Occurrences, immunoglobulin classes, and biological activities of antibodies in normal human serum that are reactive with Cryptococcus neoformans glucuronoxylomannan.

Serum obtained from normal human subjects contains antibodies reactive in an enzyme-linked immunosorbent assay with the glucuronoxylomannan (GXM) of Cryptococcus neoformans. The frequency of occurrence of class-specific antibodies among normal subjects was 28% for immunoglobulin G (IgG), 98% for IgM, and 3% for IgA. Anti-GXM antibodies with kappa light chains occurred in 98% of normal subjects, while the occurrence of lambda light chains was 28%. Each of five subjects with high levels of anti-GXM IgG antibodies had readily detectable antibodies of the IgG2 isotype; two of the five subjects had readily detectable IgG1 antibody. An examination of sera from human immunodeficiency virus-infected patients showed that human immunodeficiency virus infection was accompanied by a significant decrease in the occurrence of IgM antibodies and anti-GXM antibodies with kappa light chains; these decreases occurred early in infection when CD4 counts were still > or = 500 cells per microliter. A slight but not statistically significant decrease in the occurrence of anti-GXM IgG antibodies was seen only in patients with CD4 levels of < 200 cells per microliter. Sera from normal subjects with high levels of anti-GXM IgG antibodies were examined to identify any contribution of the antibodies to complement activation or to opsonization of the yeast cells. An analysis of the kinetics for activation and binding of C3 to the yeast cell showed no pattern of quantitative or qualitative differences between sera with high or low levels of anti-GXM IgG antibodies. Phagocytosis studies showed that the naturally occurring IgG antibodies did not contribute to opsonization of the yeast cells.

Occurrences, specificities, and functions of ubiquitous antibodies in human serum that are reactive with the Cryptococcus neoformans cell wall.

Previous studies found that normal human serum (NHS) contains an immunoglobulin G (IgG) antibody that mediates initiation of the classical complement pathway by nonencapsulated Cryptococcus neoformans. The present study used an enzyme-linked immunosorbent assay with whole nonencapsulated yeast cells as solid-phase antigens to demonstrate the presence of high levels of IgG antibody in each of 11 sera from normal adult donors. The IgG antibodies were of the IgG2 subclass. The antibody activity was blocked completely by treatment of serum with isolated yeast glucan. Treatment of serum with mannan or chitin had no effect on antibody levels. Antibody activity was adsorbed completely by treatment of serum with zymosan particles. Adsorption with intact cells of Saccharomyces cerevisiae or Candida albicans had no effect, suggesting that the glucan on S. cerevisiae or C. albicans is not surface exposed. Assessment of the opsonic requirements for phagocytosis of nonencapsulated cryptococci by monocyte-derived human macrophages (MO-M phi) showed that high levels of phagocytosis occurred when yeast cells were opsonized with NHS. Removal of anti-glucan antibody by adsorption with whole nonencapsulated cryptococci did not diminish opsonic activity. Heat-inactivated serum or anti-glucan antibody affinity purified from NHS lacked opsonic activity. Taken together, these results indicate that phagocytosis of nonencapsulated cryptococci by monocyte-derived human macrophages has an obligatory requirement for opsonic ligands of the complement system, with no contribution by the anti-glucan IgG that is found in NHS.

Accelerated decay of C3b to iC3b when C3b is bound to the Cryptococcus neoformans capsule.

Incubation of encapsulated and nonencapsulated Cryptococcus neoformans in normal human serum (NHS) leads to activation and binding of potentially opsonic fragments of complement component C3 to the yeast cells. Analysis of the molecular forms of C3 after incubation of encapsulated cryptococci in NHS showed that the percentage of bound C3 occurring as iC3b approached 100% after 8 min. The percentage of bound C3 occurring as iC3b on nonencapsulated cryptococci never exceeded 70%, even after 60 min of incubation in NHS. Conversion of C3b to iC3b was assessed further by incubating C3b-coated cryptococci for various times with a mixture of complement factors H and I at 40% of their respective physiological concentrations. Most, if not all, of the C3b on encapsulated cryptococci was converted to iC3b at a single fast rate. Conversion of C3b to iC3b on nonencapsulated cryptococci did not follow a single rate constant and appeared to have a fast and a slow component. Studies of the requirements for factors H and I in cleavage of C3b to iC3b showed steep dose-response curves for both factors in the case of encapsulated cryptococci and shallow curves with C3b bound to nonencapsulated cryptococci. Taken together, our results indicate that C3b molecules bound to encapsulated cryptococci have a uniformly high susceptibility to conversion to iC3b by factors H and I. In contrast, a significant portion of the C3b bound to nonencapsulated cryptococci is very resistant to conversion to iC3b by factors H and I.

