The human antibody response to Cryptococcus neoformans and survival in HIV-associated cryptococcal meningoencephalitis

About This Grant

ABSTRACT Cryptococcal meningoencephalitis (CM) is a devastating opportunistic infection of the central nervous system due to Cryptococcus neoformans (Cn), an encapsulated fungus that mainly occurs in people with HIV-associated immunosuppression. In 2020, globally, 4.4% of people with HIV (PWH) and <200 CD4 T cells/µl had cryptococcal antigenemia (CrAg) and 19% of all AIDS-related mortality was caused by 152,000 cases of CM. CrAg screening, antiretroviral therapy (ART), and intensified antifungal regimens have led to earlier diagnosis and improved CM outcomes. Yet, in 2019 the total economic burden of cryptococcosis in the United States was $534,594,527, and globally, mortality remains high, 13-40%. There is an urgent need for more effective therapy for CM+ PWH. While HIV-associated CD4 T cell loss is the main CM risk factor, other factors are at play since CD4 alone does not identify those who will develop CM or respond to treatment. Since antibody immunity is also impaired in PWH, we posit immunoglobulins (Igs) in human sera that bind Cn due to natural reactivity with fungal glucans, latency, and/or environmental exposure enhance control of Cn. We showed Cn capsular glucuronoxylomannan (GXM)-IgG is ubiquitous in adult and children’s sera, and PWH with reactivation (CrAg+) and disease (CM) have higher levels than those with latency (CrAg-, CM-). In marked contrast, levels of IgM/IgG/IgA that bind glucan constituents of the Cn cell wall; β-(1-3)-rare β-(1-6)-glucan (Lam(inarin), β-(1-3)-glucan (curdlan) and/or β-(1-6)- glucan (pustulan), were lower in PWH with reactivation (CrAg+) and disease (CM) than latency (CrAg-, CM-), suggesting a deficit of certain glucan-binding Igs in reactivation and disease (states of fungal growth) may impair Cn host defense. Pre-existing (natural, not elicited by infection) human Igs bind conserved glucan determinants on fungi, and IgG2 is their main subclass. We found plasma IgG2 was higher in CrAg+ than CrAg-, but lower (as was IgA) in CM+ than CM- PWH, suggesting reactivation elicits Igs that may contribute to control of Cn. The hypothesis of this application is: human Igs which target defined glucan epitopes modulate Cn virulence, enhance host control of Cn, and correlate with survival in CM+ PWH. Our specific aims are: 1. To identify Cn glucan epitopes targeted by human Igs and determine Ig antifungal activity in vitro. 2. To produce glucan-binding human monoclonal antibodies (huMabs) and identify their effects on Cn biology and functional efficacy in vitro and virulence in mice. 3. To seek associations between defined plasma glucan-specific Ig levels and survival in CM+ PWH in the Ambition CM treatment trial. This project leverages basic and translational approaches to identify human Ig-mediated effects on Cn biology and virulence characteristics and seek glucan-Ig correlates of CM survival in the landmark 844 participant Ambition trial, in which mortality was 25-29%. There is an urgent need for more effective and innovative approaches to CM therapeutics. Knowledge gained from this project will provide biomarkers of mortality risk, and the huMabs generated will be poised to advance CM therapy as novel agents to enhance antifungal drug efficacy and improve clinical CM outcomes.