Molecular characterization of KEX1, a kexin-like protease in mouse Pneumocystis carinii

The Dual Benefit of Sulfasalazine on Pneumocystis Pneumonia-Related Immunopathogenesis and Antifungal Host Defense Does Not Require IL-4Rα-Dependent Macrophage Polarization

Pneumocystis is a respiratory fungal pathogen that is among the most frequent causes of life-threatening pneumonia (PcP) in immunocompromised hosts. Alveolar macrophages play an important role in host defense against Pneumocystis, and several studies have suggested that M2 polarized macrophages have anti-Pneumocystis effector activity. Our prior work found that the immunomodulatory drug sulfasalazine (SSZ) provides a dual benefit during PcP-related immune reconstitution inflammatory syndrome (IRIS) by concurrently suppressing immunopathogenesis while also accelerating macrophage-mediated fungal clearance. The benefits of SSZ were associated with heightened Th2 cytokine production and M2 macrophage polarization. Therefore, to determine whether SSZ improves the outcome of PcP through a mechanism that requires Th2-dependent M2 polarization, RAG2-/- mice lacking interleukin 4 receptor alpha chain (IL-4Rα) on macrophage lineage cells were generated. As expected, SSZ treatment dramatically reduced the severity of PcP-related immunopathogenesis and accelerated fungal clearance in immune-reconstituted RAG2-/- mice. Similarly, SSZ treatment was also highly effective in immune-reconstituted RAG2/IL-4Rα-/- and RAG2/gamma interferon receptor (IFN-γR)-/- mice, demonstrating that neither IL-4Rα-dependent M2 nor IFN-γR-dependent M1 macrophage polarization programs were required for the beneficial effects of SSZ. Despite the fact that macrophages from RAG2/IL-4Rα-/- mice could not respond to the Th2 cytokines IL-4 and IL-13, M2-biased alveolar macrophages were identified in the lungs following SSZ treatment. These data demonstrate that not only does SSZ enhance phagocytosis and fungal clearance in the absence of macrophage IL-4Rα signaling, but also that SSZ promotes M2 macrophage polarization in an IL-4Rα-independent manner. These findings could have implications for the treatment of PcP and other diseases in which M2 polarization is beneficial.

Pneumocystis Pneumonia: Immunity, Vaccines, and Treatments

For individuals who are immunocompromised, the opportunistic fungal pathogen Pneumocystis jirovecii is capable of causing life-threatening pneumonia as the causative agent of Pneumocystis pneumonia (PCP). PCP remains an acquired immunodeficiency disease (AIDS)-defining illness in the era of antiretroviral therapy. In addition, a rise in non-human immunodeficiency virus (HIV)-associated PCP has been observed due to increased usage of immunosuppressive and immunomodulating therapies. With the persistence of HIV-related PCP cases and associated morbidity and mortality, as well as difficult to diagnose non-HIV-related PCP cases, an improvement over current treatment and prevention standards is warranted. Current therapeutic strategies have primarily focused on the administration of trimethoprim-sulfamethoxazole, which is effective at disease prevention. However, current treatments are inadequate for treatment of PCP and prevention of PCP-related death, as evidenced by consistently high mortality rates for those hospitalized with PCP. There are no vaccines in clinical trials for the prevention of PCP, and significant obstacles exist that have slowed development, including host range specificity, and the inability to culture Pneumocystis spp. in vitro. In this review, we overview the immune response to Pneumocystis spp., and discuss current progress on novel vaccines and therapies currently in the preclinical and clinical pipeline.

Combination Immunotherapy with Passive Antibody and Sulfasalazine Accelerates Fungal Clearance and Promotes the Resolution of Pneumocystis-Associated Immunopathogenesis

The pulmonary immune response protects healthy individuals against Pneumocystis pneumonia (PcP). However, the immune response also drives immunopathogenesis in patients who develop severe PcP, and it is generally accepted that optimal treatment requires combination strategies that promote fungal killing and also provide effective immunomodulation. The anti-inflammatory drug sulfasalazine programs macrophages for enhanced Pneumocystis phagocytosis and also suppresses PcP-related immunopathogenesis. Anti-Pneumocystis antibody opsonizes Pneumocystis organisms for greater phagocytosis and may also mask antigens that drive immunopathogenesis. Thus, we hypothesized that combining antibody and sulfasalazine would have the dual benefit of enhancing fungal clearance while dampening immunopathogenesis and allow the rescue of severe PcP. To model a clinically relevant treatment scenario in mice, therapeutic interventions were withheld until clear symptoms of pneumonia were evident. When administered individually, both passive antibody and sulfasalazine improved pulmonary function and enhanced Pneumocystis clearance to similar degrees. However, combination treatment with antibody and sulfasalazine produced a more rapid improvement, with recovery of body weight, a dramatic improvement in pulmonary function, reduced lung inflammation, and the rapid clearance of the Pneumocystis organisms. Accelerated fungal clearance in the combination treatment group was associated with a significant increase in macrophage phagocytosis of Pneumocystis Both passive antibody and sulfasalazine resulted in the suppression of Th1 cytokines and a marked increase in lung macrophages displaying an alternatively activated phenotype, which were enhanced by combination treatment. Our data support the concept that passive antibody and sulfasalazine could be an effective and specific adjunctive therapy for PcP, with the potential to accelerate fungal clearance while attenuating PcP-associated immunopathogenesis.

Immunization with Pneumocystis recombinant KEX1 induces robust and durable humoral responses in immunocompromised non-human primates

Infection with the opportunistic fungal pathogen, Pneumocystis jirovecii causes life-threatening pneumonia in immunocompromised individuals. In addition to HIV-1 infected patients, individuals at risk of Pneumocystis infection include those receiving immunosuppressive therapies due to transplantation, cancer or autoimmune disease. Antibiotic treatment is not always successful, and it does not prevent obstructive lung disease after clearance of the pathogen. Therefore, it is essential to develop therapeutic alternatives that are more effective against PCP. We reported that Pneumocystis recombinant protein KEX1 induces protective immunity against the development of PCP in a non-human primate model of HIV-induced immunosuppression. In this study, we tested the immunogenicity KEX1 immunization of healthy rhesus macaques and the durability of these responses during drug-induced immunosuppression using tacrolimus (FK506) and methylprednisolone. We observed that vaccination with KEX1 prior to the start of the immunosuppressive regimen generated a robust and long-lasting antibody response that was maintained throughout the immunosuppressive treatment. Furthermore, boosting with KEX1 during immunosuppression induced recall of memory responses against recombinant KEX1. The durability of the anti-KEX1 response and the ability to induce a recall response during immunosuppressive therapy provide a proof-of-concept data supporting further investigation of the KEX1 as a prophylactic vaccine to prevent PCP in drug-induced immunosuppression. This approach provides fundamental knowledge for the elaboration of therapeutic and prophylactic alternatives for PCP in patients undergoing severe immunosuppressive therapy.

