Pneumocystis elicits a STAT6-dependent, strain-specific innate immune response and airway hyperresponsiveness.

It is widely held that exposure to pathogens such as fungi can be an agent of comorbidity, such as exacerbation of asthma or chronic obstructive pulmonary disease. Although many studies have examined allergic responses to fungi and their effects on pulmonary function, the possible pathologic implications of the early innate responses to fungal pathogens have not been explored. We examined early responses to the atypical fungus Pneumocystis in two common strains of mice in terms of overall immunological response and related pathology, such as cell damage and airway hyperresponsiveness (AHR). We found a strong strain-specific response in BALB/c mice that included recruitment of neutrophils, NK, NKT, and CD4 T cells. This response was accompanied by elevated indicators of lung damage (bronchoalveolar lavage fluid albumin and LDH) and profound AHR. This early response was absent in C57BL/6 mice, although both strains exhibited a later response associated with the clearance of Pneumocystis. We found that this AHR could not be attributed exclusively to the presence of recruited neutrophils, NKT, NK, or CD4 cells or to the actions of IFN-γ or IL-4. However, in the absence of STAT6 signaling, AHR and inflammatory cell recruitment were virtually absent. Gene expression analysis indicated that this early response included activation of several transcription factors that could be involved in pulmonary remodeling. These results show that exposure to a fungus such as Pneumocystis can elicit pulmonary responses that may contribute to morbidity, even without prior sensitization, in the context of certain genetic backgrounds.

CD8 T cells modulate CD4 T-cell and eosinophil-mediated pulmonary pathology in pneumocystis pneumonia in B-cell-deficient mice.

Pneumocystis spp. pneumonia (PCP) in humans and in surrogate animal species typically occurs in the absence of CD4 T cells, as takes place during acquired immune deficiency syndrome. However, patients treated with highly active anti-retroviral therapy sometimes exhibit an exacerbation of diseases such as PCP that coincides with resurgent CD4 T cells, a phenomenon known as immune reconstitution disease. We used an animal model of PCP using the B-cell-deficient muMT mouse together with antibody-mediated depletion of various T-cell subsets to examine the role of CD4 and CD8 T cells in the development of pathology in PCP. Although overt pathology occurs in the presence of CD4 T cells only, CD8 T cells only, or both, pulmonary injury occurs via different paths, depending on the complement of T cells present. Surprisingly, profound damage occurred when only CD4 T cells were present, and this pathology coincided with enhanced recruitment and activation of eosinophils and strong type 2 cytokine polarization in the alveolar environment. In addition, CD8 T cells can act to moderate this CD4 T cell-mediated pathology, possibly by increasing the ratio of putative CD25+ suppressor CD4 T cells to CD25- effector CD4 T cells.

CD8 T cell-mediated lung damage in response to the extracellular pathogen pneumocystis is dependent on MHC class I expression by radiation-resistant lung cells.

Pneumocystis, a fungal, extracellular pathogen causes a life-threatening pneumonia in patients with severe immunodeficiencies. In the absence of CD4 T cells, Pneumocystis infection results in vigorous CD8 T cell influx into the alveolar and interstitial spaces of the lung. This response results in lung damage characterized by low pO2 and albumin leakage into the bronchoalveolar lavage fluid similar to other CD8 T cell-mediated interstitial lung diseases. How this extracellular pathogen elicits a CD8 T cell response is not clear, and it was the aim of our study to determine the Ag specificity of the recruited CD8 T cells and to determine whether MHC class I (MHC I) expression was necessary to initiate lung damage. Using an adoptive T cell-transfer model with either polyclonal wild-type CD8 T cells or transgenic influenza virus-specific CD8 T cells we found that CD8 T cell recruitment is Ag-specific and requires the continuous presence of the Pneumocystis pathogen. Bone marrow chimera experiments using Rag-1 and beta2-microglobulin-deficient mice as hosts demonstrated a requirement for MHC I expression on nonbone marrow-derived cells of the lung. This suggests either direct processing of Pneumocystis Ags by nonbone marrow-derived cells of the lung or the induction of lung damage triggered by a lung-specific autoantigen. Using perforin-, Fas-, and IFN-gamma-deficient animals, we showed that these molecules are not directly involved in the CD8-mediated lung damage. However, CD8 T cell-mediated lung damage is Ag-specific is induced by a MHC I-expressing nonbone marrow-derived cell in the lung and is dependent on the continued presence of live Pneumocystis.

Role of type I IFNs in pulmonary complications of Pneumocystis murina infection.

Despite the advent of highly active antiretroviral therapy, pulmonary complications in AIDS are a common clinical problem. Pneumocystis jiroveci infection causes a life-threatening pneumonia, especially in individuals with CD4 T cell deficiencies as occurs in AIDS. Although Pneumocystis sp. is an extracellular fungal pathogen, CD8 T cells are the predominant lymphocyte recruited to the lung in CD4-deficient humans and mice during Pneumocystis pneumonia, and we have found that these CD8 T cells are responsible for subsequent lung damage in CD4 T cell-depleted mice. Comparing CD4 T cell-depleted IFN-alpha receptor knockout (KO) mice to wild-type mice, we found that this CD8 T cell recruitment and lung damage is type I IFN (IFN-alphabeta) dependent. However, in both CD4 competent, wild-type and IFN-alpha receptor (IFNAR) KO mice, Pneumocystis infection leads to an eosinophilic granulocyte influx with bronchial epithelial changes as seen in asthma. This response is delayed in IFNAR KO mice, as is pathogen clearance. Although the inflammation is transient in wild-type animals and resolves upon Pneumocystis clearance, it is more severe and persists through day 35 postinfection in IFNAR KO mice, leading to fibrosis. In addition, IFNAR KO, but not wild-type, mice mount a Pneumocystis-specific IgE response, an indicator of allergic sensitization. Thus, in the absence of IFNAR signaling and CD4 T cells, Pneumocystis-mediated lung damage does not occur, whereas in CD4-competent animals, the absence of IFNAR signaling results in an exacerbated Th2 response, asthma-like symptoms, and fibrosis. Therefore, both CD4 T cell- and type I IFN-mediated mechanisms can determine pulmonary complications from Pneumocystis infection.