A Cryptococcus neoformans polysaccharide conjugate vaccine made with filtered polysaccharide elicits protective immunity in mice

Diseases caused by the encapsulated fungus Cryptococcus neoformans are major causes of mortality and morbidity in immunocompromised patients. Two important cryptococcal virulence factors are the polysaccharide capsule (CPS) and the secreted exopolysaccharides (EPS), both of which contain predominantly glucuronoxylomannan (GXM) polymers. Here, we evaluated the efficacy of an experimental glycoconjugate vaccine generated by linking minimally processed cryptococcal EPS with the protein carrier CRM₁₉₇. Two different adjuvants (aluminum hydroxide and Freund's adjuvant) were utilized to increase the immunogenicity and to evaluate the efficiency of vaccine protection in a mouse model of cryptococcosis. After a three-dose vaccination schedule, titers of GXM-specific antibodies and survival following lethal challenge were assessed. The protective efficacy of antibodies from sera of vaccinated mice was also evaluated in vitro, through the characterization of their enhancement of macrophage engulfment and opsonization patterns on cryptococcal cells. Antibodies elicited by the EPS-CRM₁₉₇ vaccine formulated with Freund's adjuvant showed the best opsonic capabilities as shown by the macrophage engulfment analysis and cryptococcal capsule binding patterns, which was mirrored by longer survival of this vaccine group in our challenge studies. This study confirms that an EPS-protein conjugate vaccine can elicit a protective immune response in mice and provides encouragement for the development of polysaccharide–protein conjugates for the prevention of cryptococcosis.