H2O2 induces monocyte apoptosis and reduces viability of Mycobacterium avium-M. intracellulare within cultured human monocytes

Abstract

Mycobacterium avium-M. intracellulare, an intracellular parasite of mononuclear phagocytes, rarely causes disease in immunocompetent individuals. In contrast, in human immunodeficiency virus type 1-infected patients, M. avium-M. intracellulare can infect almost every tissue and organ. This suggests that immunocompetent individuals have a protective mechanism to control or prevent the infection. How mycobacteria may be killed by the host immune response is unclear. We have recently reported that induction of apoptosis of Mycobacterium bovis BCG-infected macrophages with ATP 4؊ was associated with killing of the intracellular mycobacteria. In the present study, a long-term culture of M. avium-M. intracellulare-infected monocytes was used to further evaluate the interaction between M. avium-M. intracellulare and primary human monocytes. In our system, M. avium-M. intracellulare parasitized the human monocytes and appeared to replicate slowly over 14 days within the host cells. To examine the role of apoptotic mechanisms in survival or death of intracellular mycobacteria, M. avium-M. intracellulare-infected human monocytes were treated with a monoclonal antibody to Fas receptor (APO-1/CD95) or with various concentrations of H 2 O 2 . Although both of these exogenous agents induced monocyte apoptosis, optimal killing (65% reduction in CFU) of intracellular M. avium-M. intracellulare was observed only when M. avium-M. intracellulare-infected cells were treated with 10 mM H 2 O 2 . Fas-induced apoptosis did not affect M. avium-M. intracellulare viability. Our results suggest that not all stimuli of monocyte apoptosis induce killing of intracellular M. avium-M. intracellulare. Since release of H 2 O 2 following phagocytosis of mycobacteria has been documented, H 2 O 2 -induced apoptotic death of M. avium-M. intracellulare-infected monocytes and its association with killing of the intracellular bacilli may be a physiological mechanism of host defense against M. avium-M. intracellulare.

The multiple roles of Fas ligand in the pathogenesis of infectious diseases

“…MDM infection with Mycobacterium avium intracellulare in vitro results in MDM apoptosis in association with control of mycobacterial replication, similar to that suggested for MTB [82], and these MDMs are sensitive to both TNF-a-and Fas-mediated killing [83]. These ®ndings apply only to differentiated macrophages: freshly isolated monocytes infected with Mycobacterium avium intracellulare in vitro, although sensitive to Fas-mediated apoptosis, do not demonstrate any inhibition of mycobacterial replication following Fas-mediated killing [84].…”

Apoptosis ofMycobacterium avium-infected macrophages is mediated by both tumour necrosis factor (TNF) and Fas, and involves the activation of caspases

“…BCG-infected macrophages have been shown to undergo apoptosis following treatment of the monolayers with ATP [10,21], resulting in the killing of the intracellular bacteria. Likewise, it has been shown that exposure of M. avium-infected macrophages to hydrogen peroxide, but not to anti-Fas antibody, is associated with apoptosis and inhibition of the growth of intracellular organisms [11]. More recently, Fratazzi and colleagues [8] reported 30% apoptosis in monolayers infected with M. avium with a 90% reduction in the viability of intracellular bacteria.…”

“…Apoptosis of bacille Calmette-Guérin (BCG) infected macrophages induced by ATP has been shown to be associated with killing of intracellular bacilli [10]. In addition, adding exogenous hydrogen peroxide to macrophage culture also triggers apoptosis and killing of intracellular M. avium in macrophages [11]. In addition, Fratazzi has shown that in vitro apoptotic macrophages with intracellular M. avium can be ingested by healthy macrophages that ultimately kill the intracellular bacteria [8].…”

CD8+ T-cell-mediated response to Listeria monocytogenes taken up in the liver and replicating within hepatocytes

“…Several additional studies reported that apoptosis, but not necrosis, induced in host cells by Mycobacterium spp. replicating intracellularly contributed to death of the pathogen by an unknown mechanism (94,95). Taken together, these results indicate that apoptosis of infected cells serves a complex function in host defenses.…”