Pneumocystis jiroveci

Overview: Formerly known as Pneumocystis carinii, and classified as a protozoa, investigative tests upon this organism’s nucleic acid and biochemical composition has since placed it in the Kingdom of Fungi. Pneumocystis jiroveci is found in the lungs of mammals, where it resides in the alveoli without causing overt infection until the host's immune system becomes debilitated, causing the fungal infection Pneumocystis pneumonia or pneumocystosis. Thus, the organism causes disease in immunosuppressed individuals, including those with AIDS or undergoing chemotherapeutic treatments. P. jiroveci is specific to humans; it has not been shown to infect other animals, while other species of Pneumocystis that parasitize other animals have not been shown to infect humans.

Figure 1. This photomicrograph reveals Pneumocystis jirovecii fungi, which were present in this Giemsa-stained impression smear of rat lung tissue. Note the round cyst in the very middle of this image containing eight immature haploid neuclei, as well as numbers of freed trophozoites [1000 X].

Biology: P. jiroveci is a non-motile unicellular eukaryote, traditionally described in two different forms. The small trophic forms 1 μm (micrometres) to 5 μm in size are pleomorphic and ameboid in shape, generally represent the majority of organisms identified in the lungs of animals with P. jiroveci pneumonia, and frequently exist as large clusters. The larger cyst forms 5 μm to 8 μm in diameter are the most readily recognized forms of the organism, although representing only a small percentage of organisms in the lungs of animals with P. jiroveci pneumonia. The smooth spherical cyst form is enclosed by a thick cell wall and may contain up to eight intracystic bodies (sporozoites). These intracystic bodies may represent precursors to tropic forms which may develop upon release by the cyst, although the life-cycle has not been established and is poorly understood. 

Life Cycle: In the asexual phase, trophic forms replicate by mitosis. In the sexual phase, haploid trophic forms conjugate and produce a zygote or sporocyte (early cyst) (Figure 2). The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst). Spores exhibit different shapes (such as, spherical and elongated forms). It is postulated that elongation of the spores precedes release from the spore case. It is believed that the release occurs through a hole in the cell wall. After release, the empty spore case usually collapses, but retains some residual cytoplasm. A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist.

Figure 2. Pneumocystis jiroveci cysts.

Virulence and Pathogenicity: The attachment of P. jiroveci to lung epithelial cells, upon inhalation of the trophozoite (replicating) form, appears to be a critical step in establishing disease in the susceptible host. Attachment to lung epithelial cells (type I pneumocytes) may be mediated in part by a variety of host molecules, including fibronectin, vitronectin, laminin, and mannose receptors. Type I pneumocytes (squamous alveolar cells) are responsible for gas exchange in the alveoli and cover a majority of the alveolar surface area (>95%). Binding may be facilitated by P. jiroveci extracellular matrix receptors. In the susceptible host, attachment of P. jiroveci to alveolar epithelial cells is associated with proliferation of the organism and impaired lung cell replication, perhaps through secretion of proteolytic enzymes such as chymase or reactive oxygen species (Patel & Koziel, 2004).

Although specific virulence factors are poorly defined, P. jiroveci can induce disease in the susceptible host through a number of strategies. P. jiroveci induces selective alterations in expression and biophysical activity of lung surfactants, and may also have effects on non-epithelial lung cells such as alveolar macrophages and lung monocytes by down-regulation of important cell transcription factors. By coating itself in host-derived glycoprotein molecules from the lung microenvironment such as surfactant protein-A and a soluble form of the macrophage mannose receptor, P. jiroveci may delay recognition and destruction by the host immune system, to enable proliferation and survival (Patel & Koziel, 2004).

P. jiroveci is generally not associated with clinical disease in healthy individuals. An effective host response to P. jiroveci infection likely involves the concerted effects of cellular and molecular components of innate and adaptive immunity. The importance of adequate numbers of functional CD4⁺ T lymphocytes has been clearly established. Other cells which may contribute to an effective host response include CD8⁺ T lymphocytes, antibody producing B lymphocytes, natural killer cells and alveolar macrophages. Neutrophils can phagocytose P. jiroveci although bronchoalveolar lavage neutrophilia may contribute to organ dysfunction and disease pathogenesis. Both T helper type 1and type 2 cytokines are associated with effective clearance of P. jiroveci; these include tumor necrosis factor-α, interleukin-1 (IL-1), interleukin-1 (IL-6), interferon-γ, and granulocyte-macrophage colony-stimulating factor (GM-CSF) (Patel & Koziel, 2004).

P. jiroveci replicates extracellularly and impairs oxygen diffusion. Inflammation induced by this pathogen causes host cell damage, resulting in cell lysis and rupture. Damage to the lung basement membrane generates a characteristic foamy exudate and interstitial leukemic infiltration in the alveoli, thereby reducing alveolar capillary permeability. Granulomatous inflammation in Pneumocystis pneumonia is rare but may be seen in up to 5% of lung biopsies from human immunodeficiency virus (HIV)-infected patients.

Identification: Prior to the mid-1990s, the only way of detecting the presence of a Pneumocystis infection was through the visualization of the fungal microbe from stained respiratory tissues using Giemsa and Gomori-Grocott techniques, staining sputum, branchoalveolar fluid or lung tissue (Thomas et al., 2004). When staining, the firm-walled cyst form is most visible, but the flexible-walled trophic form outnumbers the cyst form 1:10 and the trophic form is difficult to visualize (Forbes et al., 2003). Therefore, staining is not the most convenient way of detecting infection. Polymerase chain reaction (PCR) is now used to identify the presence of the organism in medical specimens, which has increased the sensitivity of detecting the microbe.

Clinical Infections: In individuals with AIDS, highly active antiretroviral therapy (HAART) is the most common treatment, which also reduces the death and infection with opportunistic microbes. The infection occurs more frequently when the concentration of T helper cells is below 200 cells per cubic millimeter. In fact, a low T helper cell count can impair hydrogen peroxide and superoxide production by alveolar macrophages. In healthy individuals, P. jiroveci is cleared from the lungs via phagocytosis by alveolar macrophages. It is believed that immune cells recognize the pathogen by the mannose-containing residues. The symptoms of a pneumocystis pneumonia include progressive dyspnea, nonproductive cough, and low grade fever.

Treatment: Antifungal medications do not kill P. jiroveci. The antibiotic Trimethoprim-sulfamethoxazole is the primary treatment for all severity of pneumocystis pneumonia. Trimethoprim is an inhibitor of dihydrofolate reductase and sulfamethoxazole is an inhibitor of dihydropteroate synthase. The microbe stays within hosts for weeks after therapy has begun. It is of extreme importance that patients with AIDS take the preventative methods to avoid pneumocystis pneumonia because without treatment, over 85% of people with HIV develop the infection (Stringer et al., 2002).

References:

Patel, N., & Koziel, H. (2004). Pneumocystis jiroveci Pneumonia in Adult Patients with AIDS: Treatment Strategies and Emerging Challenges to Antimicrobial Therapy. Treatments in Respiratory Medicine, 3(6): 381-397.

Stringer, J.R., Beard, C.B., et al. (2002). A New Name (Pneumocystis jiroveci) for Pneumocystis from Humans. Emerging Infectious Diseases, 8: 891-896.