They were challenged on later dates with 20 g O111 LPS in 300 ul PBS i.p. responses that eliminate invading microorganisms. For hours to a few days after such an encounter, however, the cells pro-inflammatory responses to the same and other microbial molecules may be greatly reduced. This period of tolerance (also called reprogramming, since some anti-inflammatory responses are usually preserved (Henricson et al., 1993; Shnyra et al., 1998)), is usually thought to minimize inflammation-induced damage during recovery from infectious diseases. Although tolerance induced by Gram-negative bacteria or their LPS (endotoxin) has been studied most intensively, a similar phenomenon has also been noted in response to other bacterial agonists (Cross, 2002; Medvedev et al., 2006; Wang et al., 2003) and during some viral infections (Didierlaurent et al., 2008). Besifloxacin HCl Tolerance to LPS has been documented in several clinical settings: in adults with pyelonephritis (McCabe, 1963), in the alveolar macrophages of individuals who have smoked endotoxin-containing tobacco (Chen et al., 2007), and in the blood leukocytes of children with bacterial meningitis (Helminen and Vesikari, 1990a), adults with alcoholic cirrhosis (Von Baehr et al., 2000) and adults with sepsis (McCall et al., 1993; Faist et al., 1988; Munoz et al., 1991). Perhaps most significantly, tolerance in blood leukocytes has been one of the most consistently-observed correlates of infection-induced immunosuppression in humans (West and Heagy, 2002). Tolerance has also been noted in the blood cells of uninfected patients following major trauma (West and Heagy, 2002), SHCC with greater tolerance noted in those who had elevated plasma endotoxin levels (Kelly et al., 1997). In an investigation of patients with severe sepsis, the blood monocytes of patients with Gram-negative bacterial infections showed greater tolerance than did those from patients with Gram-positive infections (Munoz Besifloxacin HCl et al., 1991; McCabe, 1963). Whereas much is now known about how LPS initiates Besifloxacin HCl reprogramming in both monocyte-macrophages and neutrophils (Medvedev et al., 2006; Wysocka et al., 2001; Cross, 2002; Cavaillon et al., 2003; McCall and Yoza, 2007; Foster et al., 2007), the mechanisms that terminate the reprogrammed (tolerant) state and restore host defenses are poorly understood (McCall and Yoza, 2007; Wysocka et al., 2005). Indeed, the duration of tolerance may vary greatly. In children with type b meningitis, leukocyte hyporesponsiveness to LPS resolved as the patients recovered over a few days (Helminen and Vesikari, 1990b). In contrast, adult cancer patients who received a bolus intravenous injection of LPS remained tolerant for almost two weeks (Engelhardt et al., 1991) and the peripheral blood leukocytes of septic adults have often been reprogrammed for many days or weeks after apparently successful treatment of the inciting contamination (Weighardt et al., 2000). In septic patients, recovery from tolerance has been associated with survival (Munoz et al., 1991; Weighardt et al., 2000). Understanding how tolerance is usually terminated thus might suggest ways to decrease the duration of post-infection immunosuppression and improve outcome. Here we show that recovery from tolerance elicited by LPS or Gram-negative bacteria requires acyloxyacyl Besifloxacin HCl hydrolase (AOAH), a macrophage, dendritic cell and neutrophil lipase that inactivates LPSs by removing secondary fatty acyl chains from the lipid A moiety (Munford and Hall, 1986). The duration and degree of tolerance correlated with the presence of fully acylated LPS in peritoneal macrophages. Moreover, mice experiencing prolonged tolerance were highly susceptible to challenge. These results suggest that killing Gram-negative bacteria does not allow full restoration of homeostasis and effective host defenses: to prevent prolonged immune suppression, the dominant bacterial signal molecule, LPS, must also be inactivated. Results AOAH allows recovery from tolerance induced by LPS or Gram-negative bacteria and mice did not differ in their acute cytokine responses to the first LPS injection (see data for PBS-primed mice in Fig. 1ACC), and the mice in both groups were tolerant on day 5 Besifloxacin HCl after the initial LPS dose. In contrast, whereas wild-type mice regained their ability to respond normally to a second LPS dose within 5 to 10 days, LPS-primed mice had greatly reduced plasma IL-6, TNF and RANTES responses to a second dose for the entire 22 day period after the initial LPS exposure (Fig. 1A, B; Fig. S1). In keeping with previous reports that LPS-induced macrophage reprogramming alters responses along both the MyD88-dependent and – impartial arms of the intracellular signaling pathway downstream of TLR4 (Bagchi et al., 2007; Sato et al., 2002), responses to LPS challenge that were MyD88-dependent (TNF [Physique.