Immune response and host resistance of humans to dermatophyte infection

Cutaneous defenses against dermatophytes and yeasts

Predispositions to the superficial mycoses include warmth and moisture, natural or iatrogenic immunosuppression, and perhaps some degree of inherited susceptibility. Some of these infections elicit a greater inflammatory response than others, and the noninflammatory ones are generally more chronic. The immune system is involved in the defense against these infections, and cell-mediated immunity appears to be particularly important. The mechanisms involved in generating immunologic reactions in the skin are complex, with epidermal Langerhans cells, other dendritic cells, lymphocytes, microvascular endothelial cells, and the keratinocytes themselves all participating in one way or another. A variety of defects in the immunologic response to the superficial mycoses have been described. In some cases the defect may be preexistent, whereas in others the infection itself may interfere with protective cell-mediated immune responses against the organisms. A number of different mechanisms may underlie these immunologic defects and lead to the development of chronic superficial fungal infection in individual patients. Although the immunologic defects appear to be involved in the chronicity of certain types of cutaneous fungal infections, treatment of these defects remains experimental at the present time.

The dermatophytes

The etiologic agents of the dermatophytoses (ringworm) are classified in three anamorphic (asexual or imperfect) genera, Epidermophyton, Microsporum, and Trichophyton. Species capable of reproducing sexually belong in the teleomorphic genus, Arthroderma, of the Ascomycota. On the basis of primary habitat association, they may be grouped as geophilic (soil associated), zoophilic, and anthropophilic. Adaptation to growth on humans by most geophilic species resulted in diminished loss of sporulation, sexuality, and other soil-associated characteristics. The dermatophytes have the ability to invade keratinized tissue (skin, hair, and nails) but are usually restricted to the nonliving cornified layer of the epidermis because of their inability to penetrate viable tissue of an immunocompetent host. However, invasion does elicit a host response ranging from mild to severe. Acid proteinases, elastase, keratinases, and other proteinases reportedly act as virulence factors. The development of cell-mediated immunity correlated with delayed hypersensitivity and an inflammatory response is associated with clinical cure, whereas the lack of or a defective cell-mediated immunity predisposes the host to chronic or recurrent dermatophyte infection. Chronic dermatophytosis is mostly caused by Trichophyton rubrum, and there is some evidence that mannan produced by this fungus suppresses or diminishes the inflammatory response. Since dermatophytes cause a communicable disease, modes of transmission and control are discussed as well as a survey of recent trends in therapy. Collection of specimens, culture media, and tests for identification are also presented. Genetic studies have led to an understanding of incompatibility mechanisms, pleomorphism and variation, resistance to griseofulvin, and virulence. Molecular biology has contributed to our knowledge of the taxonomy and phylogenetic relationships of dermatophytes.

SDS-PAGE separation of dermatophyte antigens, and western immunoblotting in feline dermatophytosis

Water-soluble antigens liberated from the disrupted mycelium of nine dermatophytes (seven isolates of Microsporum canis, one each of Microsporum gypseum and Trichophyton mentagrophytes) were compared by analytical slab SDS-PAGE. No substantial differences were observed between the protein bands of the M. Canis isolates, but certain distinctive bands were apparent in the other two species examined. Western immunoblotting using M. canis-derived antigens separated by SDS-PAGE was used to investigate the humoral immune response in 79 cats with naturally-occurring dermatophytosis (72 with M. canis, six with M. gypseum and one with T. mentagrophytes) and this information was compared to results of immunoblots from 46 control (non-dermatophyte exposed) cats. Seven dominant bands (bands which occurred frequently and stained heavily) were identified in immunoblots from the dermatophyte-infected cats with apparent molecular weights varying between 39 and 120 kD. None of these bands were totally specific markers for dermatophytosis as a variable proportion of the control cats showed reactivity to all these proteins. However, most (73%) of the dermatophyte-infected cats showed reactivity to six or seven of the identified bands whereas most (80%) of the control cats showed reactivity to between zero and three of these bands (p < 0.005). Western immunoblotting could be used to select individual immunodominant antigens for further evaluation of protective (cell-mediated) immunity.

1992 AAD Award for Excellence in Education: The Integrated Basic and Clinical Science Conference Series at the University of Texas Southwestern Medical Center

This article was written to share information concerning the establishment and implementation of a successful, integrated basic and clinical science conference series for resident trainees in dermatology. We discuss the underlying philosophy and describe the concepts employed in, and the dynamics that have resulted from, this educational experiment. Finally, we include the schedule and references for the 1993-1994 academic year.

Suppression of immunity and inflammation by products produced by dermatophytes

When normal, previously uninfected hosts are exposed to dermatophytes under experimental occlusive conditions, infections develop and cell-mediated immunity is induced. Subsequent exposure to dermatophytes under the same conditions elicits an immune response that is capable of curing the infection, once occlusion is removed. Lymphocytes or monocytes involved in the immune response may produce cytokine growth factors that foster stratum corneum turnover and shedding of the fungus from the skin surface. Chronic dermatophyte infections develop when conditions of the local environment or virulence factors of the fungus outweigh the capabilities of cell-mediated immunity, or when a person does not develop cell-mediated immunity to fungal antigens. Even if immunity does develop, certain dermatophytes such as Trichophyton rubrum produce substances that diminish the immune response. One class of these substances, the mannans, can indirectly inhibit stratum corneum turnover. A nonresponsive host immune system or the suppression of the immune response by products produced by dermatophytes can prevent complete eradication of the fungus or predispose to reinfection.