The scavenger protein apoptosis inhibitor of macrophages (AIM) potentiates the antimicrobial response against Mycobacterium tuberculosis by enhancing autophagy.
Apoptosis inhibitor of macrophages (AIM), a scavenger protein secreted by tissue macrophages, is transcriptionally regulated by the nuclear receptor Liver X Receptor (LXR) and Retinoid X Receptor (RXR) heterodimer. Given that LXR exerts a protective immune response against M. tuberculosis, here we analyzed whether AIM is involved in this response. In an experimental murine model of tuberculosis, AIM serum levels peaked dramatically early after infection with M. tuberculosis, providing an in vivo biological link to the disease. We therefore studied the participation of AIM in macrophage response to M. tuberculosis in vitro. For this purpose, we used the H37Rv strain to infect THP-1 macrophages transfected to stably express AIM, thereby increasing infected macrophage survival. Furthermore, the expression of this protein enlarged foam cell formation by enhancing intracellular lipid content. Phagocytosis assays with FITC-labeled M. tuberculosis bacilli indicated that this protein was not involved in bacterial uptake; however, AIM expression decreased the number of intracellular cfus by up to 70% in bacterial killing assays, suggesting that AIM enhances macrophage mycobactericidal activity. Accordingly, M. tuberculosis-infected AIM-expressing cells upregulated the production of reactive oxygen species. Moreover, real-time PCR analysis showed increased mRNA levels of the antimicrobial peptides cathelicidin and defensin 4B. These increases were concomitant with greater cellular concentrations of the autophagy-related molecules Beclin 1 and LC3II, as well as enhanced acidification of mycobacterial phagosomes and LC3 co-localization. In summary, our data support the notion that AIM contributes to key macrophage responses to M. tuberculosis.
Innate Immunity Group, Fundació Institut d'Investigació en Ciències de la Salut Germans Trias i Pujol (IGTP), Badalona, Spain.
Lucía
L
Sanjurjo
Núria
N
Amézaga
Cristina
C
Vilaplana
Neus
N
Cáceres
Elena
E
Marzo
Marta
M
Valeri
Pere-Joan
PJ
Cardona
Maria-Rosa
MR
Sarrias
D000818:Animals; D023181:Antimicrobial Cationic Peptides / Q000378:metabolism; D051017:Apoptosis Regulatory Proteins / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D001343:Autophagy; D058491:Bacterial Load; D002460:Cell Line; D002470:Cell Survival; D005260:Female; D005786:Gene Expression Regulation; D006801:Humans; D007113:Immunity, Innate; D016209:Interleukin-8 / Q000378:metabolism; D000071518:Liver X Receptors; D008264:Macrophages / Q000166:cytology / Q000276:immunology / Q000378:metabolism; D051379:Mice; D009169:Mycobacterium tuberculosis / Q000502:physiology*; D057093:Orphan Nuclear Receptors / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism; D011971:Receptors, Immunologic / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D051116:Receptors, Scavenger; D023083:beta-Defensins / Q000378:metabolism; D054804:Cathelicidins
D023181:Antimicrobial Cationic Peptides; D051017:Apoptosis Regulatory Proteins; C117272:Cd5l protein, mouse; C106641:DEFB4A protein, human; D016209:Interleukin-8; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; D017382:Reactive Oxygen Species; D011971:Receptors, Immunologic; D051116:Receptors, Scavenger; D023083:beta-Defensins; D054804:Cathelicidins