Toll-like receptor 4 (TLR4) is the major pattern recognition receptor triggering the protective effect of a
Candida albicans
extracellular vesicle-based vaccine prototype in murine systemic candidiasis
Authors: Honorato Leandro , Bonilla Jhon J. Artunduaga , Valdez Alessandro F. , Frases Susana , Araújo Glauber Ribeiro de Sousa , Sabino Albaniza Liuane Ribeiro do Nascimento , da Silva Natalia Martins , Ribeiro Larissa , Ferreira Marina da Silva , Kornetz Julio , Rodrigues Marcio L. , Cunningham Iain , Gow Neil A. R. , Gacser Attila , Guimarães Allan J. , Dutra Fabianno F. , Nimrichter Leonardo 
Published: July 2024

Systemic candidiasis remains a significant public health concern worldwide, with high mortality rates despite available antifungal drugs. Drug-resistant strains add to the urgency for alternative therapies. In this context, vaccination has reemerged as a prominent immune-based strategy. Extracellular vesicles (EVs), nanosized lipid bilayer particles, carry a diverse array of native fungal antigens, including proteins, nucleic acids, lipids, and glycans. Previous studies from our laboratory demonstrated that
Candida albicans
EVs triggered the innate immune response, activating bone marrow-derived dendritic cells (BMDCs) and potentially acting as a bridge between innate and adaptive immunity. Vaccination with
C. albicans
EVs induced the production of specific antibodies, modulated cytokine production, and provided protection in immunosuppressed mice infected with lethal
C. albicans
inoculum. To elucidate the mechanisms underlying EV-induced immune activation, our study investigated pathogen-associated molecular patterns (PAMPs) and pattern recognition receptors (PRRs) involved in EVs-phagocyte engagement. EVs from wild-type and mutant
C. albicans
strains with truncated mannoproteins were compared for their ability to stimulate BMDCs. Our findings revealed that EV decoration with
O
- and
N
-linked mannans and the presence of
β
-1,3-glucans and chitin oligomers may modulate the activation of specific PRRs, in particular Toll-like receptor 4 (TLR4) and dectin-1. The protective effect of vaccination with wild-type EVs was found to be dependent on TLR4. These results suggest that fungal EVs can be harnessed in vaccine formulations to selectively activate PRRs in phagocytes, offering potential avenues for combating or preventing candidiasis.

IMPORTANCE

Systemic candidiasis is a serious global health concern with high mortality rates and growing drug resistance. Vaccination offers a promising solution. A unique approach involves using tiny lipid-coated particles called extracellular vesicles (EVs), which carry various fungal components. Previous studies found that
Candida albicans
EVs activate the immune response and may bridge the gap between innate and adaptive immunity. To understand this better, we investigated how these EVs activate immune cells. We demonstrated that specific components on EV surfaces, such as mannans and glucans, interact with receptors on immune cells, including Toll-like receptor 4 (TLR4) and dectin-1. Moreover, vaccinating with these EVs led to strong immune responses and full protection in mice infected with
Candida
. This work shows how harnessing fungal EVs might lead to effective vaccines against candidiasis.