Recently, accumulating evidence suggested that immunosuppression occurs after the onset of septic shock. Apoptosis of immune cells and imbalance of Th1 and Th2 are the most reported mechanisms contributing to immunosuppression[4, 5]. Our results demonstrated that Esmolol without HR reduction decreased T-lymphocytes apoptosis and drifted a shift from Th2 to Th1 in septic shock models, which attenuated sepsis-induced immunosuppression. Esmolol might alleviate T-lymphocyte apoptosis by regulating Akt/Bcl-2/Caspase-3 pathway and reduced Th0 differentiation to Th2 by inhibiting Erk1/2 activation in septic shock models. As a result, Esmolol without HR reduction restored MAP, decreased lactatemia and alleviated multiple organ injuries in our study.
Previous studies reported that immunosuppression are predominately resulted from apoptosis of monocytes, B-lymphocytes and T-lymphocytes in septic shock patients[21, 22]. In our study, CLP induced apoptosis of circulating monocytes, B-lymphocytes and T-lymphocytes, which was similar to clinical settings. The mechanism of triggering cellular apoptosis was not well studied. During septic shock, the sympathetic nervous system is over-activated and a large number of catecholamines are secreted. Lymphocytes and monocytes express adrenergic receptors. Overstimulating these cells by catecholamines through adrenergic receptors contributes to apoptosis. Our results showed that blocking β1-adrenergic receptors by Esmolol at low dose significantly reduced T-lymphocyte apoptosis.
The mechanisms of Esmolol reducing T-lymphocytes apoptosis are explored in our study. Previous studies reported that over-secretion of norepinephrine promoted cardiomyocyte apoptosis via activating β-adrenoreceptor in septic shock mice[25, 26], which could be inhibited by blockade of β1-adrenoceptor through decreasing Caspase-3 activation. An et al. reported that myocardial apoptosis was alleviated through Akt/Bcl-2 pathway activation in sepsis. Zhang et al. observed that activation of the Akt pathway reduced the expression of cleaved Caspase-3, subsequently ameliorating sepsis-induced acute lung injury. Our results revealed that Esmolol at low dose reversed CLP-induced decrease of Akt phosphorylation and Bcl-2 expression and increase of cleaved Caspase-3 in animal models. Therefore, Esmolol might inhibit apoptosis of T-lymphocytes by modulating the Akt/Bcl-2/Caspase-3 pathway.
Previous studies showed the imbalance of T-lymphocyte sub-populations in septic patients, such as an increase in Th2 counts and an augmented ratio of Th2 and Th1[5, 29, 30]. Our results revealed that both Th1 and Th2 counts were up-regulated in the septic shock models. Whereas, Th2 increased more than Th1, thus the ratio of Th2 and Th1 was increased in septic shock models. Esmolol at low dose significantly reduced Th2 counts but not Th1 counts, thus restored the ratio of Th2 and Th1.
Cytokines primarily influenced Th0 differentiation. Cytokines, such as IL-4 and IL-10, promote Th0 differentiation to Th2, mainly dependent on Stat6 activation. Erk1/2 was involved in the regulation of Stat6 activity. Previous studies have reported that inhibition of Erk1/2 activation suppressed IL-4-induced Stat6 activity. In our research, CLP increased the phosphorylation of Erk1/2. Infusion of the low dose of Esmolol reduced Erk1/2 phosphorylation. The results indicated that Esmolol might influence Th0 differentiation by inhibiting Erk1/2 activation. We further observed Stat6 expression. Infusion of Esmolol trended to decrease CLP-induced increased mRNA expression of Stat6 but didn't reach statistical significance. Indeed, mRNA expression could not reflect total protein activation. Observation of Stat6 activation is required in future work. Besides, Esmolol may modulate Th0 differentiation through Erk1/2 by other mechanisms, such as influencing cytokines secretion. As previously reported[32, 33], Erk1/2 activation up-regulated IL-4 and IL-10 production so that promote Th0 differentiation to Th2. In fact, our previous study has shown that Esmolol decreased the level of IL-10. Thus, Esmolol might modulate Th0 differentiation by decreasing relative cytokines production by inhibiting Erk1/2 activation. More studies are needed to clarify the mechanisms of Esmolol on Th0 differentiation.
In septic shock, excessive stimulation by catecholamine resulted in the reduction of β1-adrenoreceptor density on cardiomyocytes. Our results demonstrated that the mRNA and protein expression of β1-adrenoreceptor in T lymphocytes decreased in septic shock rats. Esmolol infusion at different doses didn't restore mRNA or protein expression of β1-adrenoreceptor on T-lymphocytes in septic shock rats. Thus, it seems that Esmolol exerted its effects on T-lymphocytes through downstream pathways rather than regulating the expression of its receptor.
In addition, the optimal dose of Esmolol for immunoregulation in septic shock remains unclear. Our results showed that Esmolol at high dose with HR reduction didn’t significantly decreased T-lymphocytes apoptosis and the ratio of Th2 and Th1 in septic shock models, which are not as effective as that of at low dose without HR reduction. More studies about dose-effect relationship are needed to confirm our findings.