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The immune response to fungal infections

The immune response to fungal infections

Shmuel Shoham1 and Stuart M. Levitz

"With the increasing number of immune compromised patients, fungi have emerged as major causes of human disease. Risk factors for systemic candidiasis include presence of intravascular catheters, receipt of broad-spectrum antibiotics, injury to the gastrointestinal mucosa and neutropenia. Within a species, the fungal morphotype (e.g. yeast, pseudohyphae and hyphae of Candida albicans) may be an important determinant of the host response. Whereas yeasts and spores are often effectively phagocytosed, the larger size of hyphae precludes effective ingestion.

Differentiation of CD4+ T cells along a T-helper (Th) cell type 1 (Th1) or type 2 (Th2) pathway and development of specific Th responses, is an essential determinant of the host�s susceptibility or resistance to invasive fungal infections. Development of Th1 responses is influenced by the concerted action of cytokines, such as interferon (INF)-c, interleukin (IL)-6, tumour necrosis factor (TNF)-a, and IL-12, in the relative absence of Th2 cytokines, such as IL-4 and IL-10 (Romani, 2002).

Oropharyngeal candidiasis (OPC) is among the most common mycotic infections of immunocompromised patients. Development of infection depends upon both systemic and local determinants. Risk factors for oral candidiasis include extremes in age, diabetes mellitus, particularly when glycemic control is poor, nutritional deficiencies, use of broad spectrum antibiotics and immunosuppression (especially of cell-mediated immunity) (Klein et al, 1984; Guggenheimer et al, 2000). Local factors that promote infection include dentures, salivary abnormalities, treatment with inhaled steroids, and destruction of mucosal barriers with radiotherapy for head and neck cancers or cytotoxic chemotherapy. Human immunodeficiency virus (HIV) is one of the most important predisposing conditions worldwide. AIDS patients have a particularly high incidence of mucosal candidiasis, which is often recurrent and, when it involves the esophagus, can be disabling (Sangeorzan et al, 1994). Local defence mechanisms against mucosal infection include salivary proteins, such as lactoferrin, beta-defensins, histatins, lysozyme, transferrin, lactoperoxidase, mucins, and secretory immunoglobulin A. These impair adhesion and growth of Candida in the oropharyngeal cavity.  Development of OPC has been associated with a salivary Th2-type cytokine profile (Leigh et al, 1998).

Cell-mediated immunity plays the dominant role in prevention of candidiasis at the gastrointestinal surfaces. In AIDS, development of oropharyngeal and oesophageal candidiasis correlates with declining CD4+ lymphocyte counts. OPC is also associated

with T cell immunosuppression from corticosteroid therapy, organ transplantation, cancer chemotherapy and chronic mucocutaneous candidiasis (CMC). Candida species have emerged as an important cause of bloodstream and deep tissue infections. Risk factors for candidaemia include breakdown of mucosal barriers due to cytotoxic chemotherapy and surgical procedures, neutropenia, changes in the gut flora due to antibiotics, and invasive interventions that breach the skin, such as intravenous lines and drains (Wey et al, 1989). Common sites of dissemination include the bloodstream, kidney, liver, spleen, and endovascular structures. Quantitative and qualitative abnormalities of neutrophils and monocytes are associated with systemic candidiasis. Patients with lymphoma, leukaemia, chronic granulomatous disease, and recipients of intensive cancer chemotherapy with resultant neutropenia are at increased risk for disseminated infection. Similar to the situation with Aspergillus hyphae, the large size of Candida hyphae and pseudohyphae may preclude phagocytosis.

Achieving a balance between Th1 and Th2 cytokines may be important for optimal

antifungal protection while minimizing immune-mediated damage. In vivo models indicate that T regulatory cells attenuate Th1 antifungal responses, induce tolerance to the

fungus and participate in the development of long lasting protective immunity after yeast priming (Montagnoli et al, 2002; Romani, 2004).

Dendritic cells play an important role in linking innate with adaptive immunity. Dendritic cells that ingest the yeast form induce differentiation of CD4+ T cells toward a Th1 pathway. In contrast, hyphae induce Th2 responses (d�Ostiani et al, 2000). Neutrophils, macrophages and natural killer (NK) cells also modulate adaptive responses to the fungus. Neutrophils differentially induce Th1 and Th2 responses depending on whether the exposure is to yeast or hyphae.

The syndrome of chronic disseminated candidiasis (CDC, also known as hepatosplenic candidiasis) predominantly affects patients with haematological malignancies upon recovery from neutropenia. CDC is characterized by increased serum levels of IL-10 and local production of Th2-inducing cytokines by hepatocytes and by infected mononuclear cells (Roilides et al, 1998b; Letterio et al, 2001). Thus, although neutropenia is a major predisposing factor, the propensity for persistence of the fungus in infected tissues may be a consequence of cell-mediated immune dysregulation with suppression of Th1 and overexpression of Th2 responses."

Full Article:

Keywords: fungal infections immune response candidiasis antibiotics cytokines Th cells neutropenia drjefftop advanced

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