Immunity against the opportunistic fungal pathogen Pneumocystis

IL-4 polymorphism influences susceptibility to Pneumocystis jirovecii pneumonia in HIV-positive patients

OBJECTIVES

Pneumocystis jirovecii pneumonia (PJP) is an important cause of morbidity and mortality in HIV-positive patients. Polymorphisms in immune genes are increasingly reported to influence susceptibility to fungal infections. We analysed the role of 21 single nucleotide polymorphisms from 19 candidate genes on PJP development in patients from the Swiss HIV Cohort Study.

DESIGN AND METHODS

The analysis included patients with a nadir CD4 T-cell count less than 200 cells/μl, divided into a discovery (N = 1645) and a replication (N = 1861) cohort. The associations were analysed by using cumulative incidence curves as well as competing risk regression over 18 years, starting from the estimated date of HIV infection, considering death a competing risk, with censoring at lost follow-up, and assuming the dominant mode of inheritance.

RESULTS

The minor allele of rs2243250 in IL-4 was associated with the risk of PJP in the discovery cohort (cumulative incidence 0.18 versus 0.12, P = 0.002). This association was replicated in the validation cohort (0.16 versus 0.12, P = 0.02). It was still significant in multivariate models, adjusted for HIV transmission mode, viral load, CD4 T cells slope, age, antiretroviral therapy, tobacco smoking, hepatitis C virus coinfection, year of cohort entry and PJP prophylaxis (global subhazard ratio 1.42, 95% confidence interval 1.17-1.73, P = 0.0004).

CONCLUSION

Our data suggest rs2243250, a single nucleotide polymorphism known to influence IL-4 production, is associated with susceptibility to PJP in HIV-positive patients.

Effects of clinical and environmental factors on bronchoalveolar antibody responses to Pneumocystis jirovecii: A prospective cohort study of HIV+ patients

BACKGROUND

Humoral immunity plays an important role against Pneumocystis jirovecii infection, yet clinical and environmental factors that impact bronchoalveolar antibody responses to P. jirovecii remain uncertain.

METHODS

From October 2008-December 2011 we enrolled consecutive HIV-infected adults admitted to San Francisco General Hospital (SFGH) who underwent bronchoscopy for suspected Pneumocystis pneumonia (PCP). We used local air quality monitoring data to assign ozone, nitrogen dioxide, and fine particulate matter exposures within 14 days prior to hospital admission. We quantified serum and bronchoalveolar lavage fluid (BALF) antibody responses to P. jirovecii major surface glycoprotein (Msg) recombinant constructs using ELISA. We then fit linear regression models to determine whether PCP and ambient air pollutants were associated with bronchoalveolar antibody responses to Msg.

RESULTS

Of 81 HIV-infected patients enrolled, 47 (58%) were diagnosed with current PCP and 9 (11%) had a prior history of PCP. The median CD4+ count was 51 cells/μl (IQR 15-129) and 44% were current smokers. Serum antibody responses to Msg were statistically significantly predictive of BALF antibody responses, with the exception of IgG responses to MsgC8 and MsgC9. Prior PCP was associated with increased BALF IgA responses to Msg and current PCP was associated with decreased IgA responses. For instance, among patients without current PCP, those with prior PCP had a median 73.2 U (IQR 19.2-169) IgA response to MsgC1 compared to a 5.00 U (3.52-12.6) response among those without prior PCP. Additionally, current PCP predicted a 22.5 U (95%CI -39.2, -5.82) lower IgA response to MsgC1. Ambient ozone within the two weeks prior to hospital admission was associated with decreased BALF IgA responses to Msg while nitrogen dioxide was associated with increased IgA responses.

CONCLUSIONS

PCP and ambient air pollutants were associated with BALF IgA responses to P. jirovecii in HIV-infected patients evaluated for suspected PCP.

Synthetic p55 tandem DNA vaccine against Pneumocystis carinii in rats

Pneumocystis spp. are opportunistic fungal pathogens that are closely associated with severe pneumonia and pulmonary complications in patients with impaired immunity. In this study, the antigenic epitopes of the gene encoding the 55 kDa antigen fragment of Pneumocystis (p55), which may play an important role in Pneumocystis pneumonia, were analyzed. A gene containing tandem variants of the p55 antigen was synthesized and named the tandem antigen gene (TAG). TAG's potential as a DNA vaccine was assessed in immunosuppressed rats. Immunization with p55-TAG DNA vaccine significantly reduced both the pathogen burden and lung-weight to body-weight ratios. Additionally, p55-TAG vaccination in immunosuppressed rats elicited both cell-mediated and humoral immunity.

