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).

The cytokine network and infection severity were characterized during disseminated cryptococcosis in tumor necrosis factor (TNF)- and lymphotoxin (Lt)-alpha-deficient mice. On day 16, the fungus burden was higher and median survival time was reduced, as was polymorphonuclear leukocyte infiltrate in the brains of knockout mice. TNF/Lt-alpha-deficient mice had lower levels of interleukin (IL)-6 in lungs and brains, IL-1beta, and the chemokine KC in brain and spleen and of the chemokine monocyte chemoattractant protein (MCP)-1 in spleen than control animals. In contrast, higher levels of IL-6, IL-10, and MCP-1 in plasma and higher levels of IL-12, interferon (IFN)-gamma, and nitrite/nitrate were found in all compartments of TNF/Lt-alpha-deficient mice. These data confirm that TNF or Lt-alpha is a key cytokine for the anticryptococcal response and demonstrate its major role for the induction of IL-1beta, IL-6, and KC in the brain; however, its presence is not a prerequisite for IL-12, IFN-gamma, and nitrite/nitrate production.

Cryptococcosis is an hematogenously disseminated meningoencephalitis during which the relationship between the disease severity and the immune response remains unclear. We thus analyzed, by enzyme-linked immunosorbent assay, proinflammatory (tumor necrosis factor alpha [TNF-α] and interleukin-6 [IL-6]) and anti-inflammatory (IL-10) cytokine levels in plasma at the time of diagnosis in 51 AIDS patients with culture-proven cryptococcosis. We used a murine model to determine the correlation between cytokine levels and fungal burden in blood and tissues and the kinetics of the immune response and of the formation of cerebral lesions. In AIDS patients, plasma TNF-α and IL-10, but not IL-6, levels were significantly higher in the case of fungemia or disseminated infection than in their absence, whereas the presence of meningitis had no influence on these levels. In mice, none of these cytokines were detected within the first day after inoculation. Later on, TNF-α and IL-10, but not IL-6, levels in plasma correlated significantly with the fungal burden in the blood and spleen but not the brain. In the brain, cytokine levels were low compared to those in other compartments, and tissue lesions and a degree of infection similar to those observed in humans were seen, further suggesting the relevance of this experimental model. Thus, AIDS patients with cryptococcosis produce an immune response that reflects the dissemination but not the meningeal involvement. This murine model of disseminated cryptococcosis can be used to investigate the pathophysiology of cryptococcosis and new therapeutic approaches.

Endotoxin tolerance was induced in mice following one, two or three injections of low amounts of lipopolysaccharide (LPS) before a further LPS injection, and circulating cytokines were analyzed 1.5 h and 3 h after LPS challenge. Three different patterns of cytokine production were obtained. In a first group of cytokines, including tumor necrosis factor (TNF), interleukin-6 (IL-6) and gamma interferon (IFN-γ), the reduction of plasma peak levels was already significantly pronounced after one tolerizing injection of LPS. The second group of cytokines includes the CC chemokine KC, the CXC chemokine monocyte-chemo-attractant protein-1 (MCP-1) and IL-12. The plasma levels of these cytokines were modestly reduced, and the reduction was more pronounced with increasing numbers of tolerizing injections of LPS. The third group of cytokines includes IL-1β and IL-18, the levels of which 3 h after LPS challenge (i.e. at the peak timing) remained essentially similar to those of control mice and after 1.5 h were even enhanced. Altogether, these data illustrate that, in tolerized animals, in vivo regulation of cytokine production differs greatly among different mediators and that immunoparalysis is not a general state. Furthermore, despite the presence of large amounts of IL-12 and IL-18, IFN-γ was essentially suppressed in tolerized animals.