BCG, a live attenuated vaccine from Mycobacterium bovis, is used to prevent tuberculosis in children and is one of the vaccines planned for immunization in China and most countries in the world. BCG vaccination does not usually cause serious reactions, but in rare cases, BCG enters the bloodstream and systemic dissemination occurs, which can often be fatal and is called disseminated BCG infection. Disseminated BCG infections are the most serious adverse reactions, sometimes with a familial distribution and a high number of people with a poor clinical prognosis. Disseminated BCG infections are extremely rare, with an incidence of 0.06-1.56 per million and a mortality rate of approximately 60%. Disseminated BCG infections occur in patients with established immunodeficiency diseases such as severe combined immunodeficiency disease (SCID), chronic granulomatous disease (CGD), and AIDS, in addition to patients with some as yet unidentified immunodeficiency diseases. The common feature of most patients is their inability to produce gamma interferon (IFNγ) or their non-response to IFNγ. To date, immunological and genetic analyses of affected families have identified mutations in seven different genes that control the interleukin 12 (IL12)-dependent, high-throughput IFNγ pathway. Studying the relationship between disseminated BCG infection and inherited defects in the interleukin 12/interferon gamma pathway is of great value for the prevention of disseminated infection after BCG vaccination and is a guide for clinical diagnosis and treatment.
BCG vaccine is a vaccine made from Mycobacterium tuberculosis isolated from a cow suffering from tuberculous mastitis by the Pasteur Institute in France in 1902, which has lost its virulence and pathogenicity after several generations of culture, but still preserves its antigenicity. BCG vaccination in China is arranged within a few days after the birth of infants, and the methods include scratch method, oral vaccination, and intradermal injection, which can effectively prevent childhood tuberculosis, especially for tuberculous meningitis and cornual tuberculosis. , such as anti-tuberculosis, anti-viral, anti-tumor, anti-hypersensitive disease, treatment of chronic bacterial infections, antifungal, etc. BCG vaccine is an intracellular pathogen that can survive and multiply in mononuclear phagocytes. It usually does not cause serious reactions after vaccination, but only multiplies locally, causing redness, pus or ulcers, swollen lymph nodes and other tuberculosis lesions such as caseous necrosis. However, most of them can dissipate on their own or heal quickly with appropriate anti-TB treatment. Only in some cases does it cause local ulcers and swollen lymph nodes, pus, or even serious abnormal reactions such as BCG osteomyelitis and systemic spread of BCG.
2. Disseminated BCG infection The most serious adverse reaction that occurs after BCG vaccination is disseminated BCG infection. This complication is often seen in immunocompromised children and sometimes has a familial distribution. It usually develops months to years after BCG vaccination, with localized skin or lymph nodes on the vaccinated side first becoming enlarged and ulcerated, and gradually involving internal organs, mostly the lungs, liver and spleen. The disease generally presents with prolonged fever, weight loss, and easy combination of opportunistic infections. On pathological examination, typical dry cool-like necrosis, epithelioid cells and tuberculosis granuloma composed of Langhans giant cells surrounded by lymphocytic infiltration are seen in those with normal immune function measurements; those with immune function deficiency do not have typical tuberculosis-like granulomatous lesions, but show septic granulomatous lesions. A large number of antacid mycobacteria can be found in the lesion tissue or cerebrospinal fluid. Treatment with anti-tuberculosis drugs and immune-enhancing agents such as cytokines is required and the prognosis is poor.
Mechanisms of disseminated BCG infection Host defense against BCG relies primarily on cellular immunity; therefore, individuals with inherited or acquired T-cell immunodeficiency are susceptible to BCG. Disseminated BCG infections occur in addition to patients with established immunodeficiency diseases such as severe combined immunodeficiency disease (SCID), chronic granulomatous disease (CGD), and AIDS. It also occurs in patients with a number of as yet unidentified immunodeficiency diseases, sometimes with a familial distribution. The common pathogenic mechanism in these children is an impaired IFNγ-mediated immune mechanism that is unable to produce IFNγ or does not respond to IFNγ, and the severity of the clinical presentation depends on the type of genetic defect. To date, genetic and immunological analyses of affected families have identified mutations in 10 different genes in the interleukin 12 (IL12)-dependent, high-throughput IFNγ pathway.