Common variable immune deficiency in a Pomeranian with Pneumocystis carinii pneumonia

A Pomeranian dog, 1 year- and 8 month-old neutered female, was presented with persistent respiratory distress and recurrent generalized demodicosis. Physical examination revealed cyanosis, rough respiratory sounds, multifocal alopecia and dermal erosions on the dorsal side of the forelimbs, perineal area and skin around the eyes. A severe diffuse interstitial lung pattern was observed on thoracic radiographs. The blood examination revealed neutrophilia and hypoglobulinemia. Serum immunoglobulin concentrations of IgG and IgA were low. Histopathological examination revealed severe diffuse interstitial pneumonia with Pneumocystis carinii infection. Severe lymphoid depletion was observed in the spleen and other organs with lymphoid follicles consisted mainly of CD3-positive T cells and few cells of B-cell lineage. B-cell hypoplasia with subsequent antibody deficiency was suspected.

Pattern recognition receptors in antifungal immunity

Receptors of the innate immune system are the first line of defence against infection, being able to recognise and initiate an inflammatory response to invading microorganisms. The Toll-like (TLR), NOD-like (NLR), RIG-I-like (RLR) and C-type lectin-like receptors (CLR) are four receptor families that contribute to the recognition of a vast range of species, including fungi. Many of these pattern recognition receptors (PRRs) are able to initiate innate immunity and polarise adaptive responses upon the recognition of fungal cell wall components and other conserved molecular patterns, including fungal nucleic acids. These receptors induce effective mechanisms of fungal clearance in normal hosts, but medical interventions, immunosuppression or genetic predisposition can lead to susceptibility to fungal infections. In this review, we highlight the importance of PRRs in fungal infection, specifically CLRs, which are the major PRR involved. We will describe specific PRRs in detail, the importance of receptor collaboration in fungal recognition and clearance, and describe how genetic aberrations in PRRs can contribute to disease pathology.

Pneumocystis jirovecii Rtt109, a novel drug target for Pneumocystis pneumonia in immunosuppressed humans

Pneumocystis pneumonia (PcP) is a significant cause of morbidity and mortality in immunocompromised patients. In humans, PcP is caused by the opportunistic fungal species Pneumocystis jirovecii. Progress in Pneumocystis research has been hampered by a lack of viable in vitro culture methods, which limits laboratory access to human-derived organisms for drug testing. Consequently, most basic drug discovery research for P. jirovecii is performed using related surrogate organisms such as Pneumocystis carinii, which is derived from immunosuppressed rodents. While these studies provide useful insights, important questions arise about interspecies variations and the relative utility of identified anti-Pneumocystis agents against human P. jirovecii. Our recent work has identified the histone acetyltransferase (HAT) Rtt109 in P. carinii (i.e., PcRtt109) as a potential therapeutic target for PcP, since Rtt109 HATs are widely conserved in fungi but are absent in humans. To further address the potential utility of this target in human disease, we now demonstrate the presence of a functional Rtt109 orthologue in the clinically relevant fungal pathogen P. jirovecii (i.e., PjRtt109). In a fashion similar to that of Pcrtt109, Pjrtt109 restores H3K56 acetylation and genotoxic resistance in rtt109-null yeast. Recombinant PjRtt109 is an active HAT in vitro, with activity comparable to that of PcRtt109 and yeast Rtt109. PjRtt109 HAT activity is also enhanced by the histone chaperone Asf1 in vitro. PjRtt109 and PcRtt109 showed similar low micromolar sensitivities to two reported small-molecule HAT inhibitors in vitro. Together, these results demonstrate that PjRtt109 is a functional Rtt109 HAT, and they support the development of anti-Pneumocystis agents directed at Rtt109-catalyzed histone acetylation as a novel therapeutic target for human PcP.

HIV-1 Tat increases oxidant burden in the lungs of transgenic mice

Chronic human immunodeficiency virus infection is associated with higher incidence of pulmonary complications including hypertension, vasculopathy, lymphocytic alveolitis, and interstitial pneumonitis not attributed to either opportunistic infections or presence of the virus. The Tat (transactivator of transcription) protein, a required transactivator for expression of full-length viral genes, is pleiotropic and influences expression of cellular inflammatory genes. Tat-dependent transactivation of cellular genes requires specific mediators, including NF-κB, widely recognized as sensitive to changes in cellular oxidant burden. We hypothesized that overproduction of Tat leads to increased oxidant burden and to alterations in basal inflammatory status as measured by NF-κB activation. We engineered transgenic mouse lines that express Tat (86-amino-acid isoform) in the lung under the control of the surfactant protein C promoter. Tat-transgenic mice exhibit increased pulmonary cellular infiltration, increased nitrotyrosine and carbonyl protein modifications, and increased levels of NF-κB, MnSOD, and thioredoxin-interacting protein. These data indicate that Tat increases oxidant burden and resets the threshold for inflammation, which may increase susceptibility to secondary injuries.

