Resumo

Recent studies suggested that polysaccharide fractions of identical composition with variable molecular dimensions vary in biological functions. Glucuronoxylomannan (GXM) is the major capsular component in the Cryptococcus complex. It has been consistently demonstrated that the capsule is composed of GXM molecules of variable dimensions. However, the knowledge on how GXM molecules of variable size interact to form the capsular network, as well as how molecular dimensions influences many parameters of the interaction of C. neoformans with the host, remain obscure. In this study, we fractionated GXM from culture supernatants of a serotype A strain of C. neoformans (H99) to produce samples ranging from 300 to 1kDa of molecular mass. All fractions were recognized in a dose dependent manner by a monoclonal antibody to GXM. These fractions were incubated with an acapsular mutant of C. neoformans for further analysis by immunofluorescence with four different monoclonal antibodies to GXM. Fractions of lower molecular mass (1 to 10 kDa) accumulated in surface budding sites. Higher molecular mass samples (100 to 300 kDa) were distributed into the entire cell surface. In all cases, fractionated GXM formed punctuated patterns of surface distribution, in contrast with the usual annular pattern of surface binding observed when crude GXM was used. Fractions of higher and lower molecular masses also different in their ability to bind host macrophages, although they were similar in their ability to protect C. neoformans against phagocytosis. Stimulation of mice with GXM fractions of low (10 kDa range) or high (300 kDa range) molecular masses revealed a different pattern of cytokine induction in the lung. Although both fractions were similarly effective in inducing lung TNF-alpha, the high molecular mass fraction was significantly more efficient in inducing interleukins 17 and 10. In summary, our results indicate that GXM fractions with variable molecular masses can differentially interact with cell wall components of C. neoformans and host cell receptors, in addition to a differential ability to induce host cytokine responses. These results reveal a great complexity in GXM biological functions, but clearly demonstrate that polysaccharide functions with identical compositions and variable molecular masses can be functionally distinct. CNPq, FAPERJ, Capes