Cording Mycobacterium tuberculosis Bacilli Have a Key Role in the Progression towards Active Tuberculosis, Which is Stopped by Previous Immune Response

Abstract

Cording was the first virulence factor identified in Mycobacterium tuberculosis (Mtb). We aimed to ascertain its role in the induction of active tuberculosis (TB) in the mouse strain C3HeB/FeJ by testing the immunopathogenic capacity of the H37Rv strain. We have obtained two batches of the same strain by stopping their growth in Proskauer Beck liquid medium once the mid-log phase was reached, in the noncording Mtb (NCMtb) batch, and two days later in the cording Mtb (CMtb) batch, when cording could be detected by microscopic analysis. Mice were challenged with each batch intravenously and followed-up for 24 days. CMtb caused a significant increase in the bacillary load at an early stage post-challenge (day 17), when a granulomatous response started, generating exudative lesions characterized by neutrophilic infiltration, which promoted extracellular bacillary growth together with cording formation, as shown for the first time in vivo. In contrast, NCMtb experienced slight or no bacillary growth and lesions could barely be detected. Previous Bacillus Calmette-Guérin (BCG) vaccination or low dose aerosol (LDA) Mtb infection were able to delay the progression towards active TB after CMtb challenge. While BCG vaccination also reduced bacillary load when NCMtb was challenged, LDA did not, and its proliferative lesions experienced neutrophil infiltration. Analysis of lung cytokine and chemokine profiles points to their capacity to block the production of CXCL-1 and further amplification of IL-1β, IL-17 and neutrophilic extracellular trap formation, all of which are essential for TB progression. These data highlight the key role of cording formation in the induction of active TB.

Aggregated Mycobacterium tuberculosis enhances the inflammatory response

“…Aggregated Mtb infection led to neutrophil accumulation in a mouse model [11]. Death of Mtb infected neutrophils may also enhance Mtb growth, which we have observed to be accelerated in dead infected cells [15] and may make aggregated Mtb a more virulent form of infection [12, 11, 46, 17, 10]. In support of this, we observed that Mtb aggregates occurred at a substantial frequency around a TB cavitary lesion.…”

“…Neutrophil accumulation in Mtb infection leads to inflammation and has been shown to be detrimental to the host [27, 45]. Aggregated Mtb infection led to neutrophil accumulation in a mouse model [11]. Death of Mtb infected neutrophils may also enhance Mtb growth, which we have observed to be accelerated in dead infected cells [15] and may make aggregated Mtb a more virulent form of infection [12, 11, 46, 17, 10].…”

“…The host-pathogen interactions that tip the balance to active disease are not clearly defined. Aggregation has been proposed to be associated with pathogenicity [9, 10] and increase Mtb virulence in mice and ex vivo [11, 12, 13]. Mtb aggregates have been observed in human lung tissue [7], though the fraction of bacilli in this state is unclear.…”

A High-content Screening Assay based on Automated Microscopy for Monitoring Antibiotic Susceptibility of Mycobacterium Tuberculosis Phenotypes

“…Recently, the cording phenotype has been shown to cause extensive immunopathological changes associated with active TB in C3HeB/FeJ mice (6). Mtb cords were also identified inside human alveolar macrophages obtained from patients with active TB (7) and lymphatic endothelial cells isolated from patients with extrapulmonary TB (8).…”

A high-throughput screening assay based on automated microscopy for monitoring antibiotic susceptibility of Mycobacterium tuberculosis phenotypes

“…As we have previously demonstrated, the cording phenotype represents more intrusive interaction with immune cells causing macrophages to release macrophage extracellular traps (METs) [6]. Recently, the cording phenotype has been shown to cause extensive immunopathological changes associated with active TB in C3HeB/FeJ mice [7]. Mtb cords were also identified inside human alveolar macrophages obtained from patients with active TB [8] and lymphatic endothelial cells isolated from patients with extrapulmonary TB [9].…”