Identification of Cryptococcus neoformans capsule-associated protein antigens that mediate B cell-dependent vaccine protection in mice and elicit antibody responses in humans with cryptococcosis

Abstract

Infection with the encapsulated fungal pathogen Cryptococcus neoformans causes substantial global disease burden in persons with compromised T cell function. Previously, we demonstrated that vaccines containing adjuvanted recombinant cryptococcal proteins protected mice against lethal infection by mechanisms dependent on CD4+ T cells. For the present study, we hypothesized that vaccines containing protein antigens naturally located on the surface of the cryptococcal capsule would contribute protective B cell responses. We found that B cell-deficient mice exhibited reduced survival following vaccination with any of four capsule-associated antigens: Blp4, Cpd1, Lhc1, or Krp1. To further elucidate the role of antibodies in protection, we tested serum from mice vaccinated with a live-attenuated strain (cda123) and then challenged with a virulent C. neoformans strain for IgG responses to a panel of 26 cryptococcal recombinant proteins. Strong reactivity was detected against Blp4, which we focused on because it is capsule-associated and protects mice when administered in adjuvanted subunit vaccines, by mechanisms partly dependent on B cells. Blp4 antibodies were consistently present in surviving mice, including those vaccinated with unrelated proteins or those challenged with diverse clinical isolates. Additionally, serum from Blp4-vaccinated mice promoted macrophage opsonophagocytosis of C. neoformans. Finally, IgG antibodies against Blp4 were detected in 20 of 21 serum samples from patients who survived cryptococcosis, compared with only 4 of 14 samples from healthy controls. Together, these data identify Blp4 as an antigen that elicits protective B cell responses and is a strong candidate for inclusion in subunit cryptococcal vaccines.