Version 1
: Received: 12 May 2023 / Approved: 15 May 2023 / Online: 15 May 2023 (05:30:34 CEST)
Version 2
: Received: 13 June 2023 / Approved: 14 June 2023 / Online: 14 June 2023 (04:33:00 CEST)
Anes, E.; Pires, D.; Mandal, M.; Azevedo-Pereira, J.M. ESAT-6 a Major Virulence Factor of Mycobacterium tuberculosis. Biomolecules2023, 13, 968.
Anes, E.; Pires, D.; Mandal, M.; Azevedo-Pereira, J.M. ESAT-6 a Major Virulence Factor of Mycobacterium tuberculosis. Biomolecules 2023, 13, 968.
Anes, E.; Pires, D.; Mandal, M.; Azevedo-Pereira, J.M. ESAT-6 a Major Virulence Factor of Mycobacterium tuberculosis. Biomolecules2023, 13, 968.
Anes, E.; Pires, D.; Mandal, M.; Azevedo-Pereira, J.M. ESAT-6 a Major Virulence Factor of Mycobacterium tuberculosis. Biomolecules 2023, 13, 968.
Abstract
Mycobacterium tuberculosis (Mtb) the causative agent of human tuberculosis (TB) is one of the most successfully adapted-human pathogens. Human-to-human transmission occurs at high rates through aerosols containing bacteria, but the pathogen evolved prior to the establishment of crowded populations. Mtb has developed a particular strategy to ensure persistence in the host until an opportunity for transmission arise. It has refined its lifestyle to obviate the need for virulence factors such as capsules, flagella, pilli, or toxins to circumvent mucosal barriers. Instead, the pathogen uses host macrophages, where it establishes intracellular niches, for its migration into the lung parenchyma and other tissues and for the induction of long-lived latency in granulomas. Finally, at the end of the infection cycle Mtb induces necrotic cell death of macrophages to escape to extracellular milieu and instructs a strong inflammatory response required for progression from latency to disease and transmission. Common to all these events is ESAT-6, one of the major virulence factors secreted by the pathogen. This review highlights the recent advances in understanding the role of ESAT-6 in hijacking macrophage function to establish a successful infection and transmission and its use as a target for the development of diagnostic tools, and vaccines.
Medicine and Pharmacology, Epidemiology and Infectious Diseases
Copyright:
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Received:
14 June 2023
Commenter:
Elsa Anes
Commenter's Conflict of Interests:
Author
Comment: new title: ESAT-6 a Major Virulence Factor of Mycobacterium tuberculosis There isalso aneed to develop better diagnostic tools, more sensitive, usingsimple instruments easy to handle in remote geographical areas, and cost-effective for rapid detection of Mtb.Moreover, although new methods arebeing implemented[4],currently availablemethods do not allow differentiation betweendifferent stages of infection [4-8].Mycobacterium tuberculosis (Mtb) is considered the mainly causative agent of human TB[9]. Mtbinfection primarilyaffects the lungs, a condition thatcontributes to the high transmissibility by the respiratory route[9]. Although not transmitted from persontoperson, extrapulmonary TB is a second form of disease manifestation that particularly affectslessimmunocompetent individuals, namely children or people co-infected with HIV[10-12].In addition to Mtb, other pathogens can also infect humans and cause TB with similar clinical symptoms[13-20].These, includeM. africanum,which is restricted to humans in West Africa [21-23], where it causes nearly half of all pulmonary TB [18-20] andthe animal-adapted species M. bovis, which isestimated to cause TB in about 2% of the world’s population [24,25].M.capraeis responsiblefor a smaller burden of zoonotic TB[26]. They all belong to the Mycobacterium tuberculosis species complex (MTBC) which also includes the more distant M. canettiigroup and additional speciesthat cause TB in animals such asM. microti[27], M. pinnipedii, M. orygis and M. mungi[20,23,28-30].Although each of these animal MTBC variants causes TB in its host species, they may triggerslightor no disease outside of their adapted host,especially in immunocompetent hosts[13,30,31].However, conditions such as geographic prevalence of infected animals, close humanscontacts, route oftransmission (milk, infected meet or airbornedroplets containing pathogens), and conditions such as HIV infectionor other immunosuppressive conditions should be taken in account to assessthetruethreat topublic health [32].
