Neither Classical nor Alternative Macrophage Activation Is Required for Pneumocystis Clearance during Immune Reconstitution Inflammatory Syndrome

Abstract

c Pneumocystis is a respiratory fungal pathogen that causes pneumonia (Pneumocystis pneumonia [PcP]) in immunocompromised patients. Alveolar macrophages are critical effectors for CD4؉ T cell-dependent clearance of Pneumocystis, and previous studies found that alternative macrophage activation accelerates fungal clearance during PcP-related immune reconstitution inflammatory syndrome (IRIS). However, the requirement for either classically or alternatively activated macrophages for Pneumocystis clearance has not been determined. Therefore, RAG2؊/؊ mice lacking either the interferon gamma (IFN-␥) receptor (IFN-␥R) or interleukin 4 receptor alpha (IL-4R␣) were infected with Pneumocystis. These mice were then immune reconstituted with wild-type lymphocytes to preserve the normal T helper response while preventing downstream effects of Th1 or Th2 effector cytokines on macrophage polarization. As expected, RAG2 cell responses in RAG/IFN-␥R؊/؊ mice were associated with elevated lung IFN-␥ levels, and neutralization of IFN-␥ restored the CD8 response. These data demonstrate that restricting the ability of macrophages to polarize in response to Th1 or Th2 cytokines does not impair Pneumocystis clearance. However, a cell type-specific IFN-␥/IFN-␥R-dependent mechanism regulates CD8 ؉ T suppressor cell recruitment, limits immunopathogenesis, preserves lung function, and enhances the resolution of PcPrelated IRIS. Pneumocystis is an opportunistic fungal pathogen, which is widespread throughout the human population. Vargas et al. reported that nearly 85% of children have been exposed to Pneumocystis by 20 months of age (1). The infection appears mild and self-limited in a normal host, but Pneumocystis causes life-threatening pneumonia in patients with defects in cell-mediated immunity. Pneumocystis pneumonia (PcP) remains prevalent in those with AIDS (2) and is a leading cause of death in HIV-infected patients, with a mortality rate of up to 50% in those with severe disease (3). Pneumocystis also infects patients receiving long-term immunomodulatory therapy for rheumatoid arthritis and other inflammatory diseases (4) as well as cancer patients undergoing chemotherapy (5). Furthermore, Pneumocystis infection or colonization has been associated with chronic obstructive pulmonary disease in both HIV and non-HIV patients (6).Despite the availability of antibiotics that effectively eradicate Pneumocystis, rapid clinical improvement is not always observed in patients with PcP. The host's immune response against this pathogen often contributes to severe PcP-related lung injury (6, 7). Clinical studies suggest that the intensity of pulmonary inflammation, rather than organism lung burden, correlates with the severity of PcP (8-10). Furthermore, rapid and unbalanced restoration of cell-mediated immunity following antiretroviral treatment of Pneumocystis-infected HIV patients sometimes results in exaggerated lung inflammation and leads to the clinical manifestation of immune reconstitution inflammatory syndrome (IRIS) (11). IRIS is an...

Pneumocystis infection alters the activation state of pulmonary macrophages

“…A recent study by Zhang et al demonstrates that mice lacking in either CAM or AAM responses still possess the ability to clear Pneumocystis (Zhang et al, 2015). By crossing IFNγR −/− mice or IL4rα −/− mice to RAG2 −/− animals, and then providing WT T cells by adoptive transfer, they demonstrated that in the setting of immune reconstitution inflammatory syndrome (IRIS), neither CAM or AAM were required for the clearance of Pneumocystis .…”

“…By crossing IFNγR −/− mice or IL4rα −/− mice to RAG2 −/− animals, and then providing WT T cells by adoptive transfer, they demonstrated that in the setting of immune reconstitution inflammatory syndrome (IRIS), neither CAM or AAM were required for the clearance of Pneumocystis . However, while all strains were able to clear the organism, mice that lacked a CAM response (RAG2 −/− / IFNγR −/− ) had increased Pneumocystis-related immunopathologies (Zhang et al, 2015). Our results correspond with theirs in that the lack of signaling through the IL4rα to restrict AAM responses did not impact the clearance kinetics of the organism.…”

“…There are important differences between studies, as our mice also lack IL4rα on T cells, and in the Zhang study the mice were undergoing immune reconstitution, which can in itself cause immunopathology (Bonham et al, 2008). Key elements to the response in the mice that lacked IFNγR included impaired CD8 + T cell regulatory function and elevated concentrations of IFNγ (Zhang et al, 2015). Indeed, these results support that there is a complex balance of macrophage and T cell phenotypic responses needed in order to produce an optimal situation of organism clearance and limited pulmonary pathology.…”

Immune reconstitution inflammatory syndrome associated with pulmonary pathogens

“…Lung injury associated with PCP-IRIS manifests with hypoxaemia, elevated lactate dehydrogenase and total protein in bronchoalveolar lavage fluid, weight loss and mortality [46, 47, 49]. A recent study using Rag1 −/− IFN-γR −/− or Rag1 −/− IL-4R −/− mouse models of IRIS demonstrated that the activation of M1 or M2 macrophages is not required for Pneumocystis clearance [50]. Furthermore, this study has shown that IFN-γ is a critical mediator in the exacerbated inflammatory response by decreasing the CD8 + Foxp3 + regulatory T-cell subset in PCP-IRIS [50].…”

“…A recent study using Rag1 −/− IFN-γR −/− or Rag1 −/− IL-4R −/− mouse models of IRIS demonstrated that the activation of M1 or M2 macrophages is not required for Pneumocystis clearance [50]. Furthermore, this study has shown that IFN-γ is a critical mediator in the exacerbated inflammatory response by decreasing the CD8 + Foxp3 + regulatory T-cell subset in PCP-IRIS [50]. Dectin-1 is involved in PCP-IRIS dependent lung injury, as antagonising dectin-1 signalling blocked the development of fungal IRIS [51].…”