Cryptococcus neoformans

In the past half-century, considerable progress has been made in understanding the ecology of Cryptococcus neoformans and the epidemiology of cryptococcal infections. Among the most significant observations were the division of C. neoformans strains into the varieties neoformans and gattii on the basis of phenotypic, biochemical, and genetic differences; the association of C. neoformans var. neoformans with avian excreta; and the association of C. neoformans var. gattii with eucalyptus trees. This chapter describes the distinctions between the two C. neoformans varieties and reviews the available information on the ecology of this fungal pathogen. C. neoformans strains have been divided into two varieties on the basis of phenotypic, serological, genetic, biochemical, and epidemiological criteria. Sexual reproduction of C. neoformans results in the production of haploid basidiospores. The question of which mode of reproduction is employed by C. neoformans in the environment may be important for a complete understanding of its ecology and pathogenesis. Recently, other C. neoformans characteristics have been suggested to contribute to virulence, including mannitol production and the elaboration of several enzymes that have the potential to degrade tissue, such as proteases and phospholipases.

The understanding of the epidemiology and pathogenic features for Cryptococcus neoformans infections can now be aided by the use of molecular biological techniques. In the last three decades, model systems for eukaryotic molecular biology have developed around certain fungi, such as Saccharomyces cerevisiae, Schizosaccharomyces pombe, Neurospora crassa, and Aspergillus nidulans. In the late 1980s, studies for the present foundation of molecular biology in C. neoformans began. The early development of molecular biological techniques for studying a pathogen generally involves questions of epidemiology in relationship to pathogenicity. In fact, little work has yet been performed on the genetics and molecular biology of mitochondrial DNA in C. neoformans. The use of polymerase chain reaction (PCR) technology combined with repetitive primers (arbitrary primed PCR) has allowed investigators to identify the DNA fingerprints of different fungal strains rapidly, specifically, and with a minimal number of DNA purifications. Recent work in molecular phylogeny of C. neoformans has focused on other genes and their ancestral lineages. Investigations into C. neoformans gene structures and the understanding of gene regulation in this yeast are in their infancy.

The low frequency of symptomatic cryptococcal infections, despite the presumably high frequency of exposure, suggests that physical barriers and nonspecific immunity provide adequate defenses to protect the host against infection. This chapter describes physical barriers such as skin, nasal passages, and eye, and humoral and cellular nonspecific defense mechanisms such as polymorphonuclear cells (PMNs), natural killer (NK) cells and macrophages that are believed to provide the first line of defense against cryptococcal infection. Antibody-mediated phagocytosis occurs through Fc receptors, which are constitutively expressed in macrophages, but it should be emphasized that opsonic capsule-binding antibody is not consistently present during human and animal infections. In human macrophages, complement-mediated binding is an energy-dependent process that requires three C3 complement receptors (CR1, CR3, and CR4) and actin but does not necessarily lead to phagocytosis. In summary, there is considerable evidence that macrophages from patients with AIDS are less effective against Cryptococcus neoformans than are those from HIV-seronegative individuals. The relative importance of oxidative and nonoxidative antimicrobial mechanisms of host effector cells in inhibiting and killing C. neoformans is uncertain. In vivo, it is likely that the oxidative and nonoxidative antimicrobial mechanisms cooperate to produce the antifungal effects described for host effector cells.

This chapter explores the host response to infection, with emphasis on integrating clinical observations with those from animal studies and in vitro observations. The chapter describes the host response to infection for lung, brain, and skin, since these are the organs for which most information is available. The inflammatory response to Cryptococcus neoformans infection has four unusual features. First, the inflammatory response can range in intensity from florid, intense granulomatous inflammation to the virtual absence of inflammatory cells. The variables responsible for differences in the magnitude and intensity of the inflammatory response include the immune status of the host and poorly defined characteristics of C. neoformans strains. Second, C. neoformans can be found in tissue as both an intracellular pathogen and an extracellular pathogen. Third, the type of inflammatory response is a function, in part, of the organ tissue where the infection occurs. Fourth, there is considerable variation in the type of effector cells found in inflammatory responses. A study compared the inflammatory response to pulmonary infection in CBA/J mice using two strains of C. neoformans, and strongly suggested that some of the differences observed in the human inflammatory response may be the result of unique C. neoformans strain differences in host inflammatory cell recruitment. In summary, the literature strongly supports the view that impairment of cell-mediated immunity is associated with enhanced susceptibility to C. neoformans infection.