Intrinsic Programming of Alveolar Macrophages for Protective Antifungal Innate Immunity Against Pneumocystis Infection

Invasive fungal infections, including Pneumocystis Pneumonia (PcP), remain frequent life-threatening conditions of patients with adaptive immune defects. While innate immunity helps control pathogen growth early during infection, it is typically not sufficient for complete protection against Pneumocystis and other human fungal pathogens. Alveolar macrophages (AM) possess pattern recognition molecules capable of recognizing antigenic and structural determinants of Pneumocystis. However, this pathogen effectively evades innate immunity to infect both immunocompetent and immunosuppressed hosts, albeit with differing outcomes. During our studies of mouse models of PcP, the FVB/N strain was identified as unique because of its ability to mount a protective innate immune response against Pneumocystis infection. In contrast to other immunocompetent strains, which become transiently infected prior to the onset of adaptive immunity, FVB/N mice rapidly eradicated Pneumocystis before an adaptive immune response was triggered. Furthermore, FVB/N mice remained highly resistant to infection even in the absence of functional T cells. The effector mechanism of innate protection required the action of functional alveolar macrophages, and the adoptive transfer of resistant FVB/N AMs, but not susceptible CB.17 AMs, conferred protection to immunodeficient mice. Macrophage IFNγ receptor signaling was not required for innate resistance, and FVB/N macrophages were found to display markers of alternative activation. IFNγ reprogrammed resistant FVB/N macrophages to a permissive M1 biased phenotype through a mechanism that required direct activation of the macrophage IFNγR. These results demonstrate that appropriately programmed macrophages provide protective innate immunity against this opportunistic fungal pathogen, and suggest that modulating macrophage function may represent a feasible therapeutic strategy to enhance antifungal host defense. The identification of resistant and susceptible macrophages provides a novel platform to study not only the mechanisms of macrophage-mediated antifungal defense, but also the mechanisms by which Pneumocystis evades innate immunity.

Immunization with Pneumocystis Cross-Reactive Antigen 1 (Pca1) Protects Mice against Pneumocystis Pneumonia and Generates Antibody to Pneumocystis jirovecii

Pneumocystis pneumonia (PcP) is a life-threatening infection that affects immunocompromised individuals. Nearly half of all PcP cases occur in those prescribed effective chemoprophylaxis, suggesting that additional preventive methods are needed. To this end, we have identified a unique mouse Pneumocystis surface protein, designated Pneumocystis cross-reactive antigen 1 (Pca1), as a potential vaccine candidate. Mice were immunized with a recombinant fusion protein containing Pca1. Subsequently, CD4⁺ T cells were depleted, and the mice were exposed to Pneumocystis murina Pca1 immunization completely protected nearly all mice, similar to immunization with whole Pneumocystis organisms. In contrast, all immunized negative-control mice developed PcP. Unexpectedly, Pca1 immunization generated cross-reactive antibody that recognized Pneumocystis jirovecii and Pneumocystis carinii Potential orthologs of Pca1 have been identified in P. jirovecii Such cross-reactivity is rare, and our findings suggest that Pca1 is a conserved antigen and potential vaccine target. The evaluation of Pca1-elicited antibodies in the prevention of PcP in humans deserves further investigation.

Overcoming Hurdles to Development of a Vaccine against Pneumocystis jirovecii

Development of Pneumocystis pneumonia (PCP) is a common problem among immunosuppressed individuals. There are windows of opportunity in which vaccination would be beneficial, but to date, no vaccines have made it to clinical trials. Significant hurdles to vaccine development include host range specificity, making it difficult to translate from animal models to humans. Discovery of cross-reactive epitopes is critical to moving vaccine candidates from preclinical animal studies to clinical trials.

Genome analysis of three Pneumocystis species reveals adaptation mechanisms to life exclusively in mammalian hosts

Pneumocystis jirovecii is a major cause of life-threatening pneumonia in immunosuppressed patients including transplant recipients and those with HIV/AIDS, yet surprisingly little is known about the biology of this fungal pathogen. Here we report near complete genome assemblies for three Pneumocystis species that infect humans, rats and mice. Pneumocystis genomes are highly compact relative to other fungi, with substantial reductions of ribosomal RNA genes, transporters, transcription factors and many metabolic pathways, but contain expansions of surface proteins, especially a unique and complex surface glycoprotein superfamily, as well as proteases and RNA processing proteins. Unexpectedly, the key fungal cell wall components chitin and outer chain N-mannans are absent, based on genome content and experimental validation. Our findings suggest that Pneumocystis has developed unique mechanisms of adaptation to life exclusively in mammalian hosts, including dependence on the lungs for gas and nutrients and highly efficient strategies to escape both host innate and acquired immune defenses.

Kexin-like endoprotease KexB is required for N-glycan processing, morphogenesis and virulence in Aspergillus fumigatus

Kexin-like proteins belong to the subtilisin-like family of the proteinases that cleave secretory proproteins to their active forms. Several fungal kexin-like proteins have been investigated. The mutants lacking of kexin-like protein display strong phenotypes such as cell wall defect, abnormal polarity, and, in case of Candida albicans, diminished virulence. However, only several proteins have been confirmed as the substrates of kexin-like proteases in these fungal species. It still remains unclear how kexin-like proteins contribute to the morphogenesis in these fungal species. In this study, a kexB-null mutant of the human opportunistic fungal pathogen Aspergillus fumigatus was constructed and analyzed. The ΔkexB mutant showed retarded growth, temperature-sensitive cell wall defect, reduced conidia formation, and abnormal polarity. Biochemical analyses revealed that deletion of the kexB gene resulted in impaired N-glycan processing, activation of the MpkA-dependent cell wall integrity signaling pathway, and ER-stress. Results from in vivo assays demonstrated that the mutant exhibited an attenuated virulence in immunecompromised mice. Based on our results, the kexin-like endoprotease KexB was involved in the N-glycan processing, which provides a novel insight to understand how kexin-like protein affects the cell-wall modifying enzymes and therefore morphogenesis in fungi.

Pneumocystis

Since its initial misidentification as a trypanosome some 100 years ago, Pneumocystis has remained recalcitrant to study. Although we have learned much, we still do not have definitive answers to such basic questions as, where is the reservoir of infection, how does Pneumocystis reproduce, what is the mechanism of infection, and are there true species of Pneumocystis? The goal of this review is to provide the reader the most up to date information available about the biology of Pneumocystis and the disease it produces.

Colonization by Pneumocystis jirovecii and its role in disease

Although the incidence of Pneumocystis pneumonia (PCP) has decreased since the introduction of combination antiretroviral therapy, it remains an important cause of disease in both HIV-infected and non-HIV-infected immunosuppressed populations. The epidemiology of PCP has shifted over the course of the HIV epidemic both from changes in HIV and PCP treatment and prevention and from changes in critical care medicine. Although less common in non-HIV-infected immunosuppressed patients, PCP is now more frequently seen due to the increasing numbers of organ transplants and development of novel immunotherapies. New diagnostic and treatment modalities are under investigation. The immune response is critical in preventing this disease but also results in lung damage, and future work may offer potential areas for vaccine development or immunomodulatory therapy. Colonization with Pneumocystis is an area of increasing clinical and research interest and may be important in development of lung diseases such as chronic obstructive pulmonary disease. In this review, we discuss current clinical and research topics in the study of Pneumocystis and highlight areas for future research.

