Effect of the capsular material of cryptococcus neoformans on the interplay between Mmicroglial cells and Neutrophils.

Abstract

Cryptococcal meningitis is an important opportunistic infection in immunocompromised patients. It has been well established that a distinguishing feature of this form of meningitis is a relatively low neutrophil count in the cerebrospinal fluid (CSF) compared to bacterial meningitis. There has been speculation and research undertaken previously to understand this phenomenon, however, little information is available in human studies. Furthermore, there is insufficient information on expression and function of Toll-like receptors (TLR) in the human central nervous system (CNS). The work presented here investigated the effect of the capsular material of a series of clinical isolates of Cryptococcus neoformans on neutrophil recruitment at the site of infection and determined whether downregulation occurs at the level of TLR expression. This was done in a multiple component study. Clinical information was collected from patients with cryptococcal meningitis and baseline blood and CSF investigations were performed, which included the quantification of neutrophils in CSF and blood specimens. The size of the Cryptococcus capsule was measured in each isolate and shed capsular material was quantified in individual CSF specimens. The extent of neutrophil chemotaxis inhibition by individual strains of C. neoformans was determined by using a Transwell migration assay. Toll-like receptor (TLR)2 and TLR4 gene expression induced by individual C. neoformans isolates in human microglial cells was quantified. The possible associations among these experiments were subsequently evaluated. As anticipated, a paucity of neutrophils in the CSF was observed. The cryptococcal capsule was larger in isolates of patients with lower CSF neutrophil counts. In addition, patients with lower CSF neutrophil counts shed more capsular material in the CSF. Chemotaxis inhibition occurred in close to 70% of tested isolates. The concentration of shed capsular material in this group was higher compared to the group with no chemotaxis inhibition. Patients presenting with fever had higher CSF neutrophil counts as well as elevated intracranial pressures. The majority of isolates expressed downregulation for TLR2 and TLR4 in microglial cells exposed to C. neoformans. CSF neutrophil counts were lower in this group. These findings imply that the capsular components of C. neoformans downregulated recruitment of neutrophils into the CSF. Downregulation of neutrophil recruitment was observed at the level of TLR expression.