Sepsis-induced apoptosis of the thymocytes in mice

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 ~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-α production after Gram-negative or Gram-positive bacteria injection was slightly different. E. coli induced serum TNF-α 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-α 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-α is a common denominator released and might be responsible for the thymocyte apoptosis.