Apoptosis, inflammatory bowel disease and carcinogenesis: Process of apoptosis of target cells (Part 4)
During lymphocyte maturation and activation, fluctuations in the levels of expression of Bcl-2 or Bcl-xL appear to correlate inversely with susceptibility to apoptosis. Overexpression of Bcl-2 or Bcl-xL leads to enhanced survival of immature lymphocytes and prolonged antibody responses. Conversely, knockout of Bcl-2 or Bd-xL results in reduced survival of mature or immature lymphocytes. It has also been suggested that the long life span of memory lymphocytes may be due to constitutive expression of Bcl-2 and/or Bcl-xL. Besides, lymphocytes undergo a second form of apoptotic death, that occurs not because of lacking stimulation or deficient of growth factors but as a consequence of receptor-mediated activation. This is termed activation-induced cell death (AICD) and is considered to be induced and regulated by mechanisms that are different from those that control Bcl-2-regulated programmed cell death. In particular, AICD can be induced through activation of surface death receptors (such as Fas and TNF-receptor [TNFR]). AICD is essential for the death of lymphocytes that recognize self-antigens and also plays a role in the induction of tolerance to some foreign antigens.
At the intestinal mucosal level, the apoptosis of lamina propria (LP) TLs is regulated via active and passive mechanisms. Immune responses in the mucosa are commonly characterized by major expansions of antigen-specific TLs that have potent effector function. Although this may be essential for host defense, it may also result in effector cell populations with considerable autoreactivity that can cause mucosal inflammation. To deal with this latter possibility, the mucosal immune system has evolved several strategies for the control of mucosal immune responses. Among these is the regulation of apoptosis that either occurs by an active mechanism following TCR stimulation (AICD) or by a passive mechanism following lym-phokine (for instance, IL-2) withdrawal. The active mechanism involves death receptors such as Fas and TNFR and/or their ligands (FasL and TNF), while the passive mechanisms do not. Shop with best online pharmacy and find cialis super active online pharmacy only here getting the most out of it.