Serologic responses to pneumocystis proteins in HIV patients with and without Pneumocystis jirovecii pneumonia

BACKGROUND

Immune responses to Pneumocystis jirovecii are not well understood in HIV infection, but antibody responses to proteins may be useful as a marker of Pneumocystis risk or presence of Pneumocystis pneumonia (PcP).

DESIGN

Retrospective analysis of a prospective cohort.

METHODS

Enzyme-linked immunosorbent assays of antibodies to recombinant Pneumocystis proteins of major surface glycoprotein fragments (MsgC1, C3, C8, and C9) and of antibody titers to recombinant kexin protein (KEX1) were performed on 3 sequential serum samples up to 18 months before and 3 samples after first AIDS-defining illness from Multicenter AIDS Cohort Study participants and compared between those who had PcP or a non-PcP AIDS-defining illness.

RESULTS

Fifty-four participants had PcP and 47 had a non-PcP AIDS-defining illness. IgG levels to MsgC fragments were similar between groups before first AIDS-defining illness, but the PcP group had higher levels of IgG to MsgC9 (median units/mL 50.2 vs. 22.2, P = 0.047) post-illness. Participants with PcP were more likely to have an increase in MsgC3 [odds ratio (OR): 3.9, P = 0.02], MsgC8 (OR: 5.5, P = 0.001), and MsgC9 (OR: 4.0, P = 0.007). The PcP group was more likely to have low KEX1 IgG before development of PcP (OR: 3.6, P = 0.048) independent of CD4 cell count and to have an increase in high IgG titers to KEX1 after PcP.

CONCLUSIONS

HIV-infected individuals develop immune responses to both Msg and kexin proteins after PcP. Low KEX1 IgG titers may be a novel marker of future PcP risk before CD4 cell count has declined below 200 cells per microliter.

Innate and adaptive antifungal immune responses: partners on an equal footing

Adaptive immunity has long been regarded as the major player in protection against most fungal infections. Mounting evidence suggest however, that both innate and adaptive responses intricately collaborate to produce effective antifungal protection. Dendritic cells (DCs) play an important role in initiating and orchestrating antifungal immunity; neutrophils, macrophages and other phagocytes also participate in recognising and eliminating fungal pathogens. Adaptive immunity provides a wide range of effector and regulatory responses against fungal infections. Th1 responses protect against most forms of mycoses but they associate with significant inflammation and limited pathogen persistence. By contrast, Th2 responses enhance persistence of and tolerance to fungal infections thus permitting the generation of long-lasting immunological memory. Although the role of Th17 cytokines in fungal immunity is not fully understood, they can enhance proinflammatory or anti-inflammatory responses or play a regulatory role in fungal immunity all depending on the pathogen, site/phase of infection and host immunostatus. T regulatory cells balance the activities of various Th cell subsets thereby permitting inflammation and protection on the one hand and allowing for tolerance and memory on the other. Here, recent developments in fungal immunity research are reviewed as means of tracing the emergence of a refined paradigm where innate and adaptive responses are viewed in the same light.

Anti-CD3 antibody decreases inflammation and improves outcome in a murine model of Pneumocystis pneumonia

The T cell-mediated immune response elicited by Pneumocystis plays a key role in pulmonary damage and dysfunction during Pneumocystis carinii pneumonia (PcP). Mice depleted of CD4(+) and CD8(+) T cells prior to infection are markedly protected from PcP-related respiratory deficit and death, despite progressive lung infection. However, the therapeutic effectiveness of Ab-mediated disruption of T cell function in mice already displaying clinical symptoms of disease has not been determined. Therefore, a murine model of PcP-related immune reconstitution inflammatory syndrome was used to assess whether Ab to the pan-T cell molecule CD3 is effective for reducing the severity of PcP when administered after the onset of disease. Mice that received anti-CD3 Ab exhibited a rapid and dramatic halt in the PcP-associated pulmonary function decline within 1 week after treatment, and a striking enhancement of survival rate compared with mice receiving the control Ab. Physiologic improvement in anti-CD3 treated mice was associated with a significant reduction in the number of CD4(+) and CD8(+) T cells recovered in lung lavage fluid. This effectiveness of anti-CD3 was noted whether the mice also received antibiotic therapy with trimethoprim-sulfamethoxazole. These data suggest that monoclonal Ab-mediated disruption of T cell function may represent a specific and effective adjunctive therapy to rapidly reverse the ongoing pathologic immune response occurring during active PcP. Thus, the anti-human CD3 monoclonal Ab OKT3, which is already in clinical use, has the potential to be developed as an adjunctive therapy for PcP.

Characterization of a novel ADAM protease expressed by Pneumocystis carinii

Pneumocystis species are opportunistic fungal pathogens that cause severe pneumonia in immunocompromised hosts. Recent evidence has suggested that unidentified proteases are involved in Pneumocystis life cycle regulation. Proteolytically active ADAM (named for "a disintegrin and metalloprotease") family molecules have been identified in some fungal organisms, such as Aspergillus fumigatus and Schizosaccharomyces pombe, and some have been shown to participate in life cycle regulation. Accordingly, we sought to characterize ADAM-like molecules in the fungal opportunistic pathogen, Pneumocystis carinii (PcADAM). After an in silico search of the P. carinii genomic sequencing project identified a 329-bp partial sequence with homology to known ADAM proteins, the full-length PcADAM sequence was obtained by PCR extension cloning, yielding a final coding sequence of 1,650 bp. Sequence analysis detected the presence of a typical ADAM catalytic active site (HEXXHXXGXXHD). Expression of PcADAM over the Pneumocystis life cycle was analyzed by Northern blot. Southern and contour-clamped homogenous electronic field blot analysis demonstrated its presence in the P. carinii genome. Expression of PcADAM was observed to be increased in Pneumocystis cysts compared to trophic forms. The full-length gene was subsequently cloned and heterologously expressed in Saccharomyces cerevisiae. Purified PcADAMp protein was proteolytically active in casein zymography, requiring divalent zinc. Furthermore, native PcADAMp extracted directly from freshly isolated Pneumocystis organisms also exhibited protease activity. This is the first report of protease activity attributable to a specific, characterized protein in the clinically important opportunistic fungal pathogen Pneumocystis.

