New approaches to invasive fungal infections

Sphingolipid signaling in fungal pathogens

Sphingolipid involvement in infectious disease is a new and exciting branch of research. Various microbial pathogens have been shown to synthesize their own sphingolipids and some have evolved methods to "hijack" host sphingolipids for their own use. For instance, Sphingomonas species are bacterial pathogens that lack the lipopolysaccharide component typical but instead contain glycosphingolipids (Kawahara 1991, 2006). In terms of sphingolipid signaling and function, perhaps the best-studied group of microbes is the pathogenic fungi. Pathogenic fungi still represent significant problems in human disease, despite treatments that have been used for decades. Because fungi are eukaryotic, drug targets in fungi can have many similarities to mammalian processes. This often leads to significant side effects of antifungal drugs that can be dose limiting in many patient populations. The search for fungal-specific drugs and the need for better understanding of cellular processes of pathogenic fungi has led to a large body of research on fungal signaling. One particularly interesting and rapidly growing field in this research is the involvement of fungal sphingolipid pathways in signaling and virulence. In this chapter, the research relating to sphingolipid signaling pathogenic fungi will be reviewed and summarized, in addition to highlighting pathways that show promise for future research.

Effectsof Aspergillus fumigatusgliotoxin and methylprednisolone on human neutrophils: implications for the pathogenesis of invasive aspergillosis

Aspergillus fumigatus (AF) is a ubiquitous mold and the most common cause of invasive aspergillosis (IA) in immunocompromised patients. In stem cell transplant recipients, IA now occurs most frequently in the setting of therapy with corticosteroids, including methylprednisolone (MP). We showed previously that gliotoxin (GT), an AF-derived mycotoxin, induces apoptosis in monocytes and dendritic cells, resulting in the suppression of AF-specific T cell responses. We examined the ability of GT to induce apoptosis in polymorphonuclear leukocytes (PMN) and assessed GT effects on important neutrophil functions, including phagocytic function, degranulation, myeloperoxidase activity, and the production of reactive oxygen species (ROS). In contrast to its effects on monocytes, PMN remained resistant to GT-mediated apoptosis. Although many essential neutrophil functions were unaffected, GT inhibited phagocytosis and also induced a decrease in ROS generation by PMN. In contrast, MP therapy potentiated ROS production, suggesting a mechanism that may facilitate tissue injury in IA. Distinct from its effects on untreated PMN, GT augmented ROS production in MP-treated PMN. Our results suggest that although GT may suppress the adaptive immune response, GT may also serve to increase PMN-mediated inflammation, which is likely to play an important role in tissue destruction in the setting of IA.

Voriconazole : a review of its use in the management of invasive fungal infections

Voriconazole (VFEND), a synthetic second-generation, broad-spectrum triazole derivative of fluconazole, inhibits the cytochrome P450 (CYP)-dependent enzyme 14-alpha-sterol demethylase, thereby disrupting the cell membrane and halting fungal growth. In the US, intravenous and/or oral voriconazole is recommended in adults for the treatment of invasive aspergillosis, candidaemia in non-neutropenic patients, disseminated infections caused by Candida spp., oesophageal candidiasis, and in patients with scedosporiosis and fusariosis who are refractory to or intolerant of other antifungal therapy. In Europe, intravenous and/or oral voriconazole is recommended in adults and paediatric patients of at least 2 years of age for the treatment of invasive aspergillosis, candidaemia in non-neutropenic patients, fluconazole-resistant serious invasive Candida spp. infections, scedosporiosis and fusariosis. In large randomised trials, voriconazole was an effective and generally well tolerated primary treatment for candidiasis and invasive aspergillosis in adults and adolescents. More limited data also support the use of voriconazole for the treatment of invasive fungal infections in children, in those with rare fungal infections, such as Fusarium spp. or Scedosporium spp., and in those refractory to or intolerant of other standard antifungal therapies. The availability of both parenteral and oral formulations and the almost complete absorption of the drug after oral administration provide for ease of use and potential cost savings, and ensure that therapeutic plasma concentrations are maintained when switching from intravenous to oral therapy. On the other hand, the numerous drug interactions associated with voriconazole may limit its usefulness in some patients. Further clinical experience will help to more fully determine the position of voriconazole in relation to other licensed antifungal agents. In the meantime, voriconazole is a valuable emerging option for the treatment of invasive aspergillosis and rare fungal infections, including Fusarium spp. and Scedosporium spp. infections, and provides an alternative option for the treatment of candidiasis, particularly where the causative organism is inherently resistant to other licensed antifungal agents.

Comparison of epidemiological, clinical, and biological features of invasive aspergillosis in neutropenic and nonneutropenic patients: a 6-year survey

BACKGROUND

Invasive aspergillosis is an opportunistic infection that occurs mainly among patients with prolonged neutropenia. Few data are available on invasive aspergillosis in nonneutropenic patients.

METHODS

The aim of this survey was to compare neutropenic and nonneutropenic patients who had received a diagnosis of invasive aspergillosis at our institution during a 6-year period.

