Numerous studies of various strategies for vaccination against Candida spp by multiple groups distributed internationally have been performed. Additionally, there has been an extensive effort to develop cryptococcal active and passive vaccines. Antigens used for Candida include heat killed whole organisms, attenuated live organisms, Candida enolase, the Candida cell surface iC3b receptors, mannans, beta-glucan, heat shock protein hsp90, hyphally-regulated protein (Hyr1), proteins from the Sap family (secreted aspartic proteins), and recombinant proteins from the ALS (agglutinin like sequence) gene family. Additionally, several glycoconjugate strategies have been tested experimentally. Differing adjuvant strategies have been explored also. For Cryptococcus, in addition to a variety of antigens, various immunostimulatory strategies, to be used in combination with antigens, have been explored. In nearly all preclinical studies, significant and encouraging efficacy has been found.
Coincident with the ever increasing prevalence of Candida spp in hospitalised patients and the high prevalence rates of cryptococcal infection in patients with HIV, has been a substantive increasing societal/medical interest in the development of vaccines for both of these yeasts. The high costs of human trials have been a considerable road block for robust development of fungal vaccines in general. Several million dollars are needed for a single Phase 1 trial in humans. The Swiss company Pevion completed a Phase 1 clinical trial of vaccination with a recombinant Sap2 protein for patients with recurrent vulvovaginal candidiasis (RVVC). NovaDigm Therapeutics, with a future focus on disseminated candidiasis, has completed two Phase 1 trials, and is conducting a Phase 1b/2a trial in patients with RVVC. The vaccine NDV-3 in these studies contains the recombinant antigen Als3. The results of the first six months of this NovaDigm 1b/2a trial are currently under statistical analysis. In all three of these trials, NDV-3 has elicited robust antibody responses (IgG and IgA) as well as evidence of Th1 and Th17 T-cell activation. The safety profile of NDV-3 has been comparable to similar recombinant vaccines with local site reactions but there have been no serious adverse events due to the vaccine. NovaDigm intends to assess the Hyr1 and Sap2 antigens in a combined vaccine with Als3. To our knowledge, no clinical trials have been conducted with active vaccination strategies for Cryptococcus to date.
Of additional interest is the discovery of a very high level of 3D, structural homology between the rAls3p-N, the antigen used in the NovaDigm trials, and surface adhesins of Staphylococcus aureus. In preclinical murine studies, vaccination with the Candida antigen protects against challenge with Staphylococcus aureus in septicaemic and skin infections. Subjects vaccinated with rAls3p-N in the Phase 1 and 1b/2a trials clinical trials have produced antibodies that are opsonophagocytic for Staphylococcus.
The increasing need for prevention of Candida spp infections, intensive antigen and adjuvant searches, the possible of immuno-enhancement strategies, especially for Cryptococcus, coupled with the development of newer adjuvants, and the growing interest in both governmental and private funding sources, provide considerable optimism for vaccines being successfully developed for these yeasts.