Potent inhibition of neutrophil migration by cryptococcal mannoprotein-4-induced desensitization

Cryptococcal capsular Ags induce the production of proinflammatory cytokines in patients with cryptococcal meningitis. Despite this, their cerebrospinal fluid typically contains few neutrophils. Capsular glucuronoxylomannan is generally considered to mediate the inhibition of neutrophil extravasation. In the current study, culture supernatant harvested from the nonglucuronoxylomannan-producing strain CAP67 was found to be as potent as supernatant from wild-type strains in preventing migration. We identified capsular mannoprotein (MP)-4 as the causative agent. Purified MP-4 inhibited migration of neutrophils toward platelet-activating factor, IL-8, and fMLP, probably via a mechanism involving chemoattractant receptor cross-desensitization, as suggested by its direct chemotactic activity. Supporting this hypothesis, MP-4 elicited Ca(2+) transients that were inhibited by preincubation with either fMLP, IL-8, or C5a, but not platelet-activating factor, and vice versa. Moreover, MP-4 strongly decreased the neutrophil surface expression of L-selectin and induced shedding of TNF receptors p55/p75, whereas CD11b/18 increased. Finally, MP-4 was clearly detectable in both serum and cerebrospinal fluid of patients suffering from cryptococcal meningitis. These findings identify MP-4 as a novel capsular Ag prematurely activating neutrophils and desensitizing them toward a chemoattractant challenge.

Quantitative analysis of phagocytosis of Cryptococcus neoformans by adherent phagocytic cells by fluorescence multi-well plate reader

Macrophages and monocytes are adherent phagocytic cells which play an important role in host defence against the yeast-like fungus Cryptococcus neoformans. Before, phagocytosis by adherent phagocytes could only be measured by means of microscopy or by a radioactive assay, which both have obvious disadvantages. We have developed a new, rapid and objective method to measure phagocytosis of C. neoformans by adherent phagocytes (e.g. alveolar macrophages) using a fluorescence multi-well plate reader. This method allows us to discriminate accurately between adherence and internalisation of C. neoformans by macrophages during long term incubation. In addition, the method was used to study the role of the mannose receptor in phagocytosis of the acapsular yeast in the absence of serum by human monocyte-derived macrophages (MDM).

Cryptococcus neoformans and its cell wall components induce similar cytokine profiles in human peripheral blood mononuclear cells despite differences in structure

We studied the cytokine profile of peripheral blood mononuclear cells after stimulation with various cryptococcal strains or its purified cell wall components. After 3 h of stimulation, tumor necrosis factor (TNF) alpha levels were strongly increased, whereas interferon (IFN) gamma and interleukin (IL) 10 levels were increased only slightly, or not at all (respectively). In contrast, after 18 h, TNF-alpha and IFN-gamma levels were (strongly) decreased, whereas the IL-10 levels were increased. The IL-1beta, IL-6 and IL-8 levels were equally high throughout the experiment. In order to establish which of the cryptococcal envelope components contributed most to the observed cytokine profile induced by whole cryptococci, glucuronoxylomannan, galactoxylomannan and mannoproteins were purified and partially characterized biochemically. All cryptococcal components elicited a similar cytokine pattern despite the differences in structure.

Pulmonary surfactant protein A binds to Cryptococcus neoformans without promoting phagocytosis

BACKGROUND

Evidence is accumulating that the alveolar collecting surfactant protein A (SP-A) plays an important role in the first line of defence against infiltrating pathogenic micro-organisms and viruses. The ability of SP-A to facilitate the binding and uptake of acapsular Cryptococcus neoformans by monocyte-derived macrophages, human alveolar macrophages, monocytes and polymorphonuclear leucocytes was investigated.

MATERIALS AND METHODS

Binding, competition and phagocytosis experiments were performed using a flow cytometry technique.

RESULTS

SP-A bound to both the acapsular and the encapsulated form of C. neoformans in a concentration-dependent manner. SP-A showed a threefold better binding to the acapsular yeast: this binding was partly calcium dependent and could be inhibited by mannose (ID50 = 3 mmol L-1) and glucose (ID50 = 2.1 mmol L-1) but not by galactose (ID50 = 391 mmol L-1). SP-A did not function as an opsonin in phagocytosis of acapsular C. neoformans for any of the phagocytes studied.

CONCLUSION

Our results indicate that SP-A binds in a concentration-dependent manner to both encapsulated and acapsular C. neoformans. Despite SP-A binding to the acapsular C. neoformans, phagocytosis by various phagocytes was not enhanced.