BCG: a vaccine with multiple faces

ABSTRACT

BCG has been recommended because of its efficacy against disseminated and meningeal tuberculosis. The BCG vaccine has other mechanisms of action besides tuberculosis protection, with immunomodulatory properties that are now being discovered. Reports have shown a significant protective effect against leprosy. Randomized controlled trials suggest that BCG vaccine has beneficial heterologous (nonspecific) effects on mortality in some developing countries. BCG immunotherapy is considered the gold standard adjuvant treatment for non-muscle-invasive bladder cancer. BCG vaccine has also been tested as treatment for diabetes and multiple sclerosis. Erythema of the BCG site is recognized as a clinical clue in Kawasaki disease. BCG administration in the immunodeficient patient is associated with local BCG disease (BCGitis) or disseminated BCG disease (BCGosis) with fatal consequences. BCG administration has been associated with the development of autoimmunity. We present a brief review of the diverse facets of the vaccine, with the discovery of its new modes of action providing new perspectives on this old, multifaceted and controversial vaccine.

Introduction

In 1908 Albert Calmette, a bacteriologist and Camille Guerin, a veterinarian, working at the Pasteur Institute, began an experiment to develop a vaccine by attenuating a bovine strain; Mycobacterium bovis until it lost its virulence.¹ In 1921, after 13 years, they finally introduced the vaccine obtained by serial subculturing on glycerinated bile potato medium (231 passages). The strain, known today as bacille Calmette-Guerin (BCG), not only it lost its virulence for calves and guinea pigs but both also conferred protection against virulence challenge and induced a certain degree of tuberculin sensitivity.¹ The first dose of BCG was given orally, and this route continued until 1924 when it was replaced with more immunogenic routes,¹ however in Brazil the vaccine continued to be administered orally until the 70s when it was replaced by the intradermal vaccine.² Currently the World Health Organization (WHO) recommends intradermal injection of BCG on the deltoid region,³ although some physicians in Brazil still prescribe the oral vaccine until this day.²

Over 4 billion doses administered make BCG the most widely used vaccine worldwide.⁴ Policies and practices vary across the world, with some countries, such as the United Kingdom that have had universal BCG vaccination programs, while others (including Canada, Italy, Belgium, the Netherlands and the United States) that either recommended BCG only for high-risk groups or did not advocate BCG for any purpose.⁵ BCG vaccination policies have varied by the number of doses used, the age at which vaccination was given and the methods used to deliver the vaccine.⁶

The BCG vaccine has other effects besides tuberculosis (TB) protection, including immunomodulatory properties. It is also definitively harmful in certain immunodeficient patients (Figure 1). The purpose of this article is to present a narrative review of the diverse facets of the BCG vaccine. Several comprehensive reviews of the different topics presented are available in the literature.^7–10 We conducted a search Pubmed, MEDLINE, EMBASE, Web of Science and Scopus using combination of the terms: BCG and protective vaccine, tuberculosis, complications, autoimmunity, immunodeficiency, cancer, Kawasaki disease and heterologous effects. We reviewed the abstracts and retrieved relevant articles. The BCG vaccine has different faces, just like the ancient Roman god Janus; the god of beginnings, gates, transitions, duality and endings, who usually is depicted as having two faces since he looks to the future and to the past, an analogy appropriate with the use of this vaccine.

Figure 1.

The many faces of BCG vaccine.

BCG as a protective vaccine

BCG has been recommended because of its partial protective effect against active tuberculosis, although it has greater efficacy against disseminated and meningeal disease in children than for prevention of pulmonary disease in adults.^11,12 It doesn’t protect against primary TB infection or reactivation of latent TB or even Mycobacterium tuberculosis (MTB) person to person transmission spreading.¹³ However, autopsy studies suggest BCG decreases the size of pulmonary tuberculosis foci and animal models showed a reduction of bacillary burden.¹⁴ Recently, epidemiological studies showed that BCG vaccine also protects against TB infection and that BCG has been associated with an improved microbiological response to treatment.^11–18

Although the primary goal of BCG vaccines is to protect against TB, reports also showed a significant protective effect against leprosy. The protective properties of the BCG against M. leprae is likely due to T and B-cell cross-reactivity of the mycobacterial antigens.¹⁹ BCG vaccine also protects against nontuberculous mycobacterial lymphadenitis and Buruli ulcer.²⁰