Pneumocystis colonization in immunocompetent and simian immunodeficiency virus-infected cynomolgus macaques

Pneumocystis (Pc) colonization is common among human immunodeficiency virus (HIV)-infected subjects, although the clinical consequences of Pc carriage are not fully understood. We examined the frequency of asymptomatic carriage in healthy and simian immunodeficiency virus (SIV)-infected cynomolgus macaques by use of polymerase chain reaction (PCR) and assessment of changes in the serologic response to a recombinant fragment of the Pc protein kexin (KEX1). Anti-KEX1 antibodies were detected in 95% of healthy monkeys. To create a model of natural transmission of Pc, SIV-infected monkeys were cohoused with macaques coinfected with SIV and Pc. Pc colonization occurred when the CD4(+) T cell count decreased to <500 cells/microL, despite anti-Pc prophylaxis with trimethoprim-sulfamethoxazole. Increases in anti-KEX1 antibody titers preceded detection of Pc DNA in bronchoalveolar lavage (BAL) fluid samples by use of PCR. These results demonstrate the usefulness of recombinant KEX1 in serologic studies of Pc colonization and will improve the understanding of Pc transmission and clinical consequences of Pc colonization in HIV-infected patients.

Parenchymal cell TNF receptors contribute to inflammatory cell recruitment and respiratory failure in Pneumocystis carinii-induced pneumonia

The opportunistic organism Pneumocystis carinii (Pc) produces a life-threatening pneumonia (PcP) in patients with low CD4(+) T cell counts. Animal models of HIV-AIDS-related PcP indicate that development of severe disease is dependent on the presence of CD8(+) T cells and the TNF receptors (TNFR) TNFRsf1a and TNFRsf1b. To distinguish roles of parenchymal and hematopoietic cell TNF signaling in PcP-related lung injury, murine bone marrow transplant chimeras of wild-type, C57BL6/J, and TNFRsf1a/1b double-null origin were generated, CD4(+) T cell depleted, and inoculated with Pc. As expected, C57 --> C57 chimeras (donor marrow --> recipient) developed significant disease as assessed by weight loss, impaired pulmonary function (lung resistance and dynamic lung compliance), and inflammatory cell infiltration. In contrast, TNFRsf1a/1b(-/-) --> TNFRsf1a/1b(-/-) mice were relatively mildly affected despite carrying the greatest organism burden. Mice solely lacking parenchymal TNFRs (C57 --> TNFRsf1a/1b(-/-)) had milder disease than did C57 --> C57 mice. Both groups of mice with TNFR-deficient parenchymal cells had low bronchoalveolar lavage fluid total cell counts and fewer lavageable CD8(+) T cells than did C57 --> C57 mice, suggesting that parenchymal TNFR signaling contributes to PcP-related immunopathology through the recruitment of damaging immune cells. Interestingly, mice with wild-type parenchymal cells but TNFRsf1a/1b(-/-) hematopoietic cells (TNFRsf1a/1b(-/-) --> C57) displayed exacerbated disease characterized by increased MCP-1 and KC production in the lung and increased macrophage and lymphocyte numbers in the lavage, indicating a dysregulated immune response. This study supports a key role of parenchymal cell TNFRs in lung injury induced by Pc and a potential protective effect of receptors on radiosensitive, bone marrow-derived cells.

Pneumocystis murina infection and cigarette smoke exposure interact to cause increased organism burden, development of airspace enlargement, and pulmonary inflammation in mice

Chronic obstructive pulmonary disease (COPD) is characterized by the presence of airflow obstruction and lung destruction with airspace enlargement. In addition to cigarette smoking, respiratory pathogens play a role in pathogenesis, but specific organisms are not always identified. Recent reports demonstrate associations between the detection of Pneumocystis jirovecii DNA in lung specimens or respiratory secretions and the presence of emphysema in COPD patients. Additionally, human immunodeficiency virus-infected individuals who smoke cigarettes develop early emphysema, but a role for P. jirovecii in pathogenesis remains speculative. We developed a new experimental model using immunocompetent mice to test the interaction of cigarette smoke exposure and environmentally acquired Pneumocystis murina infection in vivo. We hypothesized that cigarette smoke and P. murina would interact to cause increases in total lung capacity, airspace enlargement, and pulmonary inflammation. We found that exposure to cigarette smoke significantly increases the lung organism burden of P. murina. Pulmonary infection with P. murina, combined with cigarette smoke exposure, results in changes in pulmonary function and airspace enlargement characteristic of pulmonary emphysema. P. murina and cigarette smoke exposure interact to cause increased lung inflammatory cell accumulation. These findings establish a novel animal model system to explore the role of Pneumocystis species in the pathogenesis of COPD.

Pneumocystis: a novel pathogen in chronic obstructive pulmonary disease?

Chronic obstructive pulmonary disease (COPD) results in significant morbidity and mortality. Smoking has long been recognized as the primary risk factor for development of COPD, but factors determining the severity or pattern of disease in smokers are largely unknown. Recent interest has focused on the potential role of infectious agents and the associated host response in accelerating progression of airway obstruction or in perpetuating its progression following discontinuation of tobacco exposure. Pneumocystis jirovecii is a fungal pathogen that causes pneumonia in immunocompromised individuals. Recent evidence has linked this organism with COPD. Using sensitive molecular techniques, low levels of Pneumocystis have been detected in the respiratory tract of certain individuals and termed colonization. Several findings support the theory that colonization with Pneumocystis is involved in the "vicious circle" hypothesis of COPD in which colonization with organisms perpetuates an inflammatory and lung remodeling response. Pneumocystis colonization is more prevalent in smokers and in those with severe COPD. The presence of Pneumocystis in the lungs, even at low levels, produces inflammatory changes similar to those seen in COPD, with increases in numbers of neutrophils and CD8(+) lymphocytes. HIV-infected subjects who have had PCP develop permanent airway obstruction, and HIV-infected patients have a high prevalence of both emphysema and Pneumocystis colonization. In addition, a non-human primate model of colonization shows development of airway obstruction and radiographic emphysema. Additional studies are needed to confirm the role of Pneumocystis in the pathogenesis of COPD, given that this agent might be a treatable co-factor in disease progression.

Antigenic variation in pneumocystis

Pneumocystis is a genus containing many species of non-culturable fungi, each of which infects a different mammalian host. Pneumonia caused by Pneumocystis is a problem in immunodeficient humans, but not in normal humans. Nevertheless, it appears that Pneumocystis organisms cannot survive and proliferate outside of their mammalian hosts, suggesting that Pneumocystis parasitizes immunocompetent mammals. Residence in immunocompetent hosts may rely on camouflage perpetrated by antigenic variation. In P. carinii, which is found in rats, there exist three families of genes that appear to be designed to create antigenic variation. One gene family, which encodes the major surface glycoprotein (MSG), contains nearly 100 members. Expression of the MSG family is controlled by restricting transcription to the one gene that is linked to a unique expression site. Changes in the sequence of the MSG gene linked to the expression site occur and appear to be caused by recombination with MSG genes not at the expression site. Preliminary evidence suggests that gene conversion is the predominant recombination mechanism.

