Some of these variations have functional consequences, representi

Some of these variations have functional consequences, representing distinct molecular mechanisms that facilitate Histoplasma this website pathogenesis. The realization of Histoplasma strain diversity highlights the importance of characterizing Histoplasma virulence

factors in the context of specific clinical strain isolates. Histoplasma capsulatum is the etiologic agent of histoplasmosis, a fungal disease that can affect both immunocompromised and immunocompetent individuals. Cases of histoplasmosis occur worldwide with endemic regions present in North America, Latin America, and parts of Africa. Within the Ohio and Mississippi River valley areas, more than 80% of individuals exhibit serological evidence of infection (Edwards et al., 1969). The site of initial infection is the lung and pulmonary disease presents with a range of non-specific respiratory symptoms, the severity of which is determined by the immune status of the host and the number of infectious conidia inhaled (Rippon, 1988). From the lung, Histoplasma disseminates throughout the body, most commonly infecting organs check details populated with reticuloendothelial cells (i.e., liver, spleen, lymph nodes, and bone marrow). Progressive disseminated histoplasmosis

is the most lethal form of the disease. Within the lung, Histoplasma cells infect host macrophages. Histoplasma survives within these innate immune cells suggesting the operation of specific virulence factors designed to avert or neutralize immune defenses. In immunocompetent individuals, immune control of Histoplasma infection requires that sensitized T cells activate macrophages to kill the fungal invader (Newman, 2001). If cell-mediated immunity is inadequate, such as in AIDS patients (McKinsey et al., 1997), organ transplant patients (Freifeld et al., 2005), or individuals receiving cytokine-blocking therapies, Sorafenib in vivo the risk of progressive disseminated disease increases (Lee et al., 2002; Wood et al., 2003). Even following activation of cell-mediated immunity, infections may not be completely cleared and latent Histoplasma cells may persist constituting a reservoir of organisms that can

seed reactivation disease upon diminished immune function (Wheat, 1992; Allen & Deepe, 2006). Histoplasma belongs to a group of ascomycetes termed the dimorphic fungal pathogens, which includes Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, Sporothrix schenkii, and Penicillium marneffei. These dimorphic fungi exhibit two distinct morphologies dependent upon environmental conditions: a filamentous mold within the soil, and a yeast or spherule (Coccidioides spp.) within the mammalian host. This thermal dimorphism is not only restricted to cellular morphology but also reflects the adoption of saprophytic (mold) or parasitic (yeast) growth. The mold form is avirulent, as preventing the switch of mycelia to yeast during growth at 37 °C renders the organism unable to cause disease (Medoff et al., 1986).

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