Relevance of Multidisciplinary Research Fighting Latent/Persistent TB

Latent tuberculosis represents an enormous health, epidemiological, and biotechnological multidisciplinary challenge for the precise identification and control of this infectious disease. Efforts in human and animal infection diagnosis have revealed scientific evidence of shared biomarkers demonstrating the molecular and clinical signs of persistent infection in the natural hosts of the related M. tuberculosis complex bacteria. These findings parallelly potentiate the development of improved tools for identifying and preventing this worldwide persistent disease.

Tuberculosis (TB) is still considered a priority public health problem and remains one of the leading causes of mortality from respiratory diseases. TB continues to increase worldwide, and estimates from the World Health Organization (WHO) calculate a global infection rate of 10.4 million cases and 1.7 million deaths per year, with 25% of the total population infected [1].

The standard confirmatory diagnosis of TB is through the culture and isolation of the infectious agent, either Mycobacterium tuberculosis (Mtb) or Mycobacterium bovis (M. bovis). The latter is transmitted from bovines to humans and is considered by WHO to be an infectious zoonosis with significant repercussions on public health. Identification of the correct type of pathogen is critical for treatment, M. bovis is naturally resistant to pyrazinamide. Human and zoonotic TB are routinely diagnosed by the corresponding intradermal skin test called Tuberculin (TST) which uses complex mycobacterial protein mixtures as an immunological reagent. Although it is a widespread test for epidemiological purposes (not a gold standard), it is inaccurate due to host factors associated with immunologic status (co-infections), co-morbid conditions, previous Calmette Guerin (BCG) vaccination, or different stages of the infection [2].

Improvements in immunological TB assessment are based on blood tests using antigenic stimulus with specific virulence antigen derivatives of ancillary laboratory tests (T-SPOT.TB, QuantiFERON® TB GOLD In-Tube). These are ELISA tests, specifically Interferon-Gamma Release Assays (IGRA), based on fresh blood culture stimulus to detect secreted interferon-gamma from memory T-cells in response to specific TB antigens such as ESAT6, CFP10, EspC, and MPB83 (from Mtb and M. bovis as well) to identify those individuals with an active infection with the M. tuberculosis complex [3,4]. Despite the advances in detecting TB, it continues to spread due to the high rate of individuals with asymptomatic or latent infection. After initial infection, progression to active TB typically occurs within two years. Some people may clear the infection, but many do not, and the bacilli remain viable for many years; besides, some people never develop the disease [5]. The disease can manifest in older people or immunosuppressed individuals, and the infection exists in a spectrum of states beyond the current binary classification of latent and active TB [6].

Latent TB is currently detected in humans with immunological analysis using particular protein cocktails supported by clinical, microbiological, and molecular diagnostic tests [7,8], being the basis for the scientific demonstration of the clinical latent state of infection. Zoonotic TB transmission may occur through inhalation of aerosols been observed in workers in the livestock industry [9,10]. Thus, microbiological and immunological evidence of a subclinical bovine TB suggests a latent TB infection in animal hosts as well [11]. Taken together, with some reports of immunosuppressed people who developed a reactivated M. bovis infection are evidence of a latent M. bovis infection with important implications for the prevalence and distribution of the disease in human and bovine hosts [12].

Tuberculosis cases continue to be significantly prevalent worldwide, with serious economic and public health consequences due to its difficult eradication, driven by an endless infectious cycle in bacterial epidemiology. Both humans and bovines are considered the most susceptible natural hosts and reservoirs of the persistent disease, so scientific studies to improve and update the diagnosis of latent TB in both species have been accumulating and improving. The study of scientific evidence for recognizing persistent infection in humans and cattle has broadened the understanding of TB control. Both species exhibit similar pathological signs, such as the development of granulomatous chronic lesions and comparable immune responses, which have been crucial in identifying the persistent form of TB, contributing to deciphering the silent survival bacterial pattern in asymptomatic hosts [13,14].

In terms of prevention, the BCG (Bacille Calmette-Guérin) vaccine, originally derived from an artificially attenuated strain of M. bovis about a century ago, is still administered to humans in endemic countries as a single dose a few days after birth. However, it does not provide long-lasting protection with booster shots. Currently, about a dozen experimental vaccine substitutes are using artificially attenuated (by heat or genetic deletions) immunogenic relatives such as Mtb, M. vaccae and M. obuense strains, been at different clinical trial phases.

They are based on a combination of proteins/cell extracts from selected Mtb antigens delivered with adenovirus-originated vectors, nanoparticles, liposomes, and virus-like particles [15]. These promise to be next-generation immunostimulating candidates to combat the disease in immunocompromised people, infants, youth, and adults. Progress in improving prophylactic and therapeutic strategies continues to grow, thanks to relevant biotechnological advances in the field of diagnosis and treatment aimed at targeting pulmonary and chronic TB [16,17].

Although zoonotic TB is more difficult to control due to the lack of strict adherence to health prevention programs for animal disease, animal vaccination against bovine TB is currently being considered in countries that have a greater stake in bovine TB eradication. To date, however, no commercial brands have been released. The effectiveness of BCG in controlling bovine TB remains unclear, but it may slow disease progression reducing early onward transmission. Vaccination studies have benefitted from the introduction of more sensitive and optimized tests that can distinguish between vaccinated and infected animals. These tests, Including the automated test Xpert® MTB/RIF Ultra assay for gene amplification of specific DNA fragments (IS6110, IS1081, rpoB gene) by real-time PCR and the IGRA for identifying immune biomarkers in bovine tissue and secretory samples, which can be performed antemortem. Biotechnological advancements are currently adapting these tests for practical field applications including protein engineering, technological combinations like Immuno-PCR (I-PCR), and Point-Of-Care (POC) rapid tests [18-24].

It is only a matter of time before there is an effective form of prevention for the eradication of this infectious disease worldwide. The growing international body of multidisciplinary scientific work is generating extensive knowledge for the design of a whole range of improved and innovative biologicals to combat the resilient form of hidden TB infection in an integrative way. Most likely, within this decade, multiple effective and complementary strategies for diagnosing and preventing TB will emerge. These advances will be key in transforming TB into a fully preventable and eradicable disease.

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