Therapeutic Strategies for Tuberculosis: Progress and Lessons Learned

Tuberculosis (TB), caused by the pathogen Mycobacterium tuberculosis (Mtb), remains a significant global health threat. According to the Global Tuberculosis Report published by the World Health Organization (WHO) in 2023, 1.3 million TB-related deaths occurred worldwide in 2022, emphasizing that only after COVID-19, TB was the second leading cause of death from a single infectious agent^[1]. Since the first identification of Mtb by Robert Koch in 1882^[2], substantial progress has been made in its prevention and treatment. The advent of the Bacillus Calmette-Guérin (BCG) vaccine and the introduction of streptomycin initiated a new era in the prevention and chemotherapy of TB infection, radically transforming the landscape of TB control^[3]. These advancements have led to the development of combination therapy strategies, which form the basis of the current standard 6-month therapy regimen^[4]. Between 2015 and 2022, the total number of TB-related deaths decreased by 19%, and the global treatment success rate for newly diagnosed and relapsed TB cases reached 86% in 2021, indicating significant achievements in TB treatment worldwide^[1]. The WHO has established ambitious targets to reduce the annual incidence of TB by 10% by 2025 and 17% by 2035^[5]. However, the COVID-19 pandemic has severely hindered progress, particularly in regions with a high prevalence of TB^[6].

In the current global health landscape, TB management continues to face significant obstacles, primarily due to the emergence of drug-resistant strains, including rifampin-resistant TB (RR-TB), multidrug-resistant TB (MDR-TB), and extensively drug-resistant TB (XDR-TB). These resistant forms require complex and expensive treatment protocols that compromise the efficacy of conventional therapies^[7]. Recent statistics estimate that by 2022, approximately 410,000 cases of drug-resistant TB (MDR/RR-TB) will occur globally, with approximately 160,000 deaths related to these strains. The treatment success rate of drug-resistant TB remains low (59%), highlighting the compelling need for more effective therapeutic interventions^[1].

Therefore, innovative treatment strategies are required to overcome these challenges. The global health community is prioritizing the development of novel drug therapies and vaccines to reduce treatment duration, minimize side effects, and improve treatment outcomes^[8]. This review explores the latest progress in TB treatment, with an emphasis on chemotherapy and immunotherapy, and offers insights into potential future developments in TB treatment strategies.

First-line drugs are crucial in the treatment of newly diagnosed TB and typically form the foundation of standard anti-TB regimens. The primary drugs—isoniazid (INH), rifampicin (RIF), pyrazinamide (PZA), and ethambutol (EMB)—are often combined to maximize therapeutic efficacy and minimize the development of drug resistance^[1]. INH inhibits the synthesis of mycobacterial cell walls, making it effective against active and latent TB infections. RIF exerts bactericidal effects by blocking bacterial RNA synthesis, thereby shortening the infection period and preventing resistance^[9]. Although PZA exhibits relatively weak early bactericidal activity (EBA), it is regarded as an indispensable sterilizing drug for treating DS-TB. EMB inhibits a crucial enzyme involved in cell wall synthesis, demonstrates moderate EBA activity, and is employed as an adjunctive medication to mitigate drug resistance^[10].

The treatment regimen for patients with TB follows the guidelines established in 2010. The drugs are combined in a six-month Directly Observed Therapy, Short-course regimen. The initial two-month intensive phase is crucial for eradicating rapidly dividing bacteria, thus preventing the development of resistance. The subsequent phase targets bacteria in a dormant metabolic state to prevent a relapse^[11]. However, Mtb can develop resistance to these drugs, particularly when they are used inappropriately. Therefore, strict medication adherence coupled with drug sensitivity testing is vital for preventing the development of drug resistance^[4].

In the treatment of TB, second-line drugs are crucial when first-line drugs fail, or drug resistance develops. These drugs are primarily used to treat RR-TB/MDR-TB. Treatment selection depends on the results of drug susceptibility testing (DST)^[12]. Additionally, factors such as patient tolerance, side effects, availability, cost, and treatment history are significant when choosing appropriate medications^[13].