The epidemiology of cryptococcal infection is an important undertaking because it provides clues as to the pathogenesis of infections and permits an assessment of infection risk in a given population. Knowledge of how and where Cryptococcus neoformans infects the human host is crucial for understanding the epidemiology and pathogenesis of infection. In a study the mean time that patients were on immunosuppressive therapy before the onset of cryptococcal infection was 48 months, and mortality was 30%. Among neoplastic disorders, disseminated cryptococcal infection has been historically associated with lymphoproliferative malignancies. Cryptococcosis is a relatively common infection among renal transplant patients despite the fact that renal transplantation has the lowest incidence of fungal infection among solid organ transplantation procedures. Insights on the epidemiology and pathogenesis of infection have resulted from comparative studies of cryptococcosis in various geographic regions. Researchers have interpreted the genetic diversity of C. neoformans strains as consistent with the theory of adaptive polymorphism, which predicts that the genetic diversity of a species is proportional to the diversity of ecological sites occupied.

In the first 100 years of clinical cases of Cryptococcus neoformans, the infection was described in most organs of the human and animal host. The clinical presentations of human cryptococcosis may be varied, and because of the variety of signs and symptoms, clinical diagnosis without laboratory studies can be difficult at times in both high- and low-risk patients. Clinical manifestations by organ system and demonstrates the breadth of clinical signs attributed to C. neoformans such as central nervous system (CNS), lung and skin. The relative differences in frequency of clinical symptoms and findings are compared in patients with or without human immunodeficiency virus (HIV) infection. A new factor in liver transplants has been the use of tacrolimus and its possible effect on cryptococcosis. It is known that all the immunophilins (cyclosporine, tacrolimus, and sirolimus) have anticryptococcal activity in vitro, but the effect of these medications on human cryptococcosis in this group remains uncertain. A review of pulmonary infections in immunocompromised patients prior to the AIDS epidemic described 34 of 41 patients with pulmonary cryptococcosis who had an underlying immunocompromised condition other than HIV. The most common radiographic findings (in order of frequency of occurrence) were alveolar or interstitial infiltrates, single or multiple-coin lesions, masses, cavitary lesions, and pleural effusions. A clinical principle emphasized in this study is that immunocompromised patients with pulmonary cryptococcosis frequently have disseminated infection and always require antifungal therapy. In animals, C. neoformans strains have been identified that appear to localize/produce infection directly in skin tissue.

The benefits of prevention are obvious given that cryptococcal infections require prolonged therapy, have high mortality and morbidity despite appropriate therapy, and are virtually incurable in patients with severely impaired immune function. The inadequacy of existing therapies is further exemplified by their inability to eradicate most cryptococcal infections in patients with severe immunological impairment, such as those with late-stage human immunodeficiency virus (HTV) infection. Chemoprophylaxis involves the administration of antifungal drugs to high-risk patients to reduce the probability of primary infection or recurrence of infection. The introduction of safe and effective oral azole drugs in the early 1990s made feasible the design of effective, low-toxicity regimens for preventing cryptococcal infections. The experience with Cryptococcus neoformans infections in patients with AIDS is different from that in other patient groups in that cryptococcal infection in patients with AIDS is seldom curable. It is important to consider that non-antifungal drugs and/or dietary practices may conceivably reduce the risk of cryptococcal infection in some patients. The availability of a sensitive and specific diagnostic test for C. neoformans infection in the form of the cryptococcal antigen detection assay suggests the feasibility of screening high-risk populations for early signs of cryptococcosis. Several studies have shown that screening of high-risk patients for cryptococcal antigen can identify some cases of cryptococcal infection. The conjugate vaccine is based on the premise that a vaccine that elicits a strong antibody response will protect against C. neoformans infection in susceptible hosts.