Macrophages commit ‘defensive suicide’ after Adenovirus and Listeria infection

Posted by Kasra

Cells often kill themselves for the benefit of their lot. New forms of cell suicide are being discovered every day now.  I wrote about apoptosis, which is a rather clean form of cell suicide recently. However, necrosis which until recently seemed to be a an uncontrolled form of cell death, is now being looked at again as a form of controlled suicide. A recent publication by  Di Paolo et al in the new journal of Cell Reports sheds some light on on of these rather unusual forms of cell death. The authors call it ‘defensive suicide’.

Di Paolo et al. intravenously injected Adenovirus into the mice. They observed that the macrophages (specifically in this paper, liver macrophages) capture the virus particles. However, shortly after the macrophages died of necrosis. Interestingly, they find this phenomenon to be independent from normal mediators of cell death such as various Caspases, as well as inflammatory mediators such as MyD88, TRIF and ASC. They finally point to IRF3,  a transcription factor normally activated after certain infections. Macrophages from IRF3-/- mice did not go through necrotic death after Adenovirus infection. The authors next show the proteins upstream of IRF3 are dispensable for the necrotic death of macrophages and that IRF3 is not phosphorylated at the time of macrophage necrosis, further adding to the enigma of the mechanism. The only clue we get so far is that this mechanism is dependent on escape the of the pathogen from the phagolysosome into the cytosol. They show this by using Adenovirus and also Listeria monocytogenes  mutants that cannot escape the phagolysosome. Compared to their wildtype counterparts, the mutant intracellular pathogens do not induce necrotic death of the macrophages.

Finally, to see if this necrotic death actually has a benefit for the host, the authors deplete mice from macrophages and infect them again with Adenovirus or L. monocytogenes. They observe that without the macrophages the virus or bacterial burden is a lot higher in the liver. Thus, this mechanism could be a way of slowing down the systemic spread of infection. The macrophages might collect the pathogens that would be otherwise infecting other defenseless cells and destroy them via necrotic death. Would this mean that necrotic death better kills the intracellular pathogens compared to other forms of programmed death? Or they just go through this pathway because other pathways of programmed death are blocked by the pathogens? Considering that necrosis occurs very rapidly (within minutes), the first one seems more likely.

The possible role of IRF3 in induction of necrotic death in macrophage following intracellular infection. From Di Paolo et al. , Cell Reports, Volume 3, Issue 6, 1840-1846, 13 June 2013

The possible role of IRF3 in induction of necrotic death in macrophage following intracellular infection. From Di Paolo et al. , Cell Reports, Volume 3, Issue 6, 1840-1846, 13 June 2013

This mass suicide of macrophages is a very interesting phenomenon. It also raises many questions that have not yet been addressed. The most obvious question is the signaling mechanism by which IRF3 induces this special form of necrosis. The authors did not find any dependence on the proteins that are usually known to be upstream of IRF3. So there might be a novel mechanism involved. Another question concerns the route of infection. The authors have used intravenous injection both for Adenovirus as well as L. monocytogenes infection. However, these pathogens usually enter the body from the gut or the lungs and then reach the circulation system. Would this defensive necrosis extend to the immune cells in other tissues such the lung or the gut macrophages? Would the route of infection affect the intensity/quality of macrophage necrosis? We will hopefully get the answers in the near future!

Di Paolo NC, Doronin K, Baldwin LK, Papayannopoulou T, & Shayakhmetov DM (2013). The Transcription Factor IRF3 Triggers “Defensive Suicide” Necrosis in Response to Viral and Bacterial Pathogens. Cell reports, 3 (6), 1840-6 PMID: 23770239

Apoptosis, not so quiet after all

Posted by Kasra

Apoptosis has been conventionally regarded as a quiet and non-inflammatory event, compared to necrosis which results in release of alarmins and other danger signals inducing inflammation and immune cell recruitment. However, a recent report by Cullen et al. published in Molecular Cell suggests otherwise. They suggest that at least in one form of apoptosis, pro-inflammatory mediators are released by the apoptotic cells and can act as ‘find-me’ signals to the phagocytes to find and clear them.

Fas receptor or CD95 is among the famous apoptosis receptors. It is a member of the TNF receptor family and it induces apoptosis through Caspase-8 activation. Interestingly, Cullen et al. show that different cell types produce pro-inflammatory chemikones such as MCP-1, CXCL1 and MIP-2 when they go through apoptosis via Fas-pathway. They show that this chemokine release is NF-kappa-B mediated and independent of Caspase-8 activation. It is possible be that somewhere during the evolution, the apoptotic pathway cross-linked with the pro-inflammatory signaling pathway and found benefit in it. Accordingly, Cullen et al. show that the Fas-induced pro-inflammatory cytokine/chemokine production still occurs  even if the apoptotic pathway is inhibited showing that these pathways are separate.

Next the authors show that the supernatant from the apoptotic cells can induce migration of macrophages and neutrophils. They also pinpoint the responsible chemokine by depleting them one-by-one. They show that MCP-1 induces macrophage migration and IL-8 recruits neutrophils.

At this point it cannot be said really how inflammatory these apoptotic cells would be in vivo. There are parts of the body where apoptosis occurs constantly,  so this could potentially lead to an unwanted constant inflammation in those areas. Therefore, either different cells would have different levels of apoptotic-proinflammatory chemokine release, or local mechanisms would compensate and counteract the inflammation. More studies will help us understand how apoptotic cells can send their ‘find-me’ signals without causing too much turbulence in their tissue.

Schematic diagram of model proposed by Cullen et al. From

From Cullen et al. Molecular cell, 49 (6), 1034-48

Cullen SP, Henry CM, Kearney CJ, Logue SE, Feoktistova M, Tynan GA, Lavelle EC, Leverkus M, & Martin SJ (2013). Fas/CD95-induced chemokines can serve as “find-me” signals for apoptotic cells. Molecular cell, 49 (6), 1034-48 PMID: 23434371