Enhanced defense against Pneumocystis carinii mediated by a novel dectin-1 receptor Fc fusion protein

Pneumocystis carinii (PC) pneumonia is a leading opportunistic infection found among HIV-infected individuals worldwide. Although CD4(+) T cell deficiency clearly correlates with susceptibility to PC pneumonia, murine models of disease indicate that PC-directed Abs may prevent infection and/or inhibit growth of existing PC within the lungs. Recognition of PC by alveolar macrophages involves the beta-glucan receptor Dectin-1 and macrophage effector function against PC is enhanced by Abs derived from PC-vaccinated hosts. We developed a fusion protein consisting of the extracellular domain of Dectin-1 linked to the Fc portion of murine IgG1, which we hypothesized would enhance host recognition and opsonic phagocytosis of PC. The recombinant protein, Dectin-Fc, is dimeric and the Ag recognition site identifies beta-1,3 glucan linkages specifically and with high affinity (K(D) = 2.03 x 10(-7) M). Dectin-Fc enhances RAW264.7 macrophage recognition of the beta-glucan containing particulate zymosan in an FcgammaRII- and FcgammaRIII-dependent manner and preopsonization of PC organisms with Dectin-Fc increased alveolar and peritoneal macrophage-dependent killing of PC. SCID mice treated with a replication incompetent adenoviral vector expressing Dectin-Fc had attenuated growth of PC within the lungs, overall decreased PC lung burden, and diminished correlates of PC-related lung damage relative to SCID mice receiving a control vector. These findings demonstrate that targeting PC beta-glucan with Dectin-Fc enhances host recognition and clearance of PC in the absence of B and T cells, and suggest that FcgammaR-based targeting of PC, via cell wall carbohydrate recognition, may promote resistance against PC pneumonia in the immunodeficient host.

Current insights into the biology and pathogenesis of Pneumocystis pneumonia

The fungal infection Pneumocystis pneumonia is the most prevalent opportunistic infection in patients with AIDS. Although the analysis of this opportunistic fungal pathogen has been hindered by the inability to isolate it in pure culture, the use of molecular techniques and genomic analysis have brought insights into its complex cell biology. Analysis of the intricate relationship between Pneumocystis and the host lung during infection has revealed that the attachment of Pneumocystis to the alveolar epithelium promotes the transition of the organism from the trophic to the cyst form. It also revealed that Pneumocystis infection elicits the production of inflammatory mediators, culminating in lung injury and impaired gas exchange. Here we discuss these and other recent findings relating to the biology and pathogenesis of this intractable fungus.

Pneumocystis murina MSG gene family and the structure of the locus associated with its transcription

Analysis of the Pneumocystis murina MSG gene family and expression-site locus showed that, as in Pneumocystis carinii, P. murina MSG genes are arranged in head-to-tail tandem arrays located on multiple chromosomes, and that a variety of MSG genes can reside at the unique P. murina expression site. Located between the P. murina expression site and attached MSG gene is a block of 132 basepairs that is also present at the beginning of MSG genes that are not at the expression site. The center of this sequence block resembles the 28 basepair CRJE of P. carinii, but the block of conserved sequence in P. murina is nearly five times longer than in P. carinii, and much shorter than in P. wakefieldiae. These data indicate that the P. murina expression-site locus has a distinct structure.

Sensitized CD8+ T cells fail to control organism burden but accelerate the onset of lung injury during Pneumocystis carinii pneumonia

While CD8+ cells have been shown to contribute to lung injury during Pneumocystis carinii pneumonia (PCP), there are conflicting reports concerning the ability of CD8+ cells to kill P. carinii. To address these two issues, we studied the effect of the presence of CD8+ cells in two mouse models of PCP. In the reconstituted SCID mouse model, depletion of CD8+ cells in addition to CD4+ cells after reconstitution did not result in increased numbers of P. carinii cysts compared to the numbers of cysts in mice with only CD4+ cells depleted. This result was observed regardless of whether the mice were reconstituted with naïve or P. carinii-sensitized lymphocytes. In contrast, reconstitution with sensitized lymphocytes resulted in more rapid onset of lung injury that was dependent on the presence of CD8+ cells. The course of organism replication over a 6-week period was also examined in the CD4+-T-cell-depleted and CD4+- and CD8+-T-cell-depleted mouse model of PCP. Again, the organism burdens were identical at all times regardless of whether CD8+ cells were present. Thus, in the absence of CD4+ T cells, CD8+ T cells are a key contributor to the inflammatory lung injury associated with PCP. However, we were unable to demonstrate an in vivo effect of these cells on the course of P. carinii infection.

Contribution of T cell subsets to the pathophysiology of Pneumocystis-related immunorestitution disease

Immune-mediated lung injury is an important component of Pneumocystis pneumonia (PcP)-related immunorestitution disease (IRD). However, the individual contribution of CD4(+) and CD8(+) T cells to the pathophysiology of IRD remains undetermined. Therefore, IRD was modeled in severe combined immunodeficient mice, and specific T cell depletion was used to determine how T cell subsets interact to affect the nature and severity of disease. CD4(+) cells were more abundant than CD8(+) cells during the acute stage of IRD that coincided with impaired pulmonary physiology and organism clearance. Conversely, CD8(+) cells were more abundant during the resolution phase following P. carinii clearance. Depletion of CD4(+) T cells protected mice from the acute pathophysiology of IRD. However, these mice could not clear the infection and developed severe PcP at later time points when a pathological CD8(+) T cell response was observed. In contrast, mice depleted of CD8(+) T cells efficiently cleared the infection but developed more severe disease, an increased frequency of IFN-gamma-producing CD4(+) cells, and a prolonged CD4(+) T cell response than mice with both CD4(+) and CD8(+) cells. These data suggest that CD4(+) T cells mediate the acute respiratory disease associated with IRD. In contrast, CD8(+) T cells contributed to neither lung injury nor organism clearance when CD4(+) cells were present, but instead served to modulate CD4 function. In the absence of CD4(+) cells, CD8(+) T cells produced a nonprotective, pathological immune response. These data suggest that the interplay of CD4(+) and CD8(+) T cells affects the ultimate outcome of PcP-related IRD.

Active immunization against Pneumocystis carinii with a recombinant P. carinii antigen

Mice immunized with recombinant mouse Pneumocystis carinii antigen A12-thioredoxin fusion protein developed an antibody response that recognized P. carinii antigens, as determined by Western blotting and immunofluorescence analysis. Compared to mice immunized with thioredoxin alone, mice immunized with A12-thioredoxin had significantly reduced lung P. carinii burdens after CD4+ T-cell depletion and challenge with P. carinii.

