The importance of the priming of the lung environment by past infections is being increasingly recognized. Exposure to any given antigen can either improve or worsen the outcome of subsequent lung infections, depending on the immunological history of the host. Thus, an ability to impart transient alterations in the lung environment in anticipation of future insult could provide an important novel therapy for emerging infectious diseases. In this study, we show that nasal administration of virus-like particles (VLPs) before, or immediately after, lethal challenge with methicillin-resistant Staphylococcus aureus (MRSA) of mice i) ensures complete recovery from lung infection and near absolute clearance of bacteria within 12 hours of challenge, ii) reduces host response-induced lung tissue damage, iii) promotes recruitment and efficient bacterial clearance by neutrophils and CD11c(+) cells, and iv) protects macrophages from MRSA-induced necrosis. VLP-mediated protection against MRSA relied on innate immunity. Complete recovery occurred in VLP-dosed mice with severe combined immunodeficiency, but not in wild-type mice depleted of either Ly6G(+) or CD11c(+) cells. Early IL-13 production associated with VLP-induced CD11c(+) cells was essential for VLP-induced protection. These results indicate that VLP-induced alteration of the lung environment protects the host from lethal MRSA pneumonia by enhancing phagocyte recruitment and killing and by reducing inflammation-induced tissue damage via IL-13-dependent mechanisms.

Although acquired bone marrow failure (BMF) is considered a T cell-mediated autoimmune disease, few studies have considered contributing roles of innate immune deviations following otherwise innocuous infections as a cause underlying the immune defects that lead to BMF. Type-I-IFN signaling plays an important role in protecting hematopoiesis during systemic stress responses to the opportunistic fungal pathogen Pneumocystis. During Pneumocystis lung infection, mice deficient in both lymphocytes and type-I-IFN-receptor (IFrag−/−) develop rapidly progressing BMF associated with accelerated hematopoietic cell apoptosis. However, the communication pathway eliciting the induction of BMF in response to this strictly pulmonary infection has been unclear. We developed a conditional-null allele of Ifnar1 and used tissue-specific induction of the IFrag−/− state and found that, following Pneumocystis lung infection, type-I-IFNs act not only in the lung to prevent systemic immune deviations, but also within the progenitor compartment of the BM to protect hematopoiesis. In addition, transfer of sterile-filtered serum from Pneumocystis-infected mice as well as intra-peritoneal injection of Pneumocystis into uninfected IFrag−/− mice induced BMF. Although specific cytokine deviations contribute to induction of BMF, immune-suppressive treatment of infected IFrag−/− mice ameliorated its progression but did not prevent loss of hematopoietic progenitor functions. This suggested that additional, non-cytokine factors also target and impair progenitor functions; and interestingly, fungal β-glucans were also detected in serum. In conclusion, our data demonstrates that type-1-IFN signaling protects hematopoiesis within the BM compartment from the damaging effects of pro-inflammatory cytokines elicited by Pneumocystis in the lung and possibly at extra-pulmonary sites via circulating fungal components.

