality After the Introduction of Diphtheria-Tetanus-Pertussis Vaccine to Children Aged 6-35 Months in Guinea-Bissau: A Time for Reflection?

Background

Whole-cell diphtheria–tetanus–pertussis (DTP) and oral polio vaccine (OPV) were introduced to children in Guinea-Bissau in 1981. We previously reported that DTP in the target age group from 3 to 5 months of age was associated with higher overall mortality. DTP and OPV were also given to older children and in this study we tested the effect on mortality in children aged 6–35 months.

Methods

In the 1980s, the suburb Bandim in the capital of Guinea-Bissau was followed with demographic surveillance and tri-monthly weighing sessions for children under 3 years of age. From June 1981, routine vaccinations were offered at the weighing sessions. We calculated mortality hazard ratio (HR) for DTP-vaccinated and DTP-unvaccinated children aged 6–35 months using Cox proportional hazard models. Including this study, the introduction of DTP vaccine and child mortality has been studied in three studies; we made a meta-estimate of these studies.

Results

At the first weighing session after the introduction of vaccines, 6–35-month-old children who received DTP vaccination had better weight-for-age z-scores (WAZ) than children who did not receive DTP; one unit increase in WAZ was associated with an odds ratio of 1.32 (95% CI = 1.13–1.55) for receiving DTP vaccination. Though lower mortality compared with not being DTP-vaccinated was, therefore, expected, DTP vaccination was associated with a non-significant trend in the opposite direction, the HR being 2.22 (0.82–6.04) adjusted for WAZ. In a sensitivity analysis, including all children weighed at least once before the vaccination program started, DTP (±OPV) as the most recent vaccination compared with live vaccines or no vaccine was associated with a HR of 1.89 (1.00–3.55). In the three studies of the introduction of DTP in rural and urban Guinea-Bissau, DTP-vaccinated children had an HR of 2.14 (1.42–3.23) compared to DTP-unvaccinated children; this effect was separately significant for girls [HR = 2.60 (1.57–4.32)], but not for boys [HR = 1.71 (0.99–2.93)] (test for interaction p = 0.27).

Conclusion

Although having better nutritional status and being protected against three infections, 6–35 months old DTP-vaccinated children tended to have higher mortality than DTP-unvaccinated children. All studies of the introduction of DTP have found increased overall mortality.

Adjuvanted influenza vaccines

In spite of current influenza vaccines being immunogenic, evolution of the influenza virus can reduce efficacy and so influenza remains a major threat to public health. One approach to improve influenza vaccines is to include adjuvants; substances that boost the immune response. Adjuvants are particularly beneficial for influenza vaccines administered during a pandemic when a rapid response is required or for use in patients with impaired immune responses, such as infants and the elderly. This review outlines the current use of adjuvants in human influenza vaccines, including what they are, why they are used and what is known of their mechanism of action. To date, six adjuvants have been used in licensed human vaccines: Alum, MF59, AS03, AF03, virosomes and heat labile enterotoxin (LT). In general these adjuvants are safe and well tolerated, but there have been some rare adverse events when adjuvanted vaccines are used at a population level that may discourage the inclusion of adjuvants in influenza vaccines, for example the association of LT with Bell's Palsy. Improved understanding about the mechanisms of the immune response to vaccination and infection has led to advances in adjuvant technology and we describe the experimental adjuvants that have been tested in clinical trials for influenza but have not yet progressed to licensure. Adjuvants alone are not sufficient to improve influenza vaccine efficacy because they do not address the underlying problem of mismatches between circulating virus and the vaccine. However, they may contribute to improved efficacy of next-generation influenza vaccines and will most likely play a role in the development of effective universal influenza vaccines, though what that role will be remains to be seen.

Hypothesis: stimulation of trained immunity as adjunctive immunotherapy in cancer

Cancer immunotherapy has steadily progressed during the past decades, with checkpoint inhibitor therapy becoming the latest and one of the most promising treatments. Despite the progress, most of the patients do not respond or develop resistance, and novel additional approaches are needed to improve the clinical effectiveness of immunotherapy. Trained immunity (TI) has been described recently as a process of epigenetic and metabolic reprogramming that induces a long-term enhanced function of innate immune cells. TI is considered to have beneficial effects in improving host response to infections and vaccination, and increasing evidence suggests that TI-mediated mechanisms also have useful and potent antitumor effects. We hypothesized that novel and more effective approaches for immunotherapy in cancer may involve induction of TI, alone or in combination with current immunotherapies.