Second-line drugs, while offering more effective management of complex cases and expanding treatment options for patients with drug resistance, are often associated with a higher risk of severe side effects and increased treatment costs^[14]. These drugs are categorized into several classes: fluoroquinolones, including moxifloxacin, levofloxacin, and gatifloxacin; injectable agents, such as amikacin, capreomycin, and kanamycin; oral antibiotics, such as cyclosporine, para-aminosalicylic acid, ethionamide, prothionamide, clofazimine, and terizidone; and novel drugs like bedaquiline^[15]. Bedaquiline is a novel anti-TB drug that has shown significant efficacy in treating MDR-TB and XDR-TB. Its primary mechanism of action involves the inhibition of adenosine triphosphate (ATP) synthase of Mtb, thereby reducing ATP production and inhibiting bacterial growth^[16]. Clinical treatment typically includes a high-dose initial phase, followed by a lower maintenance-dose phase lasting up to six months. Bedaquiline has significantly improved cure rates in patients with drug-resistant TB; however, its widespread use must be managed carefully to prevent further drug resistance^[17].

Second-line anti-TB drugs remain vital in combating drug-resistant TB; however, their use poses significant challenges. Balancing the effectiveness of these treatments with the potential for severe side effects, ensuring patient adherence, and managing the high costs are critical concerns. Refining drug combinations, reducing adverse effects, and improving accessibility are essential to make these treatments more practical and widely available in diverse clinical settings.

Fluoroquinolones, play a pivotal role in TB treatment, marking a significant advancement in the medical field and demonstrating the potential for repurposing antibiotics. These drugs are highly regarded for their broad-spectrum antimicrobial activity, effectively combating various bacterial species, including gram-positive and gram-negative bacteria. Among the fluoroquinolones, moxifloxacin, a fourth-generation agent, has become indispensable for treating MDR-TB. It is crucial in clinical treatment and is the focus of clinical trials aimed at shortening the treatment duration for MDR-TB, thereby offering new hope to patients^[18]. Although levofloxacin is prescribed less frequently, it remains essential in the treatment of MDR-TB, particularly in pediatric populations, where its efficacy and safety are widely recognized. Fluoroquinolone use not only expands the treatment options for patients with TB but also illustrates innovative approaches to antibiotic use^[19]. However, their clinical application presents challenges, such as QT interval prolongation with moxifloxacin and the risk of resistance, especially in areas with a high TB burden^[20,21]. Despite these challenges, fluoroquinolones offer advantages in terms of treatment duration and side-effect profiles compared to other second-line therapies, making careful patient selection and monitoring crucial.

Linezolid, the pioneer oxazolidinone drug, has been recognized by the WHO and the Centers for Disease Control and Prevention (CDC) as a critical medication for the treatment of MDR-TB and XDR-TB. It plays a significant role in innovative six-month treatment strategies^[1]. Although the precise mechanisms of linezolid resistance remain unclear, numerous studies have identified various genetic mutations linked to this resistance in both clinical and in vitro settings. Furthermore, reports indicate that the incidence of linezolid resistance ranges from 1.9% to 10.8%, suggesting that it is relatively common^[22]. These mutations pose a challenge to the continued efficacy of these drugs. As medical research progresses, new-generation oxazolidinone drugs such as TBI-223^[23], delpazolid^[24], and sutezolid^[25] have demonstrated potential advantages over linezolid (Figure 1). These drugs reduce mitochondrial toxicity and possess broad activity against both drug-sensitive and drug-resistant TB strains, making them potentially safer and more effective alternatives to linezolid. However, these drugs, like their predecessors, may still face challenges related to the development of resistance and poor patient adherence owing to adverse effects. Further clinical studies are needed to validate these findings and fully understand the scope of their application in diverse patient groups.

Figure 1.  Comprehensive strategies for anti-TB compound development and discovery. This image was drawn using Figraw 2.0.

Second-generation bedaquiline analogs such as TBAJ-587 and TBAJ-876 have demonstrated efficacy against bedaquiline-resistant strains of Mtb, specifically those associated with the MmpS5-MmpL5 efflux system^[26,27]. Sudapyridine (WX-081), a novel anti-TB drug, was optimized based on the chemical structure of bedaquiline. In preclinical studies, sudapyridine demonstrated superior safety and antimycobacterial activity than bedaquiline, reduced the risk of cardiotoxicity, and exhibited promising inhibition against drug-resistant Mycobacterium abscessus in zebrafish models. Substitution of bedaquiline with these analogs has demonstrated their potential to decrease the development of drug resistance^[28].

