This study adds to a growing body of research revolving around the role of nucleotides in airway disease. Similar to previous work in adults, we demonstrate good performance of a whole blood P2X7 functional assay as a method to detect P2RX7 LOF alleles (). By utilizing this robust assay, we have demonstrated that a lack of P2X7 pore activity in high-risk children is associated with a reduced risk of asthma (), as well as sensitization to fewer aeroallergens (). However, the mechanisms underlying these observations are not clear. Discerning the role of P2X7 activation by extracellular ATP in concert with secondary signals including allergen exposure and/or viral infections may help determine how P2X7 activity could modulate risk of chronic conditions such as asthma.
Previous studies indicate the amount of extracellular ATP may be related to airway disease severity.25
Rather than directly measuring ATP in the airway after injurious events, our study has the strength to study the potential for differential host responses to natural in vivo
extracellular ATP fluctuations. Our results () recapitulate that considerably more contributes to P2X7
pore function than validated P2RX7
LOF alleles and illustrates the power of our functional approach to evaluate potential gene-by-environment interactions. The COAST population has already demonstrated gene-by-environment interactions including between IFNG
which may be important to in vivo
function since IFN-γ reportedly regulates P2X7
Although our current results are from pre-pubertal children, they display a varied risk of asthma by P2X7
status based on gender (). Whether the dynamics of this relationship change during and after puberty will be of great interest.
Our current results are in general agreement with findings from P2X7
knockout mice wherein low P2X7
function is protective from asthma-like symptoms.30
knockout mice demonstrate decreased cell influx into the lung after allergen or smoke challenge,30,46
and we have previously shown a decreased neutrophil infiltration in the nose during an acute cold in adults with low P2X7
Although our current study may have been strengthened if P2X7
pore assays could have been performed in early life before the earliest asthma evaluations, the high reproducibility and genetic basis of our results () indicates assays should be similar at any age and mitigate these potential concerns.
While low P2X7
pore status protection from asthma in the current COAST cohort is consistent over multiple ages, these results seem counter to the inverse relationship between P2X7
function and exacerbation risks in adults with a natural cold.47
Differences in study populations and in the pathogenesis of asthma inception compared to exacerbations may help reconcile these findings. There are significant differences in study populations: the COAST population is comprised of high risk children followed prospectively from birth, while the previous study enrolled symptomatic asthmatic adults during the peak cold season. In the children asthma was more common in boys, while in adults asthma was predominantly observed in women. It is possible the overall lack of association at age 11 years between asthma and P2X7
status may continue to change throughout puberty into adulthood and reflect the exacerbation risks observed in adults. Specifically, it is intriguing to note that a small percentage of children had both low P2X7
status and a history of HRV wheezing and that this group demonstrated the largest shift in rates of current asthma at different ages. Whether modification of P2X7
function from nucleotide activation is sufficient to alter asthma outcomes in humans has yet to be measured.
How does P2X7
influence asthma risk? While P2X7
is present in airway epithelial cells, the receptor is more highly expressed and active in immune cells, including DCs.48–50
Both nucleotides and nucleotide receptors, including P2X7
, impact DC function27,30,49–52
and loss of P2X7
function – specifically from LOF alleles detected by our pore assay – leads to a decrease in DC pore activity as well as other P2X7
-dependent functional responses.50,51
T cell maturation, including Treg
17 phenotypes, is modified by nucleotide activity upon T cells and DCs, either directly or by engaging pathways associated with P2X7
including the NLRP3 inflammasome or pannexin-1 and suggest that functional P2X7
activation may lead to a decrease in Treg
A DC-focused role of P2X7
is supportive of an amplified response to infections or allergens when co-mingled with danger signals acting as adjuvants. Our study demonstrates a potential role for monitoring host responsiveness to immunomodulatory danger signals.
sits at a balance point in the immune system in response to allergic and infectious events. It is not clear whether a single episode of P2X7
activation is sufficient to increase the risk of asthma or whether frequent stimulation is required. Moreover, P2X7
function may not always b e beneficial or harmful in the immune response; the role of P2X7
may be different when comparing disease inception to active, chronic conditions with superimposed acute events such as exacerbations. As examples, influenza virus activation of the inflammasome has been linked to P2X7
while another report suggests P2X7
may be necessary for some viruses to achieve cell entry.59
plays an active role in HRV infection or is secondary and solely responsive to cell injury could indicate when and where alterations of P2X7
function are relevant. To study these relationships, the P2X7
pore assay system described in this report is a useful tool to identify individuals at altered risk for disease and should be considered when further studying the role of danger signaling in disease pathogenesis.