Antibodies targeting Mycobacterium tuberculosis peptides inhibit mycobacterial entry to infection target cells

Potential biomarkers for evaluating the BCG vaccination response based on humoral immunity

Background

The current prophylactic tuberculosis vaccine Bacille Calmette-Guérin (BCG), was derived in the 1920s, but the humoral immune responses induced by BCG vaccination have not been fully elucidated to date. In this study, our aim was to reveal the profiles of antibody responses induced by BCG vaccination in adults and identify the potential biomarkers for evaluating the BCG vaccination response.

Methods

Proteome microarrays were performed to reveal the serum profiles of antibody responses induced by BCG vaccination in adults. ELISA was used to validate the potential biomarkers in validation cohort (79 healthy controls and 58 BCG-vaccinated subjects). Then combined panel was established by logistic regression analysis based on OD values of potential biomarkers.

Results

Multiple antigens elicited stronger serum IgG or IgM antibody responses in BCG vaccinated subjects than healthy subjects at 12 weeks post BCG vaccination; among the antigens, Rv0060, Rv2026c and Rv3379c were further verified using 137 serum samples and presented the moderate performance in assessment of the BCG vaccination response by receiver operating characteristic analysis. Furthermore, a combined panel exhibited an improved AUC of 0.923, and the sensitivity and specificity were 77.59 % and 91.14 %, respectively. In addition, the antibody response against Rv0060, Rv2026c and Rv3379c was related to the clinical background to a certain extent.

Conclusions

The novel antigens identified in our study could offer better knowledge towards developing a more efficacious vaccine based on humoral immune responses, and they could be potential biomarkers in assessments of BCG vaccination responses.

Clinical manifestations and immune response to tuberculosis

Tuberculosis is a far-reaching, high-impact disease. It is among the top ten causes of death worldwide caused by a single infectious agent; 1.6 million tuberculosis-related deaths were reported in 2021 and it has been estimated that a third of the world's population are carriers of the tuberculosis bacillus but do not develop active disease. Several authors have attributed this to hosts' differential immune response in which cellular and humoral components are involved, along with cytokines and chemokines. Ascertaining the relationship between TB development's clinical manifestations and an immune response should increase understanding of tuberculosis pathophysiological and immunological mechanisms and correlating such material with protection against Mycobacterium tuberculosis. Tuberculosis continues to be a major public health problem globally. Mortality rates have not decreased significantly; rather, they are increasing. This review has thus been aimed at deepening knowledge regarding tuberculosis by examining published material related to an immune response against Mycobacterium tuberculosis, mycobacterial evasion mechanisms regarding such response and the relationship between pulmonary and extrapulmonary clinical manifestations induced by this bacterium which are related to inflammation associated with tuberculosis dissemination through different routes.

Peptide-pulsed dendritic cells' immunomodulating effect regarding Mycobacterium tuberculosis growth in macrophages

Mycobacterium tuberculosis is one of the most successful pathogens affecting humans, being the main cause of tuberculosis. It accounts for most infectious agent-related deaths worldwide; it has been estimated that a third of the world's population are bacillus carriers. This pathogen's evolutionary adaptation is mainly due to its ability to block a host's immune system by preventing it using an effective immune response in cases of active tuberculosis. Peptide-based synthetic vaccines represent an alternative for counteracting tuberculosis; however, although peptide antigens can be identified, they are not recognised by a host's immune system. An approach using dendritic cells as immunomodulating agents for increasing synthetic peptides' antigenic capacity has thus been advanced. Dendritic cells obtained from IL to 4- and GM-CSF-treated peripheral blood mononuclear cells were pulsed with synthetic Mtb protein peptides which have been reported as participating in mycobacteria-host interactions; their amino acid sequences were modified to improve MHC-II coupling and thus increase their recognition by a host's immune system. pMHC-II/TCR interaction triggered a lymphocyte response which controlled Mtb intracellular growth in infected macrophages. This work has been aimed at contributing to understanding dendritic cells' role in Mycobacterium tuberculosis protein peptide antigen presentation, thereby increasing individuals' immune response as a means of controlling the disease.

Rv0180c contributes to Mycobacterium tuberculosis cell shape and to infectivity in mice and macrophages

Mycobacterium tuberculosis, the main causative agent of human tuberculosis, is transmitted from person to person via small droplets containing very few bacteria. Optimizing the chance to seed in the lungs is therefore a major adaptation to favor survival and dissemination in the human population. Here we used TnSeq to identify genes important for the early events leading to bacterial seeding in the lungs. Beside several genes encoding known virulence factors, we found three new candidates not previously described: rv0180c, rv1779c and rv1592c. We focused on the gene, rv0180c, of unknown function. First, we found that deletion of rv0180c in M. tuberculosis substantially reduced the initiation of infection in the lungs of mice. Next, we established that Rv0180c enhances entry into macrophages through the use of complement-receptor 3 (CR3), a major phagocytic receptor for M. tuberculosis. Silencing CR3 or blocking the CR3 lectin site abolished the difference in entry between the wild-type parental strain and the Δrv0180c::km mutant. However, we detected no difference in the production of both CR3-known carbohydrate ligands (glucan, arabinomannan, mannan), CR3-modulating lipids (phthiocerol dimycocerosate), or proteins in the capsule of the Δrv0180c::km mutant in comparison to the wild-type or complemented strains. By contrast, we established that Rv0180c contributes to the functionality of the bacterial cell envelope regarding resistance to toxic molecule attack and cell shape. This alteration of bacterial shape could impair the engagement of membrane receptors that M. tuberculosis uses to invade host cells, and open a new perspective on the modulation of bacterial infectivity.

The epidemic efficiency of tuberculosis bacilli is determined by their capacity to transmit via aerosol. Currently, the bacterial functions that favor Mycobacterium tuberculosis seeding in the lung of naïve host remain mostly unknown. Here we implemented a genome-wide approach to identify M. tuberculosis mutants deficient for seeding and early replication in the lung of mice. In addition to genes known to encode virulence factors, we identified three genes not previously described. We used complementary approaches to characterize the phenotype of a M. tuberculosis mutant with insertion within the rv0180c gene. We found that this mutant is impaired for seeding in the lung of mice and for invasion and replication in human macrophages. In macrophages, the defect relies on a lack of engagement of CR3 receptor. Although we did not detect any difference between the wild type strain and the rv0180c mutant with regard to potential CR3-ligand, we found that the bacterial cell envelope is altered in the rv0180c mutant. Our study provides new insight into bacterial genes required for early interaction of M. tuberculosis with the host and perspective to understand the bacterial functions enhancing infectivity.