An airway-to-brain sensory pathway mediates influenza-induced sickness 2023 Bin et al

Harvard, Boston.

I think this paper highlights a number of areas that could be usefully studied with respect to ME/CFS. To me, it makes sense that a lot of the symptoms we experience are a result of sickness behaviour, maybe a 'dialled-up' response to a mostly latent infection.

Of course, 'sickness behaviour' is a non-BPS term meaning the types of behaviours and signs that animals exhibit when sick - withdrawing from others, reducing activity, reducing food intake, fever (with pain and fatigue, for example, used to produce the behavioural response). The itaconate shunt to reduce energy availability to pathogens would also be an aspect of sickness behaviour. The combination of responses can help a sick animal to survive an infection, and to avoid infecting genetically-related group members.

We have seen the phenomenon instead labelled 'sickness response', which is perhaps better, as it avoids confusion with malingering.

 

The authors note that, as well as generalised responses, there can be site specific responses, such as nausea and diarrhoea in response to gut infections, and coughs in response to respiratory infections.

They say that some molecules produced by the body when administered alone to an animal can produce sickness behaviour (interferons, interleukins, TNF, eicosanoids, others). Pathogen-derived factors such as lipopolysaccharides and bacterial toxins can activate central and/or peripheral neurons. So, potentially there might be many ways to produce sickness behaviour, [and maybe many ways for things to go wrong].

They note that drugs such as aspirin and ibuprofen can dial down sickness symptoms by blocking

They note that the drugs don't eliminate sickness symptoms entirely, suggesting that there are other ways of generating the response. I find this interesting, as I have found and reported here that I think ibuprofen is helpful in damping down PEM, used acutely and as a prophylactic.

They talk about PGE2 (prostaglandin E2) as an instigator of a sickness response. Knocking out enzymes in the pathway downstream of cyclooxygenase and upstream of PGE2 can also reduce the sickness response. PGE2 is detected by four G-protein-coupled receptors (EP1-EP4). Knocking out different receptors in different parts of the body reduces different aspects of the sickness response. For example, knocking out the EP3 receptors in part of the hypothalamus in the brain reduced a fever response. PGE2 receptors elsewhere are reportedly relevant to arousal and feeding. [This might explain part of the feeding problems in some cases of ME/CFS, perhaps even in some other conditions where motivation to eat is lost.]

There's some discussion about how PGE2 might act on the brain. The possibility that there may be impacts on peripheral neurons is raised:

 

Experiment 1 - sickness responses to flu infection in mice
They infected mice with a flu virus, with the mice going on to exhibit sickness responses. PGE2 levels increased. Also:

Ibuprofen and aspirin both reduced PGE2 levels, reduced sickness responses and also increased survival rates from the infection. However, the drugs did not completely eliminate the sickness response.

They tried applying inhibitors of the 4 different receptors, but only inhibitors of EP3 made any difference to the symptoms/behaviours tracked.

Experiment 2 - do EP3 receptors in the brain or in the periphery cause sickness responses?
The EP3 receptor is expressed in some types of both central and peripheral neurons. The authors wanted to find out which receptors were causing the flu-induced sickness response.

They obtained/produced two strains of mice that could have EP3 receptors knocked out when exposed to Cre recombinase. One of the mice strains had Cre recombinase targeting central neurons, in the other, Cre recombinase targeted peripheral neurons. The mice without EP3 receptors on the peripheral neurons had sickness responses that looked like the ibuprofen-treated normal mice with the flu. The mice without EP3 receptors on the central neurons had sickness responses that looked like the normal mice with the flu - they moved less and ate less.

So, the conclusion is that it is the EP3 receptors on the peripheral neurons that are playing a significant part in producing the sickness response. It doesn't seem to make any difference whether the EP3 receptors in the central neurons are there or not (at least for sickness responses from flu in mice).

 

Experiment 3 - What type of peripheral EP3 receptors are causing sickness responses?

So, the location of the EP3 receptors that seemed to be accounting for a significant amount of the sickness response in the mice includes the vagus nerve, the nerve to the mouth and throat (glossopharyngeal), the dorsal root ganglia and possibly some others (but not neurons in the brain). The authors wanted to find out which locations were causing the sickness response.

It sounds as though the layout of these nerves in mice is a bit different to that in humans.

When the super ganglion (combining the vagal and glossopharyngeal neurons) was knocked out, the sickness responses were much reduced. The authors concluded that influenza induces behavioural changes through EP3 receptor expressed on vagal and/or glossopharyngeal sensory afferents.

 

The work described in this paper is impressive. The authors didn't stop there.

