Sepsis is a fatal disease that is frequently encountered by clinicians worldwide. When organ dysfunction occurs, the mortality rate increases[19]. Thus, effective treatments for sepsis are urgently needed. Sepsis is induced by microbial components or toxins during infection, and most animal models of sepsis are poly-microbial models that mimic intra-abdominal sepsis and typically include feces-induced peritonitis[20]. In this study, male C3H mice were injected with fresh fecal solution, imitating sepsis induced by acute peritonitis[21]. This mouse model of sepsis was originally developed by Huet and colleagues[22] and remains a reliable and internally consistent approach. In our current study, three mice in the control group were euthanized within 24 h after infection and showed signs of abdominal pain. Additionally, IL-10, NO, and ET-1 expression levels were upregulated. These results suggested the occurrence of severe inflammation, disruption of endothelial integrity, and dysregulation of blood vessels and supported that the sepsis mouse model was successfully constructed[23].
Injection of NK cells in our sepsis model improved survival rates in mice and reduced abdominal pain. Importantly, NK cells regulate immune responses, such as cytotoxicity and cytokine production, which affect DCs, macrophages, neutrophils, T cells, and B cells[24]. In our study, NK cells partially protected mice from feces-induced sepsis via downregulation of IL-10, indicating reduced inflammation, and NO[25]. This suggests that transfusion with NK cells could improve outcomes in patients with sepsis. IL-10, a characteristic cytokine marker of sepsis[26], can counterbalance the pro-inflammatory response and is substantially upregulated during sepsis. Thus, detection of IL-10 levels may be useful for evaluating the severity of sepsis[27]. In this study, IL-10 levels were decreased after transfusion with NK cells, suggesting that NK cells weakened the inflammatory response in sepsis, which may have protected the animals from death due to serious infection.
The endothelial cell lining (ECL) of the vasculature is a unique cellular system that can regulate hemostasis and vasomotor control[28]. In sepsis, severe endothelial cell dysfunction occurs, resulting in dysregulation of the ECL and disruption of vascular activity[29]. Cytokines and inflammatory factors, including NO and ET, are also upregulated and released. The function of the ECL is determined largely by the glycocalyx, which is associated with sepsis[30]. Shedding of the glycocalyx is associated with sepsis-induced organ failure and may result in endothelial release of NO or ET[29], thereby contributing to the loss of vascular reactivity, which may lead to DIC. Notably, NO metabolism plays important roles in the regulatory function of the ECL and influences vascular function. In a previous study, NO and ET expression levels were found to increase in endotoxic shock[31]. Consistent with these findings, in the current study, we also found that NO and ET levels increased plasma and tissue homogenates from septic mice. Our results also suggested that maintenance of endothelial barrier integrity may be important in the development of novel therapies for the treatment of sepsis[32]. Specifically, NK cells reduced the expression of ET-1, VEGF, and ANGPT-1, which are important pro-angiogenic factors, suggesting that NK cells may play important roles in maintaining the integrity of the vascular endothelium. Moreover, ANGPT-1, an oligomeric-secreted glycol-protein member of the vascular-specific family of growth factors[3], is essential during angiogenesis and is required for correct organization and maturation of newly formed vessels and maintenance of the structural integrity of the mature vasculature[33]. Additionally, NK cells can mediate cellular cytotoxicity[32] and produce high levels of perforin and granzymes to directly kill target cells[34], thereby exhibiting important functions in maintaining vascular integrity.
NK cells can produce angiogenic factors, such as VEGFA, to promote vascular repair and neovascularization. VEGF levels are increased after treatment with NK cells and play key pathophysiological roles in inducing barrier permeability[35]. In this study, VEGF levels were significantly elevated in septic mice, consistent with a previous report[36]. Moreover, VEGFA and VEGFR2 protein levels were significantly lower in the test group than in the control group in the spleen, liver, and BM; the opposite results were observed in the skin. These findings suggested that there may be an interaction between NK cells and the VEGFA pathway to modulate angiogenesis in sepsis. However, more samples and detailed analyses are required to confirm these findings and elucidate the mechanism. Because NK cells can induce vascular growth, remodel the vasculature, and secrete angiogenic factors[37], these cells may protect the integrity of the endothelium and promote angiogenesis. Indeed, we found that blood vessel formation increased in the kidneys and liver following treatment with NK cells; however, no differences were observed in the spleen and skin. Thus, our findings indicated that NK cells promoted angiogenesis in the kidney and liver. Further studies are needed to assess the roles of NK cells in the spleen and skin.
In this study, we found that microcirculation was improved after injection of NK cells compared with that in the control group. The pro-angiogenic effects of NK cells have been reported previously[38]; however, little is known regarding the angiogenic functions of NK cells in an inflammatory environment, such as sepsis. Our study provided insights into the different factors through which NK cells modulate sepsis. Additional work is still needed to fully elucidate these mechanisms.
In conclusion, we described the systematic response of mice with feces-induced sepsis to NK cell injection. We found that NK cells modulated angiogenesis, which may be important in the antisepsis response. Mechanistically, the functions of NK cells in sepsis may be related to the VEGFA pathway and the integrity of the endothelium may play an important role. Further studies are needed to explore the echanisms through which the endothelial system is related to sepsis and to assess the specific effects of NK cells in the antisepsis response. Overall, our results supported that NK cells may have clinical applications in the treatment of patients with sepsis.