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Is sepsis-induced apoptosis associated with macrophage dysfunction?
A Ayala1, M A Urbanich, C D Herdon
1Shock and Trauma Research Institute, Michigan State University, East Lansing, USA.
Abstract:
Apoptosis (A O) is a pathological process by which cells undergo a form of inducible nonnecrotic cellular suicide. In vitro studies suggest that changes in the rate of macrophage (Mo) A O may be associated with elevated proinflammatory cytokine secretory capacity, such as interleukin-1 beta (IL-1 beta) (via IL-1 converting enzyme activation). Furthermore, it has been reported that Mo are activated during early (0-4 hours) experimental septic insult to act as sources of proinflammatory cytokines, such as IL-1. However, with the progression of sepsis, these same cells become refractory to further stimulation (appearing dysfunctional). Nonetheless, it remains unknown if this acquired immunosuppression (dysfunction) is associated with an acceleration in macrophage A O. To determine this, male C3H/HeN mice were subjected to sepsis (cecal ligation and puncture, CLP) or sham-CLP and 4 or 24 hours thereafter Mo were isolated from the peritoneum (PMo) and liver (KMo). Macrophage monolayers were lysed either after stimulation with lipopolysaccharide (LPS) (10 microgram/mL, 24 hours) in vitro or immediately (ex vivo) before LPS stimulation and the cytoplasmic cell fraction was retained. The extent of A O was determined using a cell-death enzyme-linked immunosorbent assay, which detects the presence of cytoplasmic oligonucleosomes and changes in the propidium iodide staining intensity. The results indicate that, early after CLP (4 hours) only PMo stimulated with LPS in vitro showed evidence of increasing A O. At 24 hours (late) after the onset of sepsis, the ex vivo extent of A O in PMo was increased but it was decreased in KMo. However, the addition of LPS in vitro results in a marked increase in both septic PMo and KMo A O. This latter result suggests that the inability of Mo to release cytokines in response to stiumulants, such as LPS during late sesis (24 hours), may be because of induciton of accelerated A O in these Mo populations.
Insights
Sepsis accelerates macrophage apoptosis, especially in the peritoneum, leading to immune dysfunction. This accelerated cell death, particularly after lipopolysaccharide stimulation, explains the reduced cytokine release observed in late-stage sepsis.
Area of Science:
- Immunology
- Cell Biology
- Pathology
Background:
- Apoptosis is a form of programmed cell death.
- Macrophages (Mo) play a key role in the immune response to sepsis.
- During sepsis, Mo can become dysfunctional, leading to immunosuppression.
Purpose of the Study:
- To investigate whether acquired immunosuppression in sepsis is linked to increased macrophage apoptosis.
- To determine the role of apoptosis in macrophage dysfunction during experimental sepsis.
Main Methods:
- Male C3H/HeN mice were subjected to cecal ligation and puncture (CLP) to induce sepsis.
- Peritoneal (PMo) and liver (KMo) macrophages were isolated at 4 and 24 hours post-CLP.
- Apoptosis was measured using a cell-death ELISA and propidium iodide staining, with and without in vitro lipopolysaccharide (LPS) stimulation.
Main Results:
- Early after sepsis (4 hours), only LPS-stimulated PMo showed increased apoptosis.
- Late after sepsis (24 hours), ex vivo PMo apoptosis increased, while KMo apoptosis decreased.
- In vitro LPS stimulation markedly increased apoptosis in both PMo and KMo at 24 hours post-CLP.
Conclusions:
- Accelerated macrophage apoptosis, particularly in peritoneal macrophages, contributes to immune dysfunction during late-stage sepsis.
- The inability of macrophages to release cytokines in response to LPS during late sepsis may be due to induced apoptosis.
- Apoptosis is a key mechanism underlying macrophage dysfunction in sepsis.