The Scavenger Protein Apoptosis Inhibitor of Macrophages (AIM) Potentiates the Antimicrobial Response against Mycobacterium tuberculosis by Enhancing Autophagy

Abstract

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.

The macrophage soluble receptor AIM/Api6/CD5L displays a broad pathogen recognition spectrum and is involved in early response to microbial aggression

“…A recent report shows that transfection with AIM enhances macrophage bactericidal ability in a model of infection by Mycobacterium tuberculosis. 35 Interestingly, AIM levels increased early in mice serum (24 h) following inoculation of M. tuberculosis, which is in agreement with our results. This increase in AIM levels could, at least in part, be ascribed to PAMP-mediated triggering of AIM release into the circulation; however, the release observed in vitro is much faster than that observed in the in vivo experiments.…”

The human CD5L/AIM-CD36 axis: A novel autophagy inducer in macrophages that modulates inflammatory responses

“…Accordingly, in a previous study we observed that the chemokine IL8 was enhanced by CD5L expression in THP1 MK. 53 Further research is underway to analyze the modulation of MF chemokine/cytokine production by CD5L. Of relevance for the present study, CD5L modulation of TNF, IL1B, and IL10 were reverted by silencing of the ATG7 protein.…”

“…53 Together, these data point to the participation of CD5L in promoting autophagy. In accordance, CD5L enhanced the LC3-II/-I ratio and downmodulated AKT (Ser473) phosphorylation in both THP1 MF and PB monocytes.…”

“…In mouse models and in humans, elevated levels of CD5L occur in conditions with inflammatory components such as LPSinduced fulminant hepatitis 20 , M. tuberculosis infection, 53 obesity, 25 liver cirrhosis, 12,13,62 and allergic asthma, 63 among others. Therefore, in these scenarios, high expression of CD5L may act in an autocrine fashion on MF to modulate inflammatory responses.…”

“…52 Moreover, our previous results indicate that CD5L promotes autophagy in THP1 MK upon M. tuberculosis infection. 53 Here we further analyzed the effects of CD5L on the induction of MK autophagy by bacterial Pam3CSK4 and LPS, given that these products have been described to induce autophagy. 54 THP1 MK and PB monocytes were examined for the microtubule-associated protein 1 light chain 3 A/B (LC3)-II/LC3-I ratio as an autophagy marker in western blot of total cell lysates (Fig.…”