Activation and binding of opsonic fragments of C3 on encapsulated Cryptococcus neoformans by using an alternative complement pathway reconstituted from six isolated proteins.

Encapsulated Cryptococcus neoformans yeast cells are potent activators of the complement system. We examined the interaction of the yeast cells with an alternative complement pathway reconstituted from isolated factor D, factor B, factor H, factor I, C3, and properdin. Incubation of encapsulated cryptococci with the reconstituted pathway led to activation and binding of C3 fragments to the yeast cells that was quantitatively and qualitatively identical to that observed with normal human serum. Incubation with either normal serum or a mixture of isolated proteins led to binding of 4 x 10(7) to 5 x 10(7) C3 molecules to the yeast cells. The kinetics for activation and binding of C3 were identical, with maximum binding observed after a 20-min incubation. Immunoglobulin G was not needed for optimal activation kinetics. C3 fragments eluted from the yeast cells by treatment with hydroxylamine and subsequent analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated the presence primarily of iC3b on yeast cells incubated with either normal serum or the reconstituted pathway. Ultrastructural examination of the opsonized yeast cells showed that the cryptococcal capsule was the site for binding of C3 activated from normal serum or the reconstituted pathway, with a dense accumulation of C3 at the periphery of the capsule. Thus, incubation of encapsulated cryptococci in the reconstituted pathway led to deposition of opsonic complement fragments at a site that was appropriate for interaction with phagocyte receptors. Cryptococci opsonized with the reconstituted pathway showed a markedly enhanced interaction with cultured human monocytes compared with unopsonized yeast cells, indicating that the alternative pathway alone is opsonic for yeast cells. However, the results indicate that additional serum factors are needed for optimal opsonization of yeast cells because a 35% reduction in the number of cryptococci bound to macrophages was observed with cryptococci opsonized with the reconstituted pathway compared with that observed when yeast cells were opsonized with normal serum.

Strain variation in phagocytosis of Cryptococcus neoformans: dissociation of susceptibility to phagocytosis from activation and binding of opsonic fragments of C3.

Phagocytosis of Cryptococcus neoformans is markedly influenced by the presence of a polysaccharide capsule. We examined activation and binding of C3 fragments to eight isolates of C. neoformans. All isolates were shown to have capsules by light and electron microscopy. These strains differed in susceptibility to phagocytosis by neutrophils. Yeast cells were opsonized by incubation in normal human serum. Five strains were resistant to ingestion, two strains showed intermediate levels of resistance to ingestion, and one strain was quite sensitive to phagocytosis. Yeast cells opsonized with heat-inactivated serum (56 degrees C for 30 min) neither attached nor were ingested by neutrophils. A quantitative estimate of the amount of C3 bound to the yeast cells was determined by use of normal human serum containing 125I-labeled C3. The results showed approximately 5 X 10(6) to 10 X 10(6) C3 molecules per yeast cell regardless of whether the yeast cells were sensitive or resistant to phagocytosis. Bound C3 was eluted from the yeast cells by treatment with 0.1 M NH2OH (pH 10), and the eluted fragments were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Results of this analysis showed that little of the C3 was in the form of C3b, and there was substantial decay to iC3b, the inactive decay product of C3b. This pattern of decay was similar with all strains. Immunoelectron microscopy was used to assess the ultrastructural location of the C3 fragments bound to the yeast cells. C3 fragments were bound to the perimeter of the capsule regardless of whether the isolate was sensitive or resistant to phagocytosis. Thus, phagocytosis-sensitive and phagocytosis-resistant isolates were similar with regard to the amount, molecular form, and ultrastructural location of C3 fragments bound to the cryptococcal capsule. These results further indicate that activation of the complement cascade is necessary but not sufficient for phagocytosis of the yeast cell.

