Pulmonary natural killer cells control neutrophil intravascular motility and response to acute inflammation

Context and background

The immune system is highly complex with many layers of regulation and control, without which we would suffer from constant auto-immunity or dysregulated immune responses. One of the most important layers of regulation comes from cell-cell interactions (1). For example, natural killer (NK) cells and neutrophils are capable of complex interactions that are crucial for innate immune responses (2).

NK cells are innate immune cells that function in a similar manner to cytotoxic T cells. NK cells’ primary role is to recognise and remove virally infected cells or tumours. However, NK cells are also capable of modulating the activities of other immune cells as they can release pro-inflammatory molecules such IFN-γ and TNF-α (3).

Neutrophils are the most abundant immune cell in the blood, and are often the first immune cells to respond to infection. Indeed, the primary function of neutrophils is to remove invading microbes. To carry out these functions, neutrophils are capable of phagocytosis, degranulation and producing extracellular traps, termed NETs (reviewed in (4)).

The role of interactions between NK cells and neutrophils within the lung microenvironment are not clear. The authors aimed to address this with the use of lung-intravital microscopy to image NK cells and neutrophils in vivo and in real-time.

 

Key findings

1. Natural killer cells are resident within the lung vasculature

Flow cytometry revealed that NK cells are present within a number of tissues, including the liver and the lungs. Live, intravital microscopy of the lungs confirmed that NK cells reside in the lung vasculature. Tracking cells over time demonstrated that NK cells remain within the field of view longer than neutrophils. Furthermore, neutrophils appeared to be much more mobile than the NK cells, providing further evidence that the NK cells were more of a resident population (Fig 1).

Figure 1. Schematic animation of the interaction between neutrophils and NK cells. A resident NK cell is shown in green with a neutrophil in orange. Cells can be observed interacting with each other over various time periods, where the cells appear to sample each other. Material is transferred from the neutrophil to the NK cell.

 

2. Natural killer cells and neutrophils form interactions

Utilising intravital microscopy, the authors observed interactions between NK cells and neutrophils that could persist for over 20 minutes, though shorter interactions were much more common. Moreover, these interactions were not singular events, with over 50% of neutrophils interacting with an NK cell more than once. Precisely how these interactions occur was not clear but this represents an interesting area for further work.

3. There is a transfer of material from neutrophils to NK cells

The finding that I found most interesting was that there was a transfer of material from the neutrophil to the NK cell (Fig 2). “Patches” of the neutrophil marker used during imaging were observed in NK cells after an interaction. Labelling of the neutrophil membrane with a membrane-specific dye confirmed that this transfer contained material that was either membrane-bound or vesicle-bound rather than cytoplasmic.

 

Figure 2. Stills taken from the schematic animation. NK cells and neutrophils form interactions that can result in the transfer of material. A resident NK cell is shown in green with a neutrophil in orange. As the cells interact, some of the neutrophil membrane-bound material (dark orange) is captured and retained within the NK cell.

 

4. Neutrophil-NK cell interactions impact upon neutrophil functions, modulating motility and response to infections

Neutrophils that did not interact with NK cells had shorter tracks and increased speed compared to those that did interact. Importantly, following an interaction with an NK cell, the neutrophil appeared to “scan” the epithelium more slowly and, perhaps, more intensively. To further investigate the impact of these cell-cell interactions on neutrophil behaviour, the authors injected LPS (lipopolysaccharide) to elicit an immune response. When NK cells were depleted, there were approximately 50% more neutrophils responding to the LPS infection. This suggests that NK cells are regulating the neutrophil response to infection by controlling accumulation within the lung vasculature.

 

Why I chose this paper

The importance of observing, in real-time, how cells behave in their natural environment is hugely important. Although Drosophila and zebrafish are excellent models for imaging, they have various limitations that can only be resolved through the use of organisms more closely resembling humans. I am fascinated by the technique of intravital imaging and as the technology gets better, we will uncover surprising and fascinating science. This paper nicely demonstrates this powerful technique whilst revealing complex cell-cell interactions that raise any intriguing questions.

 

Open questions

1. Are the NK cells within the lung vasculature tissue resident or are they are a more dynamic population?
   1. It would have been interesting to perhaps see cell tracing experiments to confirm that the observed cells remain within the lung vasculature, for example with the use of photoconvertible labels.

 

2. What is the role of the NK cells in different organs, for example in the liver? Are they also interacting with neutrophils (or other immune cells) in these tissues?

 

3. How are the interactions between NK cells and neutrophils mediated?
   1. It would be interesting to see labelled tethering molecules or the knockout of proteins that could facilitate such interactions to uncover the mechanism. Alternatively, it may be that the authors have ideas as to how these interactions are occurring.

 

4. Are the neutrophils that have interacted with an NK cell better at recognising an immune challenge than those that do not interact?

 

5. What is the mechanism by which these interactions are altering the neutrophil behaviour?

 

6. Concerning the material that is transferred:
   1. Is this material actively transferred by the neutrophil or are the NK cells “sampling” the neutrophils?
   2. What is the nature of the material, could the authors speculate further on what they believe is being transferred?
   3. What is the effect of this transfer? Is this required for the control of neutrophils during an infection?
   4. Is there an increase in the number of interactions or material transferred during an infection compared to steady-state?

 

7. Did the authors investigate interactions with/between other immune cells?

 

References

1. Xie J, Tato CM, Davis MM. How the immune system talks to itself: the varied role of synapses. Immunol Rev. 2013;251(1):65–79.
2. Costantini C, Cassatella MA. The defensive alliance between neutrophils and NK cells as a novel arm of innate immunity. J Leukoc Biol. 2011;89(2):221–33.
3. Mandal A, Viswanathan C. Natural killer cells: In health and disease. Hematol Oncol Stem Cell Ther. 2015 Jun 1;8(2):47–55.
4. Rosales C. Neutrophil: A Cell with Many Roles in Inflammation or Several Cell Types? Front Physiol [Internet]. 2018 Feb 20 [cited 2019 Jun 24];9. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5826082/

 

Tags: intravital imaging, microscopy

doi: https://doi.org/10.1242/prelights.11639

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