Delamanid and PA-824 (now known as pretomanids) belong to the nitroimidazole class of drugs. WHO has emphasized the potential of delamanid after extensive clinical trials confirmed its safety and efficacy, recommending its use when other treatments have failed. This recommendation is based on rigorous data from multicenter, randomized, double-blind, placebo-controlled clinical trials^[29,30]. Research on pretomanids has highlighted their potential in treating drug-resistant TB. Studies indicate that pretomanid is a potent antimicrobial with a unique mechanism of action that targets both actively replicating and dormant forms of Mtb^[31,32]. Pretomanid has proven effective both as a monotherapy and in combination with other drugs, particularly when combined with bedaquiline and linezolid, to treat drug-resistant TB^[33,34].

TBI-166 (also called pyrifazimine) is primarily used to treat MDR-TB and XDR-TB^[35]. Recent advancements have indicated its significant efficacy and safety in preclinical and clinical settings. Following the successful completion of Phase I clinical trials that confirmed its tolerability, TBI-166 is currently undergoing Phase II trials^[36]. Studies have indicated that when combined with bedaquiline and pyrazinamide, TBI-166 has substantial bactericidal and sterilization effects in murine TB models^[37]. Additionally, It results in less skin pigmentation in vivo than clofazimine and exhibits a synergistic effect with mocozinone, significantly lowering the minimum inhibitory concentration^[38,39]. These findings suggest that TBI-166 is a promising novel therapeutic agent for TB.

Recent advancements in drug repurposing have also underscored the potential of sanfetrine and spectinamides to combat bacterial infections, particularly TB. Sanfetrinem, an orally bioavailable tricyclic carbapenem antibiotic, has demonstrated promise in Phase II clinical trials. It remains stable against clinically relevant β-lactamases and exhibits rapid bactericidal activity against various bacterial species. Recent studies have highlighted its efficacy against MDR-TB and latent TB, positioning it as a valuable candidate for combating bacterial infections, including TB^[40]. Spectinamides, a novel class of semisynthetic spectinomycin derivatives, demonstrate selective ribosomal inhibition and possess a targeted spectrum of anti-TB activity. These compounds have been engineered to circumvent the intrinsic efflux mechanisms of Mtb, thereby enhancing their efficacy against MDR/XDR-TB strains. Notably, spectinamides exhibited significant bactericidal activity without promoting cross-resistance to other antibiotics^[41,42].

Repurposing and reengineering conventional anti-TB compounds have significantly advanced TB treatment, providing agents with reduced toxicity and broader efficacy. However, challenges persist, particularly in terms of resistance, patient adherence, and access in low-resource settings. Future studies should prioritize validating these therapies in diverse populations, optimizing their combinations, and integrating them into standard treatment protocols.

The discovery of novel anti-TB drugs has increasingly relied on phenotypic screening combined with whole-genome sequencing. This strategy expands the range of druggable targets and identifies molecules that interact with multiple targets, ensuring drug compatibility with the physicochemical properties of bacteria, internal uptake, and metabolic stability^[43,44]. Recently, this approach yielded several candidate molecules with novel mechanisms of action. These candidates have demonstrated proof-of-concept efficacy in mouse models and are now progressing into advanced preclinical or clinical developmental stages^[45].

The synthesis and remodeling pathways of the Mtb cell wall are crucial targets for drug discovery. Key targets in the cell wall biosynthetic pathway, such as mycobacterial membrane protein large 3 (MmpL3) and decaprenylphosphoryl-β-d-ribose 2′-epimerase (DprE1), have several inhibitors currently under clinical development^[46,47]. New targets are also involved in the synthesis and remodeling of the Mtb cell wall, including beta-ketoacyl ACP synthase I (KasA), fatty acyl-AMP ligase FadD32, and polyketide synthase 13 (Pks13), which interact with specific compounds and demonstrated therapeutic efficacy in vivo^[48-50]. While cell wall synthesis is a promising domain for anti-TB drug discovery, these pathways may not be vulnerable in non-replicating bacteria.

The viability of both replicative and non-replicative Mtb relies on the energy produced by the components of the respiratory chain, making these components potential drug targets. Q203, which targets the QcrB unit of cytochrome bc₁–aa₃ oxidase, is currently in Phase II clinical trials^[51]. Cytochrome bd inhibitors such as ND-011992 and CK-2-63, although ineffective individually, significantly enhance the bactericidal activity of Q203 when used in combination^[52]. This combination effectively killed both the replicating and non-replicating mycobacteria by disrupting respiration and ATP homeostasis^[53].

