Management of fungal infections in neutropenic patients: more doubts than certainties?

The Role of IL-17 in Protection against Mucosal Candida Infections

Interleukin-17 (IL-17) is a proinflammatory cytokine produced by adaptive CD4+ T helper cells and innate lymphocytes, such as γδ-T cells and TCRβ+ "natural" Th17 cells. IL-17 activates signaling through the IL-17 receptor, which induces other proinflammatory cytokines, antimicrobial peptides and neutrophil chemokines that are important for antifungal activity. The importance of IL-17 in protective antifungal immunity is evident in mice and humans, where various genetic defects related to the IL-17-signaling pathway render them highly susceptible to forms of candidiasis such oropharyngeal candidiasis (OPC) or more broadly chronic mucocutaneous candidiasis (CMC), both caused mainly by the opportunistic fungal pathogen Candida albicans. OPC is common in infants and the elderly, HIV/AIDS and patients receiving chemotherapy and/or radiotherapy for head and neck cancers. This review focuses on the role of IL-17 in protection against candidiasis, and includes a brief discussion of non-Candida albicans fungal infections, as well as how therapeutic interventions blocking IL-17-related components can affect antifungal immunity.

Role of neutrophils in IL-17-dependent immunity to mucosal candidiasis

Oropharyngeal candidiasis (OPC), caused by the commensal fungus Candida albicans, is an opportunistic infection associated with infancy, AIDS, and IL-17-related primary immunodeficiencies. The Th17-associated cytokines IL-23 and IL-17 are crucial for immunity to OPC, but the mechanisms by which they mediate immunity are poorly defined. IL-17RA-deficient humans and mice are strongly susceptible to OPC, with reduced levels of CXC chemokines and concomitantly impaired neutrophil recruitment to the oral mucosa. Paradoxically, humans with isolated neutropenia are typically not susceptible to candidiasis. To determine whether immunity to OPC is mediated via neutrophil recruitment, mice lacking CXCR2 were subjected to OPC and were found to be highly susceptible, although there was no dissemination of fungi to peripheral organs. To assess whether the entire neutrophil response is IL-17 dependent, IL-17RA(-/-) and IL-23(-/-) mice were administered neutrophil-depleting Abs and subjected to OPC. These mice displayed increased oral fungal burdens compared with IL-17RA(-/-) or IL-23(-/-) mice alone, indicating that additional IL-17-independent signals contribute to the neutrophil response. WT mice treated with anti-Gr-1 Abs exhibited a robust infiltrate of CD11b(+)Ly-6G(low)F4/80(-) cells to the oral mucosa but were nonetheless highly susceptible to OPC, indicating that this monocytic influx is insufficient for host defense. Surprisingly, Ly-6G Ab treatment did not induce the same strong susceptibility to OPC in WT mice. Thus, CXCR2(+) and Gr-1(+) neutrophils play a vital role in host defense against OPC. Moreover, defects in the IL-23/17 axis cause a potent but incomplete deficiency in the neutrophil response to oral candidiasis.

Th17 cells in immunity to Candida albicans

Our understanding of immunity to fungal pathogens has advanced considerably in recent years. Particularly significant have been the parallel discoveries in the C-type lectin receptor family and the Th effector arms of immunity, especially Th17 cells and their signature cytokine, IL-17. Many of these studies have focused on the most common human fungal pathogen, Candida albicans, which is typically a commensal microbe in healthy individuals but causes various disease manifestations in immunocompromised hosts, ranging from mild mucosal infections to lethal disseminated disease. Here, we discuss emerging fundamental discoveries with C. albicans that have informed our overall molecular understanding of fungal immunity. In particular, we focus on the importance of pattern recognition receptor-mediated fungal recognition and subsequent IL-17 responses in host defense against mucosal candidiasis. In light of these recent advances, we also discuss the implications for anticytokine biologic therapy and vaccine development.

