Development of an HK-fbp1-based vaccine platform against Cryptococcus neoformans

Cryptococcal meningitis remains a leading cause of death among HIV/AIDS patients, and no antifungal vaccine is available in clinical use. We previously showed that mice immunized with a heat-killed preparation of Cryptococcus neoformans ( Cn ) fbp1 Δ mutant (HK-fbp1) are protected against cryptococcosis and other invasive mycoses, suggesting that HK-fbp1 is an intriguing vaccine candidate for further study. Recent studies have reported that heat-killed wild type H99 (HK-H99) can also offer some protection in other vaccination models. Here we set out to decipher the differential protective capacity of HK-fbp1 versus HK-H99 across multiple conditions. We observed that HK-fbp1 confers superior protection in both prophylactic and therapeutic intranasal immunization settings than HK-H99, which was associated with greater activation of pulmonary CD4⁺ T cells and stronger airway IFN- γ production. In addition, HK-fbp1 preparations were highly stable, retaining full protective efficacy even after 5 years of frozen storage. Mice intranasally immunized with two-dose HK-fbp1 were protected against challenge by up to 10⁶ H99 cells, a 100-fold increase over the commonly used lethal inoculum. Addition of a CpG adjuvant ODN to HK-fbp1 (HK-bp1/ODN) led to a 5-fold reduction of the intranasal vaccine inoculum required for protection, and itssingle-dose intramuscular vaccination induced protection persisted for at least 3 months post vaccination. Together, these findings highlight the robust protective capacity and formulation versatility of HK-fbp1, supporting its further development as a stable, dose-sparing antifungal vaccine candidate.

Vaccines against viral and bacterial infections have had a transformative impact in our fight against infectious diseases. However, there are no fungal vaccines currently approved for clinical use. This study provides a full characterization of a potent whole cell fungal vaccine we recently developed based on heat-killed Cryptococcus neoformans fbp1 Δ mutant (HK-fbp1) and its adjuvanted version. Our data demonstrates that HK-fbp1 and its adjuvanted version have superior protection advantage than heat-killed wild type H99. Importantly, the HK-fbp1 is highly stable with its preparation remained protective potency after years of frozen storage. Including a CpG adjuvant ODN further reduced the antigen requirement in the intranasal route, and provided HK-fbp1 a highly durable protection after single intramuscular vaccination. These clinically relevant attributes of a successful vaccine, including high stability, dose-sparing potential, and durable efficacy following a single dose, strengthen the rationale for developing HK-fbp1 as a versatile whole-cell antifungal platform.