Subunit vaccine candidates are designed to boost BCG-primed responsesor to induce specific immune responsesas examples of the therapeutic vaccinesunder developmentto be used in conjugation with antibiotic treatment[140-143]. ESAT- 6-based subunit vaccines, already in clinical trials, include TB/FLU-04L and use a live-attenuated influenza A virus vector. Other subunit vaccines in clinical evaluationsuch as H6, H56:IC31and GamTBvacare provided innon-viral delivery systems and are based on fusion immunogenic proteins, including ESAT-6 together with adjuvants[142,144-146].An intranasallyadministeredsubunit vaccinecombiningESAT-6 and cyclic dimeric adenosine monophosphate (c-di-AMP) was designed to promote macrophageautophagy via the STING pathwaywith impact on pathogen killing and humoral and cellular immune responses[147].Other fusion proteins, suchas dodicin-ESAT-6, result in the increased expression of the costimulatory molecules CD80/CD86 and the antigen presenting machinery MHC-II in a mouse model of the infection[148].In the improvement of these combinations with ESAT-6in subunit vaccine candidates the chaperone-like protein HtpGMtb ofMtbhas been studiedand structure-based designstudies points formore effective antigen properties forimmunization[149-151].
However, the challenges associated with subunit ESAT-6 based subunit vaccines is that they will virtuallyoverride all modern available immunodiagnostics (IGRAs and skin tests) thereby limiting the ability to distinguish immunized from infected people. To overcome this ESAT-6 free IGRA tests are under development aimed at differentiatingESAT-6 subunit responsesin vaccinated individuals. These assays are based on the combination of a cocktail of proteins that are part of ESX-1 operon including the CFP-10 chaperon but excluding ESAT-6. Preliminary results show promising effects on theinductionIFNγ and the chemokine biomarker IP-10 allowing distinguish subunit-vaccinated from non-vaccinated individuals[75].An ESAT-6/CFP-10-based skin test, C-Tb, has revealed similar sensitivity for active TB compared withtuberculin skin test (TST) and QuantiFERON-TB-Gold-In-Tube (QFT), but with limited sensitivity in children and HIV-infected individuals[154].The diagnosis of TB in children as in HIV-infected individuals is often difficult due to several factors,including the frequent extrapulmonary TB,arecurrentassociatedsputum smear-negative and the fact that that they have a lowhumoral response to mycobacterial antigens using conventionalenzyme-linked immunosorbent assays (ELISA) for TB immunodiagnosis[51,155].To overcome this,immunodiagnostic methods using antigen combinations are being developed, using “cocktails” ofPhoP, ESAT-6, CFP-10and the latency-associated antigen Acr-1. This combination of antigens includesproteins that are associated with the different stages of disease progression[156]and has been shown tosignificantly improvethe sensitivity.Recently, another cocktail usingbiosynthetically derived peptides of ESAT-6 and Ag-85allowed thedetection ofirrespective specific-IgG in patients’ sera, providing a reliable diagnosis of active TB in children[157].Other situations were extrapulmonary TB is frequent is in bovine TB and the corresponding zoonotic form in humans. The available methods to distinguish M.bovis from Mtb are based in polymerase chain reaction and genomic sequencing. This imposes limitations requiring bacilli isolation difficult to obtain in this context and the lack of technicalcapacity in high burden countries[32].
