Exercise induced eosinophil responses: normal cell counts with a
marked decrease in responsiveness
To the Editor,
Type II inflammation is characterized by elevated blood eosinophils
which makes these cells an important diagnostic and treatment target in,
for instance, severe asthma. Therefore, blood eosinophil numbers are a
main inclusion criterion for many clinical studies that investigated the
treatment of eosinophilic asthma with anti-IL5(Rα). However, there is no
consensus on cut-off values for blood eosinophils during inclusion, as
evidenced by a high variability between studies, ranging from 150 to 400
cells/µL. Moreover, the range of blood eosinophils in a healthy
population, without confounding factors for increased blood eosinophils,
is 30 – 330 cells/µL in males and 30 – 310 cells/µL in females. This
implies that the cut-off values used for clinical studies greatly
overlap with blood eosinophil counts that are found in the healthy
population. This inherently poses a problem as eosinophil blood counts
seem to be inadequate to use for diagnosing eosinophilic disease.
This overlap in eosinophil counts between patients and the healthy
population limits the application of eosinophil numbers for
discriminating between health and several inflammatory diseases. A more
promising approach in diagnosing eosinophilic disease is to combine
eosinophil numbers with their activation status. Unfortunately, there is
surprisingly little evidence that blood eosinophil counts correlate with
their activation status and/or responsiveness in vivo in disease. This
lack of correlation can be caused by ex vivo activation and/or their
homing to the lung leaving behind non-activated cells in the blood.
To circumvent ex vivo activation, we analyzed blood eosinophils
activation status directly after venipuncture with a fast, automated,
point-of-care, mobile flow cytometer (AQUIOS CL, Beckman Coulter). As
exercise can be used as a model to modulate eosinophil numbers in a
healthy setting, we studied whether eosinophil blood counts correlate
with their activation status and their responsiveness to formyl peptides
in a cohort of long-distance runners participating in a
mass-participation trail run (22, 29 or 43 km). The study was approved
by medical research ethics committee Oost-Nederland (NL79864.091.22).
After written informed was obtained, venous blood samples were collected
from 35 athletes before, directly after and 24 hours after exercise. The
eosinophil activation status was assessed by combining automated flow
cytometry with a 5-dimensional algorithm-based gating.
An acute leukocytosis with eosinopenia was present directly
post-exercise, which is in agreement with previous research. These
numbers normalized 24 hours after exercise (figure 1A, 1C ).
Compared to before exercise, eosinophils showed a more activated
phenotype (increased CD11b and decreased CD62L) directly after exercise
which also normalized within 24 hours. In marked contrast to acute
inflammation, such as caused by SARS-CoV2 infection, this eosinopenia
directly after exercise did not lead to refractoriness to
fNLF-stimulation. However, after the normalization of eosinophil counts
24 hours after exercise, the eosinophils were refractory for activation
by fNLF (figure 1B, 1C ). This clearly showed a complete
dissociation between blood eosinophil numbers and their relative
activation status.
Our results illustrate that the eosinophil blood compartment is not
adequately characterized by solely counting cell numbers (‘quantity’) as
normalized numbers do not necessarily reflect normalization of their
activation status (‘quality’). This finding is not limited to measuring
the state of type II immunity in eosinophilic disease, but probably also
applies to other infectious/inflammatory conditions and to
non-pathological settings. Our data call for a re-evaluation of using
blood eosinophil counts as an adequate representation of the eosinophil
compartment’s state. Until recently, determining the activation status
of the eosinophil compartment was complicated by ex vivo artifacts
already starting at the moment of venipuncture. Now with the
availability of fast, automated, point-of-care flow cytometry, it is
feasible to measure both the quantity and quality of
eosinophils in a wide scope of health and disease settings.