RESULTS

Among the 88 cases of invasive aspergillosis analyzed here, 12 were histologically proven, 52 were probable, and 24 were possible. Forty-seven percent of cases were diagnosed in the intensive care unit, and 40% were diagnosed in hematology units. Neutropenia was a risk factor for 52 patients (59%), most of whom had hematological or solid malignancies. Among the 36 nonneutropenic patients (41%), the main underlying conditions were steroid-treated chronic obstructive pulmonary disease, asthma, rheumatoid arthritis, giant-cell arteritis, and microvascular disorders; 10 patients were recipients of solid-organ transplants, and 1 patient was seropositive for human immunodeficiency virus. The distribution of proven and probable invasive aspergillosis was similar for neutropenic and nonneutropenic patients. The mortality rate was 71.5% overall and was significantly higher among nonneutropenic patients than among neutropenic patients (89% vs. 60%; P<.05). Compared with neutropenic patients, nonneutropenic patients were significantly less likely to have symptoms of invasive aspergillosis and more likely to have frequent intercurrent pneumonia due to another microorganism. The sensitivity of mycological examination of bronchoalveolar lavage fluid specimens was higher for nonneutropenic patients than for neutropenic patients (85% vs. 58%; P<.05), whereas the sensitivity of antigenemia was the same for the 2 populations (65% vs. 64%). Findings on thoracic computed tomographs were similar, except that segmental areas of consolidation occurred more frequently among neutropenic patients.

CONCLUSION

This survey at a whole institution underlines the high number of cases of invasive aspergillosis among nonneutropenic patients, with an overall mortality rate that was significantly higher than that for neutropenic patients.

Bloodstream infection after umbilical cord blood transplantation using reduced-intensity stem cell transplantation for adult patients

Bloodstream infection (BSI) is a significant problem after cord blood transplantation (CBT). However, little information has been reported on BSI after reduced-intensity CBT (RI-CBT). We retrospectively reviewed the medical records of 102 patients. The median age of the patients was 55 years (range, 17-79 years). Preparative regimens comprised fludarabine 125 to 150 mg/m 2 , melphalan 80 to 140 mg/m 2 , or busulfan 8 mg/kg and total body irradiation 2 to 8 Gy. Prophylaxis against graft-versus-host disease comprised cyclosporin or tacrolimus. BSI developed within 100 days of RI-CBT in 32 patients. The cumulative incidence of BSI was 25% at day 30 and 32% at day 100. The median onset was day 15 (range, 1-98 days). Causative organisms included Pseudomonas aeruginosa (n = 12), Staphylococcus epidermidis (n = 11), Staphylococcus aureus (n = 6), Enterococcus faecium (n = 4), Enterococcus faecalis (n = 4), Stenotrophomonas maltophilia (n = 4), and others (n = 7). Of the 32 patients with BSI, 25 (84%) died within 100 days after RI-CBT. BSI was the direct cause of death in 8 patients (25%). Univariate analysis failed to identify any significant risk factors. BSI clearly represents a significant and fatal complication after RI-CBT. Further studies are warranted to determine clinical characteristics, identify patients at high risk of BSI, and establish therapeutic strategies.

TLRs govern neutrophil activity in aspergillosis

Polymorphonuclear neutrophils (PMNs) are essential in initiation and execution of the acute inflammatory response and subsequent resolution of fungal infection. PMNs, however, may act as double-edged swords, as the excessive release of oxidants and proteases may be responsible for injury to organs and fungal sepsis. To identify regulatory mechanisms that may balance PMN-dependent protection and immunopathology in fungal infections, the involvement of different TLR-activation pathways was evaluated on human PMNs exposed to the fungus Aspergillus fumigatus. Recognition of Aspergillus and activation of PMNs occurred through the involvement of distinct members of the TLR family, each likely activating specialized antifungal effector functions. By affecting the balance between fungicidal oxidative and nonoxidative mechanisms, pro- and anti-inflammatory cytokine production, and apoptosis vs necrosis, the different TLRs ultimately impacted on the quality of microbicidal activity and inflammatory pathology. Signaling through TLR2 promoted the fungicidal activity of PMNs through oxidative pathways involving extracellular release of gelatinases and proinflammatory cytokines while TLR4 favored the oxidative pathways through the participation of azurophil, myeloperoxidase-positive, granules and IL-10. This translated in vivo in the occurrence of different patterns of fungal clearance and inflammatory pathology. Both pathways were variably affected by signaling through TLR3, TLR5, TLR6, TLR7, TLR8, and TLR9. The ability of selected individual TLRs to restore antifungal functions in defective PMNs suggests that the coordinated outputs of activation of multiple TLRs may contribute to PMN function in aspergillosis.

Anti-Aspergillus fumigatus efficacy of pentraxin 3 alone and in combination with antifungals

The collectin pentraxin 3 (PTX3) is an essential component of host resistance to pulmonary aspergillosis. Here we examined the protective effects of administration of PTX3 alone or together with deoxycholate amphotericin B (Fungizone) or liposomal amphotericin B (AmBisome) against invasive aspergillosis in a murine model of allogeneic bone marrow transplantation. PTX3, alone or in combination with the polyenes, was given intranasally or parenterally either before, in concomitance with, or after the intranasal infection with Aspergillus fumigatus conidia. Mice were monitored for resistance to infection and parameters of innate and adaptive T-helper immunity. The results showed the following: (i) complete resistance to infection and reinfection was observed in mice treated with PTX3 alone; (ii) the protective effect of PTX3 was similar or superior to that observed with liposomal amphotericin B or deoxycholate amphotericin B, respectively; (iii) protection was associated with accelerated recovery of lung phagocytic cells and T-helper-1 lymphocytes and concomitant decrease of inflammatory pathology; and (iv) PTX3 potentiated the therapeutic efficacy of suboptimal doses of either antimycotic drug. Together, these data suggest the potential therapeutic use of PTX3 either alone or as an adjunctive therapy in A. fumigatus infections.