1. IL12-dependent IFNγ pathway Upon stimulation of macrophages and dendritic (DC) cells by B. bifidum, secreted IL12 and IL23 induce IFNγ production by Th1 cells and natural killer (NK) cells, a process that requires co-stimulation by IL18. IFNγ activates macrophages to kill intracellular microbial infections through a number of mechanisms. In addition IL12 secretion by macrophages also requires co-stimulation by Toll-like receptors (TLR) [7]. the NEMO/NFκB signaling pathway also has a co-stimulatory effect on IFNγ and IL12 production.
IL12 is a cytokine secreted by phagocytes and DC cells responding to microbial stimuli and consists of a heterodimeric molecule consisting of two polypeptide chains, p40 and p35, linked by disulfide bonds. iFNγ, IL4 and CD40/CD40L increase the ability of cells to produce IL12. IL12 specifically recognizes the membrane surface of NK cells, activated T and B cells expressing the The IL12 receptor consists of two polypeptide chains, IL12Rβ1 and IL12β2. Upon binding of IL12 to the receptor, IL12R dimerizes, activating the activated protein tyrosine kinases (Jak2 and Tyk2) coupled to it, phosphorylating tyrosine sites in specific regions of the intracellular segment of the receptor signal transduction chain, which then acts as “The activated STAT4 monomer polymerizes and forms a homodimer that crosses the nuclear membrane to the nucleus, where it recognizes its specific target sequence on DNA and initiates specific gene transcription. It induces the proliferation of activated T cells and enables the development and differentiation of Th0 cells along the Th1 pathway and the production of IFNγ and tumor necrosis factor (TNFα) to enhance the cell-mediated immune response. Thus IL-12 helps to fight against many kinds of infections, especially bacterial infections and intracellular parasitic infections.
Interleukin 23 (IL23) is a heterodimeric cytokine consisting of two polypeptide chains, IL12p40 and p19. IL23 receptor consists of IL12Rβ1 and IL23R chains. il23 is secreted by activated human macrophages and dendritic cells. il23 has similar effects to IL12 and induces IFNγ production.
Interleukin 18 (IL18), also produced by antigen-presenting cells, is widely distributed in the body, has multiple immunomodulatory functions, and is a member of the interleukin 1 (IL1) family, but its biological function is similar to that of IL-12. IL18 receptor consists of two chains (IL18R1 and IL18R2) and is transmitted via IL1 receptor-associated kinase (IRAK), MyD88 IL12 induces the expression of IL18 receptor in addition to the cellular production of IFN-γ. IL18 has antitumor effects by promoting Th1 cell development, proliferation and differentiation as well as enhancing NK cell activity, and together with IL12 and IL23 promotes the induction of IFNγ production.
IFNγ is a homodimeric molecule expressed on most nucleated cells and is produced by activated antigen-specific type I helper T cells (Th1 cells) and natural killer (NK) cells. iFNγ interacts with its cell surface receptor (IFNγR) to regulate the expression and function of a variety of genes. iFNγR consists of two transmembrane proteins, IFNγR1 and IFNγR2. IFNγR1 is the ligand-binding chain and IFNγR2 is the signaling chain that acts as a signal transducer. upon binding of IFNγ to the receptor, Jak1 and Jak2, which are coupled to IFNγR1 and IFNγR2, respectively, are activated, with subsequent phosphorylation of the tyrosine at position 440 of IFNγR1. This leads to tyrosine phosphorylation at position 701 of the cellular signal transducer and activator of transcription-1 (STAT1) anchored at this site. The phosphorylated STAT1 binds less strongly to the IFNγR1 chain, dissociates to form homo- or heterodimers and translocates to the nucleus where it binds to the γ-activating sequence in the promoter region of the IFNγ-inducible gene and begins transcription.
