During infection with gram-negative bacteria, exposure of immune cells to lipopolysaccharide (LPS) from the bacterial cell membrane induces a rapid cytokine response which is essential for the activation of host defenses against the invading pathogens. Administration of LPS to mice induces a state of hyporesponsiveness, or tolerance, characterized by reduced cytokine production upon subsequent LPS challenge. In the model of experimental Salmonella enterica serovar Typhimurium infection of mice, we assessed the question of whether complete LPS tolerance induced by repetitive doses of LPS interfered with cytokine production and host defense against gram-negative bacteria. Although production of various cytokines in response to serovar Typhimurium was attenuated by LPS pretreatment, LPS-tolerant mice showed improved antibacterial activity, evidenced by a prolongation of survival and a continuously lower bacterial load. We attribute this protective effect to three independent mechanisms. (i) Peritoneal accumulation of leukocytes in the course of LPS pretreatment accounted for enhanced defense against serovar Typhimurium during the first 6 h of infection but not for decreased bacterial load in late-stage infection. (ii) LPS-tolerant mice had an increased capacity to recruit neutrophilic granulocytes during infection. (iii) LPS-tolerant mice showed threefold-increased Kupffer cell numbers, enhanced phagocytic activity of the liver, and strongly improved clearance of blood-borne serovar Typhimurium. These results demonstrate that despite attenuated cytokine response, acquired LPS tolerance is associated with enhanced resistance to infections by gram-negative bacteria and that this effect is mainly mediated by improved effector functions of the innate immune system. Endotoxin, or lipopolysaccharide (LPS), a glycolipid of the cell membranes of gram-negative bacteria, is one of the most potent stimulators of immune responses known. The immune system responds to LPS with a systemic production of proinflammatory cytokines, which recruit and activate immune cells to eliminate invading pathogens (40). Although these cytokines are indispensible for the efficient control of the growth and dissemination of the pathogen (7,10,17), an excessive inflammatory response is potentially autodestructive and may lead to microcirculatory dysfunction, causing tissue damage, septic shock, and eventually death (3, 14). The phenomenon of endotoxin tolerance is known from animal models of "sterile infection" induced by LPS: after an initial low dose of LPS, animals are protected against the detrimental consequences of a subsequent high dose of LPS. This protection is associated with an attenuated cytokine response to LPS (11) due to a downregulation of macrophage responsiveness (12).The value of endotoxin tolerance induction as a mean of sepsis prophylaxis was studied in animal models of endotoxic shock or polymicrobial sepsis. In these models, protection by tolerance induction was ascribed to the decreased proinflammatory response, resulting i...