The development of new chemical entities remains critical for anti-TB drug discovery. Aminoacyl-tRNA synthetases, which are essential enzymes for protein synthesis, are emerging targets, with GSK3036656 and DDD02049209 targeting mycobacterial leucine-tRNA and lysine-tRNA synthetases, respectively^[54,55]. GSK839 targets Mtb tryptophan synthetase^[56], whereas GSK2556286 modulates the cholesterol metabolic pathway, potentially shortening the treatment duration^[57]. Furthermore, the Clp protease system has emerged as a promising antibacterial target, with recent advances exhibiting significant bactericidal effects, particularly through strategies, such as BacPROTACs, which target essential Clp components for degradation^[58]. BVL-GSK098 (alpibectir), a novel chemical entity currently under early clinical evaluation, operates through a unique transcriptional regulatory mechanism by targeting the VirS regulator, thus offering a potential breakthrough in the treatment of drug-resistant TB^[59]. Although these developments have demonstrated notable efficacy, including substantial potency against Mtb, they remain in the preliminary stages of research.

Targeted Protein Degradation (TPD) is gaining attention as a novel approach to antimicrobial drug discovery. TPD in eukaryotic systems typically involves proteolysis-targeting chimeras (PROTACs) that promote ubiquitination and proteasomal degradation. Recent research has utilized TPD in bacterial systems using proteolytic complexes such as ClpC1P1P2^[60]. BacPROTACs, a specific type of PROTAC, facilitate the degradation of bacterial model proteins by directly linking target proteins to bacterial proteases^[58]. This approach differs from traditional inhibition methods and exhibits promise in improving the efficacy of current antibiotics.

Recent studies have highlighted significant advancements in anti-TB treatment regimens, particularly for MDR-TB- and XDR-TB. WHO has endorsed the BPaLM regimen, an entirely oral, six-month treatment protocol for patients with MDR/RR-TB, which includes bedaquiline, pretomanid, linezolid, and moxifloxacin^[61]. For patients with pre-extensively drug-resistant (pre-XDR-TB), a moxifloxacin-excluding BPaL regimen is recommended^[62]. These novel regimens not only reduce treatment costs but also improve patient compliance and outcomes through shorter treatment durations and reduced drug volumes^[61]. However, the emergence of resistance to these novel agents and the potential for adverse effects, especially with linezolid, remain significant challenges.

Clinical trials such as TB-PRACTECAL and ZeNix have demonstrated that these short-duration regimens offer higher cure rates and fewer adverse events than traditional treatments^[63,64]. For instance, the ZeNix trial specifically emphasized optimizing linezolid dosing to balance efficacy with toxicity, which is crucial for long-term patient adherence and minimizing adverse effects^[64]. Global research organizations such as Médecins Sans Frontières (MSF) and the UNITE4TB consortium are promoting more efficient clinical trial methodologies and a global clinical trial network to accelerate the development of new anti-TB drugs and treatment regimens. Furthermore, studies have revealed that a four-month regimen including RIF and Moxifloxacin is as effective as the standard six-month treatment regimen that has been used for over 40 years for DS-TB^[65]. This finding marks a significant shift towards shorter and more manageable treatment protocols, which could improve global TB control efforts. However, the long-term efficacy and potential development of resistance to these shorter regimens warrant continuous monitoring and further research.

Personalized treatment duration has become feasible using risk stratification algorithms that integrate patient and pathogen characteristics^[66]. For example, in TB clinical trials, risk stratification tools classify patients into various risk groups based on criteria, such as HIV status, sex, baseline cavitary disease, body mass index, and month 2 culture status. These algorithms provide a nuanced understanding of disease progression, allowing treatment protocols to be customized to the specific needs of different patient subgroups, thereby optimizing the treatment duration and ensuring a more personalized approach to TB care^[67].

Dose optimization is crucial in TB treatment, as it ensures effective bacterial eradication while minimizing drug toxicity and resistance development. Recent studies have highlighted several advances in this domain. For instance, mathematical modeling and simulations have been employed to guide the optimal dosing of moxifloxacin and linezolid against MDR-TB^[68]. RIFASHORT and other high-dose RIF trials aimed to optimize the dosage and exposure to RIF or Rifabutin to maximize the reduction in the burden of Mtb or achieve treatment shortening^[69]. In the ZeNIX study, the minimum effective dose of linezolid combined with bedaquiline and pretomanid was identified to address the frequent adverse events observed in the NIX-TB trial, leading to linezolid discontinuation^[64]. Future research must integrate pharmacokinetic (PK) and pharmacodynamic (PD) data to optimize individualized TB treatment regimens. Studies on population PKs and Bayesian dose adjustment in TB treatment have demonstrated significant potential for optimizing dose regimens for anti-TB drugs^[70]. Additional clinical trials are required to validate the safety and efficacy of these optimized doses, particularly in diverse patient populations.