Heterologous effects of BCG immunization

The immune memory response to previously encountered pathogens can sometimes alter the response to and the course of infection of an unrelated pathogen by a process known as heterologous, nonspecific immunity. This phenomenon has been observed with BCG. The WHO Special Advisory Group systematic review showed that neonatal BCG vaccination is associated with a 30% reduction in all-cause mortality (RR 0.70; CI 95%, 0.49–1.01).²¹ Randomized controlled trials suggest that BCG vaccine also has beneficial heterologous effects on mortality in developing countries.^21–29 A similar effect on all-cause mortality was documented in the UK and the USA following the introduction of universal BCG immunization for the prevention of TB in the 1950s. The benefit of BCG vaccination is conferred as early as 7 days of life.²⁷ Studies have reported a clinical benefit with BCG vaccination in the prevention of pneumonia.²⁵ and the risk of pneumonia related deaths by 50%.³⁰

Adaptive features of innate immunity described as trained immunity have been documented after BCG vaccination.²³ BCG induces trained immunity and nonspecific protection from infection through epigenetic reprogramming of innate immune cells, with the effect lasting up to one year.^31,32 BCG vaccine induces an increase in proinflammatory cytokine production by NK cells in response to mycobacteria and unrelated pathogens with a memory-like effect.³² Important lines of evidence from experimental studies in mice indicate that BCG protects against viral pathogens.³³ In a recent review, studies supporting the nonspecific beneficial effects for leishmaniasis and malaria are described.³⁴ A beneficial effect of BCG on vaccine responses has also been reported.²²

The presence of a maternal BCG scar is associated with an increased proinflammatory immune response in infants.³⁵ BCG-vaccinated infants have decreased infectious illness unrelated to BCG during the first three months of life if the mother had a history of BCG vaccination.³⁶Animal studies have shown conflicting results of BCG modulating atherosclerosis development,^36–38 in a mouse hyperlipidemic model, BCG reduces plasma non-HDL cholesterol levels and delays atherosclerotic lesion formation.³⁸ This effect is a result of accelerated hepatic uptake of cholesterol enriched lipoprotein remnants, reduced intestinal cholesterol absorption and reduced foam cell formation.³⁸

BCG vaccination prevents hyperoxic lung injury in a neonatal rat model,³⁹ in this study, BCG improved alveolar surface area and fibrosis compared with hyperoxia-exposed placebo group, and the protective effects were attributed to preserved IL-13 expression levels and upregulation of NFkB1, FGF.BP1 and VEGF-A genes.³⁹

The concept of a protective effect of BCG against cancer is not new.^40–45 BCG immunization may have a protective effect against cancer in later life by enhancing the development of the immune system and there are a number of experimental studies in animals showing the inhibitory effect of BCG on tumor growth.^40,41 A recent meta-analysis suggests that early vaccination with BCG might be associated with a reduced risk of leukemia,⁴⁵ and a retrospective review showed that childhood BCG vaccination was associated with a lower risk of lung cancer development.⁴⁶ Prior immunization with BCG has been associated with prolongation of survival in patients who later contract melanoma.⁴³ BCG vaccination prevents diabetes in non-obese diabetic mice, however no evidence of prevention of diabetes has been found in case-control studies in humans.^47–50 BCG vaccination has also been hypothesized to prevent Alzheimer disease derived from studies in a mouse model.^51,52

BCG as a diagnostic tool

BCG vaccination usually results in an ulcer that develops into a characteristic scar, and a previous scar can be used as evidence of past vaccination. Tuberculin conversion usually takes place within 6–8 weeks after BCG vaccination.⁵³ BCG scars and a positive tuberculin skin test (TST) reaction are easily measurable markers of BCG-induced cell-mediated immunity.⁵⁴ BCG induced TST reactivity has been considered a surrogate of protective immunity by some authors.^55,56 The presence of a scar could be important since having a scar and a TST response after BCG vaccination has been associated with lower mortality risk.^57–59 Furthermore, some researchers have suggested that revaccination of scar-negative children should be considered.⁶⁰ However other groups consider that the presence of a scar does not diagnose protection against tuberculosis and failure to develop a scar might be an indication of poor technique;⁶¹ this is still a controversial issue.

