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

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 <i>Cryptococcus neoformans</i> mutant strain <i>fbp1</i>Δ 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 <i>fbp1</i>Δ (HK-<i>fbp1</i>) can elicit an interferon gamma (IFN-γ)-dependent Type 1 immune response and provide protection against <i>C. neoformans</i> and its sibling species <i>C. gattii</i>. However, we have yet to decipher the immunogenic factor(s) expressed by the <i>fbp1</i>∆ 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-<i>fbp1</i> vaccine-mediated protection as acapsular HK-<i>fbp1</i> 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-<i>fbp1</i>. Deletion of Cig1 in the <i>fbp1</i>Δ 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 <i>fbp1</i>Δ mutant resulted in reduced protection in vaccination survival studies at lower vaccine inoculum doses compared to HK-<i>fbp1</i>. In aggregate, these findings demonstrate Cig1 is an antigen contributing to the immunogenicity of HK-<i>fbp1</i> that may be utilized to further optimize the HK-<i>fbp1</i> fungal vaccine as a tool in the arsenal against invasive fungal infections.