2. Mutation types that predispose to disseminated BCG infection Disseminated BCG infection is a typical disease caused by defects in the IL12/IFNγ pathway. Specific types of mutations include: mutations in the R1 or R2 genes encoding the IFNγ receptor, resulting in the absence of IFNγ receptor expression or function; deletion of the gene encoding the IL12 (IL12-p40) induction chain and failure to produce IL12; mutations in the genes encoding the IL12 and IL23 receptor β1 chains, resulting in the absence of IL12Rβ1 gene expression on the cell surface and thus failure to respond to IL12 mutations in the gene encoding the signal transduction molecule STAT1, which diminishes the ability to respond to IFNγ; mutations in the gene encoding TYK2, which results in failure to respond to IL12; and mutations in the gene encoding NEMO, which affects the production of cytokines such as IL12 and IFNγ. In addition, animal experiments have shown that STAT4, Myd88, and IRAK4 defects also render mice susceptible to a variety of pathogens, including Mycobacterium tuberculosis. The severity of the phenotype correlates with the type of genetic defect and these defects are described in detail below.
IFNγR1 defects: IFNγR1 defects are classified as complete or partial defects. Patients with complete IFNγR1 defects have no IFNγ receptor expression on the cell surface, and in rare cases, although IFNγR1 expression is normal, they lack the IFNγ binding site and do not respond to IFNγ at all. patients with complete IFNγR1 defects tend to develop BCG or other pathogenic mycobacterial infections before the age of 3 years, and most cases die with a poor prognosis for anti-mycobacterial drug therapy. Some IFNγR1-deficient patients express the IFNγR1 receptor on the surface of monocytes, but the affinity of the receptor for the ligand is significantly reduced, and there is a partial response to IFNγ. Symptoms are milder and easier to treat than in complete defects.
IFNγR2 deficiency: IFNγR2 deficiency is also divided into complete and partial deficiency. IFNγR2 is the IFNγ receptor signaling chain and is an important determinant of the cellular response to IFNγ. Patients with complete IFNγR2 deficiency have cells that are completely unresponsive to IFNγ and have clinical symptoms as severe as those of complete IFNγR1 deficiency, predisposing them to severe outbreaks of infection in early childhood and ineffective treatment. Some IFNγR2-deficient patients have cells that are partially responsive to IFNγ stimulation, and therefore the infections they suffer are often curable.
Complete IL12-p40 deficiency leads to IL12 and IL23 deficiency: alterations in either subunit of IL12 (p40 or p35) may affect the body’s immune protection. IL12-p40-deficient patients cannot produce IL12, so lymphocytes cannot secrete IFNγ after stimulation by Mycobacterium or Salmonella antigens in vitro and are susceptible to pathogenic Mycobacterium and Salmonella, with more severe symptoms than IL12Rβ1 deficiency and a 38% mortality rate.
Complete IL12Rβ1 deficiency results in the inability to respond to IL12 and IL23: the mutation results in the absence of IL12Rβ1 chain expression on the surface of activated T cells and therefore a complete lack of response to IL12 and IL23. Antigen (BCG) stimulation of peripheral blood lymphocytes in patients leads to a significant decrease in IFNγ production. patients with IL12Rβ1 defects have milder clinical manifestations than complete IFNγR1 or IFNγR2 defects, have a partial response to IFNγ treatment, have curable infections and have a later onset of disease.
STAT1 deficiency: Complete STAT1 deficiency is a very rare autosomal recessive form of STAT1 mutation that fails to produce interferon-stimulated gene factor 3 (ISGF3) consisting of phosphorylated STAT1/STAT2/p48 and gamma-activating factor (GAF) consisting of phosphorylated STAT1/STAT1 homodimer, and responds to IFNγ Poor or no response, with reduced production of IL12 and IFNγ.
Tyk2 deficiency: Tyk2 induces Jak2 and Tyk2 tyrosine phosphorylation via IL12R signaling and subsequent activation of several STATs including STAT4. transfer of phosphorylated STAT4 dimers to the nucleus is important for IFNγ production. Patients exhibit defects in several cytokine signaling pathways, including the IL12 pathway, and the addition of wild-type Tyk2 to patient T cells corrects the cytokine signaling defect.
NEMO defects: NFκB must regulatory protein (NEMO) is an important component of the NFκB signaling pathway that binds together the IKK complex kinases IKKα, IKKβ and the regulatory subunit ELKS to control signaling through the receptor. mutations in NEMO are X-linked recessive and reduce IL12 and IFNγ production, which can lead to severe mycobacterial disease.