Relationship of pneumocystis antibody response to severity of chronic obstructive pulmonary disease

Pneumocystis colonization has been associated with severity of chronic obstructive pulmonary disease (COPD). The relationship of Pneumocystis antibody status to COPD severity has not been investigated, but antibody levels might relate to both colonization susceptibility and COPD progression. We investigated anti-Pneumocystis antibody titers and airway obstruction in a cohort of patients with COPD. Undetectable anti-Pneumocystis antibody titer was an independent predictor of more-severe airway obstruction, although use of inhaled corticosteroids is a possible confounder of this effect.

Immune response to fungal infections

The immune mechanisms of defence against fungal infections are numerous, and range from protective mechanisms that were present early in evolution (innate immunity) to sophisticated adaptive mechanisms that are induced specifically during infection and disease (adaptive immunity). The first-line innate mechanism is the presence of physical barriers in the form of skin and mucous membranes, which is complemented by cell membranes, cellular receptors and humoral factors. There has been a debate about the relative contribution of humoral and cellular immunity to host defence against fungal infections. For a long time it was considered that cell-mediated immunity (CMI) was important, but humoral immunity had little or no role. However, it is accepted now that CMI is the main mechanism of defence, but that certain types of antibody response are protective. In general, Th1-type CMI is required for clearance of a fungal infection, while Th2 immunity usually results in susceptibility to infection. Aspergillosis, which is a disease caused by the fungus Aspergillus, has been the subject of many studies, including details of the immune response. Attempts to relate aspergillosis to some form of immunosuppression in animals, as is the case with humans, have not been successful to date. The defence against Aspergillus is based on recognition of the pathogen, a rapidly deployed and highly effective innate effector phase, and a delayed but robust adaptive effector phase. Candida albicans, part of the normal microbial flora associated with mucous surfaces, can be present as congenital candidiasis or as acquired defects of cell-mediated immunity. Resistance to this yeast is associated with Th1 CMI, whereas Th2 immunity is associated with susceptibility to systemic infection. Dermatophytes produce skin alterations in humans and other animals, and the essential role of the CMI response is to destroy the fungi and produce an immunoprotective status against re-infection. The resolution of the disease is associated with a delayed hypersensitive response. There are many effective veterinary vaccines against dermatophytoses. Malassezia pachydermatis is an opportunistic yeast that needs predisposing factors to cause disease, often related to an atopic status in the animal. Two species can be differentiated within the genus Cryptococcus with immunologic consequences: C. neoformans infects predominantly immunocompromised hosts, and C. gattii infects non-immunocompromised hosts. Pneumocystis is a fungus that infects only immunosupressed individuals, inducing a host defence mechanism similar to that induced by other fungal pathogens, such as Aspergillus.

Immune responses to Pneumocystis murina are robust in healthy mice but largely absent in CD40 ligand-deficient mice

Pneumocystis is a pathogen of immunocompromised hosts but can also infect healthy hosts, in whom infection is rapidly controlled and cleared. Microarray methods were used to examine differential gene expression in the lungs of C57BL/6 and CD40 ligand knockout (CD40L-KO) mice over time following exposure to Pneumocystis murina. Immunocompetent C57BL/6 mice, which control and clear infection efficiently, showed a robust response to infection characterized by the up-regulation of 349 primarily immune response-associated genes. Temporal changes in the expression of these genes identified an early (Week 2), primarily innate response, which waned before the infection was controlled; this was followed by primarily adaptive immune responses that peaked at Week 5, which coincided with clearance of the infection. In conjunction with the latter, there was an increased expression of B cell-associated (Ig) genes at Week 6 that persisted through 11 weeks. In contrast, CD40L-KO mice, which are highly susceptible to developing severe Pneumocystis pneumonia, showed essentially no up-regulation of immune response-associated genes at Days 35-75. Immunohistochemical staining supported these observations by demonstrating an increase in CD4+, CD68+, and CD19+ cells in C57BL/6 but not CD40L-KO mice. Thus, the healthy host demonstrates a robust, biphasic response to infection by Pneumocystis; CD40L is an essential upstream regulator of the adaptive immune responses that efficiently control infection and prevent development of progressive pneumonia.

