Mouse Bone Marrow Sca-1 ⁺ CD44 ⁺ Mesenchymal Stem Cells Kill Avirulent Mycobacteria but Not Mycobacterium tuberculosis through Modulation of Cathelicidin Expression via the p38 Mitogen-Activated Protein Kinase-Dependent Pathway

Abstract

Mycobacterium tuberculosis primarily infects lung macrophages. However, a recent study showed that M. tuberculosis also infects and persists in a dormant form inside bone marrow mesenchymal stem cells (BM-MSCs) even after successful antibiotic therapy. However, the mechanism(s) by which M. tuberculosis survives in BM-MSCs is still not known. Like macrophages, BM-MSCs do not contain a well-defined endocytic pathway, which is known to play a central role in the clearance of internalized mycobacteria. Here, we studied the fate of virulent and avirulent mycobacteria in Sca-1 ϩ CD44 ϩ BM-MSCs. We found that BM-MSCs were able to kill avirulent Mycobacterium smegmatis and Mycobacterium bovis BCG but not the pathogenic species M. tuberculosis. Further mechanistic studies revealed that pathogenic M. tuberculosis dampens the antibacterial response of BM-MSCs by downregulating the expression of the cationic antimicrobial peptide cathelicidin. In contrast, avirulent mycobacteria were effectively killed by inducing the Toll-like receptor 2/4 (TLR2/4) pathway-dependent expression of cathelicidin, while small interfering RNA (siRNA)-mediated cathelicidin silencing increased the survival of M. bovis BCG in BMMSCs. We also showed that M. bovis BCG infection caused increased expression levels of MyD88, phospho-interleukin-1 receptor-associated kinase 4 (pIRAK-4), and the p38 mitogen-activated protein kinase (MAPK) signaling pathway. Further downstream investigations demonstrated that IRAK-4 -p38 activation increased the nuclear translocation of NF-B, which subsequently induced the expression of cathelicidin and the cytokine interleukin-1␤ (IL-1␤), resulting in the decreased survival of M. bovis BCG. On the other hand, inhibition of TLR2/4, pIRAK-4, p38, and NF-B nuclear translocation decreased cathelicidin and IL-1␤ expression levels and therefore increased the survival of avirulent mycobacteria. This is the first report that demonstrates that virulent mycobacteria manipulate the TLR2/4 -MyD88 -IRAK-4 -p38 -NF-B-Camp-IL-1␤ pathway to survive inside bone marrow stem cells.

Anatomic and Cellular Niches for Mycobacterium tuberculosis in Latent Tuberculosis Infection

“…Mycobacterium tuberculosis infected the CD271 + bone marrow mesenchymal stem cells via scavenger receptors MARCO and SR-B1, and mesenchymal stem cells controlled intracellular MTB using autophagic mechanisms [72]. Mesenchymal stem cells killed avirulent mycobacteria but not MTB via modulation of cathelicidin expression [73]. Moreover, CD271 + bone marrow mesenchymal stem cells harboring MTB are localized in the hypoxic niche in both mice and humans, a critical microenvironmental factor that is known to induce dormancy [74].…”

Mycobacterium tuberculosis LprE Suppresses TLR2-Dependent Cathelicidin and Autophagy Expression to Enhance Bacterial Survival in Macrophages

“…LL-37 is active against a wide range of Gram-positive and -negative bacteria (6). Previous findings, including ours, have shown that LL-37 is able to kill nonpathogenic and pathogenic mycobacteria under in vitro and in vivo conditions (7)(8)(9). The LL-37 peptide present in neutrophil extracellular traps is engulfed by macrophages to enhance intracellular killing of mycobacteria.…”

“…Intracellular survival assay M. tuberculosis, MtbΔLprE, MtbΔLprE::LprE, Msm::pSMT3, Msm::LprE, MsmΔ5043, and MsmΔ5043::LprE strains were grown to mid-exponential phase. THP-1 monocytes were treated with PMA (20 nM) for 18 h and then infected at MOI 10 as described previously (7,22). Bacterial colonies were enumerated after 3 and 21 d for M. smegmatis and M. tuberculosis, respectively.…”

Mesenchymal Stromal Cells as Potential Antimicrobial for Veterinary Use—A Comprehensive Review

“…In vivo studies using allogenic murine MSCs have also implicated cathelicidin as a key AMP for bacterial killing (Johnson et al, 2017). Cathelicidin acts through TLR2/4-IRAK-4-dependent pathways in order to establish effective killing of mycobacteria (Naik et al, 2017). Interestingly, M. tuberculosis (Mtb) has developed a survival mechanism that disrupts this pathway and suppresses the antimicrobial effect of BM-MSCs via downregulation of CAMP gene expression (Naik et al, 2017).…”