BCG can provide an indirect diagnostic tool in Kawasaki disease (KD), an acute febrile illness that affects young children. with most cases diagnosed before 5 years of age. KD is a challenging diagnosis that relies on clinical signs and symptoms. Interestingly, erythema and induration of the BCG site is increasingly recognized as a significant clinical clue described by Kawasaki for the first time in 1970.⁶² To this day, BCG site erythema has been considered to be an important specific sign of the disease, particularly in infants.^63,64 (Figure 2). Kakisaka et al. report an 11 month-old boy with redness on the BCG inoculation site reported as Human Herpes Virus type 6 infection (although the patient presented many features of KD), and recently, BCG erythema was observed in a 7 month-old infant with measles without KD.^65,66 A survey conducted in Japan identified BCG erythema in 49.9% of KD patients, with a particularly high prevalence in more than 70% of children aged 3–20 months-old.⁶⁷ In Mexico, among 399 patients, only 24.3% presented BCG reaction in the inoculation site, probably due to ethnic differences or type of BCG vaccine used.^68,69 The interval from BCG vaccination to the onset of KD seems to be important (less than 806 months after vaccination).⁶⁹ Tseng et al. studied the BCG reaction pattern and found that the intensity of the reaction (with the more severe form described as “bull’s eye pattern”) correlated with the severity of the illness coronary abnormalities and elevated aspartate aminotransferase. These authors suggested it as a potential biomarker of the disease.⁷⁰ BCG erythema in KD is histologically characterized by infiltration of T cells and macrophages with expression of IL-1β, TNF-α, IFN-γ and IL-2 suggestive of a delayed hypersensitivity reaction.^71,72 The BCG reactivation in KD has been hypothetically attributed to cross-reactivity between mycobacterial heat shock protein (hsp) 65 and a human homolog hsp63.⁷³ Autoantibodies to peroxiredoxin have been found in KD, and a homolog of peroxiredoxin is present in BCG.⁷⁴ BCG immunization has been used in animal models to induce coronary arteritis.^75,76 Kadowaki et al. were the first to describe erythema and induration in both the BCG and PPD inoculation site in KD.⁷⁷ Based on this observation, the tuberculin skin test has been suggested to be of value in the diagnosis of incomplete forms of KD.⁷⁸ It was reported from Italy that 11 patients with Kawasaki disease had a positive tuberculin intradermal test compared to controls.⁷⁹ However, in a later study in the US, nine patients with Kawasaki disease showed no reaction to intradermal tuberculin, and the authors attribute the difference in results to the use of different tuberculin products.⁸⁰ The consensus is that erythema in BCG vaccination site is a specific and useful sign in KD particularly in infants.⁸¹

Figure 2.

Erythema in BCG in Kawasaki Disease, note the variety of lesions of this useful diagnostic sign.

BCG as treatment

BCG immunotherapy is considered the gold standard adjuvant treatment for non-muscle-invasive bladder cancer with more than 3 million cancer treatments annually.^82–84 The exact mechanism of action of intravesical BCG is not completely understood but it is believed that the antitumor activity of BCG is derived from a local nonspecific immunological boost that recruits immunocompetent cells.⁸⁵ BCG induces tumor cells in the bladder to secrete cytokines and chemokines and to be subsequently killed by cytotoxic cells. BCG also increases the expression of MHC II molecules and ICAM-1 in tumor cells after instillation.^86–90

BCG is also listed as an intralesional treatment for inoperable stage III in-transit melanoma in the National Comprehensive Cancer Network Guidelines. Reports of 50% regression of injected lesions and 17% regressions of non-injected lesions in immunocompetent patients have been reported, with a role of γδT cells in destroying the malignant cells⁹¹

In the late 1990s epidemiological studies reported a decreased prevalence of allergic asthma in children immunized with BCG vaccine.⁹² A prospective cohort study assessed that BCG vaccination improved lung function while reducing the use of rescue medications in patients with asthma compared with placebo.⁹³ This hygiene hypothesis states that BCG neonatal vaccination alleviates the symptoms of asthma in both animal models and human beings by increasing the secretion of Th1 cytokines.⁹³

BCG exerts neuroprotection in a mouse model of Parkinson´s disease and experimental autoimmune encephalomyelitis.^94–97

Neonatal BCG vaccination alleviates LPS induced neurobehavioral impairment and neuroinflammation in mice,⁹⁴ two studies have suggested that BCG decrease activity in Magnetic Resonance Imaging (MRI) in relapsing-remitting Multiple Sclerosis (MS).^98,99 Clinically Isolated Syndrome (CIS) is the first symptomatic neurologic episode caused by demyelination in the CNS, which may or may not be a precursor of MS (the conversion rate from CIS to MS rates ranges from 30% to 82%).^100,101 Ristori et al. reported results of a clinical trial of BCG vaccine in patients with CIS.¹⁰⁰ Following a single dose of BCG or placebo participants underwent monthly MRI and they found that the mean number of total gadolinium enhancing lesions was substantially lower in subjects receiving BCG vs. placebo.¹⁰⁰ In a recent study, Extended Freeze Drying preparations of BCG (inactivated BCG) was used in the experimental autoimmune encephalomyelitis mouse model and it attenuated central nervous system inflammation via plasmacytoid dendritic cells inducing suppressive IL-10 secreting Tregs.¹⁰² To this day, BCG to treat CIS or MS still is not recommended, as its efficacy is not established.¹⁰¹