New Insights Contributing to the Development of Effective Vaccines and Therapies to Reduce the Pathology Caused by hRSV

Human Respiratory Syncytial Virus (hRSV) is one of the major causes of acute lower respiratory tract infections (ALRTI) worldwide, leading to significant levels of immunocompromisation as well as morbidity and mortality in infants. Its main target of infection is the ciliated epithelium of the lungs and the host immune responses elicited is ineffective at achieving viral clearance. It is thought that the lack of effective immunity against hRSV is due in part to the activity of several viral proteins that modulate the host immune response, enhancing a Th2-like pro-inflammatory state, with the secretion of cytokines that promote the infiltration of immune cells to the lungs, with consequent damage. Furthermore, the adaptive immunity triggered by hRSV infection is characterized by weak cytotoxic T cell responses and secretion of low affinity antibodies by B cells. These features of hRSV infection have meant that, to date, no effective and safe vaccines have been licensed. In this article, we will review in detail the information regarding hRSV characteristics, pathology, and host immune response, along with several prophylactic treatments and vaccine prototypes. We will also expose significant data regarding the newly developed BCG-based vaccine that promotes protective cellular and humoral response against hRSV infection, which is currently undergoing clinical evaluation.

Boosting efferocytosis in alveolar space using BCG vaccine to protect host against influenza pneumonia

Efferocytosis by alveolar phagocytes (APs) is pivotal in maintenance of lung homeostasis. Increased efferocytosis by APs results in protection against lethal acute lung injury due to pulmonary infections whereas defective efferocytosis by APs results in chronic lung inflammation. In this report, we show that pulmonary delivery of Bacillus Calmette-Guerin (BCG) significantly enhances efferocytosis by APs. Increased efferocytosis by APs maintains lung homeostasis and protects mice against lethal influenza pneumonia. Intranasally treated wild type C57Bl/6 (WT) mice with BCG showed significant increase in APs efferocytosis in vivo compared to their PBS-treated counterparts. All BCG-treated WT mice survived lethal influenza A virus (IAV) infection whereas all PBS-treated mice succumbed. BCG-induced resistance was abrogated by depleting AP prior to IAV infection. BCG treatment increased uptake, and digestion/removal of apoptotic cells by APs. BCG significantly increased the expression of TIM4 on APs and increased expression of Rab5 and Rab7. We demonstrated that increased efferocytosis by APs through pulmonary delivery of BCG initiated rapid clearance of apoptotic cells from the alveolar space, maintained lung homeostasis, reduced inflammation and protected host against lethal IAV pneumonia.

Evaluation of trained immunity by β-1, 3 (d)-glucan on murine monocytes in vitro and duration of response in vivo

The β-1, 3 (d)-glucan (β-glucan) present in the cell wall of Candida albicans induces epigenetic changes in human monocytes resulting in primed macrophages exhibiting increased cytokine responsiveness to reinfection. This phenomenon is referred to as trained immunity or innate immune memory. However, whether β-glucan can reprogramme murine monocytes in vitro or induce lasting effects in vivo has yet to be elucidated. Thus, purified murine spleen-derived monocytes were primed with β-glucan in vitro and assessed for markers of differentiation and survival. Important macrophage cell markers during monocyte-to-macrophage differentiation were downregulated and survival enhanced due to partial inhibition of apoptosis. Increased survival and not the β-glucan training effect explained the elevated production of tumour necrosis factor-α (TNFα) and interleukin-6 (IL-6) induced by subsequent lipopolysaccharide (LPS) challenge. In vivo, 4 days after systemic administration of β-glucan, mice were more responsive to LPS challenge as shown by the increased serum levels of TNFα, IL-6 and IL-10, an effect shown to be short lived as enhanced cytokine production was lost by day 20. Here, we have characterised murine macrophages derived from β-glucan-primed monocytes based on their surface marker expression and for the first time provide evidence that the training effect of β-glucan in vivo declines within a 3-week period.

