Effect of polysaccharide capsule and methods of preparation on human lymphocyte proliferation in response to Cryptococcus neoformans

Abstract

Cryptococcus neoformans is a pathogenic encapsulated yeast that infects patients that have defective cell-mediated immunity, including AIDS patients. Whole cryptococcal organisms that are killed by heating stimulate normal human lymphocytes to proliferate. However, strains of C. neoformans vary widely in virulence and therefore in their ability to cause disease in humans. To determine the effect of virulence factors such as the cryptococcal capsule, serotype, and the state of the organisms on the lymphocyte response to C. neoformans, human peripheral blood mononuclear cells were stimulated with C. neoformans in vitro and lymphocyte proliferation was determined. The major determinant of the lymphocyte response to C. neoformans was the amount of polysaccharide present. The response was greater after stimulation by minimally encapsulated strains (strains C3D, 68, and 613) than by heavily encapsulated strains (strains 6 and 145). A heavily encapsulated strain (strain 6) did not suppress the response to an acapsular mutant (strain 67). However, the response to an acapsular strain was suppressed by the addition of purified polysaccharide. Human lymphocytes responded to both serotypes of C. neoformans var. neoformans. The antigen responsible for lymphocyte stimulation was preserved despite various techniques of inactivation, including heat, paraformaldehyde fixation, irradiation, and mechanical disruption. Finally, lymphocytes responded equally to live and killed organisms. These results suggest that capsular polysaccharide, a known virulence factor, may suppress the human lymphocyte response to C. neoformans during an infection. Lymphocytes could respond to C. neoformans regardless of the viability of the organism, and they could also respond to disrupted organisms. We speculate that lymphocyte proliferation in vitro could be related to the protective immune response in host defense to C. neoformans and that it is suppressed by virulence factors of C. neoformans.