L18- Fungi – Host-Pathogen Interactions

Fungal pathogens pose a significant global health risk, particularly as they relate to host-pathogen interactions.
The World Health Organization (WHO) published a fungal pathogens priority list in 2022, highlighting under-recognition, morbidity, mortality risks, and increasing drug resistance as critical concerns.

Annually, approximately 68 million deaths are reported worldwide, with leading causes accounting for about 57% of these fatalities.
A significant portion of these deaths is linked to non-communicable diseases and injuries.

Criteria for Prioritization: WHO identified 10 criteria to score and prioritize 19 different fungi based on their potential health impact (e.g., transmission, mortality rates).

Immunocompromised Populations: Fungi have always presented a risk to those with weakened immune systems, which has increased with advancements in therapies for cancer and organ failure.
Environmental Resilience: Fungi are proficient recyclers and their viability is influenced by climate change, as it lowers the thermal barrier for many species.
Changing Interactions with Humans: Increased global mobility alters the geographic distribution of fungal species, raising the risk of invasive species (e.g., amphibian chytridiomycosis, white-nose syndrome).

Aging Populations: As populations age, vulnerability to fungal infections increases.
Rising Drug Resistance: Traditional antifungal treatments face growing resistance, complicating treatment options.

The understanding of how fungal pathogens interact with host immune systems is still developing.
Example: Cryptococcus pathogens exemplify certain immune evasion tactics.
Species of Interest:
- Cryptococcus neoformans: Prominent since HIV/AIDS, responsible for ~112,000 deaths in 2020, primarily among immunocompromised individuals.
- Cryptococcus gattii: Identified as a distinct species causing disease in immunocompetent hosts; noted for outbreaks in regions like Vancouver Island in the late 1990s.

Titanisation is a crucial process in the immune evasion of Cryptococcus species, where cells grow larger and avoid phagocytosis.
Titan cells can be significantly larger than normal cells, and even polyploid, making them less susceptible to immune responses.

Drop or micro cells formed by Cryptococcus contribute to its pathogenicity, with these smaller cells being more resilient during infections.

Entry: Cryptococcus typically enters through the lungs, where it is vulnerable to phagocytosis.
Immune Evasion: Once inside the host, Cryptococcus employs various tactics to evade immune responses, such as inducing an anti-inflammatory reaction instead of the expected pro-inflammatory responses.

Pathogens must navigate nutritional deprivation imposed by host macrophages, which limit access to essential metals (Fe, Zn, Cu, Mn).
Fungal pathogens can exhibit sophisticated adaptations, such as utilizing the glyoxylate cycle for converting fats to glucose in nutrient-scarce environments.

Candida auris: This fungal pathogen emerged in 2009 and has notable resistance patterns against many antifungals, leading to high mortality rates in healthcare settings.
- Its rapid spread across continents raises alarms about its virulence.

The Amoeboid Predator-Fungal Animal Virulence Hypothesis suggests that interactions with environmental phagocytes can shape virulence traits, providing insights into how fungi evolve to escape immune detection.

Fungal pathogens represent a growing public health concern exacerbated by environmental and societal changes.
Understanding the mechanisms of pathogenicity, immune evasion, and host susceptibility is crucial for developing effective responses to fungal infections.
Further Reading: For a deeper understanding, reference materials suggested include various articles from Nature Reviews, PloS Pathogens, and the WHO fungal priority pathogens report.