Dynamic colonisation by different Pneumocystis jirovecii genotypes in cystic fibrosis patients

Although asymptomatic carriers of Pneumocystis jirovecii with cystic fibrosis (CF) have been described previously, the molecular epidemiology of P. jirovecii in CF patients has not yet been clarified. This study identified the distribution and dynamic evolution of P. jirovecii genotypes based on the mitochondrial large-subunit (mt LSU) rRNA gene. The mt LSU rRNA genotypes of P. jirovecii isolates in 33 respiratory samples from CF patients were investigated using nested PCR and direct sequencing. Three different genotypes were detected: 36.3% genotype 1 (85C/248C); 15.1% genotype 2 (85A/248C); 42.4% genotype 3 (85T/248C); and 6% mixed genotypes. Patients studied during a 1-year follow-up period showed a continuous colonisation/clearance cycle involving P. jirovecii and an accumulative tendency to be colonised with genotype 3.

Exogenous heat-killed Escherichia coli improves alveolar macrophage activity and reduces Pneumocystis carinii lung burden in infant mice

Pneumocystis carinii is an opportunistic fungal pathogen that causes life-threatening pneumonia in immunocompromised individuals. Infants appear to be particularly susceptible to Pneumocystis pulmonary infections. We have previously demonstrated that there is approximately a 3-week delay in the clearance of Pneumocystis organisms from pup mouse lungs compared to that in adults. We have further shown that there is approximately a 1-week delay in alveolar macrophage activation in pups versus adult mice. Alveolar macrophages are the primary effector cells responsible for the killing and clearance of Pneumocystis, suggesting that pup alveolar macrophages may be involved in the delayed clearance of this organism. Alveolar macrophages cultured in vitro with Pneumocystis alone demonstrate little to no activation, as indicated by a lack of cytokine production. However, when cultured with lipopolysaccharide (LPS) or zymosan, cytokine production was markedly increased, suggesting that pup alveolar macrophages are specifically unresponsive to Pneumocystis organisms rather than being intrinsically unable to become activated. Furthermore, pup mice treated with aerosolized, heat-killed Escherichia coli in vivo were able to clear Pneumocystis more efficiently than were control mice. Together, these data suggest that while pup alveolar macrophages are unresponsive to P. carinii f. sp. muris organisms, they are capable of activation by heat-killed E. coli in vivo, as well as LPS and zymosan in vitro. The lack of response of pup mice to P. carinii f. sp. muris may reflect protective mechanisms specific to the developing pup lung, but ultimately it results in insufficient clearance of Pneumocystis organisms.

Interleukin-23 (IL-23)-IL-17 cytokine axis in murine Pneumocystis carinii infection

Host defense mechanisms against Pneumocystis carinii are not fully understood. Previous work in the murine model has shown that host defense against infection is critically dependent upon host CD4(+) T cells. The recently described Th17 immune response is predominantly a function of effector CD4(+) T cells stimulated by interleukin-23 (IL-23), but whether these cells are required for defense against P. carinii infection is unknown. We tested the hypothesis that P. carinii stimulates the early release of IL-23, leading to increases in IL-17 production and lung effector CD4(+) T-cell population that mediate clearance of infection. In vitro, stimulation of alveolar macrophages with P. carinii induced IL-23, and IL-23p19 mRNA was expressed in lungs of mice infected with this pathogen. To address the role of IL-23 in resistance to P. carinii, IL-23p19-/- and wild-type control C57BL/6 mice were infected and their fungal burdens and cytokine/chemokine responses were compared. IL-23p19-/- mice displayed transient but impaired clearance of infection, which was most apparent 2 weeks after inoculation. In confirmatory studies, the administration of either anti-IL-23p19 or anti-IL-17 neutralizing antibody to wild-type mice infected with P. carinii also caused increases in fungal burdens. IL-17 and the lymphocyte chemokines IP-10, MIG, MIP-1alpha, MIP-1beta, and RANTES were decreased in the lungs of infected IL-23p19-/- mice in comparison to their levels in the lungs of wild-type mice. In IL-23p19-/- mice infected with P. carinii, there were fewer effector CD4(+) T cells in the lung tissue. Collectively, these studies indicate that the IL-23-IL-17 axis participates in host defense against P. carinii.