BCG vaccine has also been suggested as treatment for diabetes,^103,104 Faustman et al. demonstrated that the BCG vaccine can lower blood glucose levels to a near normal range in patients with established type I diabetes mellitus and can improve glycemic control in this group of patients.^103–105 These effects are attributed to an accelerated glucose consumption in immune cells due to increased glycolysis and reduced oxidative phosphorylation.^105,106 Induction of TNF through BCG vaccination has the desired cellular immunologic effects of selective death of insulin autoreactive T cells and expansion of beneficial regulatory T cells.^107,108 The immunomodulatory effect of BCG in autoimmune and allergic diseases still awaits definitive studies.

BCG has also been used to treat recurrent respiratory papillomatosis reducing relapses by modifying the balance of effector and regulatory T cells subsets.^109,110 Recently, Intralesional BCG has also been used successfully for recurrent multiple warts.¹¹¹

Complications of BCG immunization

The risks of BCG immunization are attributable mainly to the fact that it is a live, attenuated vaccine. Immunization leads to a usually asymptomatic but bacteremic infection and within 8–12 weeks a cellular immune response to mycobacterial antigens can be detected.^112,113 Autopsy studies of BCG-immunized children who have died of other causes show that acid-fast organisms and granulomas are distributed widely in many organs.¹¹⁴

During the early years of BCG use a tragic event was documented, an accident known as the Lübeck disaster occurred in the years 1929 to 1933. In this tragic event, 251 neonates received three oral doses of the BCG vaccine contaminated with MTB; 173 infants developed TB and 72 died.¹¹⁵

The prevalence of BCG-associated complications in the general population has been estimated at around 1 in 2500 vaccines for localized complications (BCGitis) and 1 in 100,000 for disseminated BCGosis. In immunodeficient patients BCG administration is associated with local BCG-disease (BCGitis) or disseminated BCG disease (BCGosis) with fatal consequences (Figure 3).

Figure 3.

Cutaneous manifestation of disseminated BCG in a SCID patient.

BCGosis has been recognized in children with HIV infection,^116,117 studies in humans and macaques have suggested that BCG may increase susceptibility to HIV infection through inducing trained immunity and activation of CD4 T cells.^118,119 The induction and expansion of CD4+ T cells may serve as targets for HIV replication.¹²⁰ A particular complication observed in patients with HIV is the presence of BCG-induced immune reconstitution inflammatory syndrome after retroviral therapy.¹²¹

Defects impairing cellular immunity, phagocytic function and IFN-γ mediated immunity have been classically associated with BCG vaccine complications.^122,123 Data on 349 BCG vaccinated patients with severe combined immunodeficiency revealed BCG complications in 51% of them, with 34% presenting disseminated disease (33,000-fold increase over the general population).¹²³ Involvement of extra regional lymph nodes, skin and lungs was the most common clinical presentation.¹²³ Age at BCG vaccination appeared to be a strong predictor for BCG-associated complications, with patients vaccinated within the first month of life having a higher risk.¹²³ BCGitis and BCGosis are also a feared common complication of patients with chronic granulomatous disease.^124,125.

In the mid-1990s a study of patients with idiopathic disseminated BCG disease, inborn errors of immunity to mycobacteria were discovered.^126,127 These patients are normally resistant to most other microbes. Susceptibility to mycobacterial disease including BCG has led to a phenotype referred to as Mendelian susceptibility to mycobacterial disease (MSMD).¹²⁸ Mutations of 14 different genes have shown to cause MSMD (IL12B, IL12RB1, ISG15, TYK2, IRF8, SPPL2A, CYBB, IFNGR1, IFNGR2, STAT1, NEMO, il12rb2, il23r, sspl2A).^129,130 Clinical manifestations are highly variable, from severe, fatal, early-onset forms to circumscribed, late-onset forms that improve with age.¹²⁹ Remarkably, the study of these diseases has permitted the dissection of the essential pathways to control mycobacteria.¹³⁰ Interestingly, BCG disease appears to protect against environmental mycobacteria illness in patients with IL-12 deficiency or IL-12R deficiency.¹³¹ As new inborn errors of immunity are described, new forms of BCG susceptibility and complications are recognized, as recently reviewed with the examples of STAT-1 GOF, APDS1 and APDS2 patients.¹³²

