RIPK3/Fas-Associated Death Domain Axis Regulates Pulmonary Immunopathology to Cryptococcal Infection Independent of Necroptosis

Abstract

Fas-associated death domain (FADD) and receptor interacting protein kinase 3 (RIPK3) are multifunctional regulators of cell death and immune response. Using a mouse model of cryptococcal infection, the roles of FADD and RIPK3 in anti-cryptococcal defense were investigated. Deletion of RIPK3 alone led to increased inflammatory cytokine production in the Cryptococcus neoformans-infected lungs, but in combination with FADD deletion, it led to a robust Th1-biased response with M1-biased macrophage activation. Rather than being protective, these responses led to paradoxical C. neoformans expansion and rapid clinical deterioration in Ripk3−/− and Ripk3−/−Fadd−/− mice. The increased mortality of Ripk3−/− and even more accelerated mortality in Ripk3−/−Fadd−/− mice was attributed to profound pulmonary damage due to neutrophil-dominant infiltration with prominent upregulation of pro-inflammatory cytokines. This phenomenon was partially associated with selective alterations in the apoptotic frequency of some leukocyte subsets, such as eosinophils and neutrophils, in infected Ripk3−/−Fadd−/− mice. In conclusion, our study shows that RIPK3 in concert with FADD serve as physiological “brakes,” preventing the development of excessive inflammation and Th1 bias, which in turn contributes to pulmonary damage and defective fungal clearance. This novel link between the protective effect of FADD and RIPK3 in antifungal defense and sustenance of immune homeostasis may be important for the development of novel immunomodulatory therapies against invasive fungal infections.

Cryptococcus neoformans-Host Interactions Determine Disease Outcomes

“…Rather than being protective, this robust host response was deleterious and is associated with paradoxical fungal growth and rapid clinical deterioration (Figure 2). These findings showed that excessive inflammation can mediate tissue damage and host disease during cryptococcal infection [93]. Furthermore, the balance between Th1 and Th17 immune responses plays important roles in optimizing clearance and minimizing inflammatory damage to the host tissues during fungal infections.…”

“…Although generation of the Th1/Th17 response and subsequent M1 activation play a critical role in controlling fungal growth, excessive immune responses can become destructive and cause lung immunopathology following fungal infection. Recent studies demonstrated that FADD and RIPK3 proteins, which are mediators of death receptor-triggered extrinsic apoptosis, play a crucial immune regulatory role in preventing excessive inflammation during C. neoformans infection [93]. Deletion of RIPK3 and FADD led to a robust Th1-biased response with M1-biased macrophage activation, which is accompanied by marked upregulation of cytokines like TNF-α, IL-1α, IL-1β, IL6, and IFN-γ (Figure 2).…”

“…Collectively, these data define an important role for DC in regulating the initial innate and adaptive response following fungal infections. [93].…”

Resistance and Tolerance to Cryptococcal Infection: An Intricate Balance That Controls the Development of Disease

“…Host resistance during cryptococcal infection is characterized by the expression of pro-inflammatory cytokines, recruitment of inflammatory DCs, and generation of Th1/Th17 immune responses that is followed by classical activation of macrophages (50, 51, 119, 121–126). However, excessive inflammation and robust Th1/Th17 responses that provide sterilizing immunity can induce severe pathology and damage to the host (59, 127–133). It has been proposed that a tightly regulated combination of pro-inflammatory and anti-inflammatory stimuli is crucial for effective control of fungal infection (134–136).…”

“…Fas-associated death domain (FADD) and receptor interacting protein kinase 3 (RIPK3): The FADD protein is a key mediator of death receptor-triggered extrinsic apoptosis, which plays a crucial immune regulatory role at the site of infection and prevents excessive inflammation (127). Deletion of RIPK3 in combination with FADD led to a robust Th1-biased response with M1-biased macrophage activation, yet this host response was deleterious in a mouse model of cryptococcal infection.…”