Discussion
Although a bunch of evidence is available regarding the association
between NK cell cytotoxicity and female fertility
(18), distinct opinions were formulated
regarding the relationship between the cytotoxic activity of pNK and RM
(19-21). In this study, the
cytotoxic-related parameters, such as the percentage, immunophenotyping,
and cytotoxicity of pNK cells in RM patients were evaluated.
Interestingly, our results showed that NK cytotoxicity in the RM group
was significantly lower than that in the control group. Moreover, the RM
group showed a lower MFI of granzyme
B and higher MFI of inhibitory receptors of pNK compared to that in the
control group. These opposite results questioned the role of
the
cytotoxicity-related function of pNK cells in the occurrence of RM.
During implantation, the first step is the attachment of the embryo to
the surface of the endometrium, followed by its migration and invasion
until it is immersed in the depth of the uterine tissue
(22). Uterine NK cells are a key factor
in regulating the invasion of trophoblasts by secreting proteolytic
enzymes such as matrix metalloproteinase-2 (MMP2) and MMP9
(23), and reducing extravillous
trophoblast cell apoptosis (24).
Nevertheless, it is still not clear whether the recruited pNK cells can
regulate the pregnancy outcome or reflect what is happening at the
maternal-fetal interface (19,
25, 26).
Some scientists reported that patients with RM are accompanied with high
pNK cells ratio (3,
20) or cytotoxicity
(4). However, other scientists reported
contradictory results indicating the same NK cell ratio
(27) or lower pNK cytotoxicity
(28) in
RM patients compared with the
fertile control, and our results are actually consistent with these
results. A large cohort study suggested that the relationship between
the subsequent pregnancy outcome and pNK cytotoxicity is not linear, and
both patients with low and high pNK cytotoxicity tend to have high
miscarriage rate (6). Although pNK cells
contribute in creating an immune tolerant environment for placentation,
they should still be equipped with a certain cytotoxic potential to
fight infections during normal conditions
(29). Therefore, both RM patients with
higher pNK cytotoxicity and that with lower pNK cytotoxicity compared to
fertile control should be taken into serious consideration.
NK cytotoxicity is controlled by the complex interaction between
inhibitory and activating receptors (11,
30). Indeed, NK cells exert their
cytotoxicity by secreting cytotoxic granules when the activating
receptor signals are predominant (31). In
order to explore the potential mechanism for low pNK cytotoxicity in RM
patients, the expressions of receptors and cytotoxic granules in the pNK
cells were also evaluated. However, our results showed no significant
difference about their percentage between the RM and control groups,
while the significant differences were observed in the MFI of granzyme
B, CD158a, and CD158b in the pNK cells between the RM and control
groups, which was consistent with the tendency for the cytotoxicity of
pNK in the RM group. Thus, the predominant inhibitory signal might
result in a low cytotoxicity in the RM group. It is also worth noting
that some other pathways can actually be used by NK cells to kill
infected or transformed cells. For instance, they can kill cells by the
activation of the death receptor pathway, such as FasL-Fas signaling
pathway and
TNF-related
apoptosis-inducing ligand (TRAIL) (32,
33). However, this was not the aim of our
present work, and further studies should consider these different
pathways used by the pNK cells to kill the target cells.
Our present work contains the following limitations that should not be
underestimated. Firstly, this is a retrospective study. Secondly,
several factors that affect the level and function of peripheral blood
NK cells were not considered, including mental stress
(34, 35),
exercise (36,
37) and age. Although the patients with
diseases that can influence peripheral blood NK cells were excluded, it
is possible that some undetected autoimmune diseases or infections
affected the results.