Mannoprotein Cig1 contributes to the immunogenicity of a heat-killed F-box protein Fbp1 Cryptococcus neoformans vaccine model

TL;DR

This study identifies Cig1, a mannoprotein, as a key factor in the immunogenicity of a heat-killed Cryptococcus neoformans vaccine candidate.

Contribution

The study reveals that Cig1 contributes to the protective immune response induced by the HK-fbp1 vaccine.

Findings

Cig1 is regulated by Fbp1 and contributes to the immunogenicity of the HK-fbp1 vaccine.

Deletion of Cig1 reduces recruitment of antifungal T cells and cytokine production.

Loss of Cig1 diminishes vaccine protection at lower inoculum doses.

Abstract

Currently, no fungal vaccine exists for clinical use, while fungal infections are responsible for over 1.5 million deaths every year. Our previous studies identified a Cryptococcus neoformans mutant strain fbp1Δ as a potential vaccine candidate. This strain contains deletion of the F-box protein Fbp1, a key subunit of the SCF E3 ligase complex necessary for ubiquitin-mediated proteolysis. Vaccination with heat-killed fbp1Δ (HK-fbp1) can elicit an interferon gamma (IFN-γ)-dependent Type 1 immune response and provide protection against C. neoformans and its sibling species C. gattii. However, we have yet to decipher the immunogenic factor(s) expressed by the fbp1∆ mutant that are responsible for the induction of the protective immune response. In this study, we have identified that the capsule plays an important role in HK-fbp1 vaccine-mediated protection as acapsular HK-fbp1 cells showed diminished protection against wild-type challenge. Additionally, our studies have shown that Cytokine Inducing Glycoprotein 1 (Cig1), a GPI-anchored mannoprotein, is regulated by Fbp1 and contributes to the immunogenicity of HK-fbp1. Deletion of Cig1 in the fbp1Δ background resulted in decreased recruitment of antifungal effector T cells and diminished production of protective inflammatory cytokines by the host. Furthermore, loss of Cig1 in the fbp1Δ mutant resulted in reduced protection in vaccination survival studies at lower vaccine inoculum doses compared to HK-fbp1. In aggregate, these findings demonstrate Cig1 is an antigen contributing to the immunogenicity of HK-fbp1 that may be utilized to further optimize the HK-fbp1 fungal vaccine as a tool in the arsenal against invasive fungal infections.

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