In this study, we constructed a CLP-induced sepsis mouse model and assessed the immune status of sepsis from the perspectives of inflammatory responses and cellular immunity. To the best of our knowledge, this is the first study investigating sepsis-related KLF3 expression in T lymphocytes.
Immune homeostasis plays a crucial role in determining the pathophysiology and clinical outcomes of sepsis. The inability to eliminate pathogens in the early stages of a systemic inflammatory response can result in an immune regulation imbalance. Moreover, excessive release of proinflammatory and anti-inflammatory molecules may lead to organ failure, tissue damage, and immunodeficiency [13]. CLP is currently considered the gold standard for sepsis induction [14]. In CLP, cecum perforation results in bacterial peritonitis, followed by translocation of mixed enteric bacteria into the blood compartment [14]. At the onset of sepsis, bacteremia triggers systemic activation of the inflammatory response, followed by septic shock, multiorgan dysfunction, and finally death. In the current study, our mice demonstrated clinical signs of sepsis in the early stage after CLP, which was accompanied by an abrupt release of proinflammatory and anti-inflammatory factors in massive amounts. In addition to the systemic inflammatory response, our mice also demonstrated multiple organ damage. Because the kidneys are more susceptible to changes in pressure and perfusion than other organs [17], kidney damage occurred in the early post-CLP stage and persisted thereafter; in contrast, liver, heart, and lung damage gradually intensified as sepsis progressed after CLP.
In our septic mice, changes in KLF3 expression and those in overall cytokine levels demonstrated opposite trends; this effect was consistent with the role of inflammation suppressor of KLF3. Similar results were noted in the lipopolysaccharide (LPS)-treated bone marrow-derived macrophages and Klf3−/− mice [10]: The Klf3−/− mice demonstrated a heightened, prolonged inflammatory response to LPS treatment; KLF3, in turn, was noted to directly repress the expression of the NF-κB family member RelA/p65—which plays a key role in driving the inflammatory response under physiological and pathophysiological conditions [10]. Notably, in a study on the programming of marginal zone B-lymphocyte fate, KLF3 was suggested to act as a downstream target of B-lymphocyte activating factor-mediated NF-κB signaling [4]. In another study, KLF3 was a specific target isolated in the microarray screening of NF-κB target genes in pre-B lymphocytes [18]. Taken together, these results revealed that KLF3 is a fundamental suppressor of inflammation, which cooperates with NF-κB signaling.
When sepsis occurs, the functions and numbers of immune cells demonstrate dynamic alterations. In a persistent state of sepsis, irreversible immunosuppression occurs systemically. During sepsis development, increases in apoptosis and inhibitory immune checkpoint molecule expression lead to significant reductions in the T-lymphocyte number, which is particularly noteworthy because it occurs during life-threatening infection when clonal expansion of lymphocytes should be occurring [19]. Despite the administration of broad-spectrum antibiotics and implementation of aggressive source control measures, invading pathogens cannot be eradicated in many patients with sepsis, making them more susceptible to nosocomial organisms. In this study, we induced an immunosuppressive state in our mice. In addition to an increase in apoptosis accompanied by severe depletion of T lymphocytes, our mice demonstrate downregulation of cellular activation and cytokine secretion. Similar results were reported by a rapid postmortem clinical study: LPS-stimulated splenocytes demonstrated more suppressed cytokine production in patients who died of sepsis than in patients without sepsis. The spleen demonstrates upregulation of selected inhibitory receptors with concomitant downregulation of activation pathways [20]. This possibly is the state of T-lymphocyte exhaustion occurring with chronic antigenic stimulation [19]. KLF3 was previously believed to be a regulator of lymphocyte quiescence [7, 21]. Notably, in the current study, KLF3 expression increased gradually during the immunosuppression phase. Both T-lymphocyte exhaustion and quiescence indicate decreased T-lymphocyte activation; however, quiescence is distinct from anergy in that quiescent cells are responsive to activating stimuli but resistant to apoptosis [21]. As such, the potential role of KLF3 in the regulation of T-lymphocyte function appears to vary depending on the cellular and pathophysiological contexts. In other words, the regulation of T-lymphocyte function is a complex physiological process, and KLF3 alone is insufficient for sustaining the stability of the T-lymphocyte status. As such, further research on the roles of KLF3 and the underlying mechanisms is warranted. Nevertheless, alterations in KLF3 expression may facilitate dynamic immune status monitoring and guide individualized immunomodulatory therapy. For instance, regulating KLF3 expression in T lymphocytes may aid in alleviating immunosuppression in sepsis.