Autophagy-independent role for Atg5 in M. tuberculosis infection

This is a second paper about neutrophils in Nature this week (advanced online publication). Quite interesting and surprising. It showed that autophagy molecule Atg5 has unique [autophagy-independent] role in protecting against neutrophil-driven immunopathology during M. tuberculosis lung infection.

Initially, the authors showed that myeloid cell-specific deletion of Atg5 (LysM-Cre Atg5^fl/fl) made mice highly susceptible to M. tuberculosis infection (that was expected based on earlier studies).

Unexpectedly and surprisingly, however, mice singly deficient for other autophagy components showed normal response to M. tuberculosis infection, implying unique, autophagy-independent role for Atg5 during M. tuberculosis infection.    

Indeed, LysM-Cre Atg5^fl/fl mice showed more severe M. tuberculosis-associated immunopathology and neutrophil infiltration (even though LysM-Cre Atg5^fl/fl mice did not harbor substantially more M. tuberculosis).

Furthermore, LysM-Cre Atg5^fl/fl mice depleted of neutrophils were protected from lung pathology during M. tuberculosis infection.

Finally, neutrophil-specific deletion of Atg5 (MRP8-Cre Atg5^fl/fl mice) confirmed pathological role of Atg5-deficient neutrophils during M. tuberculosis infection. 

In summary, this study provides evidence for autophagy-independent role of Atg5 deficiency in neutrophils during M. tuberculosis infection. It appears that neutrophils recruited to the sites of M. tuberculosis infection contribute to tissue pathology rather than provide protection in absence of Atg5. Earlier it was assumed that Atg5 played a protective role during infection purely via its involvement in autophagy formation. However, this new study point to a more complex role of Atg5.

David Usharauli

Neutrophils prepare niche in the lung for breast cancer metastasis and colonization

Tumors are known to hijack and exploit body's normal function for its own purpose. This is especially true for immune system which often is a reverse-target for growing tumors. 

For example, new study published by journal Nature this week showed that neutriphil-derived leukotrienes [small lipid molecules] support lung metastasis and colonization by primary breast cancer cells.

Two reasons why this study was accepted in Nature: (1) to promote newly established The Francis Crick Institute based in London [Nature is a UK journal], (2) positive results with Zileuton (Zyflo®), an orally active inhibitor of 5-lipoxygenase and leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄).

First, using mouse model, the authors [it has just 2 authors, very rare] showed that metastasis of breast cancer cells to the lung were supported by neutrophils (genetic depletion of neutrophils in G-CSF^-/- or Ela2-Cre-DTA mice as well as antibody-mediated neutrophil depletion reduced tumor metastasis).

Next, the authors found that neutrophil-conditioned medium supported tumor growth in vitro.

It turned out that this tumor-promoting neutrophil-conditioned medium contained high levels of leukotrienes, an active lipid metabolites of 5-lipoxygenase enzyme pathway. 

Finally, using 5-lipoxygease deficient mouse or Zileuton (Zyflo®) treatment, the authors showed that blocking leukotrienes activity reduced lung metastasis (though Zileuton (Zyflo®) treatment was surprisingly more effective compared to KO mouse).

In summary, this study suggests that anti-leukotriene therapy already available in the market could provide anti-cancer benefits.

David Usharauli

Gut microbiota controls clinical severity of sickle-cell disease via neutrophil "ageing"

Our knowledge of the role of gut microbiome in human health and disease is expanding on a daily basis.

For example, just few days ago journal Nature published a study where the authors showed that in mouse model of sickle-cell disease (Hba^-/- Hbb ^-/-) the presence gut microbiome influenced clinical severity via neutrophil "ageing".      

In sickle-cell disease, neutrophils expressing Mac-1 capture sickle red blood cells that leads to vaso-occlusion and tissue damage. It appears that "ageing", or mature neutrophils, defined as CD62L^low CXCR4^high population, express more Mac-1 and those neutrophils can capture more RBCs per cell basis (that contributes to development of sickle-cell disease symptoms). [the authors does not discuss the difference between "ageing", "senescence" and "mature" neutrophils].

Next, the authors showed that germ-free mice or antibiotic treated mice have reduced number of "ageing" neutrophils.

Using BM chimera, the authors found that absence of MyD88 adaptor molecule or TLR4 receptor also mimicked the effect of antibiotic treatment on neutrophil "ageing".

Finally, the authors found that antibiotic treatment of sickle-cell disease model mice reduced number of circulating "ageing" Mac-1 expressing neutrophils and showed reduction in tissue damage.

In summary, this study proposed a novel therapeutic path for treatment for sickle-cell disease by modulation of gut microbiota and neutrophil maturation ("ageing").

David Usharauli

Tumor converts neutrophils into metastasis-promoting cells via γδ T cell-derived IL-17

Immune system supposed to defend the body from infectious agents and genetically transformed cells (tumors). Usually, at the end of each immune response, that can be quite damaging (immune phase), immune cells will be involved in tissue healing, regeneration or remodeling (adaptation phase). However, sometimes infectious agents or tumors will circumvent these steps and jump directly to adaptation phase and will recruit immune cells to carry out their "agenda" at the expense of the host.

For example, we can speculate that tumor's "agenda" would be to grow and expand (metastasize). However, neither of these possible without local and distant tissue remodeling and its readiness to accommodate (accept) tumor cells. Here is where local immune cells become involved.

New study in journal Nature points to one of those possibilities. The authors showed that in a mouse model of mammary tumor metastasis, neutrophils promotes tumor lung metastasis via IL-17 produced by circulating γδ-T cells.

I would like to point out that the authors have used tumor transplantation model (to have shorter experiments) that is obviously very different from spontaneously arising tumors. Nevertheless, they found that lung or lymph node metastasis of skin transplanted tumor was reduced with neutrophil depletion using α-Ly6G antibody (this antibody supposedly selectively depletes neutrophils since they express it at high level).

Interestingly, tumor metastasis were also reduced in the recipients devoid of adaptive immune system and correlated with reduction of IL-17 and G-CSF (granulocyte colony-stimulating factor) serum levels.

Finally, the authors showed that γδ-T cell depletion or genetic deficiency reduced lung and lymph node metastasis of transplanted tumor.

In summary, these results suggest that tumor cells exploit not yet identified pathways within immune system (γδ-T cells / IL-17 / neutrophils axis) to prepare distant tissues to accommodate tumor colonies coming from original tumor niche. Only by understanding how immune cells interact with normal tissues during or after immune response, could we design ways to block such metastasis.

In general this paper is OK, especially if one considers other papers (two recent papers in JEM) corroborating the idea of IL-17's involvement in tumor initiation and metastasis. However, some data are not clear or not well explained, so not a Nature caliber paper, in my view. For example, in Fig. 3a, treatment with α-IL-17A did not modify IL-17 level in the serum. Also, the authors did not explain why they thought CD8 T cells were protective against metastasis in this model when tumor-bearing RAG KO hosts, which lack CD8 T cells, did not show increased metastasis (Fig. 2d versus. Fig. 3g)?


David Usharauli