Cell mediated immunity to fungi: a reassessment

Neutrophils and macrophages work in concert as inducers and effectors of adaptive immunity against extracellular and intracellular microbial pathogens

Emerging data suggest new facets of the concerted participation of neutrophils and macrophages in antimicrobial immunity. The classical view is that DCs and macrophages are the inducers of adaptive antimicrobial immunity, but there is evidence for neutrophil participation in this task as cytokine and chemokine producers and APCs. On the other hand, the concept that the T(H)1 response is only associated with control of infections by intracellular pathogens through activation of macrophages by IFN-gamma, and the T(H)17/IL-17 axis is only involved in protection against extracellular pathogens through mobilization and activation of neutrophils is simplistic: There is evidence suggesting that T(H)1 and T(H)17 responses, separately or in parallel, may use macrophages and neutrophils against infections by extracellular and intracellular microbial pathogens. Opsonization by pathogen-specific Igs enhances the antimicrobial capabilities of neutrophils and macrophages in infections by extracellular and intracellular microbes. The functional partnership between macrophages and neutrophils as inducers and effectors of adaptive antimicrobial immunity conforms to their affiliation with the myeloid phagocyte system and reveals a strategy based on the concurrent use of the two professional phagocytes in the adaptive defense mechanisms. Starting from a common myeloid precursor in the bone marrow, macrophages and neutrophils split during differentiation but come together at the infectious foci for a cooperative strategy that uses modulator and effector activities to attack invading microbial pathogens.

Massive induction of innate immune response to Candida albicans in the kidney in a murine intravenous challenge model

In the experimental Candida albicans intravenous challenge model, the kidney is one of the main organs involved in disease. In progressive infection, fungal burdens are found to increase over time, with rapid increases occurring from 24 h postinfection. Renal transcriptional responses were analyzed at this time in the kidneys of mice infected by either a virulent or an attenuated C. albicans strain, allowing comparison of host responses in progressive and nonprogressive infection. The results of this study demonstrate that both infections share a common transcriptional response, consisting of functions associated with the acute-phase reaction. In addition, challenge with the virulent strain led to a massively increased expression of cytokine genes, other innate response genes and genes suggestive of initiation of the adaptive immune response. This immune response to C. albicans infection, which occurs only in progressive infection, may contribute to development of sepsis and, ultimately, host death.

Therapy of murine pulmonary aspergillosis with antibody-alliinase conjugates and alliin

Aspergillus fumigatus is an opportunistic fungal pathogen responsible for invasive aspergillosis in immunocompromised individuals. The high morbidity and mortality rates as well as the poor efficacy of antifungal agents remain major clinical concerns. Allicin (diallyl-dithiosulfinate), which is produced by the garlic enzyme alliinase from the harmless substrate alliin, has been shown to have wide-range antifungal specificity. A monoclonal antibody (MAb) against A. fumigatus was produced and chemically ligated to the enzyme alliinase. The purified antibody-alliinase conjugate bound to conidia and hyphae of A. fumigatus at nanomolar concentrations. In the presence of alliin, the conjugate produced cytotoxic allicin molecules, which killed the fungus. In vivo testing of the therapeutical potential of the conjugate was carried out in immunosuppressed mice infected intranasally with conidia of A. fumigatus. Intratracheal (i.t.) instillation of the conjugate and alliin (four treatments) resulted in 80 to 85% animal survival (36 days), with almost complete fungal clearance. Repetitive intratracheal administration of the conjugate and alliin was also effective when treatments were initiated at a more advanced stage of infection (50 h). The fungi were killed specifically without causing damage to the lung tissue or overt discomfort to the animals. Intratracheal instillation of the conjugate without alliin or of the unconjugated monoclonal antibody significantly delayed the death of the infected mice, but only 20% of the animals survived. A limitation of this study is that the demonstration was achieved in a constrained setting. Other routes of drug delivery will be investigated for the treatment of pulmonary and extrapulmonary aspergillosis.

Vitamin D for treatment and prevention of infectious diseases: a systematic review of randomized controlled trials

OBJECTIVE

To review the existing human controlled intervention studies of vitamin D as adjunctive therapy in settings of infection and provide recommendations for design and implementation of future studies in this field on the basis of the evidence reviewed.

METHODS

We conducted a systematic review of randomized controlled clinical trials that studied vitamin D for treatment or prevention of infectious diseases in humans. Studies from 1948 through 2009 were identified through search terms in PubMed and Ovid MEDLINE.

RESULTS

Thirteen published controlled trials were identified by our search criteria. Ten trials were placebo controlled, and 9 of the 10 were conducted in a rigorous double-blind design. The selected clinical trials demonstrated substantial heterogeneity in baseline patient demographics, sample size, and vitamin D intervention strategies. Serious adverse events attributable to vitamin D supplementation were rare across all studies. On the basis of studies reviewed to date, the strongest evidence supports further research into adjunctive vitamin D therapy for tuberculosis, influenza, and viral upper respiratory tract illnesses. In the selected studies, certain aspects of study design are highlighted to help guide future clinical research in the field.

CONCLUSION

More rigorously designed clinical trials are needed for further evaluation of the relationship between vitamin D status and the immune response to infection as well as for delineation of necessary changes in clinical practice and medical care of patients with vitamin D deficiency in infectious disease settings.