TB immunotherapy is an innovative approach that uses immunomodulatory substances to modulate the immune system, thereby eliciting a targeted immune response against the pathogen^[71]. This therapeutic strategy comprises two primary components: immunomodulation and immune reconstitution. Immunomodulation aims to enhance the innate immune capabilities of the body while mitigating harmful immune reactions^[72]. This is achieved using immunomodulators, which can either stimulate or suppress specific immune responses as needed. In contrast, immune reconstitution involves stem cell transplantation to restore impaired cellular immunity or augment the immune function of a patient^[73]. The primary objective of TB immunotherapy is to achieve a therapeutic breakthrough that significantly reduces the treatment duration while maintaining high efficacy. Such advancements would not only streamline the treatment process but also improve patient outcomes, providing an efficient defense against TB.

Vitamin D and its derivative 1α,25-dihydroxyvitamin D3 (1α,25(OH)2 D3) play a significant role in the immune defense against TB^[74]. Studies have confirmed that 1α,25(OH)₂D3 can effectively inhibit the proliferation of Mtb, reduce the release of inflammatory cytokines, and enhance the immune response by promoting the expression of Interleukin-32 (IL-32) or autophagy-related proteins such as Beclin-1 and autophagy-related 5 (ATG5)^[75,76]. Moreover, 1α,25(OH)2D3 not only exhibits a chemotactic effect on polymorphonuclear leukocytes, enhancing immune cell movement, but also promotes the efferocytic capacity of macrophages by upregulating ArfGAP with SH3 Domain, Ankyrin Repeat and PH Domain 2 (ASAP2) expression^[77]. This dual action improves both the phagocytic and bactericidal capabilities of macrophages, potentially enhancing the immune response against Mtb. Vitamin D levels in patients with TB are reportedly positively correlated with CD4+ and CD8+ T cell expression and negatively correlated with disease severity^[78]. This correlation suggests that vitamin D deficiency may be associated with increased susceptibility to and exacerbation of TB. Although the results of in vitro experiments are promising, clinical trial outcomes are inconsistent. Vitamin D supplementation reportedly does not reduce the risk of TB in children^[79]. Such inconsistencies may be attributed to variables, such as clinical trial design, patient heterogeneity, dosage variations, treatment duration, and different study objectives^[80]. Therefore, further research is necessary to delineate the role of vitamin D in TB treatment and optimize its application strategy.

The use of corticosteroids for TB treatment remains controversial. Although they are commonly employed to alleviate severe inflammation, they may also suppress critical immune responses, potentially affecting therapeutic outcomes. Careful consideration is required to weigh the benefits and risks associated with corticosteroid use in this context^[81,82]. Although corticosteroids are not directly involved in cytokine therapy, which aims to enhance the immune response against TB by increasing cytokine levels, they may influence the function of immune cells, such as natural killer (NK) cells and T cells, which play crucial roles in the immune response^[83]. The specific role of corticosteroids in host-directed treatment strategies remains unclear, although evidence suggests that corticosteroids may be beneficial.

Clinical trials have demonstrated that corticosteroids, such as prednisone and dexamethasone (glucocorticoid receptor agonists), can effectively reduce the proinflammatory response triggered by Mtb infection. This effect is primarily attributed to the significant inhibition of Interleukin-6 (IL-6), Interleukin-8 (IL-8), monocyte chemoattractant protein-1 (MCP-1), and tumor necrosis factor-α (TNF-α) production^[84,85]. These findings have prompted the consideration of corticosteroids as a supplemental intervention alongside standard antibiotic regimens for various forms of TB, including pleural effusion, TB lymphadenitis, and pericarditis, and for managing TB immune reconstitution inflammatory syndrome (TB-IRIS) in patients with HIV co-infection^[86].

Although preliminary studies suggest that the use of corticosteroids as adjuvant therapy in the early stages of the disease may be beneficial, long-term studies have not revealed a significant difference in efficacy compared with the use of anti-TB drugs alone^[87]. In summary, corticosteroids may play an important role in reducing inflammation and improving the prognosis of patients with TB; however, their use should be strictly based on clear indications and under medical supervision.