Complement and Fc function are required for optimal antibody prophylaxis against Pneumocystis carinii pneumonia

Pneumocystis carinii is an opportunistic fungal pathogen that causes P. carinii pneumonia (PCP) in the immunocompromised host. We investigated the role of antibody Fc-mediated function in passive prophylaxis against the development of PCP in SCID mice. By comparison of anti-mouse P. carinii immunoglobulin G1 monoclonal antibody (MAb) 4F11(G1) and its F(ab')2 derivative in an intranasal immunoprophylaxis model, we determined that Fc-mediated function is required for maximum effect of this antibody. Comparison of efficacy of antibody prophylaxis in SCID mice depleted of complement to that in nondepleted mice demonstrated that complement fixation by MAb 4F11(G1) is also necessary for optimal effect of passively administered antibody, although residual protection was observed in complement-depleted SCID mice. The necessity of complement for optimal PCP prophylaxis by MAb 4F11(G1) suggests that complement may play a role in antibody-mediated protection against development of PCP.

CD4+ T cell-independent DNA vaccination against opportunistic infections

Depletion or dysfunction of CD4+ T lymphocytes profoundly perturbs host defenses and impairs immunogenicity of vaccines. Here, we show that plasmid DNA vaccination with a cassette encoding antigen (OVA) and a second cassette encoding full-length CD40 ligand (CD40L), a molecule expressed on activated CD4+ T lymphocytes and critical for T cell helper function, can elicit significant titers of antigen-specific immunoglobulins in serum and Tc1 CD8+ T cell responses in CD4-deficient mice. To investigate whether this approach leads to CD4+ T cell-independent vaccine protection against a prototypic AIDS-defining infection, Pneumocystis (PC) pneumonia, we used serum from mice vaccinated with PC-pulsed, CD40L-modified DCs to immunoprecipitate PC antigens. Kexin, a PC antigen identified by this approach, was used in a similar DNA vaccine strategy with or without CD40L. CD4-deficient mice receiving DNA vaccines encoding Kexin and CD40L showed significantly higher anti-PC IgG titers as well as opsonic killing of PC compared with those vaccinated with Kexin alone. Moreover, CD4-depleted, Kexin-vaccinated mice showed a 3-log greater protection in a PC challenge model. Adoptive transfer of CD19+ cells or IgG to SCID mice conferred protection against PC challenge, indicating a role of humoral immunity in the protection. The results of these studies show promise for CD4-independent vaccination against HIV-related or other opportunistic pathogens.

Inhibition of surfactant activity by Pneumocystis carinii organisms and components in vitro

This study examines the direct inhibitory effects of Pneumocystis carinii (Pc) organisms and chemical components on the surface activity and composition of whole calf lung surfactant (WLS) and calf lung surfactant extract (CLSE) in vitro. Incubation of WLS suspensions with intact Pc organisms (10(7) per milligram of surfactant phospholipid) did not significantly alter total phospholipid levels or surfactant protein A content. Incubation with intact Pc organisms also did not impair dynamic surface tension lowering in suspensions of WLS or centrifuged large surfactant aggregates on a bubble surfactometer (37 degrees C, 20 cycles/min, 0.5 and 2.5 mg phospholipid/ml). However, exposure of WLS or CLSE to disrupted (sonicated) Pc organisms led to severe detriments in activity, with minimum surface tensions of 17-19 mN/m vs. <1 mN/m for surfactants alone. Extracted hydrophobic chemical components from Pc (98.8% lipids, 0.1 mM) reduced the surface activity of WLS and CLSE similarly to sonicated Pc organisms, whereas extracted hydrophilic chemical components from Pc (primarily proteins) had only minor effects on surface tension lowering. These results indicate that in addition to surfactant dysfunction induced by inflammatory lung injury and edema-derived inhibitors in Pc pneumonia, disrupted Pc organisms in the alveolar lumen also have the potential to directly inhibit endogenous and exogenous lung surfactants in affected patients.

Pneumocystis carinii activates the NF-kappaB signaling pathway in alveolar epithelial cells

Pneumocystis carinii pneumonia (PcP) is a clinically important infection of immunocompromised patients. Although the interaction of Pneumocystis with the alveolar epithelium has been well documented, very little information regarding the epithelial response to Pneumocystis is currently available. In order to study Pneumocystis-epithelium interactions, a murine cell line derived specifically from an alveolar epithelial cell (AEC) was utilized. The coculture of murine AECs with mouse Pneumocystis induced a dose- and time-dependent release of the CXC chemokine MIP-2. Importantly, the specific removal of Pneumocystis from the preparation, or the pretreatment of AECs with sulfasalazine, a potent and specific inhibitor of NF-kappaB, nearly completely abrogated the chemokine response to Pneumocystis. Since the murine MIP-2 promoter contains consensus kappaB binding sequences, the ability of Pneumocystis to stimulate NF-kappaB signaling in AECs was examined. Pneumocystis stimulation of an AEC line stably transfected with a kappaB-dependent reporter construct triggered the NF-kappaB signaling pathway and reporter production. These data were confirmed in gel shift assays, providing direct evidence that Pneumocystis induced the nuclear translocation of the p50/p65 heterodimeric form of NF-kappaB. Maximal NF-kappaB activation was dependent upon direct contact with viable Pneumocystis organisms. These data demonstrate that Pneumocystis activates NF-kappaB signaling in AECs and establish a reporter cell line for studying NF-kappaB activation in AECs. Given the global regulatory functions of the NF-kappaB family, these findings suggest that Pneumocystis directly alters AEC gene expression in a manner that promotes pulmonary immune and inflammatory responses.

Passive immunization of neonatal mice against Pneumocystis carinii f. sp. muris enhances control of infection without stimulating inflammation

Pneumocystis carinii is an opportunistic fungal pathogen that causes life-threatening pneumonia in immunocompromised individuals. Infants appear to be particularly susceptible to infection with Pneumocystis. We have previously shown that there is a significant delay in clearance of the organisms from the lungs of neonatal mice compared to adults. Since alveolar macrophages are the effector cells responsible for killing and clearance of Pneumocystis, we have examined alveolar macrophage activity in neonatal mice. We found that alveolar macrophage activation is delayed about 1 week in Pneumocystis-infected neonates compared to adults. Opsonization of the organism by Pneumocystis-specific antibody resulted in increased clearance of the organism in neonatal mice; however, there was decreased expression of activation markers on neonatal alveolar macrophages and reduced levels of cytokines associated with macrophage activation. Mice born to immunized dams had significant amounts of Pneumocystis-specific immunoglobulin G in their lungs and serum at day 7 postinfection, whereas mice born to naive dams had merely detectable levels. This difference correlated with enhanced Pneumocystis clearance in mice born to immunized dams. The increase in specific antibody, however, did not result in significant inflammation in the lungs, as no differences in numbers of activated CD4+ cells were observed. Furthermore, there was no difference in cytokine or chemokine concentrations in the lungs of pups born to immune compared to naive dams. These findings indicate that specific antibody plays an important role in Pneumocystis clearance from lungs of infected neonates; moreover, this process occurs without inducing inflammation in the lungs.