Other defects that may cause disseminated BCG infection: Myd88 myeloid differentiation factor88 (MyD88), a member of the Toll/IL-lR family and the death domain family, is a key junction molecule in the Toll-like receptor (TLR) signaling pathway and is used in experimental mycobacteria. In experimental infections, MyD88 deficiency causes mice to show susceptibility to many pathogens. STAT4 is an important component of the IL12/IFNγ pathway, mainly in lymphoid and myeloid tissues, and regulates cell proliferation and differentiation at the molecular level, playing an important role in inflammation of the lung and immune diseases. STAT4 activation induces Th1 polarization and induces a range of gene expression, and STAT4-deficient mice exhibit susceptibility to intracellular bacterial infections.
Treatment and prevention of disseminated BCG infection Disseminated BCG infection is a Mycobacterium bovis infection and can be treated symptomatically with anti-TB drugs such as rifampicin, isoniazid, streptomycin, and ethambutol. Since most of the children with this disease have immunodeficiency and are prone to multiple opportunistic fungal or bacterial infections, most of these children lack response to treatment and have a poor prognosis. The type of immunodeficiency is identified as early as possible in these children, and in addition to routine immune function tests, special immune tests such as IL12/IFNγ pathway and genetic testing are required. Then, according to the type of immunodeficiency and the clinical situation, empirical antimicrobial therapy and targeted immunotherapy will be performed, presuming the type of opportunistic infectious agent. In children with IL-12 receptor deficiency and partial IFNγ receptor deficiency, IFNγ therapy may be added to conventional anti-infection, and bone marrow transplantation is required for combined immunodeficiency disease and complete IFNγ receptor gene deficiency. Because systemic disseminated BCG disease is a rare disease, prevention is more difficult. More feasible preventive measures are: intermarriage between close relatives is strictly prohibited; BCG vaccination should be withheld for newborns with suspected hereditary diseases in both parents, and the decision to vaccinate or not should be made after investigation; BCG vaccination is prohibited for adults treated with immunosuppressive drugs. The ideal prevention is genetic counseling, prenatal diagnosis, and newborn screening, and BCG should be avoided in newborns with abnormal screening or a family history of suspected immunodeficiency disease. If possible immunodeficiencies in individuals are identified early, disseminated BCG disease can be avoided. On the other hand children with abnormal reactions after BCG vaccination should be closely followed up and monitored for early detection, diagnosis and treatment of BCG infection and possible immunodeficiency diseases.
Conclusion BCG vaccine is a live attenuated vaccine of Mycobacterium bovis and is effective in preventing tuberculosis in children. Most infants show local reactions at the vaccination site 2 to 3 weeks after vaccination, and there is generally no systemic reaction. A small number of infants may spread to local lymph nodes, and very few children develop distant or systemic disseminated infections. The development of disseminated BCG infection is mainly related to the patient’s immune status and genetic factors. This complication is often seen in children with congenital or acquired immunodeficiency and has a familial distribution, and most patients have a poor prognosis. Genetic disorders associated with defective FNγ production or response are common causal mechanisms, manifested as an impaired IFNγ-mediated immune mechanism that fails to produce IFNγ or does not respond to IFNγ. Treatment can be applied with anti-tuberculosis drugs and cytokines, but most patients are treated ineffectively and have a high mortality rate.
The IL12-dependent high-throughput IFNγ pathway has been shown to be important in protecting the host against intracellular microbial infections including Mycobacterium and Salmonella, and defects in this pathway are a major cause of disseminated BCG infections, and the severity of the phenotype correlates with the type of genetic defect. Identified mutations in IFNGR1, IFNGR2, IL12-p40, IL12Rβ1, STAT1, TYK2 and NEMO lead to increased susceptibility to Mycobacterium infection. In addition, STAT4, Myd88, and IRAK4 defects also cause susceptibility to a variety of pathogens, including Mycobacterium tuberculosis, in mice. Performing IL12/IFNγ pathway as well as genetic testing for early detection of possible immune defects in individuals is extremely important for the prevention, diagnosis, and treatment of disseminated BCG disease, and is an important direction for future research.