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Sepsis-induced apoptosis of the thymocytes in mice
1Department of Microbiology and Immunology, College of Medicine, National Cheng Kung University, Tainan, Taiwan, Republic of China.
Abstract:
Intraperitoneal injection of Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa) induces thymic atrophy in mice. The thymus weight, cell number, and viability began to decrease at 3 h, and reached their lowest level at 72 h. The thymocyte death was associated with DNA fragmentation of approximately 200 base pairs in ladder form. The kinetic study on histopathology revealed the process of thymocyte death and thymic atrophy. Flow-cytometric analysis showed that CD4+CD8+ thymocytes decreased predominantly. LPS caused thymocyte apoptosis, but only in LPS-responder mice, unlike Gram-negative bacteria that induced apoptosis in both LPS-responder (C3H/HeN) and LPS-nonresponder (C3H/HeJ). Gram-positive bacteria Streptococcus pneumoniae also caused apoptosis in LPS-nonresponder (C3H/HeJ) and LPS-responder mice (B6). The kinetics of serum TNF-alpha production after Gram-negative or Gram-positive bacteria injection was slightly different. E. coli induced serum TNF-alpha peak at 1 h in B6 mice, whereas S. pneumoniae induced a peak at 6 h in C3H/HeJ and at 9 h in B6 mice. Similarly, S. pneumoniae induced thymocyte apoptosis around 9 to 12 h, which was 6 to 9 h later than that observed with E. coli in B6 mice. Anti-TNF-alpha Ab completely blocked the E. coli-induced thymocyte apoptosis, but was only partially inhibitory on the S. pneumoniae-induced thymocyte apoptosis. Furthermore, thymocyte apoptosis induced by E. coli was inhibited by cycloheximide or actinomycin D. These data indicate that both Gram-negative and Gram-positive bacteria could induce thymus atrophy via apoptosis, and that TNF-alpha is a common denominator released and might be responsible for the thymocyte apoptosis.
Insights
Bacterial infections, including Gram-negative and Gram-positive types, trigger thymus atrophy and thymocyte apoptosis in mice. Tumor necrosis factor-alpha (TNF-alpha) plays a key role in this process, mediating bacterial-induced thymus apoptosis.
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- Bacterial infections can significantly impact immune organ function.
- The thymus is crucial for T-cell development and immune response.
- Understanding the mechanisms of bacterial-induced immune suppression is vital.
Purpose of the Study:
- To investigate the effects of Gram-negative and Gram-positive bacteria on thymus structure and function.
- To elucidate the role of thymocyte apoptosis and tumor necrosis factor-alpha (TNF-alpha) in bacterial-induced thymus atrophy.
Main Methods:
- Intraperitoneal injection of various bacteria (E. coli, K. pneumoniae, P. aeruginosa, S. pneumoniae) into mice.
- Assessment of thymus weight, cell count, viability, and histopathology.
- Flow cytometry for thymocyte subset analysis (CD4+CD8+).
- Analysis of serum TNF-alpha levels and inhibition studies using anti-TNF-alpha antibodies and protein synthesis inhibitors.
Main Results:
- Gram-negative and Gram-positive bacteria induced significant thymic atrophy and thymocyte apoptosis in mice.
- CD4+CD8+ thymocytes were predominantly affected.
- Bacterial lipopolysaccharide (LPS) induced apoptosis only in LPS-responder mice, while whole bacteria induced apoptosis in both responder and non-responder mice.
- Serum TNF-alpha levels increased following bacterial injection, with different kinetics for Gram-negative and Gram-positive bacteria.
- Anti-TNF-alpha antibodies partially or completely blocked bacterial-induced thymocyte apoptosis.
- Cycloheximide and actinomycin D inhibited E. coli-induced thymocyte apoptosis.
Conclusions:
- Both Gram-negative and Gram-positive bacteria induce thymus atrophy through apoptosis.
- TNF-alpha is a critical mediator of bacterial-induced thymocyte apoptosis.
- The response to bacterial components like LPS differs between responder and non-responder mice, but whole bacteria elicit a response in both.
- These findings highlight a conserved mechanism of immune suppression by bacteria impacting the thymus.