Differential induction of apoptosis in lymphoid tissues during sepsis: variation in onset, frequency, and the nature of the mediators.

Apoptosis (Ao), is a process by which cells undergo a form of nonnecrotic cellular suicide. Although for most cells this is a constitutive process, it can be induced in immature and differentiating immune cell populations by stress mediators associated with inflammation. This inducible form of A(o) is referred to as programmed cell death. However, it is not clear whether hematopoietic cell populations such as the thymus and bone marrow are induced to undergo A(o) during polymicrobial sepsis. To assess this, thymocytes, bone marrow cells, or splenocytes (as a source of comparative nonhematopoietic cells) were harvested from C3H/HeN mice at 1, 4, or 24 hours after cecal ligation and puncture (CLP; to induce polymicrobial sepsis) or sham-CLP (Sham). The results showed that mixed bone marrow cells ex vivo, although not to the same extent as thymus, showed a marked increase in the percentage of cells in A(o), increased endonuclease activity, and a significant decrease in cell yield at 24 hours but not at 4 hours after CLP. Similar changes were not evident in splenocytes. Phenotypic, as well as morphologic assessment, indicated that most of the increase in apoptotic cells in the thymus was associated with the immature T cells (CD4+CD8+) and CD8-CD4- cells. In contrast, the increase in bone marrow cell A(o) was associated with only the B220+ cells, with no significant contribution from myeloid cells. Treatment of CLP mice in vivo with either RU-38486 or PEG-(rsTNF-R1)2 was unable to reverse the increased A(o) in the bone marrow of these animals. Taken together, these findings indicate that A(o) as a process induced by polymicrobial sepsis is not limited to the thymus, but can also be detected in the bone marrow. However, unlike thymic A(o), bone marrow is not affected directly/indirectly by glucocorticoids or tumor necrosis factor released during sepsis.