Host Defense Mechanisms and the Superficial Fungal Infections

Review on host-pathogen interaction in dermatophyte infections

Dermatophytosis is a common superficial fungal infection of the skin and its appendages caused by dermatophytes. Recent times have witnessed a dynamic evolution of dermatophytes driven by their ecology, reproduction, pathogenicity and host immune response, influenced by population migration and socioeconomic status. Dermatophytes establish infection following successful adherence of arthroconidia to the surface of keratinized tissues. The proteolytic enzymes released during adherence and invasion not only ascertain their survival but also allow the persistence of infection in the host. While the cutaneous immune surveillance mechanism, after antigen exposure and presentation, leads to activation of T lymphocytes and subsequent clonal expansion generating effector T cells that differentially polarize to a predominant Th17 response, the response fails to eliminate the pathogen despite the presence of high levels of IFN-γ. In chronic dermatophytosis, antigens are a constant source of stimulus promoting a dysregulated Th17 response causing inflammation. The host-derived iTreg response fails to counterbalance the inflammation and instead polarizes to Th17 lineage, aggravating the chronicity of the infection. Increasing antifungal resistance and recalcitrant dermatophytosis has impeded the overall clinical remission. Human genetic research has the potential to generate knowledge to explore new therapeutic targets. The review focuses on understanding specific virulence factors involved in pathogenesis and defining the role of dysregulated host immune response against chronic dermatophytic infections for future management strategies.

Unusual Inflammatory Tinea Infections: Majocchi's Granuloma and Deep/Systemic Dermatophytosis

PURPOSE OF REVIEW

Inflammatory tinea is an uncommon group of dermatophyte entities that predominantly cause fungal infection of the skin and hair. This review intends to present all of the available evidence regarding its epidemiology, etiopathogenesis, clinical features, and diagnostic methods as well as treatments recommended for various inflammatory tinea infections. This article provides a review of Majocchi's granuloma and dermatophytic or Hadida's disease.

RECENT FINDINGS

The new phylogenetic classification of dermatophytes includes nine genera, and those that affect humans are Trichophyton, Microsporum, Epidermophyton, and Nannizzia. Furthermore, molecular advancements have revealed impaired antifungal immune responses caused by inflammatory tinea, which are detailed in this article.

SUMMARY

The common denominator in these pathologies is the presence of impaired immune responses and, consequently, an impaired inflammatory response by the host. It is necessary to be familiar with these immunological characteristics in order to use the appropriate diagnostic methods and to provide adequate treatment.

Dermatophytosis and the immune response

Dermatophytes are eliminated from the skin by a cell-mediated immune reaction. Immunity is acquired by active infection. The inflammatory reaction that ensues may increase the proliferatory activity of keratinocytes, causing the fungus to be sloughed from the skin surface. Nonspecific mechanisms of defense prevent invasion into the dermis and bloodstream even in the absence of immunity. Serum inhibitory factor robs fungi of iron, an essential nutrient. The cell walls of the organism activate complement through the alternative pathway and inhibit fungal growth. Polymorphonuclear leukocytes adhere to opsonized and unopsonized hyphae to inhibit growth of the dermatophyte and perhaps damage or kill it. The fungas secrets keratinases and other enzymes that allow the dermatophyte to burrow deeper into the stratum corneum. Mannan from the cell wall of Trichophyton rubrum and a lipophilic toxin associated with it might inhibit cell-mediated immunity and keratinocyte proliferation.

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.

Majocchi's granuloma

Majocchi's granuloma (nodular granulomatous perifolliculitis) is a well recognized but uncommon infection of dermal and subcutaneous tissue by fungal organisms (dermatophytes) usually limited to the superficial epidermis. The organism usually associated with Majocchi's granuloma is Trichophyton rubrum; however, other dermatophytes including T. mentagrophytes (variety granulosum), T. epilans, T. violaceum, M. audouinii, M. gypseum, M. ferrugineum, and M. canis may be the causative agent. A review of 17 cases revealed not only the variety of possible organisms but also a marked variation from the usual hyphal forms. The morphologic variations including yeast forms, bizarre hyphae, mucinous coatings, and the Splendore-Hoeppeli phenomenon may be factors which allow the dermatophytes to persist and grow in an abnormal location. Also, there is evidence that Majocchi's granuloma may occur in two distinct groups of patients.