The potential application of NSAIDs in TB treatment is being actively explored, particularly for their role in alleviating inflammation and modulating immune responses, which may present new avenues for therapeutic strategies against TB^[88]. In Mtb infections, the balance of the host inflammatory response is crucial for disease outcome. lipoxin A4 (LXA4) and leukotriene B4 (LTB4) are particularly important in regulating inflammation. LXA4 helps maintain balanced inflammation, whereas LTB4 may promote excessive inflammatory responses^[89,90].

In clinical treatment, aspirin (a common NSAID) helps regulate the inflammatory process triggered by Mtb infection by promoting LXA4 production, limiting neutrophil activity, and reducing TNF-α release. This anti-inflammatory effect is important for the treatment of TB meningitis (TBM)^[91]. Other NSAIDs that inhibit cyclooxygenase 1 (COX1) and cyclooxygenase 2 (COX2) have shown promise for TB treatment. For instance, in murine models, ibuprofen reportedly reduces Mtb burden and lung inflammation. Furthermore, indomethacin modulates the proliferation of CD4 +, CD8 +, and regulatory T cells (Tregs) in vitro, indicating its potential to mitigate TB-associated inflammation^[92-94]. However, the clinical application of NSAIDs is limited by their gastrointestinal and renal side effects, necessitating the development of novel, selective COX2 inhibitors to minimize these adverse reactions^[95]. Future research should prioritize designing safer and more effective NSAID formulations or identifying alternative therapeutic agents that provide anti-inflammatory benefits while reducing the risk of side effects.

Statins have demonstrated significant potential as therapeutic agents for modulating the adaptive immune responses to Mtb infections. While they primarily inhibit 3-Hydroxy-3-Methylglutaryl Coenzyme A (HMG-CoA) reductase, which leads to cholesterol reduction, they also possess anti-inflammatory and immunomodulatory properties^[96]. These effects are mediated, in part, by the activation of Kruppel-like factor (KLF) transcription factors, which reduce the production of pro-inflammatory cytokines and influence T cell function^[97].

Epidemiological studies have suggested that statin use is associated with a lower incidence of TB, implying its potential role in reducing the risk of active disease^[98]. In murine models, statins reportedly lower the cholesterol content on the cell membrane, thereby enhancing phagosome maturation and key autophagic processes, leading to a reduction in the lung bacterial burden and mitigating disease severity^[99]. Furthermore, statins have synergistic effects with first-line anti-TB drugs such as INH, RIF, and PZA, thereby improving their efficacy^[100]. Continued research is essential to better understand these mechanisms and evaluate the clinical effectiveness of statins in TB treatment, which could unlock their potential in the development of novel immunotherapeutic strategies.

Metformin, widely recognized as the first-line therapy for type 2 diabetes, has also demonstrated potential for treating TB beyond its primary role in reducing blood glucose levels^[101]. The anti-TB mechanisms of metformin include autophagy induction^[102], oxidative stress modulation^[103], efficacy enhancement of anti-TB drugs^[104], and inflammatory response attenuation^[105]. When used in combination with conventional anti-TB medications, metformin not only reduces inflammatory changes in histopathology but also enhances the secretion of interferon by T cells while concurrently suppressing chronic inflammatory gene activity^[106]. Furthermore, clinical evidence indicates that metformin improves sputum culture conversion and may reduce mortality risk during TB treatment^[107,108]. However, the limitations of the existing studies, including their predominantly observational nature and focus on diabetic populations, warrant a cautious interpretation of these findings. Future research should focus on elucidating the precise molecular pathways through which metformin exerts its effects on Mtb and on assessing its long-term outcomes in diverse patient cohorts. Such studies will contribute to a more nuanced understanding of the potential role of metformin in TB therapy and inform the development of targeted treatment regimens.

Interleukin 2(IL-2), a pivotal immunomodulatory cytokine, has demonstrated notable potential as an adjunctive therapy for TB. It primarily enhances cell-mediated anti-infective immune responses and bolsters T-helper 1-type (Th1-type) immune responses. In mice infected with drug-sensitive Mtb H37Rv, the combined application of IL-2 and granulocyte-macrophage colony-stimulating factor (GM-CSF), along with the antibacterial drugs INH and RIF significantly reduced the bacterial colony count in the lungs and spleen compared to monotherapy with antibacterial drugs^[109]. Clinical studies have confirmed the safety of combining IL-2 with anti-TB drugs to effectively boost the immune response, particularly in patients with MDR-TB, effectively boosting the immune response^[110]. Additionally, a multicenter, large-sample, randomized controlled trial was conducted in China evaluated the efficacy of IL-2 as adjunct therapy for DS-TB. The findings revealed that although IL-2 may improve sputum culture conversion and cavity closure rates in the early phase of treatment, its long-term benefits, including relapse rates after the completion of standard anti-TB therapy, were not significant^[111]. These results suggest that IL-2 treatment strategies require further optimization and individualized adjustments.

