Secretion of the C3 component of complement by peritoneal cells cultured with encapsulated Cryptococcus neoformans

Abstract

Two isolates of Cryptococcus neoformans were identified as being widely divergent in pathogenic potential after intratracheal infection of mice. These isolates differed in their ability to upregulate capsule synthesis when grown under tissue culture conditions, and this property correlated with virulence. We postulated that differential capsule synthesis may cause differential stimulation of macrophages to produce products such as complement components. To test this hypothesis, heat-killed yeast cells were incubated with normal mouse peritoneal cells (PC) before the level of C3 secreted was determined. Cryptococcal stimulants were grown on mycological agar, which does not promote capsule synthesis, or in RPMI 1640 at 37°C in an atmosphere of 5% CO 2 , which stimulates capsule synthesis, to determine the role that the capsule plays in the induction of C3 secretion. C3 levels were elevated in cultures containing cryptococci grown in RPMI 1640 at 37°C in an atmosphere of 5% CO 2 , and the level of C3 detected was correlated with the amount of capsule expressed by the yeast cell stimulant. Nonencapsulated mutants of C. neoformans did not stimulate C3 secretion. Purified capsular polysaccharide (glucuronoxylomannan [GXM]) also stimulated the PC to secrete C3. Two signals were required before GXM stimulated C3 secretion. The second signal was identified as endotoxin present in small amounts (0.06 ng per ml) in tissue medium. Endotoxin may provide a priming stimulus for PC to express receptors or other cytokines needed for effective stimulation of C3. These experiments show that enhancement of C3 secretion by C. neoformans is due to GXM and is correlated with the virulence of the cryptococcal isolate.

Cryptococcal capsular glucuronoxylomannan reduces ischaemia‐related neutrophil influx

“…In those studies the effects of GXM evolved in the absence of serum and were therefore independent of complement activation. However, as several data indicate that GXM presumably activates the complement system [12,[54][55][56], the resulting generation of the chemoattracting factor C5a in the bloodstream might be an additional mechanism underlying the observed interference with leucocyte migration. The presence of massive amounts of C5a in the bloodstream would theoretically prevent neutrophils from responding properly to other chemoattracting substances present in the inflamed tissues.…”

“…The presence of massive amounts of C5a in the bloodstream would theoretically prevent neutrophils from responding properly to other chemoattracting substances present in the inflamed tissues. Although whole, encapsulated cryptococci clearly induce complement activation [13,[57][58][59], the ability of purified GXM to activate complement has not been unambiguously demonstrated [12,60], and data on GXMrelated C5a generation are currently not available. Evidence Figure 5 Effect of GXM treatment on myocardial infarct size.…”

“…It has been hypothesized that the presence of GXM in the bloodstream prevents leucocytes from responding appropriately to chemoattractants owing to a combination of its own intrinsic chemoattracting properties [9,10] and modulation of chemokine receptors [11]. Furthermore, as GXM possibly induces complement activation [12,13], the generation of massive amounts of chemotactic C5a in the circulation could hypothetically prevent neutrophils to respond to chemotactic stimuli in the inflamed tissues.…”

Complement and fungal pathogens: an update

“…GXM was identified as the stimulatory substance to induce the production of C3 in peritoneal cells. 116 The complement receptors CR1, CR3 and CR4 are the main effector molecules to mediate binding and phagocytosis of cryptococci, as demonstrated by monoclonal antibodies against the three receptors that block binding of the yeast cells. 117,118 A ranking for the CRs demonstrates the dominance of CR3.…”

Strain-dependent effects of environmental signals on the production of extracellular phospholipase by Cryptococcus neoformans

“…It has been suggested that the pathogenicity of a particular C. neoformans isolate is dictated by the quantitative expression of its combined virulence traits, including the ability to grow at 37 ‡C, production of polysaccharide capsule and melanin [3]. Cryptococcal capsule synthesis is upregulated in some C. neoformans strains when grown in low iron medium (LIM) and under tissue culture conditions [4,5]. One such well-characterised strain is C. neoformans var.…”