ACQUIRED-IMMUNITY IN EXPERIMENTAL MURINE ASPERGILLOSIS IS MEDIATED BYMACROPHAGES

A number of studies have substantiated the pivotal role of innate defe nse mechanisms in protection against invasive aspergillosis. However, experiments demonstrating increased resistance to lethal intravenous ( i.v.) infection with Aspergillus fumigatus conidia in cortisone-treate d or untreated mice preinfected with a sublethal dose of conidia and p rotection of turkeys inoculated subcutaneously with a killed A. fumiga tus germling vaccine against subsequent aerosol challenge led us to sp eculate that acquired immunity may also contribute to host defense aga inst Aspergillus infection. Five-week-old male BALB/c mice were inocul ated i.v. with 1.0 x 10(4) viable conidia or saline and challenged i.v . with 1.0 X 10(6) conidia after 7, 15, or 21 days. No protection agai nst challenge was found after 7 days. However, significant and reprodu cible protection was observed after 15 and 21 days. Mortality was redu ced from 90% in control mice to 53% in preinfected mice 40 days after challenge (P = 0.0002). Increased survival was correlated with decreas ed content of chitin in lungs, liver, and kidneys 4 and 7 days after c hallenge (P < 0.05). Mice were again inoculated with 1.0 X 10(4) conid ia or saline, and after 21 days, 1.0 x 10(8) or 2.0 x 10(8) splenocyte s were transferred to naive syngeneic recipients; 2.0 x 10(8) immune s plenocytes conferred significant protection (P = 0.0001) against i.v. challenge with 1.0 x 10(6) conidia, and mortality decreased from 83 to 48% 40 days after challenge. Transfer of immune serum offered no prot ection despite the presence of antibody against a hyphal homogenate of A. fumigatus, which was absent in the sera of control mice. Protectio n by immune splenocytes was maintained after selective depletion of T cells but was abolished after removal of plastic-adherent splenocytes. Adherent cells were characterized as macrophages by using morphologic al criteria, nonspecific esterase, and MAC-1 monoclonal antibody. Prod uction of hydrogen peroxide by peritoneal and splenic macrophages from preinfected mice was the same as and lower than, respectively, that f rom uninfected controls. However, phagocytosis of conidia by peritonea l or splenic macrophages from mice preinfected i.v. or intratracheally was significantly increased after 2 and 3 h of coculture compared wit h that from uninfected animals, whereas in vitro killing of conidia by splenic macrophages was unaltered. Peritoneal or splenic macrophages from control or preinfected mice failed to kill hyphae in vitro. Killi ng of hyphae by polymorphonuclear leukocytes was not significantly dif ferent between mice preinfected i.v. and uninfected controls. Taken to gether, the results indicate that acquired immunity mediated by activa ted macrophages can be demonstrated in experimental murine aspergillos is. Although the mechanism is present biologically, its relevance agai nst the invasive hyphal form of A. fumigatus is doubtful.