PGE2 suppresses NK activity in vivo directly and through adrenal hormones: Effects that cannot be reflected by ex vivo assessment of NK cytotoxicity

Abstract

Surgery can suppress in vivo levels of NK cell cytotoxicity (NKCC) through various mechanisms, including catecholamine-, glucocorticoid (CORT)-, and prostaglandin (PG)-mediated responses. However, PGs are synthesized locally following tissue damage, driving proinflammatory and CORT responses, while their systemic levels are often unaffected. Thus, we herein studied the role of adrenal factors in mediating in vivo effects of PGs on NKCC, using adrenalectomized and sham-operated F344 rats subjected to surgery or PGE2 administration. In vivo and ex-vivo approaches were employed, based on intravenous administration of the NK-sensitive MADB106 tumor line, and based on ex-vivo assessment of YAC-1 and MADB106 target-line lysis. Additionally, in vitro studies assessed the kinetics of the impact of epinephrine, CORT, and PGE2 on NKCC. The results indicated that suppression of NKCC by epinephrine and PGE2 are short lasting, and cannot be evident when these compounds are removed from the in vitro assay milieu, or in the context of ex-vivo assessment of NKCC. In contrast, the effects of CORT are long-lasting and are reflected in both conditions even after its removal. Marginating-pulmonary NKCC was less susceptible to suppression than circulating NKCC, when tested against the xenogeneic YAC-1 target line, but not against the syngeneic MADB106 line, which seems to involve different cytotoxicity mechanisms. Overall, these findings indicate that elevated systemic PG levels can directly suppress NKCC in vivo, but following laparotomy adrenal hormones mediate most of the effects of endogenously-released PGs. Additionally, the ex-vivo approach seems limited in reflecting the short-lasting NK-suppressive effects of catecholamines and PGs.

In vivo suppression of NK cell cytotoxicity by stress and surgery: Glucocorticoids have a minor role compared to catecholamines and prostaglandins

“…Specifically, NK cells were shown to create immunological synapses with MADB106 cells in the lungs [33], marginating pulmonary NK cells were shown to efficiently kill MADB106 cells [31, 34, 35], and selective in vivo depletion of NK cells increased LTR and lung metastases by more than a hundred fold [27, 36-40]. Furthermore, we have shown that several stress paradigms, or the administration of stress-related factors (catecholamines and their agonists, and prostaglandins), can increase LTR in normal F344 rats, but not in NK-depleted rats, indicating that increased LTR in normal rats under these conditions reflects an in vivo suppression of NK-mediated activity [26, 27].…”

“…For obvious reasons, in vitro studies alone are insufficient to conclude about the in vivo effects of these stress factors, and the ex-vivo approach may distort previous in vivo effects, as cytotoxicity is tested in vitro following the removal of all endogenous factors and in artificial conditions that do not simulate the in vivo milieu and its complex processes [30, 31]. …”

“…This index of LTR is highly sensitive to alterations in in vivo levels of NKCC. Specifically, NK cells were shown to create immunological synapses with MADB106 cells in the lungs [33], marginating pulmonary NK cells were shown to efficiently kill MADB106 cells [31, 34, 35], and selective in vivo depletion of NK cells decreased in vivo killing of MADB106 tumor cells, and increased MADB106 LTR and lung metastases by 20- to 200-fold [27, 36-40]. Most importantly, comparing effects of stress or of other in vivo manipulations between normal rats and rats that are temporarily depleted of NK cells in vivo (see Methods) distinguishes between effects that are mediated through alterations in NKCC and those that involve other or additional mechanisms.…”

The misleading nature of in vitro and ex vivo findings in studying the impact of stress hormones on NK cell cytotoxicity

“…However, significant inconsistencies are prevalent between in vitro, ex-vivo , and in vivo findings regarding the nature and direction of the effects of specific stress hormones or stress paradigms on NK cell cytotoxicity (NKCC) [1]. For example, epinephrine was reported to suppress NKCC in vitro , both in human and animal blood [2–5], through activating NK cell adrenergic receptors and the consequent increase in intracellular cAMP levels [3]. However, ex-vivo human and animal studies reported contradictory results; many have demonstrated that administration of epinephrine, acute stress exposure, or exercise enhances NKCC [6–11], whereas some have reported suppression of NKCC [12–14].…”

“…In vitro studies showed that prostaglandin-E2 (PGE2) can markedly suppresses NK activity [19, 29, 30], and in vivo studies reported deleterious impacts of PGs on resistance to cancer metastases [22], which is allegedly mediated through in vivo suppression of NK cells [31]. However, in a recent study in rats, we have provided evidence indicating that a direct in vivo suppressive effect of PGE2 on NKCC cannot be evident in an ex-vivo assessment of NKCC [5]…”