Immunity and tolerance to infections in experimental hematopoietic transplantation

Strain dependent variation of immune responses to A. fumigatus: definition of pathogenic species

For over a century microbiologists and immunologist have categorized microorganisms as pathogenic or non-pathogenic species or genera. This definition, clearly relevant at the strain and species level for most bacteria, where differences in virulence between strains of a particular species are well known, has never been probed at the strain level in fungal species. Here, we tested the immune reactivity and the pathogenic potential of a collection of strains from Aspergillus spp, a fungus that is generally considered pathogenic in immuno-compromised hosts. Our results show a wide strain-dependent variation of the immune response elicited indicating that different isolates possess diverse virulence and infectivity. Thus, the definition of markers of inflammation or pathogenicity cannot be generalized. The profound understanding of the molecular mechanisms subtending the different immune responses will result solely from the comparative study of strains with extremely diverse properties.

Neutrophils mediate maturation and efflux of lung dendritic cells in response to Aspergillus fumigatus germ tubes

Invasive aspergillosis is a life-threatening complication of neutrophil deficiency or dysfunction. Neutropenia has previously been associated with enhanced influx of CD11b-expressing conventional dendritic cells to the lungs in response to Aspergillus species, but whether neutrophils directly modulate the function of dendritic cells in this infection is not known. We hypothesized that, in the setting of intrapulmonary challenge with Aspergillus, neutrophils promote the maturation and traffic of lung conventional dendritic cells to draining mediastinal lymph nodes. We report that neutropenia results in a marked accumulation of dendritic cells in the lungs of mice challenged with Aspergillus but greatly diminishes their egress to mediastinal lymph nodes independent of neutrophil microbicidal functions. Furthermore, the phenotype of lung dendritic cells was more immature in neutropenic animals than in nonneutropenic mice exposed to the microorganism. Consistent with this, coincubation with neutrophils greatly enhanced the upregulation of costimulatory molecules on dendritic cells exposed to Aspergillus in vitro, a process that was dependent on cell contact and the dendritic cell receptor DC-SIGN. Taken together, our data support an immunomodulatory cross talk between neutrophils and dendritic cells in the context of host response to Aspergillus that promotes the maturation and efflux of lung dendritic cells.