Today’s post looks at the complex interactions between three types of immune cells: NK cells, neutrophils and T helper cells. It was found that NK cells can control the functions of neutrophils. NK cells interacted with neutrophils using a receptor called NKG2A leading to lower pro-inflammatory activities. Simultaneously, it was reported that neutrophils could control the pro-inflammatory activities T cells via a mediator called, thromboxane A2. Thus, an interesting axis of NK cells-Neutrophils-T cells may help control inflammation.
- NK cells can cause neutrophils to become more regulatory via NKG2A and control colitis.
- Neutrophils control T cell traffic from lymph nodes during vaccination via thromboxane A2.
Take home message: An NK-neutrophil-T cell axis could be playing a role during the first signs of inflammation in IBD.
NK cells are innate immune cells, which have the capacity to destroy infected and defective cells. This function is regulated by receptors that interact with ligands on the opposing cell that either activate and inhibit the NK cell. The sum total of all activating and inhibitory signals that they receive determines their cytotoxicity. In a recent article by Hall et al., published in Mucosal Immunology, it was found that NK cells have a surprisingly important protective function during intestinal inflammation that is mediated by an interaction of the inhibitory receptor, NKG2A, with local neutrophils. Simultaneously, Yang et al., in a recent publication of Journal of Experimental Medicine, determined that neutrophils regulate the adaptive immune response using Thromboxane A2, controlling both the location and magnitude of the T cell responses. Together, these articles suggest a possible camaraderie between these cells.
Using two different models of NK cell depletion, including antibody-mediated depletion and NK cell deficient mice, Hall et al. determined that NK cells must have a protective role in the dextran sodium sulfate (DSS) model of colitis. In this model, ingestion of DSS leads to destabilization of the intestinal mucus allowing bacteria to interact with the underlying epithelium causing an acute inflammation and induction of an adaptive immune response. They found that without NK cells; the mice had more colon damage and infiltration, increased pro-inflammatory cytokine production and more neutrophils in both the colon and intestinal draining lymph nodes (mesenteric lymph nodes).
The authors were particularly interested in the effects on neutrophils and further explored this avenue. They found that the loss of NK cells during DSS colitis lead to more activated neutrophils with increased CD69 expression, magnified reactive oxygen species (ROS) production, and increased amounts of IL-6 and IL-17A as determined by intracellular cytokine staining.
Using a transwell culture system, they were able to determine that the effects generally required direct cell-cell contact between the neutrophils and the NK cells. They also found that the status of the NK cell was crucial. DSS colitis primed NK cells were far more effective at suppressing and lead to more neutrophils expressing low IL-6 and high IL-10.
The next logical step was to determine how this was happening, which the authors did. As NK cells are known to express activating and inhibitory receptors, they checked their expression. They found that the expression of the inhibitory receptor NKG2A was the most strikingly affected by DSS colitis and was highly upregulated on many NK cells. Thus, they continued their neutrophil-NK cell co-culture experiments with an anti-NKG2A antibody and found that blocking antibody could mostly eliminate the suppressive effects of the DSS-primed NK Cells. Confirming this data, in vivo experiments using anti-neutrophil and anti-NKG2A antibodies showed that loss of NK cells was comparable to blocking the NKG2A. These results allowed them to conclude that NKG2A was the important factor.
I agree that NKG2A is the important factor. However, it wasn’t clear to me how this was working. Was there an interaction between NKG2A and MHC class I on the neutrophil? Was this interaction causing a signal cascade in the neutrophil? Or did the interaction lead to changes in the NK cell, which allowed it to express other factors like anti-inflammatory cytokines? It made me curious to see what would happen in an experiment where the NK cells were, for example, IL-10 deficient. Furthermore, when does the NK cell achieve its super suppressive powers? And is it possible that the NK cell is pro-inflammatory in the beginning of colitis? They look at day 7, but no earlier. As DSS colitis is a transient model, which begins to clear up at day 8, at day 7 anti-inflammatory programs would be active.
While Hall et al. was looking at NK cell influences on neutrophils, Yang et al. was investigating neutrophil effects on the adaptive immune response during footpad vaccination. At first glance, one may think that vaccination may have nothing to do with colitis. However, many lessons learned through vaccination research can be applied to immune responses in the intestine during intestinal trauma. A vaccine includes a foreign antigen combined with adjuvant to stimulate the innate immune system, which leads to a strong adaptive immune response specific for the antigen. Trauma in the gastrointestinal system includes the same elements: damage allows the exposure of bacterial products and luminal antigens to underlying immune cells in the lamina propria. The bacterial products function as an adjuvant, while an adaptive immune response can form against the copious luminal antigens.
Yang et al. found that during a footpad vaccination, there were two waves of neutrophils to the lymph nodes. The first wave included neutrophils that arrived in the local draining lymph node via the lymphatics, within 15 minutes of the injection, presumably after first visiting the site of injection. This influx peaked at two hours and then was more or less gone by 24 hours. The second wave was at day three and was characterized by neutrophils entering the local lymph node via the blood. A series of blocking experiments suggested that the first wave was controlled by prostanoids and the second wave by both prostanoids and chemokines.
In a not so logical fashion, the authors then looked at how the loss of neutrophils was affecting the resulting T cell response against the vaccine antigen. They found that without neutrophils, the numbers of responding T cells were increased in lymph nodes, especially in the distal lymph nodes with 75% of the reactivity being found there. The distal accumulation and increased responsiveness was found to be dependent on the loss of neutrophil prostenoid release, specifically, thromboxane A2. Thromboxane A2 is better known as a vasoconstrictor and platelet aggregator generated from prostaglandin H2. In this case, the authors suggest that it keeps the T cells in the lymph nodes while simultaneously increasing their random movements, preventing their interactions with dendritic cells.
The second article that I read was not the most reader-friendly of all articles and it didn’t mention the immunological consequences of increased distal T cell responsiveness. It would’ve been interesting if they used their antibodies and thromboxane A2 receptor antagonist to investigate changes in a disease setting. However, what I found interesting was that both articles dealt with neutrophils having regulatory functions. The first describes a pathway for generating suppressive neutrophils via NK cells during intestinal inflammation (measured 7 days after the colitis initiation) and the second describes (also 7 days after vaccination) neutrophils preventing T cell responses and accumulation in the distal lymph nodes.
To bring it back to IBD, these kinds of studies are essential to determine the underlying immunological networks that can play a role in creating the unwanted immune responses in IBD. It could very well be that the next best therapy or preventive for IBD is increasing the effects of NK cell-derived NKG2A or magnifying the thromboxane A2 response of neutrophils.
Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment.
Hall, L. J., Murphy, C. T., Quinlan, A., Hurley, G., Shanahan, F., Nally, K., & Melgar, S. (2013). Natural killer cells protect mice from DSS-induced colitis by regulating neutrophil function via the NKG2A receptor. Mucosal Immunology. doi:10.1038/mi.2012.140
Yang, C. W., & Unanue, E. R. (2013). Neutrophils control the magnitude and spread of the immune response in a thromboxane A2-mediated process. The Journal of Experimental Medicine. doi:10.1084/jem.20122183