NK cells interact with eosinophils
We next observed that there was a positive relationship between the
total number of nasal lavage NK cells and that of eosinophils 1, 6 and
24 hrs after allergen challenge (Figure 3, A-C ). No significant
correlation was observed between NK cells and other leukocyte
populations (data not shown). In vitro assays previously showed
that peripheral blood NK cells are able to trigger apoptosis of
autologous blood eosinophils (12). To determine whether soluble
mediators were involved in NK cell-induced eosinophil apoptosis, we
incubated freshly isolated eosinophils from healthy donors (4 hrs, 37°C)
with supernatants from past autologous NK cell-eosinophil co-cultures
kept at -80°C (Fig 4, A ). These supernatants did not increase
basal levels of eosinophils apoptosis, suggesting the need for a direct
contact between eosinophil and NK cells. Based on their CD56 expression,
two primary subsets of NK cells are found in human subjects, namely
CD56dim and CD56bright (10) . The
CD56bright NK cells are commonly described as
cytokine-producing cells with poor cytotoxic ability, whereas
CD56dim NK cells are best known for their potent
cytotoxic activity. To assess the role of one particular subtype is
involved in eosinophil apoptosis, NK cells from healthy donors were
sorted by FACS, based on their CD56 expression and incubated with
autologous eosinophils (4 hrs, 37°C) (Fig 4, B, left panel ).
Intriguingly, eosinophil apoptosis was not modified when eosinophils
were incubated alone with either the CD56dim or
CD56bright NK cell subset, suggesting a combined
interaction of both subpopulations while inducing eosinophil apoptosis
(Fig 4, B, right panel ).
Upon activation, eosinophils release many mediators, including highly
reactive oxygen species that can damage surrounding tissues. Besides the
potential pro-resolving effect of NK cells on eosinophils, activator
effects by NK cells on eosinophils have also been reported (13). We
first observed that isolated eosinophils were by far the most important
producers of superoxide anion among different leukocyte populations
involved in type 2 allergic inflammation, at rest and when activated
(Fig 4, C ). In contrast, NK cells were unable to release
superoxide anion (Fig 4, C ). We next wanted to assess superoxide
release by eosinophils at different time points after co-culture with NK
cells. In the presence of NK cells, superoxide release from eosinophils
was significantly reduced after 1 hr of co-incubation. This inhibitor
effect was overwhelmed after 4 hrs of co-incubation (Fig 4, D ),
confirming the partial activator effect of NK cells on eosinophils (13).