Mycobacterium avium Biofilm Attenuates Mononuclear Phagocyte Function by Triggering Hyperstimulation and Apoptosis during Early Infection

Abstract

Mycobacterium avium subsp. hominissuis is an opportunistic human pathogen that has been shown to form biofilm in vitro and in vivo. Biofilm formation in vivo appears to be associated with infections in the respiratory tract of the host. The reasoning behind how M. avium subsp. hominissuis biofilm is allowed to establish and persist without being cleared by the innate immune system is currently unknown. To identify the mechanism responsible for this, we developed an in vitro model using THP-1 human mononuclear phagocytes cocultured with established M. avium subsp. hominissuis biofilm and surveyed various aspects of the interaction, including phagocyte stimulation and response, bacterial killing, and apoptosis. M. avium subsp. hominissuis biofilm triggered robust tumor necrosis factor alpha (TNF-␣) release from THP-1 cells as well as superoxide and nitric oxide production. Surprisingly, the hyperstimulated phagocytes did not effectively eliminate the cells of the biofilm, even when prestimulated with gamma interferon (IFN-␥) or TNF-␣ or cocultured with natural killer cells (which have been shown to induce anti-M. avium subsp. hominissuis activity when added to THP-1 cells infected with planktonic M. avium subsp. hominissuis). Time-lapse microscopy and the TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) assay determined that contact with the M. avium subsp. hominissuis biofilm led to early, widespread onset of apoptosis, which is not seen until much later in planktonic M. avium subsp. hominissuis infection. Blocking TNF-␣ or TNF-R1 during interaction with the biofilm significantly reduced THP-1 apoptosis but did not lead to elimination of M. avium subsp. hominissuis. Our data collectively indicate that M. avium subsp. hominissuis biofilm induces TNF-␣-driven hyperstimulation and apoptosis of surveilling phagocytes, which prevents clearance of the biofilm by cells of the innate immune system and allows the biofilm-associated infection to persist.

Identification of Bicarbonate as a Trigger and Genes Involved with Extracellular DNA Export in Mycobacterial Biofilms

“…The ability of nontuberculous mycobacteria (NTM) to form biofilm creates challenges for treatment efficacy and host clearance (7, 8, 10, 11). We recently reported that NTM contain extracellular DNA (eDNA) in their biofilm matrix that significantly contributes to colonization, persistence, and drug tolerance of these biofilms, suggesting a novel antivirulence target for NTM (23).…”

“…Surveilling macrophages that encounter M. avium subsp. hominissuis biofilm in vitro become hyperstimulated and undergo early, rapid apoptosis in a tumor necrosis alpha (TNF-α)-dependent manner (11), which could explain why the aggregates and biofilms are not cleared by the immune system.…”

Biofilm Formation by Clinical Isolates and Its Relevance to Clinical Infections

“…Cultures of M. avium complex in with genetically-attenuated biofi lm formation demonstrated reduced invasion of bronchial epithelial cells (Yamazaki et al 2006 ), whereas wild-type M. avium complex biofi lms resisted clearance by, and induced apoptosis in, human mononuclear phagocytes more rapidly than their planktonic cognate (Rose and Bermudez 2014 ). Curiously, M. abscessus biofi lms were signifi cantly enhanced in the presence of necrotic neutrophils, an environment mimicking the namesake clinical syndrome caused by M. abscessus (Malcolm et al 2013 ).…”

Proteomic-Based Approach To Gain Insight into Reprogramming of THP-1 Cells Exposed to Leishmania donovani over an Early Temporal Window

“…In the present study, we have adopted a quantitative proteomics-based approach to gain insight into the reprogramming of the THP-1 cell line, an acute monocytic leukemia-derived human cell line, exposed to L. donovani for different time periods. Activated THP-1 cells have long been used as a versatile model system to study inflammatory responses, host cell apoptosis, and autophagy behavior in response to intracellular pathogens (16,17). This model may not completely replicate the in vivo conditions after infection, but it is a well-established in vitro model system to study the host-Leishmania interface (18).…”