GM-CSF is a glycoprotein secreted by various immune cells, exhibiting significant immunomodulatory and immunostimulatory functions. Recent studies have revealed that GM-CSF not only enhances the survival and differentiation of macrophages but also regulates their metabolic state, thereby enabling them to effectively restrict Mtb infection through mechanisms such as promoting autophagy and balancing inflammatory and anti-inflammatory responses^[112]. Clinical trials have revealed that GM-CSF when combined with anti-TB chemotherapy drugs, is safe and tolerable and hastens bacterial clearance from patient sputum^[113]. Particularly in the treatment of MDR-TB, combined immunotherapy with GM-CSF and IL-2 improves survival rates and reduces bacterial loads in murine models, thereby enhancing the efficacy of first-line anti-TB drugs^[109]. Moreover, the gene-therapeutic form of GM-CSF, recombinant GM-CSF adenovirus (AdGM-CSF), has the potential to reduce the pulmonary bacterial burden in mouse models^[114]. Collectively, these findings suggest that GM-CSF can not only act as an immunomodulator to enhance the effectiveness of anti-TB medications but may also contribute to shortening the treatment duration and improving therapeutic outcomes for drug-resistant TB.

Expanding upon the functions of previously discussed cytokines, several additional interleukins contribute to the modulation of the immune response against TB, highlighting their potential as complementary targets for therapeutic development. Interleukin 24 (IL-24), a member of the Interleukin 10 (IL-10) cytokine family, inhibits Mtb by activating CD8 + T cells and relies on the early involvement of neutrophils to promote the production of Interferon-gamma (IFN-γ). In murine models, IL-24 demonstrates anti-TB effects, suggesting its potential as a novel therapeutic agent^[115]. IL-32, produced by T, NK, and epithelial cells, engages in various immune responses and induces macrophages to produce inflammatory cytokines, thereby enhancing the clearance of Mtb^[116]. Studies in transgenic mice have revealed that the expression of human IL-32γ by type II alveolar epithelial cells significantly reduces pulmonary Mtb load, indicating the role of IL-32 in preventing Mtb infection and its potential as a new immunotherapeutic approach^[117]. Interleukin 12 (IL-12), a promoter of Th1 cell differentiation, has been investigated for its potential use in the development of TB vaccines^[118]. Interleukin 4 (IL-4) and IL-10 may play dual roles in the immunomodulation of TB^[119]. The clinical application prospects for these cytokines are broad and may involve the combined use of various drugs and immunomodulatory therapies to improve their therapeutic efficacy.

In the field of TB prevention and treatment, significant progress has been made in developing therapeutic vaccines, particularly in their role as adjuncts to chemotherapy. Inactivated vaccines prepared using heat treatment or chemical means to inactivate Mtb are primarily used for disease treatment rather than prevention^[120]. For instance, in mouse models, the Mycobacterium vaccae vaccine demonstrated protective effects against TB infection and increased the rate of negative bacterial sputum results in clinical studies^[121,122]. Similarly, the M. indicus pranii (MIP) vaccine has demonstrated effectiveness in clinical trials by increasing the rate of negative bacterial sputum results in patients with TB by activating innate and T cell-mediated immune responses^[123]. The DAR-901 vaccine (also known as Mk) has the potential to increase the rate of negative bacterial sputum results and enhance Th1 cytokine responses when used in combination with chemotherapy^[124]. However, the RUTI vaccine, derived from specially treated Mtb H37Rv, has shown limited effectiveness as a standalone treatment but effectively stimulates cellular immune responses in patients with latent TB infection (LTBI) when combined with chemotherapy^[125].

Subunit vaccines, which utilize the cellular components of Mtb, are used to provide adjuvant treatment for patients with TB and to prevent LTBI. BCG polysaccharide nucleic acid(BCG-PSN, trade name: SIQIKANG) is the only vaccine approved for TB immunotherapy and is also used to treat other immunodeficiency diseases^[126]. In addition to BCG-PSN, recent advancements in subunit vaccines such as H56:IC31 and ID93+GLA-SE have demonstrated significant potential. The H56:IC31 vaccine has demonstrated safety and immunogenicity in adults who have successfully completed treatment for drug-sensitive pulmonary TB, suggesting its utility as an adjunct therapy^[112]. Furthermore, the ID93+GLA-SE vaccine candidate, particularly in its thermostable formulation, elicited strong immune responses in healthy adults, which is crucial for its potential widespread use in regions with limited cold chain infrastructure^[127].

