Update on the Mechanisms of Immune Suppression of Injury and Immune Modulation

Abstract

Major trauma results in massive impairment of immunologic reactivity, the clinical consequence of which consists in the high susceptibility of the traumatized individual toward serious infection. Whereas parts of the immune system are stimulated within a systemic, nondiscriminant, excessive whole‐body inflammation, other functions within the complex of cell‐mediated immunity (CMI) are dramatically paralyzed. Immune abnormalities in the aftermath of trauma occur in a sequence of states of cellular activation and within a complex order of events that is not yet well understood. Traumatic stress is causing disintegration of the intact monocyte (Mφ)–T cell interaction, which is associated with profound changes in Mφ forward‐regulatory capacities and substantial depression of T cell function. Extensive tissue destruction results in the generation of numerous stimuli, such as phagocytosis, immune complexes, complement split products, and endo‐ and exotoxins, all of which contribute to excessive Mφ activation. Mφ then rapidly produce and release prostaglandin E 2 (PGE 2), a powerful endogenous immune suppressant. PGE 2 is an inhibitor of T cell mitogenesis, interleukin 2 (IL‐2) production, and IL‐2 receptor expression; and it has a massive impact on the quality of B cell antibody synthesis. Most importantly, PGE 2 represents an important cofactor for the induction of T‐helper lymphocyte (T H) activity toward the T H 2 direction. T H 2 cells are associated with the synthesis of immunosuppressive cytokines, such as IL‐4 and IL‐10. Although immunosuppressive substrates are inhibitory for T H 1 cells—the functional carriers of CMI—they support T H 2 activity, which predisposes the host to develop infection. The endogenous ability of the organism to survive overwhelming trauma is insufficient and requires major exogenous support. Immune modulatory interventions, depending on the immune abnormalities seen in the traumatized host, should be started as early as possible after trauma in a preventive fashion to protect against organ tissue destruction. Ideally, it should protect all cellular host defense compartments from hyperactivation as well as from exhaustion. We do believe that only a combination of drugs can effectively control the posttraumatic dyshomeostasis of the various cell systems.