State of the Field: Cytotoxic Immune Cell Responses in C. neoformans and C. deneoformans Infection

Abstract

Cryptococcus neoformans is an environmental pathogen that causes life-threatening disease in immunocompromised persons. The majority of immunological studies have centered on CD4⁺ T-cell dysfunction and associated cytokine signaling pathways, optimization of phagocytic cell function against fungal cells, and identification of robust antigens for vaccine development. However, a growing body of literature exists regarding cytotoxic cells, specifically CD8⁺ T-cells, Natural Killer cells, gamma/delta T-cells, NK T-cells, and Cytotoxic CD4⁺ T-cells, and their role in the innate and adaptive immune response during C. neoformans and C. deneoformans infection. In this review, we (1) provide a comprehensive report of data gathered from mouse and human studies on cytotoxic cell function and phenotype, (2) discuss harmonious and conflicting results on cellular responses in mice models and human infection, (3) identify gaps of knowledge in the field ripe for exploration, and (4) highlight how innovative immunological tools could enhance the study of cytotoxic cells and their potential immunomodulation during cryptococcosis.

Keywords: CD4+ T-cells; CD8+ T-cells; Cryptococcus neoformans; advanced immunodeficiency virus (AIDS); cryptococcal meningitis; cytotoxic cells; fungal infection; gamma/delta T-cells; human immunodeficiency virus (HIV); natural killer cells.

Figure 1

Murine and Human CD8⁺ T-cell Sensing, Signaling, and Response to C. neoformans and C. deneoformans. Summary of the current body of literature regarding the mechanism of (A) murine and (B) human CD8⁺ T-cell targeting, degranulation, and cytokine secretion after exposure to C. neoformans or deneoformans. (A) Despite depletion of murine CD8⁺ T-cells in the lung and systemic circulation leading to increased CFUs, the exact receptor(s) and fungal ligand(s) which trigger cytotoxicity and the cytolytic proteins secreted are unknown. During infection, infiltrating pulmonary and systemic CD8⁺ T-cells are robust producers of IFN-γ. (B) Similar to murine CD8⁺ T-cells, the exact receptor(s) and fungal ligand(s) which facilitate cytotoxicity of human CD8⁺ T-cells are unknown. Upon exposure to fungal cells, human CD8⁺ T-cells increase transcription of the cytolytic protein granulysin and depletion of granulysin abrogates CD8⁺ T-cell anti-cryptococcal effects. The mechanism by which granules containing granulysin are selectively secreted, over those containing perforin or other cytolytic molecules, remains to be determined. IL-15 stimulation enhances human CD8⁺ T-cell anti-cryptococcal effects while IL-2R expression increases upon coculture with fungal cells. However, the cytokines released by activated CD8⁺ T-cells during human cryptococcal meningitis have not been identified.

Figure 2

Murine and Human NK cell Sensing, Signaling, and Response to C. neoformans and C. deneoformans. Summary of the current body of literature regarding the mechanism of (A) murine and (B) human NK cell targeting, degranulation, and cytokine secretion after exposure to C. neoformans or deneoformans. (A) Murine NK cells interact with encapsulated Cryptococcus cells via “microvilli” or cellular protrusions, though the receptor or integrin interacting with the fungal cell capsule remains unknown. Though anti-cryptococcal activity of murine NK cells has been documented, the mechanism is undetermined but likely involves intracellular signaling pathways which are not mediated by DAP12 motifs. Murine NK cells are robust producers of the inflammatory cytokine IFN-γ during infection; the production of other inflammatory cytokines is unknown. (B) Human NK cell activating receptor NKp30 recognizes cryptococcal antigen β-1,3-glucan on the fungal cell wall which leads to the phosphorylation of intracellular tyrosine motifs by Src family kinases Fyn and Lyn and increase perforin transcription. Stimulation with IL-12 enhances NKp30 expression on NK cells from donors with HIV. The downstream signaling cascade remains to be elucidated. Granules containing granulysin and perforin are transported to the NK cell membrane via Erg5-kinesin. Perforin is released at the immunological synapse facilitating NK cell anti-cryptococcal effects and reducing fungal growth. However, it is unclear how perforin penetrates the Cryptococcus fungal cell wall. Upon exposure to Cryptococcus cells, NK cells increase secretion of chemoattractant IP-10 and reduce secretion of the inflammatory cytokine TNF-α and activating cytokine GM-CSF.