DNA vaccines have also contributed to the prevention and treatment of TB. These vaccines effectively activate humoral immunity, Th1 cell immunity, and cytotoxic T lymphocyte (CTL) responses by encoding protective antigens of Mtb and using eukaryotic cell expression platforms^[128]. In particular, the Ag85a/b chimeric DNA vaccine has exhibited significant promise in preclinical studies. This vaccine induces a robust Th1-type immune response, as evidenced by a substantial increase in CD4 + and IFN-γ + T cells in mouse models of TB. This enhanced immune response is associated with a marked reduction in the lung bacterial load and improvement in lung pathology, suggesting that the vaccine could potentially mitigate TB-induced lung injury^[129,130]. Despite these promising results, several challenges remain for the development and application of TB vaccines. One major limitation is the variability in the immune responses observed across different animal models, which raises concerns regarding the efficacy of these vaccines in humans. Additionally, the long-term safety and stability of TB vaccines require thorough evaluation before their widespread clinical adoption. Ongoing research is focused on optimizing vaccine delivery systems and adjuvants to enhance the immunogenicity and effectiveness of vaccines against TB.

Cell therapy is a revolutionary immunotherapeutic method that targets Mtb and its host macrophages by activating and expanding autologous or allogeneic immune effector cells ex vivo^[131]. This treatment approach effectively kills pathogens and regulates the immune imbalance, thereby enhancing immune function. In terms of cell type selection, mesenchymal stem cells, γδT cells, cytokine-induced killer (CIK) cells, and natural killer T (NKT) cells significantly enhance the immune response to TB and improve the killing effect on Mtb^[132-135]. Recent advances in these cellular therapies focus on boosting the host anti-infection capabilities, such as activating and expanding immune cells and investigating the protective role of Vγ2Vδ2 T cells^[136]. Additionally, invariant natural killer T cells (iNKT), mucosal-associated invariant T cells (MAIT), and regulatory T cells demonstrate potential in the immunotherapy of TB^[137-139]. However, despite these advances, several challenges remain. Variability in patient responses, potential risks of immune-related adverse effects, and the high cost of cell therapies are significant barriers to their widespread clinical application. Moreover, the complex and labor-intensive cell extraction, expansion, and reinfusion processes limit the scalability of these treatments. Therefore, additional high-quality clinical studies are required to evaluate the efficacy and safety of cell therapies.

Despite significant achievements in the prevention and treatment of TB, numerous challenges remain. These challenges are particularly pronounced in the pursuit of the ambitious goals set forth by WHO in the "End TB Strategy," which aims to reduce the incidence and mortality rates of TB by 80% and 90%, respectively, by 2030. However, the global spread of the COVID-19 pandemic has not only diverted attention from TB control but has also undermined the feasibility of achieving these goals. Prior to the pandemic, reaching these targets was a formidable task owing to the complexity of TB treatment regimens, including long-term multidrug combination therapy and the increasing severity of antibiotic resistance. To overcome these obstacles, an urgent need for new strategies in clinical trial design and drug regimen innovations exists.

Recent advances in chemotherapy and immunotherapy have instilled optimism for the future treatment of TB. These innovative breakthroughs provide a solid foundation for exploring new strategies that may simplify the treatment process and enhance the therapeutic effects. Nevertheless, the control of TB still faces significant challenges, including maintaining the momentum of TB research and development, optimizing the efficiency and safety of treatment regimens, and eradicating LTBI. The scientific community remains cautiously optimistic about significantly reducing the TB burden through continuous innovation and collaboration; however, this optimism must be matched with a profound understanding of the current scale and complexity of the task.

Achieving the TB control targets established by the WHO for 2030 requires concerted global efforts. This effort must not only continue to advance the frontiers of scientific exploration but also fundamentally modify the methods of TB treatment and control. It involves the development of new drugs and treatment regimens, as well as the implementation of comprehensive strategies to address the social determinants of health. An integrated approach comprising chemotherapy and human-directed immunotherapy will provide new perspectives for the clinical management of patients with TB.