HIV infection results in a complex immunodeficiency due to loss of CD4؉ T cells, impaired type I interferon (IFN) responses, and B cell dysfunctions causing susceptibility to opportunistic infections such as Pneumocystis murina pneumonia and unexplained comorbidities, including bone marrow dysfunctions. Type I IFNs and B cells critically contribute to immunity to Pneumocystis lung infection. We recently also identified B cells as supporters of on-demand hematopoiesis following Pneumocystis infection that would otherwise be hampered due to systemic immune effects initiated in the context of a defective type I IFN system. While studying the role of type I IFNs in immunity to Pneumocystis infection, we discovered that mice lacking both lymphocytes and type I IFN receptor (IFrag Pneumocystis is a ubiquitous extracellular pulmonary fungal pathogen with strict species specificity. It is likely contracted via airborne transmission from often transiently infected individuals and commonly causes few or unspecific symptoms in otherwise healthy individuals leading to immunity (reviewed in references 1 and 2). However, Pneumocystis can cause severe and progressive interstitial pneumonia in patients with impaired acquired immunity with mortality rates up to 60% (3). While the total number of functional CD4 ϩ T cells critically determines increased susceptibility to Pneumocystis lung infection, patients with B cell defects are also at risk. In this regard, Pneumocystis pneumonia (PCP) is an AIDSdefining condition during HIV disease progression and commonly occurs when CD4 ϩ T cell counts drop below 200 cells/l (4). Furthermore, immune suppressive and cell ablative therapy following solid-organ transplantation, autoimmunity, or cancer treatment reduce CD4 ϩ T cell and/or B cell numbers and impair functions in non-HIV patients (reviewed in references 5 and 6). Drug regimens that predispose to severe Pneumocystis infections include high-dose glucocorticoid and B cell ablative treatments with rituximab (7-11). In addition, low-grade Pneumocystis infection is found in patients with potentially subtle immune suppressions such as young infants, HIV-positive patients receiving HAART (highly active antiretroviral therapy), or patients receiving low-dose and inhaled glucocorticoids (12)(13)(14). This can promote bronchial hyperreactivity, is associated with sudden infant death syndrome (SIDS) and exacerbation of chronic obstructive lung diseases (15)(16)(17)(18)(19).Pneumocystis colonization also intensifies signs of systemic inflammation (20, 21). Thus, Pneumocystis may act as a profound comorbidity factor that may also enhance secondary systemic disease manifestations associated with chronic pulmonary diseases and HIV infection such as osteoporosis or bone marrow dysfunctions (22-28). Immunity to Pneumocystis requires the presence of functional CD4 ϩ T cells to stimulate antigen-specific immune globulin production by B cells and macrophage-mediated phagocytosis (4,(29)(30)(31)(32)(33)(34)(35)(36)(37)(38). In addition, early innate t...

Although acquired bone marrow failure (BMF) is considered a T cell-mediated autoimmune disease, possible innate immune defects as a cause for systemic immune deviations in response to otherwise innocuous infections, have not been extensively explored. In this regard we recently demonstrated an important role of type-I-IFNs in protecting hematopoiesis during systemic stress responses to the opportunistic fungal pathogen Pneumocystis in lymphocyte-deficient mice. Mice deficient in both lymphocytes and type-I-IFN-receptor (IFrag−/− mice) develop rapidly progressing BMF due to accelerated bone marrow cell apoptosis associated with innate immune deviations in the bone marrow in response to Pneumocystis lung infection. However, the communication pathway between lung and bone marrow eliciting the induction of BMF in response to this strictly pulmonary infection has been unclear. Here we report that absence of an intact type-I-IFN-system during Pneumocystis lung infection not only causes BMF in lymphocyte-deficient mice but also transient bone marrow stress in lymphocyte-competent mice. This is associated with an exuberant systemic IFN-γ response. IFNγ neutralization prevented Pneumocystis lung infection-induced bone marrow depression in type-I-IFN-receptor-deficient (IFNAR−/−) mice, and prolonged neutrophil survival time in bone marrow from IFrag−/− mice. IL-1β and upstream regulators of IFNγ, IL-12 and IL-18, were also upregulated in lung and serum of IFrag−/− mice. In conjunction there was exuberant inflammasome-mediated caspase-1-activation in pulmonary innate immune cells required for processing of IL-18 and IL-1β. Thus, absence of type-I-IFN-signaling during Pneumocystis lung infection may result in deregulation of inflammasome-mediated pulmonary immune activation causing systemic immune deviations triggering BMF in this model.

Pulmonary hypertension subsequent to an infectious disease can be due to vascular structural remodeling or to functional alterations within various vascular cell types. In our previous mouse model of Pneumocystis-associated pulmonary hypertension, we found that vascular remodeling was not responsible for observed increases in right ventricular pressures. Here, we report that the vascular dysfunction we observed could be explained by an enhanced response to endothelin-1 (20% greater reduction in lumen diameter, P ≤ 0.05), corresponding to an up-regulation of similar magnitude (P ≤ 0.05) of the endothelin A receptor in the lung tissue. This effect was potentially augmented by a decrease in production of the pulmonary vasodilator adrenomedullin of almost 70% (P ≤ 0.05). These changes did not occur in interferon-γ knockout mice similarly treated, which do not develop pulmonary hypertension under these circumstances. Surprisingly, we did not observe any relevant changes in the vascular endothelial nitric oxide synthase vasodilatory response, which is a common potential site of inflammatory alterations to pulmonary vascular function. Our results indicate the diverse mechanisms by which inflammatory responses to prior infections can cause functionally relevant changes in vascular responses in the lung, promoting the development of pulmonary hypertension.