Monocyte CD14: a multifunctional receptor engaged in apoptosis from both sides

Like all other immune system cells, monocytes and macrophages may undergo apoptotic cell death in response to specific triggers and mediators or as a consequence of aging. However, factors inducing apoptosis and the involved cellular and molecular mechanisms are much better investigated and understood for lymphocytes. Th2 cell-derived cytokines such as interleukin-4 (IL-4) are able to induce monocyte apoptosis most effectively. This process is preceded by down-regulation of the CD14 surface receptor. Mediators such as lipopolysaccharide (LPS) suppress and postpone apoptosis in parallel with up-regulation of CD14. Macrophages are rather resistant against apoptotic damage, and factors able to evoke apoptosis in monocytes are often ineffective in macrophages. Resistance of macrophages against apoptotic triggers may be beneficial for inflammatory processes where macrophages are engaged and needed as phagocytes for ingestion and removal of moribund cells. The multifunctional CD14 receptor of monocytes/macrophages is supposed to be involved in the apoptotic network on both sides: as a surface molecule of monocytes that can promote survival and antagonize apoptosis and as a recognition receptor of macrophages that enables or supports interaction with apoptotic cells.