Dectin-2 is a Syk-coupled pattern recognition receptor crucial for Th17 responses to fungal infection

Innate immune cells detect pathogens via pattern recognition receptors (PRRs), which signal for initiation of immune responses to infection. Studies with Dectin-1, a PRR for fungi, have defined a novel innate signaling pathway involving Syk kinase and the adaptor CARD9, which is critical for inducing Th17 responses to fungal infection. We show that another C-type lectin, Dectin-2, also signals via Syk and CARD9, and contributes to dendritic cell (DC) activation by fungal particles. Unlike Dectin-1, Dectin-2 couples to Syk indirectly, through association with the FcRγ chain. In a model of Candida albicans infection, blockade of Dectin-2 did not affect innate immune resistance but abrogated Candida-specific T cell production of IL-17 and, in combination with the absence of Dectin-1, decreased Th1 responses to the organism. Thus, Dectin-2 constitutes a major fungal PRR that can couple to the Syk–CARD9 innate signaling pathway to activate DCs and regulate adaptive immune responses to fungal infection.

Early-expressed chemokines predict kidney immunopathology in experimental disseminated Candida albicans infections

BACKGROUND

The mouse intravenous challenge model of Candida albicans infection is widely used to determine aspects of host-fungus interaction. We investigated the production of cytokines in the kidneys and spleen of animals up to 48 h after challenge with virulent and attenuated isolates and related these responses to semi-quantitative estimations of histopathological changes in the kidney.

METHODOLOGY/PRINCIPAL FINDINGS

Progression of Candida albicans infection of the kidney in response to highly virulent fungal strains was characterized by higher levels of host cellular infiltrate, higher lesion densities and greater quantities of fungal elements at 24 and 48 h, and by higher kidney concentrations of IL-1beta, MCP-1, KC, IL-6, G-CSF, TNF, MIP-2 and MIP-1beta, among the immune effectors measured. Levels of the chemokine KC as early as 12 h after challenge correlated significantly with all later measurements of lesion severity. Early renal IL-6 and MIP-1beta concentrations also correlated with subsequent damage levels, but less significantly than for KC. All chemokines tested appeared in kidney homogenates, while most of the cytokines were undetectable in kidney and spleen homogenates. GM-CSF and IL-10 showed inverse correlations with measures of lesion severity, suggesting these alone may have exerted a defensive role. Spleen levels of KC at all times showed significant associations with kidney lesion measurements.

CONCLUSIONS/SIGNIFICANCE

Elevated chemokine levels, including KC, represent the earliest responses to C. albicans infection in the mouse kidney. Fungal strains of low mouse virulence stimulate a lower innate response and less host infiltrate than more virulent strains. These findings are consistent with immunopathological damage to kidneys in the mouse C. albicans infection model and with growing evidence implicating some TLR pathways as the main point of interaction between fungal surface polysaccharides and leukocytes.

Host responses to a versatile commensal: PAMPs and PRRs interplay leading to tolerance or infection by Candida albicans

The molecular interactions between commensal microorganisms and their host are basically different from those triggered by pathogens since they involve tolerance. When the commensal is genetically equipped to become an opportunistic pathogen, as is the case with Candida albicans, the picture becomes more complex. In this case, the balance between protection and invasion depends on host reactivity to altered microbial expression of ligands interacting with innate immune sensors. Based on experimental evidence obtained with C. albicans, we discuss the different molecular processes involved in the sensing of this important opportunistic human pathogen by a panel of pattern recognition receptors (PRRs) according to the numerous pathogen-associated molecular patterns (PAMPs) that can be exposed at its surface. Beneficial or deleterious immune responses that either maintain a commensal state or favour damage by the yeast result from this dynamic interplay.

Regulation of interleukin-12/interleukin-23 production and the T-helper 17 response in humans

Interleukin-12 (IL-12) and IL-23 share a common chain. Yet, their production in response to pathogens is differentially regulated, and their functions are distinct and often antithetic. IL-12 is involved in the induction or amplification of the T-helper (Th) type 1 response, whereas IL-23 has been associated with the generation of the Th17 response and IL-17 production. Mycobacterium tuberculosis and yeast zymosan induce IL-23, but in the absence of other stimuli, no IL-12 is induced in human dendritic cells (DCs). The stimulation of IL-23 by M. tuberculosis was mostly explained by the triggering of Toll-like receptor (TLR2) and the cytoplasmic receptor nucleotide oligomerization domain (NOD)-containing protein 2, whereas zymosan induces IL-23 primarily by stimulating the beta-glucan receptor dectin-1 alone or in combination with TLR2. IL-23, IL-6, transforming growth factor (TGF-beta1), and IL-1beta in supernatants from activated human DCs induce human naive CD4(+) T cells to produce IL-17. These data are consistent with various recent reports that TGF-beta is an inducer of IL-17 production both in human and in mouse cells. However, IL-1 is necessary in combination with some or all of the other cytokines to induce IL-17 production in human T cells. The ability of various stimuli to induce Th17 cells depends not only on their induction of IL-23, IL-6, and TGF-beta production in DCs but also on their ability to activate directly or indirectly the inflammasome and to induce IL-1beta.