Susceptibility of the human pathogenic fungi Cryptococcus neoformans and Histoplasma capsulatum to gamma-radiation versus radioimmunotherapy with alpha- and beta-emitting radioisotopes.

UNLABELLED

Fungal diseases are difficult to treat in immunosuppressed patients and, consequently, new approaches to therapy are urgently needed. One novel strategy is to use radioimmunotherapy (RIT) with fungal-binding monoclonal antibodies (mAbs) labeled with radionuclides. However, many fungi manifest extreme resistance to gamma-radiation, such that the doses of several thousand gray are required for 90% cell killing, whereas for mammalian cells the lethal dose is only a few gray. We compared the susceptibility of human pathogenic fungi Cryptococcus neoformans (CN) and Histoplasma capsulatum (HC) to external gamma-radiation and to the organism-specific mAbs 18B7 and 9C7, respectively, conjugated to (213)Bi and (188)Re radionuclides.

METHODS

CN and HC cells were irradiated with up to 8,000 Gy ((137)Cs source, 30 Gy/min). RIT of CN with (213)Bi- and (188)Re-labeled specific mAb and of HC with (188)Re-labeled specific mAb used 0-1.2 MBq per 10(5) microbial cells. After irradiation or RIT, the cells were plated for colony-forming units (CFUs). Cellular dosimetry calculations were performed, and the pathway of cell death after irradiation was evaluated by flow cytometry.

RESULTS

Both CN and HC proved to be extremely resistant to gamma-radiation such that significant killing was observed only for doses of >4,000 Gy. In contrast, these cells were much more susceptible to killing by radiation delivered with a specific mAb, such that a 2-logarithm reduction in colony numbers was achieved by incubating them with (213)Bi- and (188)Re-labeled mAb 18B7 or with (188)Re-9C7 mAb. Dosimetry calculations showed that RIT was approximately 1,000-fold more efficient in killing CN and approximately 100-fold more efficient in killing HC than gamma-radiation. Both gamma-radiation and RIT caused cell death via an apoptotic-like pathway with a higher percentage of apoptosis observed in RIT-treated cells.

CONCLUSION

Conjugating a radioactive isotope to a fungal-specific antibody converted an immunoglobulin with no antifungal activity into a microbicidal molecule. RIT of fungal cells using specific antibodies labeled with alpha- and beta-emitting radioisotopes was significantly more efficient in killing CN and HC than gamma-radiation when based on the mean absorbed dose to the cell. These results strongly support the concept of using RIT as an antimicrobial modality.