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

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.