title: An acidic microenvironment increases NK cell killing of Cryptococcus neoformans and Cryptococcus gattii by enhancing perforin degranulation.
abstract: Cryptococcus gattii and Cryptococcus neoformans are encapsulated yeasts that can produce a solid tumor-like mass or cryptococcoma. Analogous to malignant tumors, the microenvironment deep within a cryptococcoma is acidic, which presents unique challenges to host defense. Analogous to malignant cells, NK cells kill Cryptococcus. Thus, as in tumor defense, NK cells must kill yeast cells across a gradient from physiologic pH to less than 6 in the center of the cryptococcoma. As acidic pH inhibits anti-tumor activities of NK cells, we sought to determine if there was a similar reduction in the anticryptococcal activity of NK cells. Surprisingly, we found that both primary human NK cells and the human NK cell line, YT, have preserved or even enhanced killing of Cryptococcus in acidic, compared to physiological, pH. Studies to explore the mechanism of enhanced killing revealed that acidic pH does not increase the effector to target ratio, binding of cytolytic cells to Cryptococcus, or the active perforin content in effector cells. By contrast, perforin degranulation was greater at acidic pH, and increased degranulation was preceded by enhanced ERK1/2 phosphorylation, which is essential for killing. Moreover, using a replication defective ras1 knockout strain of Cryptococcus increased degranulation occurred during more rapid replication of the organisms. Finally, NK cells were found intimately associated with C. gattii within the cryptococcoma of a fatal infection. These results suggest that NK cells have amplified signaling, degranulation, and greater killing at low pH and when the organisms are replicating quickly, which would help maintain microbicidal host defense despite an acidic microenvironment.
authors.affiliation: Department of Microbiology, Immunology and Infectious Diseases, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Anowara
authors.initials: A
authors.lastname: Islam
authors.forename: Shu Shun
authors.initials: SS
authors.lastname: Li
authors.forename: Paul
authors.initials: P
authors.lastname: Oykhman
authors.forename: Martina
authors.initials: M
authors.lastname: Timm-McCann
authors.forename: Shaunna M
authors.initials: SM
authors.lastname: Huston
authors.forename: Danuta
authors.initials: D
authors.lastname: Stack
authors.forename: Richard F
authors.initials: RF
authors.lastname: Xiang
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Kelly
authors.forename: Christopher H
authors.initials: CH
authors.lastname: Mody
mesh_terms: D002448:Cell Adhesion; D015550:Cell Degranulation; D002460:Cell Line; D002478:Cells, Cultured; D060833:Cellular Microenvironment; D002540:Cerebral Cortex / Q000276:immunology / Q000378:metabolism / Q000382:microbiology / Q000473:pathology; D003453:Cryptococcosis / Q000276:immunology / Q000378:metabolism / Q000382:microbiology / Q000473:pathology; D056285:Cryptococcus gattii / Q000276:immunology* / Q000502:physiology; D003455:Cryptococcus neoformans / Q000276:immunology* / Q000502:physiology; D003602:Cytotoxicity, Immunologic; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007694:Killer Cells, Natural / Q000276:immunology* / Q000378:metabolism; D008168:Lung / Q000276:immunology / Q000378:metabolism / Q000382:microbiology / Q000473:pathology; D020935:MAP Kinase Signaling System; D054353:Perforin / Q000378:metabolism*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D015854:Up-Regulation; D014779:Virus Replication; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D054353:Perforin; D018631:ras Proteins