Administration of Endotoxin Associated with Lipopolysaccharide Tolerance Protects Mice against Fungal Infection

Abstract

Lipopolysaccharide (LPS) pretreatment of mice resulted in a significantly enhanced survival after disseminated Cryptococcus neoformans infection. The survival was associated with reduced fungal burden in tissues. LPS-pretreated mice had lower levels of cytokines in blood, spleen, and lungs and higher levels in brain. Pentoxifylline abolished the beneficial effect of LPS pretreatment.Whether the induction of endotoxin tolerance leads to an enhanced sensitivity to infection remains a challenging question. The data reported in the literature are rather controversial. Deleterious effects of endotoxin tolerance were suggested by a study in which pretreatment of mice with lipopolysaccharide (LPS) resulted in a lower titer of plasma interferon after challenge with Newcastle disease virus (26). Although this result may suggest a reduced ability of the animals to control viral infections, this question was not addressed in that study. Similarly, using a peritonitis model, it was reported that monophosphoryl lipid A pretreatment attenuated the levels of circulating cytokines and reduced the neutrophil margination in tissues (22). The efficiency of this pretreatment, however, was not addressed in terms of outcome. It was also reported that a few hours after LPS exposure, pretreated mice had an impaired capacity to clear Pseudomonas aeruginosa from their lungs (13). Injection of heat-killed gram-negative bacteria in baboons 12 h before an intravenous infusion of viable bacteria led to increased lung injury (25). In in vitro experiments it was observed that preexposure of macrophages to LPS resulted in reduced leishmanicidal activity (24). Furthermore, macrophages previously exposed to LPS are less responsive to various types of stimuli, including bacterial superantigens (16) and Staphylococcus aureus, Streptococcus pyogenes, or zymosan (2). In contrast, endotoxin tolerance was shown to be beneficial in preventing mortality after thermal injury (7), in protecting FIG. 1. Survival curves for control and LPS-pretreated (2.5 g for 2 days) mice following intravenous infection with 2 ϫ 10 6 C. neoformans (Cn) cells. The data, expressed as percent surviving mice, are the cumulative results of four different experiments including a total of 51 mice in each group. Median survival times for control and LPS-tolerant mice were 22 and Ͼ49 days, respectively. The significance of the Kaplan-Meier survival curves was assessed by the Mantel-Cox log rank test (P Ͻ 0.0001).

Immune cells and oxidative stress in the endotoxin tolerance mouse model

“…However, that finding has not been confirmed, and some investigators have detected an increase in mortality (4,5). Comparing these studies is difficult because of the different models of tolerance, bacterial infection and fungal infection involved (4,5,14,15). However, as in our study, a preponderance of more recent reports have shown that LPS tolerance offers a protective effect in sepsis (4,5,14,15).…”

“…Comparing these studies is difficult because of the different models of tolerance, bacterial infection and fungal infection involved (4,5,14,15). However, as in our study, a preponderance of more recent reports have shown that LPS tolerance offers a protective effect in sepsis (4,5,14,15). We believe that the LPS tolerance model is characterized by changes in the immune system, and that these changes help it remain correctly regulated during sepsis.…”

“…This suggests that LPS tolerance is effective in preventing death from sepsis in a wide range of disease severity. Some studies in the literature report that LPS tolerance provides protection against infections (4,14,15). However, that finding has not been confirmed, and some investigators have detected an increase in mortality (4,5).…”

“…Previous studies have presented conflicting findings regarding cytokine production during lethal LPS challenges, as well as regarding the extent to which tolerance provides resistance to infection (4). Most of these differences are due to differences in the model used, doses used to establish tolerance, injection times, administration routes, and the time of delay before restimulation with LPS or infection (4)(5)(6)14,15). In the present study, we established and characterized a reliable murine model of endotoxin tolerance in which we applied subcutaneous injection of LPS for inducing tolerance.…”

“…In the present study, we established and characterized a reliable murine model of endotoxin tolerance in which we applied subcutaneous injection of LPS for inducing tolerance. While intraperitoneal injections are commonly reported in the literature, they cannot exclude the chance that protection against CLP is a local effect, perhaps activating local macrophages and the large number of neutrophils located in the peritoneal cavity (4)(5)(6)14,15). Because stress produces glucocorticoids that down-regulate proinflammatory cytokines, we waited for two days after establishing tolerance before challenging animals with LPS or CLP (4,16).…”