Commenter: Elsa Anes
Commenter's Conflict of Interests: Author
There is also a need to develop better diagnostic tools, more sensitive, usingsimple instruments easy to handle in remote geographical areas, and cost-effective for rapid detection of Mtb. Moreover, although new methods are being implemented [4], currently availablemethods do not allow differentiation betweendifferent stages of infection [4-8].Mycobacterium tuberculosis (Mtb) is considered the mainly causative agent of human TB [9]. Mtbinfection primarily affects the lungs, a condition thatcontributes to the high transmissibility by the respiratory route[9]. Although not transmitted from person to person, extrapulmonary TB is a second form of disease manifestation that particularly affects lessimmunocompetent individuals, namely children or people co-infected with HIV [10-12]. In addition to Mtb, other pathogens can also infect humans and cause TB with similar clinical symptoms [13-20].These, include M. africanum, which is restricted to humans in West Africa [21-23], where it causes nearly half of all pulmonary TB [18-20] and the animal-adapted species M. bovis, which is estimated to cause TB in about 2% of the world’s population [24,25].M.caprae is responsible for a smaller burden of zoonotic TB [26]. They all belong to the Mycobacterium tuberculosis species complex (MTBC) which also includes the more distant M. canettii group and additional species that cause TB in animals such as M. microti [27], M. pinnipedii, M. orygis and M. mungi [20,23,28-30]. Although each of these animal MTBC variants causes TB in its host species, they may trigger slight or no disease outside of their adapted host, especially in immunocompetent hosts [13,30,31]. However, conditions such as geographic prevalence of infected animals, close humans contacts, route of transmission (milk, infected meet or airborne droplets containing pathogens), and conditions such as HIV infection or other immunosuppressive conditions should be taken in account to assess the true threat to public health [32].
Subunit vaccine candidates are designed to boost BCG-primed responses or to induce specific immune responses as examples of the therapeutic vaccinesunder development to be used in conjugation with antibiotic treatment [140-143]. ESAT- 6-based subunit vaccines, already in clinical trials, include TB/FLU-04L and use a live-attenuated influenza A virus vector. Other subunit vaccines in clinical evaluationsuch as H6, H56:IC31 and GamTBvac are provided innon-viral delivery systems and are based on fusion immunogenic proteins, including ESAT-6 together with adjuvants [142,144-146].An intranasally administered subunit vaccinecombining ESAT-6 and cyclic dimeric adenosine monophosphate (c-di-AMP) was designed to promote macrophage autophagy via the STING pathway with impact on pathogen killing and humoral and cellular immune responses [147]. Other fusion proteins, suchas dodicin-ESAT-6, result in the increased expression of the costimulatory molecules CD80/CD86 and the antigen presenting machinery MHC-II in a mouse model of the infection [148]. In the improvement of these combinations with ESAT-6 in subunit vaccine candidates the chaperone-like protein HtpGMtb ofMtb has been studied and structure-based designstudies points for more effective antigen properties forimmunization [149-151].
However, the challenges associated with subunit ESAT-6 based subunit vaccines is that they will virtually override all modern available immunodiagnostics (IGRAs and skin tests) thereby limiting the ability to distinguish immunized from infected people. To overcome this ESAT-6 free IGRA tests are under development aimed at differentiatingESAT-6 subunit responses in vaccinated individuals. These assays are based on the combination of a cocktail of proteins that are part of ESX-1 operon including the CFP-10 chaperon but excluding ESAT-6. Preliminary results show promising effects on theinduction IFNγ and the chemokine biomarker IP-10 allowing distinguish subunit-vaccinated from non-vaccinated individuals [75].An ESAT-6/CFP-10-based skin test, C-Tb, has revealed similar sensitivity for active TB compared with tuberculin skin test (TST) and QuantiFERON-TB-Gold-In-Tube (QFT), but with limited sensitivity in children and HIV-infected individuals [154]. The diagnosis of TB in children as in HIV-infected individuals is often difficult due to several factors,including the frequent extrapulmonary TB, a recurrentassociated sputum smear-negative and the fact that that they have a low humoral response to mycobacterial antigens using conventional enzyme-linked immunosorbent assays (ELISA) for TB immunodiagnosis [51,155]. To overcome this,immunodiagnostic methods using antigen combinations are being developed, using “cocktails” of PhoP, ESAT-6, CFP-10 and the latency-associated antigen Acr-1. This combination of antigens includesproteins that are associated with the different stages of disease progression [156] and has been shown tosignificantly improve the sensitivity. Recently, another cocktail using biosynthetically derived peptides of ESAT-6 and Ag-85 allowed the detection of irrespective specific-IgG in patients’ sera, providing a reliable diagnosis of active TB in children[157].Other situations were extrapulmonary TB is frequent is in bovine TB and the corresponding zoonotic form in humans. The available methods to distinguish M.bovis from Mtb are based in polymerase chain reaction and genomic sequencing. This imposes limitations requiring bacilli isolation difficult to obtain in this context and the lack of technicalcapacity in high burden countries [32].