Zebra Fish Lacking Adaptive Immunity Acquire an Antiviral Alert State Characterized by Upregulated Gene Expression of Apoptosis, Multigene Families, and Interferon-Related Genes

To investigate fish innate immunity, we have conducted organ and cell immune-related transcriptomic as well as immunohistologic analysis in mutant zebra fish (Danio rerio) lacking adaptive immunity (rag1^−/−) at different developmental stages (egg, larvae, and adult), before and after infection with spring viremia carp virus (SVCV). The results revealed that, compared to immunocompetent zebra fish (rag1^+/+), rag1^−/− acquired increased resistance to SVCV with age, correlating with elevated transcript levels of immune genes in skin/fins and lymphoid organs (head kidney and spleen). Gene sets corresponding to apoptotic functions, immune-related multigene families, and interferon-related genes were constitutively upregulated in uninfected adult rag1^−/− zebra fish. Overexpression of activated CASPASE-3 in different tissues before and after infection with SVCV further confirmed increased apoptotic function in rag1^−/− zebra fish. Concurrently, staining of different tissue samples with a pan-leukocyte antibody marker showed abundant leukocyte infiltrations in SVCV-infected rag1^−/− fish, coinciding with increased transcript expression of genes related to NK-cells and macrophages, suggesting that these genes played a key role in the enhanced immune response of rag1^−/− zebra fish to SVCV lethal infection. Overall, we present evidence that indicates that rag1^−/− zebra fish acquire an antiviral alert state while they reach adulthood in the absence of adaptive immunity. This antiviral state was characterized by (i) a more rapid response to viral infection, which resulted in increased survival, (ii) the involvement of NK-cell- and macrophage-mediated transcript responses rather than B- and/or T-cell dependent cells, and (iii) enhanced apoptosis, described here for the first time, as well as the similar modulation of multigene family/interferon-related genes previously associated to fish that survived lethal viral infections. From this and other studies, it might be concluded that some of the characteristics of mammalian trained immunity are present in lower vertebrates.

A single, low dose of a cGMP recombinant BCG vaccine elicits protective T cell immunity against the human respiratory syncytial virus infection and prevents lung pathology in mice

Human respiratory syncytial virus (hRSV) is a major health burden worldwide, causing the majority of hospitalizations in children under two years old due to bronchiolitis and pneumonia. HRSV causes year-to-year outbreaks of disease, which also affects the elderly and immunocompromised adults. Furthermore, both hRSV morbidity and epidemics are explained by a consistently high rate of re-infections that take place throughout the patient life. Although significant efforts have been invested worldwide, currently there are no licensed vaccines to prevent hRSV infection. Here, we describe that a recombinant Bacillus Calmette-Guerin (BCG) vaccine expressing the nucleoprotein (N) of hRSV formulated under current good manufacture practices (cGMP rBCG-N-hRSV) confers protective immunity to the virus in mice. Our results show that a single dose of the GMP rBCG-N-hRSV vaccine retains its capacity to protect mice against a challenge with a disease-causing infection of 1×10⁷ plaque-forming units (PFUs) of the hRSV A2 clinical strain 13018-8. Compared to unimmunized infected controls, vaccinated mice displayed reduced weight loss and less infiltration of neutrophils within the airways, as well as reduced viral loads in bronchoalveolar lavages, parameters that are characteristic of hRSV infection in mice. Also, ex vivo re-stimulation of splenic T cells at 28days post-immunization activated a repertoire of T cells secreting IFN-γ and IL-17, which further suggest that the rBCG-N-hRSV vaccine induced a mixed, CD8⁺ and CD4⁺ T cell response capable of both restraining viral spread and preventing damage of the lungs. All these features support the notion that rBCG-N-hRSV is a promising candidate vaccine to be used in humans to prevent the disease caused by hRSV in the susceptible population.

Unravelling the nature of non-specific effects of vaccines-A challenge for innate immunologists

Epidemiological observations have shown that vaccines can influence morbidity and mortality more than can be ascribed to target-disease immunity. A growing number of immunological studies have helped identify possible biological mechanisms to explain these so-called nonspecific effects (NSE) of vaccines, including heterologous T-cell reactivity and innate immune memory or 'trained innate immunity', which involves epigenetic reprogramming of innate immune cells. Here, we review the epidemiological evidence for NSE as well as human, animal and in vitro immunological data that could explain these NSE, and discuss priorities for future epidemiologic and immunologic studies to further unravel the biology and optimize the benefits of current and new vaccines.