Phylogenetic identification of Pneumocystis murina sp. nov., a new species in laboratory mice

Pneumocystisis a fungal genus that contains multiple species. One member of the genus that has not been formally analysed for its phylogenetic relationships and possible species status is thePneumocystisfound in laboratory mice,Pneumocystis murinasp. nov. (type strain ATCC PRA-111T=CBS 114898T), formerly known asPneumocystis cariniif. sp.muris. To advance research in this area, approximately 3000 bp of additional DNA sequence were obtained from the locus encoding rRNAs. This sequence and others were used to determine genetic distances betweenP. murinaand other members of the genus. These distances indicated thatP. murinaDNA is most similar to that of the species ofPneumocystisfound in laboratory rats. Nevertheless,P. murinais at least as diverged from these otherPneumocystisspecies as species in other fungal genera are from each other. The 18S rRNA gene sequence divergence exhibited byP. murinacould not be ascribed to accelerated evolution of this gene as similar levels of divergence were observed at seven other loci. When five genes were used to construct phylogenetic trees for fivePneumocystistaxa, includingP. murina, all the trees had the same topology, indicating that genes do not flow among these taxa. The gene trees were all strongly supported by statistical tests. When sequences from the rRNA-encoding locus were used to estimate the time of divergence ofP. murina, the results indicated thatP. murinais as old as the mouse. Taken together, these data support previous recognition of multiple species in the genus and indicate thatP. murinais a phylogenetic species as well.

Expression and complexity of the PRT1 multigene family of Pneumocystis carinii

Pneumocystis carinii has a multigene family, PRT1, that encodes proteins with homology to KEX2-like proteases. PRT1 genes cluster with MSG genes near the telomeres and, like MSG, PRT1 proteins seem to be surface-expressed. The clustering of PRT1 and MSG genes suggested that expression of the two multigene families might be coordinated. Studying gene expression in P. carinii has been hampered by the lack of a culture system, and by lack of clonality in P. carinii populations in naturally infected rats, the host of this fungus. Heterogeneity can be reduced, however, by low-dose intratracheal inoculation, which can produce P. carinii populations dominated by organisms derived from a single progenitor. To study PRT1 expression, nude rats were inoculated with approximately 10 P. carinii each. The clonality of the P. carinii populations from inoculated rats was assessed by analysis of the UCS locus, a site in the genome that is known to be very heterogeneous in naturally infected rats, but nearly homogeneous in rats infected by low-dose intratracheal inoculation. Each of the populations had the same MSG gene at the UCS locus in at least 80 % of the organisms. To investigate PRT1 gene expression, RNA was amplified using primers that amplify numerous PRT1 genes. Seventy-four cloned cDNAs were sequenced, including at least 12 clones from each population of P. carinii. Many differently expressed PRT1 sequences were identified in each population, and a total of 45 different sequences were detected. However, the same PRT1 sequence was present in 15 of 74 plasmids and was found in 3 of the 5 P. carinii populations, suggesting that some PRT1 genes may be either more commonly expressed or expressed at a higher level. These data show that many members of the PRT1 gene family can be expressed in populations of P. carinii derived from few progenitors and suggest that the regulation of this family is different from that governing expression of the MSG gene family.

Epitope mapping of a protective monoclonal antibody against Pneumocystis carinii with shared reactivity to Streptococcus pneumoniae surface antigen PspA

Pneumocystis carinii is an opportunistic fungal pathogen that causes pneumonia in the immunocompromised host. A protective monoclonal antibody (MAb) termed 4F11 generated against mouse-derived P. carinii was shown by indirect immunofluorescence assay (IFA) to bind surface antigens of P. carinii derived from multiple host species, including humans. We have identified multiple epitopes recognized by MAb 4F11 in two recombinant mouse P. carinii antigens. The epitopes mapped have similar proline content and positive charge distribution. The consensus 8-mer epitope recognized by MAb 4F11 is K/RPA/RPK/QPA/TP. Immune sera raised against intact mouse P. carinii recognized native antigens affinity purified with MAb 4F11 and a recombinant antigen reactive with MAb 4F11. Database searches for short, nearly exact matches to the mapped MAb 4F11 epitopes identified a bacterial surface antigen, Streptococcus pneumoniae PspA, with a similar proline-rich region. In an IFA, MAb 4F11 detected antigens on the S. pneumoniae surface, and Western blotting identified a protein in S. pneumoniae lysates consistent with the M(r) of PspA. A fragment of the S. pneumoniae PspA gene was cloned and sequenced, and the deduced amino acid sequence contained a region with strong similarity to the MAb 4F11 epitopes identified in P. carinii. The PspA recombinant polypeptide was recognized by MAb 4F11 in a Western blot. The ability of MAb 4F11 to recognize similar proline-rich epitopes may explain its ability to recognize P. carinii derived from multiple hosts and will permit testing of the epitopes recognized by this antibody in immunization against P. carinii.

TNF Receptor Signaling Contributes to Chemokine Secretion, Inflammation, and Respiratory Deficits duringPneumocystisPneumonia

CD8(+) T cells contribute to the pathophysiology of Pneumocystis pneumonia (PcP) in a murine model of AIDS-related disease. The present studies were undertaken to more precisely define the mechanisms by which these immune cells mediate the inflammatory response that leads to lung injury. Experimental mice were depleted of either CD4(+) T cells or both CD4(+) and CD8(+) T cells and then infected with Pneumocystis: The CD4(+)-depleted mice had significantly greater pulmonary TNF-alpha levels than mice depleted of both CD4(+) and CD8(+) T cells. Elevated TNF-alpha levels were associated with increased lung concentrations of the chemokines RANTES, monocyte chemoattractant protein 1, macrophage-inflammatory protein 2, and cytokine-induced neutrophil chemoattractant. To determine whether TNFR signaling was involved in the CD8(+) T cell-dependent chemokine response, TNFRI- and II-deficient mice were CD4(+) depleted and infected with Pneumocystis: TNFR-deficient mice had significantly reduced pulmonary RANTES, monocyte chemoattractant protein 1, macrophage-inflammatory protein 2, and cytokine-induced neutrophil chemoattractant responses, reduced inflammatory cell recruitment to the alveoli, and reduced histological evidence of PcP-related alveolitis as compared with infected wild-type mice. Diminished pulmonary inflammation correlated with improved surfactant activity and improved pulmonary function in the TNFR-deficient mice. These data indicate that TNFR signaling is required for maximal CD8(+) T cell-dependent pulmonary inflammation and lung injury during PcP and also demonstrate that CD8(+) T cells can use TNFR signaling pathways to respond to an extracellular fungal pathogen.

The role of the Aspergillus niger furin-type protease gene in processing of fungal proproteins and fusion proteins

We have characterized growth and protein processing characteristics of Aspergillus niger strains carrying a disrupted allele of the previously cloned and characterized kexB gene [Appl. Environ. Microbiol. 66 (2000) 363] encoding a furin-type endoprotease. Deletion of the single-copy gene confirms it to be non-essential but disruptant strains exhibit a morphologically distinct phenotype characterized by hyperbranching. Processing of homologous pro-proteins and fusion proteins comprised of a heterologous protein fused down-stream of glucoamylase and separated at the fusion junction by an endoproteolytic cleavage site was compared in wildtype and mutant strains of A. niger. We show that maturation of the native glucoamylase requires KexB, whereas maturation of aspergillopepsin does not. The processing of fusion proteins carrying Lys-Arg requires KexB, although alternative endoproteases are capable of cleaving protein fusions at sites adjacent to Lys-Arg.