In vitro inhibition of fungal activity by macrophage-mediated sequestration and release of encapsulated amphotericin B nanosupension in red blood cells

The efficacy of antifungal treatment has been diminished by the biodistribution limitations of amphotericin B (AmB) due to its pharmacological profile, as well as the severe side effects it causes. A cellular drug-delivery system, which incorporates human erythrocytes (RBCs) loaded with an AmB nanosuspension (AmB-NS), is developed in order to improve antifungal treatment. AmB-NS encapsulation in RBCs is achieved by using hypotonic hemolysis, leading to intracellular AmB amounts of 3.81 +/- 0.47 pg RBC(-1) and an entrapment efficacy of 15-18%. Upon phagocytosis of AmB-NS-RBCs, leukocytes show a slow AmB release over ten days, and no alteration in cell viability. This results in an immediate, permanent inhibition of intra- and extracellular fungal activity. AmB-NS-RBC-leukocyte-mediated delivery of AmB is efficient in amounts 1000 times lower than the toxic dose. This drug-delivery method is effective for the transport of water-insoluble substances, such as AmB, and this warrants consideration for further testing.

Les relations pharmacocinétique-pharmacodynamique des antifongiques. Conséquences pour le suivi thérapeutique

Despite pharmacokinetic-pharmacodynamic relationships were clearly evidenced for antifungal drugs by the use of experimental models, few target plasma concentrations could be determined from studies performed in patients. The main causes explaining this lack of data are reviewed and the possible use in humans of the parameters obtained from animal models is discussed.

A systematic review of the antifungal effectiveness and tolerability of amphotericin B formulations

OBJECTIVE

A systematic review was performed to compare the effectiveness and tolerability of lipid-based amphotericin B (AmB) formulations and conventional AmB in the treatment of systemic fungal infections.

METHODS

The literature and unpublished studies were searched using MEDLINE, EMBASE, Biological Abstracts, AIDSLINE, CANCERLIT, CRD database, Cochrane Controlled Trials Register, and other databases. Search terms included: amphotericin, liposom*, lipid*, colloid*, antifungal agents, and mycoses. Studies were selected according to predetermined criteria. The outcome measures reviewed were efficacy, mortality, renal toxicity, and infusion-related reactions. Meta-analyses and number-needed-to-treat (NNT) analyses were performed.

RESULTS

Seven studies (8 publications) met the entry criteria. Meta-analysis showed that lipid-based formulations significantly reduced all-cause mortality risk by an estimated 28% compared with conventional AmB (odds ratio [OR], 0.72; 95% CI, 0.54 to 0.97). There was no significant difference in efficacy between the lipid-based formulations and conventional AmB (OR, 1.21; 95% CI, 0.98 to 1.49). AmB lipid complex (ABLC) and liposomal AmB (L-AmB) significantly reduced the risk of doubling serum creatinine by an estimated 58% (OR, 0.42; 95% CI, 0.33 to 0.54). There was no significant reduction in risk of infusion-related reactions with lipid-based formulations, although this was difficult to interpret given the lack of consistent control of confounding factors. Comparing the lipid-based formulations with conventional AmB, the overall NNT to prevent 1 death was 31. The NNT to prevent a doubling of serum creatinine for both ABLC and L-AmB compared with conventional AmB was 6.

CONCLUSIONS

This study demonstrates advantages with lipid-based formulations over conventional AmB in terms of reduced risk of mortality and renal toxicity. Future trials in patients with proven fungal infection should control for factors such as premedication, infusion rates, fluid preloading, sodium/potassium supplementation, and concomitant medication.

Infection control issues after bone marrow transplantation

Bone marrow transplant recipients are at increased risk of a variety of infections from endogenous and exogenous sources. Various practices, including the use of prophylactic agents and a controlled environment are often employed in the name of infection control. Comparable data are lacking because of non-unified definitions of infection and endpoints. Antibiotic-resistant bacteria and fungi are being isolated more frequently as a result of selective pressure.