Neonatal Bacillus Calmette-Guérin vaccination alleviates lipopolysaccharide-induced neurobehavioral impairments and neuroinflammation in adult mice

The Bacillus Calmette-Guérin (BCG) vaccine is routinely administered to human neonates worldwide. BCG has recently been identified as a neuroprotective immune mediator in several neuropathological conditions, exerting neuroprotection in a mouse model of Parkinson's disease and slowing the progression of clinically isolated syndrome in patients with multiple sclerosis. The immune system is significantly involved in brain development, and several types of neonatal immune activations exert influences on the brain and behavior following a secondary immune challenge in adulthood. However, whether the neonatal BCG vaccination affects the brain in adulthood remains to be elucidated. In the present study, newborn C57BL/6 mice were injected subcutaneously with BCG (10⁵ colony forming units) or phosphate-buffered saline (PBS). A total of 12 weeks later, the mice were injected intraperitoneally with 330 µg/kg lipopolysaccharide (LPS) or PBS. The present study reported that the neonatal BCG vaccination alleviated sickness, anxiety and depression-like behavior, lessened the impairments in hippocampal cell proliferation and downregulated the proinflammatory responses in the serum and brain that were induced by the adult LPS challenge. However, BCG vaccination alone had no evident influence on the brain and behavior in adulthood. In conclusion, the neonatal BCG vaccination alleviated the neurobehavioral impairments and neuroinflammation induced by LPS exposure in adult mice, suggesting a potential neuroprotective role of the neonatal BCG vaccination in adulthood.

Harnessing the beneficial heterologous effects of vaccination

Clinical evidence strongly suggests that certain live vaccines, in particular bacille Calmette-Guérin (BCG) and measles vaccines, can reduce all-cause mortality, most probably through protection against non-targeted pathogens in addition to the targeted pathogen. The underlying mechanisms are currently unknown. We discuss how heterologous lymphocyte activation and innate immune memory could promote protection beyond the intended target pathogen and consider how vaccinologists could leverage heterologous immunity to improve outcomes in vulnerable populations, in particular the very young and the elderly.

Long-term activation of the innate immune system in atherosclerosis

Efforts to reverse the pathologic consequences of vulnerable plaques are often stymied by the complex treatment resistant pro-inflammatory environment within the plaque. This suggests that pro-atherogenic stimuli, such as LDL cholesterol and high fat diets may impart longer lived signals on (innate) immune cells that persist even after reversing the pro-atherogenic stimuli. Recently, a series of studies challenged the traditional immunological paradigm that innate immune cells cannot display memory characteristics. Epigenetic reprogramming in these myeloid cell subsets, after exposure to certain stimuli, has been shown to alter the expression of genes upon re-exposure. This phenomenon has been termed trained innate immunity or innate immune memory. The changed responses of 'trained' innate immune cells can confer nonspecific protection against secondary infections, suggesting that innate immune memory has likely evolved as an ancient mechanism to protect against pathogens. However, dysregulated processes of immunological imprinting mediated by trained innate immunity may also be detrimental under certain conditions as the resulting exaggerated immune responses could contribute to autoimmune and inflammatory diseases, such as atherosclerosis. Pro-atherogenic stimuli most likely cause epigenetic modifications that persist for prolonged time periods even after the initial stimulus has been removed. In this review we discuss the concept of trained innate immunity in the context of a hyperlipidemic environment and atherosclerosis. According to this idea the epigenome of myeloid (progenitor) cells is presumably modified for prolonged periods of time, which, in turn, could evoke a condition of continuous immune cell over-activation.

Immune System Disorders and Epigenetics

Our understanding of epigenetics in complex diseases is rapidly advancing and increasingly influencing the practice of medicine. Much is known about disruption of chromatin-modifying enzymes in malignant disease, but knowledge of irregular epigenetics in immune-driven disorders is just emerging. Epigenetic factors, such as DNA or histone modifications, are indispensable for precise gene expression in diverse immune cell types. Thus a disruption of epigenetic landscapes likely has a large impact on immune homeostasis. Moreover, the low concordance rates for most autoimmune diseases suggest that epigenetics contribute to immune tolerance disturbance. Here we review the important role of epigenetics for initiation, maintenance, tolerance, and training of immune responses. We discuss evolving evidence that DNA/histone modifications and chromatin-modifying enzymes are altered in immune-based diseases. Furthermore, we explore the potential of small molecules targeting epigenetic machinery, some of which are already used in oncology, as a way to reset the immune response in disease.