Discussion
The body immune system plays an important role in sepsis like double-edged sword, which protects against invading pathogens, while its overreaction can cause body organ injury. During sepsis, the invasion of a large number of pathogens not only causes the body’s immune overreaction, but also causes a large number of lymphocytes to consume and cause immune paralysis. As Hotchkiss RS described that the T cells were the band conductor of the orchestra, and all immune components worked in harmony to play the best role[30]. Due to the intertwined complex relationship, Th17, Treg and their balance proportion has been attracted more and more attention, which is essential for immune homeostasis[31]. Although Treg represent less than 10% of circulating CD4+T cells, it can limit the overreaction of anti-infective effector cells, protect surrounding normal tissues from damage, and maintain the body’s immune balance [32, 33]. Th17 terminally differentiated cells fulfill opposite Treg’s functions; IL-17 can promote the occurrence of inflammation in sepsis[34]. Studies have found that Th17 can appear in lung tissue to help eliminate bacteria in the bacterial infection model, including Klebsiella pneumoniae that is resistant to carbapenem[35], which is very important to resistance in sepsis bacterial infection. Th17 cause autoimmunity and inflammation, whereas Treg inhibit these phenomena and maintain immune homeostasis. The two are functionally antagonistic to each other, but also inhibit each other in differentiation, even switch between each other[36]. Thus, unraveling the mechanisms that affect the Th17/Treg balance is critical to understand autoimmunity and tolerance better. Limited data had shown the ratio of Th17/Treg might be related to the clinical severity and prognosis of sepsis [37], [38], [39].
The length of stay in intensive care units (LSICU) is an essential factor for quality assessment and a strong determinant for the total cost of ICU admission, of which septic patients represent a major part[40]. Age was an independent risk factor of many diseases, and some researches showed that the odds for death in ICU patients with sepsis increased with age with the maximal rate of increase [46]. Age is a factor affecting the length of stay in ICU in patients with sepsis, and there is a positive correlation between age and length of stay in ICU. Clinical scores except age, such as the APACHE II[41], SOFA[42] have been widely used in clinical practice to predict outcome in critically ill patients; the early risk-stratification of these patients and their prognosis, as well as accurate monitoring of clinical treatment effects. Th17/Treg ratio can predict 28-day mortality in sepsis patients with ARDS [43]. There is currently an apparent lack of an accurate biomarker for predicting sepsis patients the length of stay in ICU. In the present study, positive correlations were observed between the Th17/Treg ratio and the length of stay in ICU, indicating that the higher the Th17/Treg ratio, the longer the length of stay in ICU in sepsis patients. The Th17/Treg ratio, APACHE II score, and SOFA score were independent predictors of the length of stay in ICU in sepsis patients, the Th17/Treg ratio in combination with the APACHE II score increased the AUC for predicting the length of stay in ICU. Taken together, our findings strongly suggest that the Th17/Treg ratio can reflect the intensity of the inflammatory response in sepsis, and might be a potential indicator for the length of stay in ICU in sepsis patients.
CD28 is the primary costimulatory molecule for naive CD4+ conventional T cell activation[44], binding to B7 ligands leads to increased duration and magnitude of T cell responses, enhanced survival and glucose metabolism, and acquisition of migratory properties[45]. CD28 activates integrin-mediated adhesion of T cells and promotes actin polymerization. Selective blockade of CD28-mediated T cell costimulation protects rhesus monkeys against acute fatal experimental autoimmune encephalomyelitis(EAE)[46]. In septic patients, there is a strong positive correlation between CD28 expression on CD4+ T cells and Th17/Treg ratio; we speculate that CD28 may be able to change Th17/Treg ratio then short the length of stay in ICU.
The mice sepsis model showed that the clinical score and Th17/Treg ratio of septic mice were the highest on the third day after CLP. After administration of CD28 antibody, the 7-day survival rate of mice increased, the Th17/Treg ratio of spleen cells decreased, and the expression of inflammatory factors in peripheral serum also decreased. Simultaneously, the spleen’s length, weight and index were improved, and the histopathological damage were alleviated. These suggested the decrease of apoptosis of spleen cells in mice. It is speculated that CD28 antibody may affect the balance between Th17 and Treg through ROR-γt and FoxP3, thereby improving the survival rate of sepsis mice.
The function of T cells is related to cell death, which can be divided into accidental cell death (ACD) and regulatory cell death (RCD). The signal cascade reactions involved in regulating cell-related functions are mainly RCD, RCD involving effector molecules, which have unique biochemical characteristics, morphological characteristics and immunological consequences [47]. RCD mode includes apoptosis, necroptosis, pyroptosis, ferroptosis, entotic cell death, and so on.[48]. T cell dysfunction is manifested specifically in increased T cell apoptosis, decreased proliferative capacity and decreased reactivity or unresponsive state[49]. Our data showed that the administration of CD28 antibody to CLP processed mouse could reduce the apoptosis of splenocytes. Western blot in tissues confirmed that the expression of pro-apoptotic protein BCL-2 decreased and the expression of anti-apoptotic protein BAX increased in mouse spleen under the action of CD28 antibody.