Recent developments in mycobacterial research

Correlation Between Serum Tumor Necrosis Factor-Alpha (TNF-α) and Clinical Severity of Tuberculosis: A Hospital-Based Study

AIM

The main aim of this study was to assess the correlation between serum tumor necrosis factor-alpha (TNF-α) levels the and clinical severity of tuberculosis.

METHODS

This was a hospital-based case-control prospective study and was conducted at the Sher-i-Kashmir Institute of Medical Sciences, a tertiary care hospital in the northern part of India, from May 2016 to May 2018. The subjects were recruited in the study considering inclusion and exclusion criteria. All patients with pulmonary tuberculosis as well as patients with extrapulmonary tuberculosis were included as subjects and a clinical severity score based on anemia, weight loss, presence of hypoxia, and radiological features was calculated and compared with TNF-α levels. Age- and sex-matched healthy individuals were recruited as controls.

RESULTS

A total of 75 subjects comprising 50 cases and 25 controls were taken for the study. There were 34 (68.0%) patients with elevated TNF-α levels while only 16 (32.0%) patients had normal TNF-α levels. And, TNF-α levels were normal in 21 (84%) control subjects as compared to tuberculosis (TB) patients. Such difference in serum TNF-α levels between cases and controls was statistically significant (p<0.05). The mean serum TNF-α level in TB cases was 1265.63 pg/mL, while the mean serum TNF-α level in controls was 312.06 pg/mL. The difference in serum TNF-α levels between the two groups was statistically significant (p<0.01). We observed a significant increase in serum TNF-α levels with the increase in clinical severity score.

CONCLUSION

Serum TNF-α levels were significantly associated with increased severity of TB.

Interaction of the sensor module of Mycobacterium tuberculosis H37Rv KdpD with members of the Lpr family

The genetic and biochemical mechanisms by which Mycobacterium tuberculosis senses and responds to the complex environment that it encounters during infection and persistence within the host remain unknown. In a number of bacterial species, the Kdp signal transduction pathway appears to be the primary response to environmental osmotic stress, which is primarily mediated by K+ concentration in bacteria. We show that kdp encodes for components of a mycobacterial signalling pathway by demonstrating the K+ dependence of kdpFABC expression in both M. tuberculosis H37Rv and Mycobacterium smegmatis. To identify proteins of M. tuberculosis that participate in this signalling pathway, we used the N-terminal sensing module of the histidine kinase KdpD as bait in a yeast two-hybrid screen. We show that the sensing domain of KdpD interacts specifically with two membrane lipoproteins, LprJ (Rv1690) and LprF (Rv1368). Overexpression of lprF and lprJ alleles in mycobacterial kdpF-lacZ reporter strains enabled us to identify alleles that modulate kdpFABC expression. By exploiting the yeast three-hybrid system, we have found that the histidine kinase domain of KdpD forms ternary complexes with LprF and LprJ and the sensing module of KdpD. Our results establish a role for membrane proteins in the Kdp signalling pathway and suggest that LprF and LprJ function as accessory or ligand-binding proteins that communicate directly with the sensing domain of KdpD to modulate kdp expression.

Mycobacterium tuberculosis WhiB3 interacts with RpoV to affect host survival but is dispensable for in vivo growth

Previous work established that the principal sigma factor (RpoV) of virulent Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex, restores virulence to an attenuated strain containing a point mutation (Arg-515-->His) in the 4.2 domain of RpoV. We used the 4.2 domain of RpoV as bait in a yeast two-hybrid screen of an M. tuberculosis H37Rv library and identified a putative transcription factor, WhiB3, which selectively interacts with the 4.2 domain of RpoV in virulent strains but not with the mutated (Arg-515-->His) allele. Infection of mice and guinea pigs with a M. tuberculosis H37Rv whiB3 deletion mutant strain showed that whiB3 is not necessary for in vivo bacterial replication in either animal model. In contrast, an M. bovis whiB3 deletion mutant was completely attenuated for growth in guinea pigs. However, we found that immunocompetent mice infected with the M. tuberculosis H37Rv whiB3 mutant strain had significantly longer mean survival times as compared with mice challenged with wild-type M. tuberculosis. Remarkably, the bacterial organ burdens of both mutant and wild-type infected mice were identical during the acute and persistent phases of infection. Our results imply that M. tuberculosis replication per se is not a sufficient condition for virulence in vivo. They also indicate a different role for M. bovis and M. tuberculosis whiB3 genes in pathogenesis generated in different animal models. We propose that M. tuberculosis WhiB3 functions as a transcription factor regulating genes that influence the immune response of the host.