Host defense mechanisms in Mycobacterium avium infection

Abstract

Mycobacterial infections constitute a major health problem with Mycobacterium tuberculosis causing tuberculosis that kills about 2 million people each year, and nontuberulous mycobacteria like M. avium causing opportunistic infections in immunocompromised individuals. The M. avium complex is a group of slow-growing mycobacteria that causes a disseminated infection in susceptible individuals, posing a significant threat to these patients. Mycobacterial infections are notoriously hard to treat and also demonstrating increasing resistance to several antibiotics. There are no efficient vaccines against these infections and treatment regimes are elaborate and costly. For these reasons, the search for novel therapeutic strategies continues to be pertinent. To this end, it is important to have an in-depth understanding of the immune responses triggered and the killing mechanisms employed by host cells in counteracting mycobacterial infections in mammalian systems. The objective of this work was to study inflammatory responses and killing mechansims involved in the control mycobacterial infections. We established a mouse infection model of M. avium to investigate host defense mechanisms to mycobacterial infections. In our model we show that M. avium strain 104 causes a chronic infection in mice and describe inflammatory and adaptive immune effector mechanisms that contribute to containment of M. avium infection (paper I). Quantification of mycobacterial loads in cells and tissues is elaborate thus we established a method using real-time PCR quantification of mycobacterial DNA to replace organ colony counts. Our results correlated with the traditional colony forming unit (CFU) counts (paper II). We investigated the role of antibacterial protein lipocalin (Lcn2) in M. avium infections using wild type and Lcn2 knockout mice. Lcn2 inhibited growth of extracellular M. avium in culture and in blood of infected mice, but not in macrophages and in tissues during long-term infection. Thus we investigated how this protein trafficked in cells in relation to M. avium in murine macrophages. We found that M. avium induced Lcn2 expression in a myeloid differentiation protein 88 (Myd88)-dependent fashion, but intracellular M. avium was efficient in evading the bactericidal properties of Lcn2 by trafficking to different compartments from Lcn2 and maintain a chronic infection in macrophages (paper III). We also investigated the role of the stress-related protein Kelch-like ECH-associated protein 1 (Keap1) in regulating inflammatory signaling and autophagy in the killing of M. avium. We found here that Keap1 was recruited to M. avium phagosomes together with the protein light chain 3 (LC3), a marker for autophagosomes. Keap1 knockdown also upregulated pattern recognition receptor induced inflammatory responses that culminated in decreased bacterial survival in macrophages (paper IV). Together, we have described a mouse M. avium infection model, added new aspects to the intracellular lifestyle of mycobacteria and provided new evidence on how the inflammatory response is orchestrated and regulated to ensure bacterial growth inhibition. This study also suggests new areas that can be exploited in the search for intervention strategies against mycobacterial diseases.