Investigation of the host-mediated proteolytic response to pulmonary Mycobacterium Avium Complex infection

Abstract

The incidence of pulmonary NTM infection is increasing rapidly. In the UK, Mycobacterium avium complex (MAC) is the most common pulmonary NTM infection outside the setting of cystic fibrosis lung disease. Patients with pulmonary MAC infection develop cavitating lung disease or nodular bronchiectasis. Prolonged treatment is required, frequently not tolerated, and often associated with progressive lung destruction. A large body of evidence suggests the tissue damage that occurs during tuberculous lung disease is driven by host-derived matrix metalloproteinases (MMPs). Unlike Mycobacterium tuberculosis (MTb) infection, the mechanisms of tissue destruction during MAC infection are not well-characterised. Here, I aim to elucidate if MMPs are upregulated during MAC infection, and how MMP upregulation is governed in an in vitro cell infection model. Clinically relevant primary human monocytes, primary human monocyte-derived macrophages (MDMs), and a bronchial epithelial cell line were infected with clinical isolates of disease-causing Mycobacterium avium and the production of pro-inflammatory cytokines and MMPs were examined. As M. avium primarily infects macrophages, production of MMPs and the mechanisms governing MMP upregulation in M. avium-infected MDMs were the main focus of much of this work.

Interestingly, unlike MTb or other chronic pulmonary pathogens such as Pseudomonas aeruginosa or Haemophilus influenzae, I found that direct infection with clinically isolated, disease-causing M. avium did not drive secretion of MMP9/gelatinase B by infected mononuclear cells from healthy donors; instead M. avium infection suppressed total MMP-9 release from infected MDMs. M. avium drove functionally unopposed MMP-7/matrilysin, and MMP-1/interstitial collagenase, which was previously thought to be an MTb-specific response. M. avium infection had no overall effect on anti-protease tissue inhibitor of metalloproteinase-1 (TIMP-1) and drove increased net MMP activity from MDMs. M. avium infection activated multiple signalling pathways within MDMs, including the p38, extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) mitogen-activated protein kinases (MAPK), the phosphatidylinositol-3 kinase (PI3K)/Akt, and nuclear factor kappa-lightchain-enhancer of activated B cells (NF-κB) pathways, and induced activator protein -1 (AP-1) nuclear translocation and DNA binding. M. avium-induced MMP-1 secretion by MDMs was dependent on ERK MAPK, JNK MAPK, and PI3K signalling. MDMMMP-7 secretion was also ERK- and JNK-dependent. Interestingly, inhibition of p38 MAPK and prostaglandin had no effect on MMP-1 or MMP-7 secretion. Inhibition of NF-κB signalling reduced M. avium-induced total MMP-1 release for each MDM donor, though this did not reach statistical significance. Furthermore, MDM MMP-1 secretion was unaffected by doxycycline, a broad-spectrum MMP inhibitor, though was completely suppressed by the corticosteroid dexamethasone.

In summary, M. avium infection of primary human mononuclear cells increases production of MMP-1 and MMP-7 but not MMP-9 or TIMP-1. Unopposed MMP-1/-7 secretion may drive the destructive pulmonary pathophysiology that characterises pulmonary MAC infection. Additionally, regulation of the host response to M. avium diverges from MTb, with p38-independent/PI3K-dependent MMP-1 secretion. This divergence in MMP response and intracellular signalling may necessitate deviation when considering potential adjunctive patient therapies for MTb and M. avium in order to reduce tissue damage in patients.

Date of Award	Jul 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Cecilia O'Kane (Supervisor), Ronan McMullan (Supervisor) & Gunnar Neels Schroeder (Supervisor)