Characterization of transmission of Pneumocystis carinii f. sp. muris through immunocompetent BALB/c mice

By using mouse models, it has been shown that Pneumocystis carinii f. sp. muris can be transmitted to immunocompetent mice that are exposed to immunosuppressed mice with active P. carinii pneumonia. We sought to determine whether P. carinii f. sp. muris could be transmitted between normal mice. The rationale for these experiments was to demonstrate whether the normal host could serve as the reservoir of organisms that produce Pcp when the organism is acquired by the immunosuppressed host. Under the conditions of these experiments, normal mice are able to be infected by brief cohousing with P. carinii-infected SCID mice. There was active replication of organisms in the normal host such that the organism could be transmitted to other normal mice, again with active replication. Mice that had seroconverted after exposure to P. carinii-infected SCID mice were more resistant to infection when reexposed. Infection in normal mice was well tolerated with minimal effects on dynamic lung compliance. We speculate, based on these results, that transmission from normal host to normal host, as an asymptomatic or minimally symptomatic infection, could be a way to maintain this opportunistic pathogen in the environment.

Characterization of thioredoxin reductase genes (trr1) from Pneumocystis carinii and Pneumocystis jiroveci

We have characterized the thioredoxin reductase (trr1) genes from Pneumocystis carinii and Pneumocystis jiroveci, and have demonstrated that multiple copies of an approximately 500 base pair fragment of the trr1 gene are present in P. carinii, but not in P. jiroveci. Thioredoxin reductases encoded by the full-length genes have predicted molecular weights of approximately 35,000 and show high homology to yeast Trr1. An NADPH-binding domain with a putative redox active site CAVC as well as an flavin-adenine dinucleotide-binding domain are highly conserved in both proteins, which were 85% identical. The multicopy trr1 gene fragments in P. carinii are not transcribed or expressed. Duplication of the gene fragment likely occurred in conjunction with duplication of the kexin homologue, protease-1, which is located immediately upstream of the trr1 gene. Thioredoxin reductase, an enzyme implicated in the growth, survival and pathogenicity of certain microbes, could be a potential target for therapeutic intervention in Pneumocystis infection.

A single-copy gene encodes Kex1, a serine endoprotease of Pneumocystis jiroveci

We have cloned and characterized the kex1 gene of Pneumocystis jiroveci. Unlike the case for Pneumocystis carinii, in which the homologous PRT-1 genes are multicopy, kex1 is a single-copy gene encoding a protein homologous to fungal serine endoproteases, which localize to the Golgi apparatus. Thus, substantial biological differences can be seen among Pneumocystis species.

Passive intranasal monoclonal antibody prophylaxis against murine Pneumocystis carinii pneumonia

Passive antibody immunoprophylaxis is one method used to protect patients against infection if they are unable to mount an adequate active immune response. Topical application of antibody may be effective against infections at mucosal sites. Using a SCID mouse model of Pneumocystis carinii pneumonia, we were able to demonstrate protection against an airborne challenge with P. carinii by intranasal administration of antibody. Immunoglobulin M (IgM) monoclonal antibodies to an epitope shared by mouse and human P. carinii organisms reduced organism numbers by more than 99% under the conditions described. An IgG1 switch variant of one of the IgM monoclonal antibodies was also protective. These experiments provide a model for exploring the utility of this approach in protecting at-risk patients from infection with P. carinii.

Pneumocystis carinii: genetic diversity and cell biology

As an important opportunistic pulmonary pathogen, Pneumocystis carinii has been the focus of extensive research over the decades. The use of laboratory animal models has permitted a detailed understanding of the host-parasite interaction but an understanding of the basic biology of P. carinii has lagged due in large part to the inability of the organism to grow well in culture and to the lack of a tractable genetic system. Molecular techniques have demonstrated extensive heterogeneity among P. carinii organisms isolated from different host species. Characterization of the genes and genomes of the Pneumocystis family has supported the notion that the family comprises different species rather than strains within the genus Pneumocystis and contributed to the understanding of the pathophysiology of infection. Many of the technical obstacles in the study of the organisms have been overcome in the past decade and the pace of research into the basic biology of the organism has accelerated. Biochemical pathways have been inferred from the presence of key enzyme activities or gene sequences, and attempts to dissect cellular pathways have been initiated. The Pneumocystis genome project promises to be a rich source of information with regard to the functional activity of the organism and the presence of specific biochemical pathways. These advances in our understanding of the biology of this organism should provide for future studies leading to the control of this opportunistic pathogen.

Functional glycosylphosphatidylinositol anchor signal sequences in the Pneumocystis carinii PRT1 protease family

Pneumocystis carinii is fungus which is a frequent cause of severe pneumonia in immunocompromised individuals. The P. carinii genome contains the PRT1 subtelomeric multigene family that encodes a kexin-like serine protease which is expressed on the surface of P. carinii. Analysis of the sequence of the carboxy-terminal sequence of many copies of PRT1 showed that they contained motifs characteristic of a glycosylphosphatidylinositol (GPI) anchor signal sequence. The ability of the C-terminal sequences of PRT1 to direct the addition of a GPI anchor was tested. CD14, a GPI-anchored monocyte glycoprotein antigen, was used as the basis of a heterologous system. CD14 was truncated to remove the carboxy-terminal sequences responsible for GPI-anchor addition. Addition of carboxy-terminal sequences from PRT1 restored high-level surface expression to the truncated CD14. Further, the majority of CD14-PRT1 recombinant protein was removed from the cell membrane by treatment with GPI-specific phospholipase C. These results suggest that the carboxy-terminal residues of most of the members of the PRT1 family of proteases have the potential to form a functional GPI-attachment signal.

Identification of Pneumocystis carinii in the lungs of infants dying of sudden infant death syndrome

BACKGROUND

Recently Pneumocystis carinii has been identified in a significant number of infants diagnosed as having died from sudden infant death syndrome (SIDS) in South America and Europe.

METHODS

We examined lung sections of 79 infants who died with a diagnosis of SIDS in Rochester, NY, and Connecticut for the presence of P. carinii.

RESULTS

Organisms with a characteristic silver stain appearance for P. carinii were identified in 14% of the lung sections.

CONCLUSIONS

These data suggest that a possible link between some cases of SIDS and infection with P. carinii should be further evaluated and that infection of young infants may serve as an important reservoir for human P. carinii.

Genetics of surface antigen expression in Pneumocystis carinii

This article reviews the molecular genetic data pertaining to the major surface glycoprotein (MSG) gene family of Pneumocystis carinii and its role in surface variation and compares this fungal system to antigenic variation systems in the protozoan Trypanosoma brucei and the bacteria Borrelia spp.