Chemokine-mediated recruitment of NK cells is a critical host defense mechanism in invasive aspergillosis

Immunity against the opportunistic fungal pathogen Pneumocystis

Species of the genus Pneumocystis exist as opportunistic fungal pathogens and are associated with severe pneumonia and pulmonary complications in immunocompromised individuals. Although prophylactic therapy for Pneumocystis has significantly decreased the overall incidence of infection, more than 80% of cases in current patient populations are considered breakthrough cases. In the HIV-infected population, in the years following the initiation of highly active antiretroviral therapy (HAART), significant reductions in the incidence of Pneumocystis infection were observed, although trends over the last several years suggest that the incidence of Pneumocystis has plateaued rather than decreased. Furthermore, with the more prominent usage of immunosuppressive therapies, the frequency of Pneumocystis infection in the HIV-negative population, such as those with hematologic malignancies and those who have undergone transplantation, has risen significantly. Investigating host defense mechanisms against P. carinii has historically been problematic due to the difficulty in achieving continuous in vitro propagation of proliferating Pneumocytis organisms. Nevertheless, clinical and experimental studies have documented that host defense against Pneumocystis involves a concerted effort between innate, cell-mediated (T lymphocyte) and humoral (B lymphocyte) responses. However, the pulmonary environment is a tissue site where heightened inflammatory responses can often lead to inflammation-mediated injury, thereby contributing to the pathogenesis of Pneumocystis infection. Accordingly, clearance of Pneumocystis from the pulmonary environment is dependent on a delicate equilibrium between the inflammatory response and immune-mediated clearance of the organism. Furthermore, innate and adaptive responses against Pneumocystis are strikingly similar to those against other medically-important fungi, thus providing additional evidence that Pneumocystis exists as a fungal organism.

The pathogenesis of fatal outcome in murine pulmonary aspergillosis depends on the neutrophil depletion strategy

Aspergillus fumigatus causes invasive disease in severely immunocompromised hosts but is readily cleared when host innate defenses are intact. Animal models for evaluation of therapeutic strategies to combat invasive aspergillosis that closely mimic human disease are desirable. We determined optimal dosing regimens for neutrophil depletion and evaluated the course of infection following aerosol infection in mice by determining survival, organ fungal burden, and histopathology in mice in which neutropenia was induced by three methods, administration of granulocyte-depleting monoclonal antibody RB6-8C5 (MAb RB6), administration of cyclophosphamide, and administration of both agents. Administration of either individual agent resulted in a requirement for relatively high conidial inocula to achieve 100% mortality in both BALB/c and C57BL/6 mice, although the infection appeared to be somewhat more lethal in C57BL/6 mice. Death following induction of neutropenia with MAb RB6 occurred when a relatively low fungal burden was present in the lung and may have been related to the inflammatory response associated with neutrophil recovery. In contrast, administration of both agents reduced the lethal inoculum in each mouse strain by approximately 1 log(10), and C57BL/6 mice that received both agents had a higher fungal burden and less inflammation in the lung at the time of death than BALB/c mice or mice of either strain that received MAb RB6 alone. Our data suggest that the relationship among fungal burden, inflammation, and death is complex and can be influenced by the immunosuppression regimen, the mouse strain, and the inoculum.

Cytokines in host defense againstAspergillus: recent advances

Many aspects of antimicrobial host responses are orchestrated by a complex network of cytokines and their receptors. This review focuses on recent progress in our understanding of the function of cytokines in innate immune responses to Aspergillus. TNF, a recognition cytokine, has been shown to be required for initiation of the innate response in the mouse model of invasive aspergillosis. Several recruitment cytokines play critical roles in mediating influx of specific leukocytes to the site of infection in invasive aspergillosis. Among these, the ELR + subset of CXC chemokines and their receptor CXCR2 are critical to neutrophil recruitment, while CCL3/macrophage inflammatory protein (MIP)-1alpha and CCL2/ monocyte chemoattractant protein (MCP)-1 are critical to recruitment of monocyte-lineage leukocytes and NK cells, respectively. Of the activation cytokines, those associated with the Th-1 phenotype, including interleukin (IL)-12, IL-18, and interferon-gamma (IFN-gamma), are critical to protective responses to the infection. Conversely, the Th2-phenotype cytokines IL-4 and IL-10 contribute to progression of infection. Modulation of the immune response to Aspergillus by manipulating these mediators remains intriguing as a potential adjunctive treatment in patients with invasive aspergillosis.

Early activation of NK cells after lung infection with the intracellular bacterium, Francisella tularensis LVS

Francisella tularensis is a gram-negative intracellular bacterium that has been classified as a Category A biothreat because of its ability to induce deadly pneumonic tularemia when inhaled. In the present study, an experimental model of F. tularensis LVS intranasal infection was used to study the immune cells involved in cytokine secretion in the lungs after infection. Dramatic increases in the numbers of cells secreting IFN-gamma were observed 72 h after intranasal infection of BALB/c and C57BL/6 mice with sublethal (1000 CFU) or lethal (10,000 CFU) doses of F. tularensis LVS and the cells primarily responsible for this IFN-gamma expression were identified as CD11b+ DX5+ NK cells. The findings were further confirmed in C57BL/6 mice showing that cells responsible for IFN-gamma secretion in the lungs were CD11b+ DX5+ NK1.1+. NK cell depletion studies showed a decrease in the percentage of IFN-gamma secreting cells, due not only to a diminished proportion of IFN-gamma secreting NK cells, but also to a reduced percentage of T cells secreting IFN-gamma. The results indicate that IFN-gamma is secreted in response to respiratory infection with F. tularensis LVS, and that NK cells are the early responders responsible for IFN-gamma secretion.

Fungal infections in bone marrow transplant patients

PURPOSE OF REVIEW

Invasive fungal infections have become the leading infectious cause of death in recipients of hematopoietic cell transplantation. Several factors have led to a renaissance in the study of invasive fungal infections. The growing incidence of both commonly encountered as well as emerging pathogens and the lethality of these infections coupled with the unprecedented number of available broad-spectrum antifungal drugs has lent a renewed vigor and enthusiasm to attempts to understand the pathogenesis of these diseases and, by doing so, improve prevention, diagnosis, and treatment. The following is a review of the primary research published from 2003 to the present that is pertinent to invasive fungal infection in the setting of hematopoietic cell transplantation.

RECENT FINDINGS

The main themes of published primary research during 2003 to the present include the efficacy and tolerability of antifungal prophylaxis, epidemiologic analyses of risk factors following nonmyeloablative preparative regimens, and more-detailed analyses of nonmyeloid immune responses.

SUMMARY

Although few definitive recommendations emerged from the studies during the review period, these investigations do contribute to a greater understanding of the immunobiology of invasive fungal infection and of the utility and limitations of newer antifungal agents in the prophylaxis or treatment of invasive fungal infection.