Immunity and tolerance to Aspergillus fumigatus.

The inherent resistance to diseases caused by Aspergillus fumigatus suggests the occurrence of regulatory mechanisms that provide the host with adequate defence without necessarily eliminating the fungus or causing unacceptable levels of host damage. Efficient responses to the fungus require different mechanisms of immunity. Dendritic cells (DCs) are uniquely able to decode the fungus-associated information and translate it into qualitatively different T helper (Th) and regulatory (Treg) cell responses. A division of labour occurred between functionally distinct Treg that were coordinately activated by a CD28/B.7-dependent costimulatory pathway after exposure of mice to Aspergillus conidia. Early in infection, inflammation was controlled by the expansion, activation and local recruitment of CD4+CD25+ Treg capable of suppressing neutrophils through the combined actions of interleukin (IL10) and cytotoxic T lymphocyte antigen 4 (CTLA4) on indoleamine 2,3-dioxygenase (IDO). The levels of IFNgamma produced in this early phase set the subsequent adaptive stage by conditioning the IDO-dependent tolerogenic program of DCs and the subsequent activation and expansion of tolerogenic Treg, which produced IL10 and transforming growth factor (TGF)beta, inhibited Th2 cells, and prevented allergy to the fungus. Thus, regulation is an essential component of the host response in infection and allergy to the fungus, and its manipulation may allow the pathogen to overcome host resistance and promote disease.