Experiments 4 - identifying which particular neuron types are affecting sickness response
They sequenced RNA from single nodose, jugular and glossopharyngeal cells, looking for the expression of the Pteger3 gene that encodes the EP3 receptor. They found 6 neuron clusters with high expression of Pteger3.

They did some work with mice where different groups of neuron types could be knocked out, concluding that:

NP9 - that's a subset of naso-pharyngeal neurons (so not part of the vagus nerve)
(They also checked TRPV1 neurons - I'm not sure if they have something to do with the TRP channels that the Griffith team are interested in? These researchers found that the TRPV1 neurons didn't seem to affect sickness response to flu in mice.)

 

Experiments 5 - how are the NP9 neurons affecting disease progression and viral transcript levels?
Flu infection produced similar levels of PGE2 in the plasma and in the lungs (BALF - lung wash fluid) in the various types of mice with different neurons knocked out. The authors conclude from that that it is the levels of PGE2 that are detected (I assume, attach to the receptors) that is important.

I thought this was interesting. In the mice that had the NP9 neurons knocked out, they had lower appetite reduction, less activity level reduction and higher survival compared to the normal mice. The results looked similar to normal mice given ibuprofen. The mice had lower viral levels in both the upper airways and the lungs, but it looks as though the virus persisted in the lungs for longer, although was eventually cleaned up. Just noting that - perhaps the authors will discuss that later.

If you look at these charts, check out the neuron groups that were tested. The NP9 results are shown in the right column.



And here's the chart of the viral titres.

 

Experiment 6 - are neurons in other parts of the body that express Gabra-1 contributing to the sickness response (to flu in mice)?
No. The authors checked by ablating the NP-9 (Gabra-1 expressing neurons) in the naso-pharangeal area, which would leave any other neurons that express Gabra-1 intact. With the NP-9 neurons gone, the mice reacted to the flu infection as if they had been given ibuprofen.

A couple of other things were done, including making sure that vagal neurons were not involved. it seems like a careful study that validates the claim the authors make:

 

Discussion
So the following can reduce the sickness response:
* NSAIDS like ibuprofen and EP3 receptor antagonists
* Ptger3 gene knockout
* ablation of the GABRA1 nasopharyngeal neurons
* transection of the glossopharyngeal nerve.

The authors discuss the finding that eliminating the sickness behaviour improved survival in these experiments. Sickness behaviour survives because it can provide a survival advantage to the individual when dealing with some pathogens, and may provide an advantage to the group for many pathogens by isolating the infected member.

The authors also note that all of the methods reduced the sickness response, but didn't eliminate it. So, there are also other things going on. The authors suggest that there are multiple sensory pathways to detect peripheral infections, and possibly different sensors for different pathogen types.


My random thoughts:
1. I'd like to see a blinded ibuprofen trial. I think, from earlier tracking of my symptoms, that ibuprofen might reduce ME/CFS symptoms. I guess that's something that we wouldn't want to do long term, but it could be a clue.

2. It's possible that infection sensing neurons are responding chronically in ME/CFS. Maybe a mostly latent infection of the type that most people have (EBV, herpes) close to the naso-pharyngeal neurons is triggering a message of 'we have an infection here, shut everything else down while we deal with it' to the brain. Maybe it's a problem because the latent infection is in or near these neurons? Maybe the neurons are signalling when there isn't any infection at all?

3. This paper might mean that treatments based on the vagal nerve have less basis, if the sickness response is important in ME/CFS.

4. It would be great to see some investigation of the NP9 GABRA1 expressing neuron type and other sickness response signalling neurons in humans, and specifically in people with ME/CFS

5. Perhaps the infection sensing neurons are actually under-responsive in ME/CFS during an infection. I say that because of the graphs of viral transcription over time in this study that showed the virus persisting longer at a low level when the sickness response was attenuated. Maybe someone took ibuprofen and that squashed the immune response too much. Maybe someone has a genetic predisposition to a low sickness response, which could be a survival advantage in some circumstances. But, perhaps for that one infection, for whatever reason, the acute infection isn't cleared properly, leaving a smouldering or latent infection close to the signalling neurons. Which might then take us to 2.

6. I wonder if this is related to the sore throats that I and some others get. I often get a sore throat just briefly at the beginning of PEM.

7. GABRA1 encodes a gamma-aminobutyric acid (GABA) receptor. I think this has come up in some papers.


Do say if you think I have misinterpreted something. What do you think about the ME/CFS=chronic sickness response theory?

 

Yes, the treatment approach would depend a lot on whether the sickness response was to an actual infection, or just something stuck in the 'on' position.

The sickness response does cause real changes in the body, not just the feelings of pain and fatigue. For example, if the itaconate shunt is switched on, there is a something like an inefficient system of producing energy, that produces waste products. There are lots of immune changes that could have downstream effects.