Cryptococcal Vaccine Development: Antigen Selection and Immunological Mechanisms of Protection

Abstract

Fungal pathogens Cryptococcus neoformans and C. gattii, the etiologic agents of cryptococcosis, cause over 100,000 deaths worldwide annually. Currently, there are no licensed fungal vaccines. Our lab previously demonstrated the ability of recombinant cryptococcal antigens delivered in glucan particles (GPs) to protect mice against an otherwise lethal C. neoformans strain KN99 challenge. We expanded upon this to recombinantly express 23 cryptococcal protein antigens and test them in GP-vaccines in BALB/c and C57BL/6 mice. Most protective candidate vaccine antigens shared significant sequence homology to other cryptococcal proteins. Intra-proteome homology enhanced vaccine efficacy and stimulated cross-reactive antibody and IFNγ responses to multiple protein family members. This suggests a benefit to selecting protein-family antigens for cryptococcal vaccine development. cda1Δ2Δ3Δ (cda123), a live-attenuated vaccine deleted of three cryptococcal chitin deacetylases, induces a protective Th1 response in the lungs of mice. While both GP-based and cda123 vaccines extended the survival of C. gattii strain R265-infected mice compared to unvaccinated groups, fungal clearance was not as robust as that seen following challenge with KN99. To identify deficiencies in the vaccine response following R265 infection, we developed a KN99-R265 coinfection model in cda123-vaccinated mice. Fungal burden in the lungs and cytokine production point to passive immune evasion by C. gattii R265: the presence of R265 in the lungs did not dampen the protective Th1 response to KN99 or inhibit the clearance of KN99. In all, these studies utilize novel variations on vaccination-challenge models to identify immunologic correlates of protection for subunit and live-attenuated cryptococcal vaccines.