As neonatal BCG is included in the vaccination programs in many countries, it is important to identify and avoid administration of BCG to infants who have immunodeficiencies. To protect these vulnerable patients strategies are to defer the administration of BCG for 2–6 months after delivery or to institute routine newborn screening for severe combined immunodeficiency.¹³⁰

BCG complication has also been observed when BCG is used to treat diseases. In BCG intravesical therapy for superficial forms of bladder cancer, systemic complications affect roughly 5% of patients and can manifest months or years after the last intravesical BCG instillation.

Autoimmune phenomena after intravesical instillation of BCG are rare but occur, mainly arthritis,¹³³ the most common form is reactive arthritis that is usually asymmetric and predominantly affects large joints. The autoimmune response (conjunctivitis-urethritis-arthritis; Reiter syndrome) after BCG instillation seems to occur particularly in genetically susceptible individuals such as those with HLA-B27 (60%).^133–135 Again, a molecular mimicry mechanism has been suggested as the cause for these complications with the mycobacterium heat shock protein proteoglycan (hsp65) sharing homology with a mammalian cartilage proteoglycan link protein.¹³⁶ It is possible that CD4+ T cells specific for an antigen shared by cartilage and mycobacteria are activated by antigen presenting cells mounting the shared peptide on a class II molecule. Intravesical BCG vaccine administration has also been associated with autoimmune diseases such as Henoch-Schoenlein pupura and Guillain Barre syndrome.^137–140

In experimental models BCG can accelerate autoimmune diseases.¹⁴¹ Cases of endophthalmitis, uveitis, vitiligo, meningitis, aortitis, or mycotic aneurysms have been reported as well as central nervous system vasculitis.^142–144 BCG administration has also been related to the pathogenesis of Takayasu arteritis or KD.^59,145Recently, a study in Finland showed that discontinuing universal BCG vaccination did not changed the incidence rates of KD, however the mean age at diagnosis increased, suggesting that the pathogenesis of KD may be associated with immunological priming by BCG vaccination.¹⁴⁶

Concluding remarks and future perspectives

BCG protects young children against miliary TB and tuberculous meningitis, but is less effective in adults, with insufficient and inconsistent protection against pulmonary TB, the major source of TB transmission. BCG does not confer life-long protective immunity against TB.¹⁴⁷ There is an ongoing debate related to revaccination of BCG following vaccination in infancy, particularly among TST-negative adolescents as they move into adulthood, the period of highest risk of pulmonary TB.^148–151 BCG also protects against Mycobacterium leprae and potentially enhances immunity against a variety of microorganisms through heterologous immunity induction.

The WHO recommendation is to give a single dose to all healthy neonates at birth,¹⁵² however infants with severe combined immunodeficiency are asymptomatic at birth and they are a population at risk to develop fatal disseminated BCG infection. Newborn screening programs for primary immunodeficiency diseases enable early diagnosis of potentially affected children and prevent administration of live attenuated vaccines including BCG, but unfortunately these programs are not widely available. This situation compels the medical community to support the implementation of newborn screening programs worldwide and defer the administration of the vaccine until the diagnoses of primary immunodeficiency diseases and HIV are ruled out. This policy is of greatest importance in least developed countries where the burden of TB and HIV is the highest, but the resources for general newborn screening are frequently not available. Postponement of BCG immunization until an age when usually clinical manifestations of an immunodeficiency become apparent is a viable solution.

The emergence of drug-resistant MTB strains and the high prevalence of HIV infection have significantly complicated TB prognosis and treatment. These facts highlight the need for new and more effective vaccines.¹⁵³ Efforts have been made to develop new vaccines that include subunit vaccines (attenuated viral vector or adjuvanted fusion protein) and whole cell vaccines (genetically attenuated MTB, recombinant BCG, killed MTB or M. vaccae).^154–156 Local pulmonary mucosal BCG delivery has proven to reduce TB disease where standard intradermal injection fails in animal models.¹⁵⁶

BCG possesses immunomodulatory properties that have only started to be revealed. Different strains of BCG may have different immunomodulatory effects as well as complications, including autoimmune diseases.^53,157 While there is still no replacement for the licensed BCG vaccine, the discovery of the new effects and modes of action of this multifaceted and controversial vaccine continues.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.