Interaction of Pneumocystis carinii with dendritic cells and resulting host responses to P. carinii

To assess the interaction of Pneumocystis carinii with dendritic cells (DCs), and the consequences of the response of the host immune system to P. carinii antigens when DC are pulsed with P. carinii, murine DC were pulsed with P. carinii, and the resultant P. carinii host responses assessed in vitro and in vivo. P. carinii interacted with murine bone marrow-derived DC in vitro in part via mannose receptors. DC pulsed with P. carinii did not demonstrate increased expression of the cell surface markers MHC II, CD40, CD54, CD80 (B7.1), and CD86 (B7.2). The release of interleukin (IL)-4 was increased, but there was no increase in the release of interleukin (IL)-12p40, IL-10, tumor necrosis factor-alpha, IL-6, and nitrite compared with naive DC. In vivo administration of DC pulsed with P. carinii induced a P. carinii-specific response, generating CD4+ cells that proliferated and released IL-4, but not interferon-gamma, in response to P. carinii-pulsed DC in vitro. In vivo administration of DC pulsed with P. carinii also induced P. carinii-specific immunoglobulin (Ig)G1, IgG2a, and IgG2b, but not IgG3, antibodies in serum, and lung lavage fluid. Finally, CD4+ depleted mice immunized with DC pulsed with P. carinii demonstrated suppression of lung growth of P. carinii after intratracheal challenge with P. carinii at 3 and 16 weeks after immunization. These observations provide insight into DC-P. carinii interactions, and support the concept that a vaccine that includes DC pulsed with P. carinii can mount a humoral and T helper 2-type cellular response to P. carinii sufficient to suppress the growth of P. carinii in the lung.

Granulomatous Pneumocystis jiroveci Pneumonia in a Patient with Chronic Lymphocytic Leukemia: A Literature Review and Hypothesis on Pathogenesis

BACKGROUND

Pneumocystis jiroveci pneumonia occurs frequently in patients with immunodeficiency syndromes, especially AIDS. Approximately 5% of AIDS patients have atypical granulomatous histology.

CASE REPORT/METHODS

A 75-year-old woman with chronic lymphocytic leukemia was treated with alemtuzumab (campath-1H) 3 times weekly for 12 weeks. After completion of therapy she presented with dyspnea, hypoxemia, and bilateral infiltrates. Bronchoscopy with biopsy revealed Pneumocystis organisms with granulomatous history. She responded well to trimethoprim-sulfamethoxazole.

RESULTS/LITERATURE REVIEW

Our literature review identified 19 patients without AIDS who had granulomatous Pneumocystis infection. These patients often had nodular infiltrates on x-rays and negative bronchoalveolar lavage study findings. Most patients required open lung biopsies. Histologic specimens frequently revealed necrosis. These patients responded well to therapy.

CONCLUSION

The limited information available from these studies suggests that these patients have immune reconstitution-like syndrome related to either increasing numbers of CD4+ lymphocytes following therapeutic suppression or impaired modulation of CD4+ function. This unusual clinical presentation may delay diagnosis and effective therapy.

Pneumocystis jiroveci infection in patients with hyper-immunoglobulin E syndrome

The hyper-immunoglobulin E syndrome is a primary immunodeficiency characterized by recurrent pyogenic skin and lung abscesses, dermatitis, and elevated serum immunoglobulin E levels. Pneumocystis jiroveci (formerly Pneumocystis carinii) is not typically associated with hyper-immunoglobulin E syndrome. We identified 7 patients with hyper-immunoglobulin E syndrome with P. jiroveci detected in respiratory or pulmonary pathology specimens. In 5 patients it was the sole pathogen, and in 2 other patients it contributed to a polymicrobial etiology. No consistent prophylaxis was given, and there have been no recurrences on long-term follow-up. Our experience suggests that P. jiroveci can cause pneumonia in patients with hyper-immunoglobulin E syndrome both with and without chronic lung disease.

The host response of CD28-deficient mice to Pneumocystis infection

Pneumocystis infection leads to a life threatening pneumonia in susceptible individuals. While depletion or dysfunction of CD4+T cells is a key determinant of susceptibility to Pneumocystis, the host response that leads to resolution of infection or lung injury is less well understood. We had previously shown that mice deficient in the T cell costimulatory molecule CD28 are susceptible to infection with Pneumocystis. A detailed analysis revealed that they clear Pneumocystis with delayed kinetics. This is associated with an influx of naïve CD8+ T cells. Depletion of CD8+ T cells did not alter organism burden, suggesting these cells are not responsible for clearance. Analysis of the cytokine milieu demonstrated a consistent increase in mRNA for IL-10 and IFN-gamma in the CD28-deficient mice. These data suggest that CD28 function in important in the efficiency of the host response to Pneumocystis pneumonia.