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Innate and adaptive immunity in Candida albicans infections and saprophytism

Innate and adaptive immunity in Candida albicans infections

and saprophytism

Luigina Romani


"Underlying acquired immunity to the fungus Candida albicans is usually present in adult

immunocompetent individuals and is presumed to prevent mucosal colonization progressing to symptomatic infection. Exploration of immunological events leading to Candida resistance or susceptibility has indicated the central role of the innate and

adaptive immune systems, the relative contribution of which may vary depending on the site of the primary infection. Nevertheless, acquired resistance to infection results from the development of Th1 responses. Cytokines produced by Th1 cells activate phagocytic cells to a candidacidal state. In contrast, cytokines produced by Th2 cells inhibit Th1 development and deactivate phagocytic effector cells. Because reciprocal influences have been recognized between innate and adaptive Th immunity, it appears that an integrated immune response determines the life-long commensalism of the fungus at the mucosal level, as well as the transition from mucosal saprophyte to pathogen. However, if the ability of C. albicans to establish a disseminated infection

involves neutropenia as a major predisposing factor, its ability to persist in infected tissues or to behave as a commensal may involve primarily downregulation of host cell-mediated adaptive immunity. As a commensal, C. albicans may be endowed with the ability to elude the host�s immunological surveillance, thus allowing its persistence on mucosal surfaces. Th1 and Th2 CD41 T-cells develop from a common, naïve CD41 T-cell precursor, and several parameters have been shown to influence the pathway of differentiation of CD41 T-cell precursors. Among these, cytokines appear to play

a major role, acting not only as modulators of antifungal effector functions but also as key regulators in the development of the different Th subsets from precursor Th cells. Studies in mice have shown that development of protective Anticandidal Th1 responses requires the concerted actions of several cytokines, such as interferon (IFN)-g, transforming growth factor (TGF)-b, interleukin (IL)-6 [31], tumor necrosis factor (TNF)-a, and IL-12, in the relative absence of inhibitory Th2 cytokines, such as IL-4 and IL-10, which inhibit development of Th1 responses. Early in infection, neutralization of Th1 cytokines (IFN-g and IL-12) leads to the onset of Th2 rather than Th1 responses, while neutralization of Th2 cytokines (IL-4 and IL-10) allows development of Th1- rather than Th2-cell responses. TNF/lymphotoxin

(LT)-a and IL-6 deficiencies render mice highly susceptible to C. albicans infections. Studies in humans have reinforced this concept, by showing that acquired immunity to C. albicans correlates with the expression of local or peripheral Th1 reactivity, whereas susceptibility to the infection seen in thermally injured patients, in patients with human immunodeficiency virus (HIV) infection, or in patients with chronic mucocutaneous or hepatosplenic candidiasis correlates with a biased Th2 response to the fungus. Altogether these data demonstrate that susceptibility to primary and secondary C. albicans infections in cytokine-deficient mice correlates with the failure to develop anticandidal, protective Th1 responses and with the occurrence of unprotective IL-4- and IL-10-producing Th2 cells. However, an important immunoregulatory role has been attributed to neutrophils recently. Neutrophils, more than macrophages, were endowed with the ability to produce directive cytokines such as IL-10 and IL-12. Most importantly, IL-12 appeared to be released in response to a low-virulence Candida strain that initiates Th1 development in vivo, but IL-10 was released in response to a virulent strain. Human neutrophils also produced bioactive IL-12 in response to a mannoprotein fraction of C. albicans, capable of inducing Th1 cytokine expression in peripheral blood mononuclear cells. By producing directive cytokines such as IL-10 and IL-12, neutrophils influenced antifungal Th-cell development, as evidenced by the inability of neutropenic mice to mount protective anticandidal Th1 responses. Production of IL-12 by neutrophils occurred independently of TNF-a and IFN-g. It was impaired upon iron overload but increased upon in vitro priming with IL-4 through upregulation of IL-4 receptor expression. Human studies confirm the multiple and complex role neutrophils have in candidiasis. First, risk factors for invasive fungal infections are not the same in all neutropenic patients. Secondly, chronic systemic candidiasis initiated by neutropenia may persist in spite of normal neutrophil counts and adequate antifungal therapy. Third, some patients, particularly transplant recipients who have adequate or even normal neutrophil counts, may be at high risk for invasive mycoses."

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Keywords: immunity Candida albicans infections saprophytism immunocompetent infection cytokines mucosal drjefftop advanced

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