Role of the capsule in phagocytosis of Cryptococcus neoformans.

The capsule is closely associated with the virulence of Cryptococcus neoformans. The capsule inhibits phagocytosis by macrophages, monocytes, and neutrophils. Studies in our laboratory have shown that incubation of encapsulated cryptococci in normal human serum leads to deposition of large amounts of C3 fragments at the surface of the yeast and lesser amounts of IgG within the capsule. Thus, the capsule mediates two biologic activities with opposing effects. It is our current view that phagocytosis of the yeast is dependent on a balance between the antiphagocytic action of cryptococcal polysaccharide and the ability of the yeast to focus opsonically active complement fragments and the IgG at the capsular surface.

Activated neutrophils exhibit enhanced phagocytosis of Cryptococcus neoformans opsonized with normal human serum.

We studied the effect of agents that activate neutrophils on phagocytosis of C. neoformans. The amount of CR3 on the surface of neutrophils was used as a marker for neutrophil activation. Surface CR3 was estimated by flow cytometry using phycoerythrin-labelled anti-CR3 (anti-Leu-15) monoclonal antibody. Phagocytosis was determined by incubation of neutrophils with cryptococci that had been preincubated with normal human serum. We found that treatment of neutrophils with (i) the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine, (ii) zymosan activated serum, (iii) supernatant fluid from a mixed leucocyte culture, or (iv) supernatant fluid from human leucocytes cultured with phytohaemagglutinin produced a dose-dependent increase in CR3 density. These agents also markedly enhanced phagocytosis of opsonized cryptococci in a parallel dose-dependent fashion. These results indicate that phagocytosis of cryptococci opsonized with normal human serum is markedly enhanced by treatment of neutrophils with reagents that stimulate neutrophils. Our results demonstrate that neutrophils activated in an appropriate manner are capable of efficient phagocytosis of encapsulated cryptococci. This potential phagocyte activity may account in part for the high natural resistance to cryptococcosis.

Activation and binding of C3 by Candida albicans.

Interaction with components of the complement system is an important aspect of the pathogenesis of infection by Candida albicans. The key role of C3 as an opsonic ligand and as an element in amplification of complement activation led us to examine several factors that influence the activation and binding of C3 cleavage fragments to the yeast. Activation and binding of C3 were determined by use of normal human serum containing 125I-labeled C3. Incubation of yeast-phase cells in 20% serum led to deposition of 2.5 X 10(5) to 3.0 X 10(5) molecules of C3 per yeast cell. Binding of C3 was absent in serum that was heat inactivated for 30 min at 37 degrees C or in serum that was chelated with EDTA. Chelation of serum with EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] reduced binding of C3 fragments by 31%. These results suggest that the alternative complement pathway is the primary mechanism for activation and binding of C3 fragments to C. albicans. Bound C3 could be partially removed (50%) by treatment with 1.0 M hydroxylamine. In contrast, 1.0 M hydroxylamine removed 98% of the C3 fragments bound to encapsulated Cryptococcus neoformans. These results suggest that ester bonds are the primary mechanism for binding C3 to C. neoformans, whereas binding of C3 to C. albicans involves both ester and amide bonds. Monoclonal antibodies specific for C3c and an iC3b neoantigen were used to identify the fragment of C3 that was bound to C. albicans. The results showed that the primary fragment bound to the yeast was C3b. An examination of the kinetics of activation and binding of C3 fragments showed that activation and binding were very rapid. Near-maximal binding occurred after a 2.5- to 5-min incubation period. In contrast, activation and binding of C3 fragments to C. neoformans proceeded at a slower rate, with maximal binding requiring 10 to 20 min. These results indicate that activation and binding of C3 fragments by the yeasts C. albicans and C. neoformans differ in several important characteristics.

Activation of the complement system by Cryptococcus neoformans leads to binding of iC3b to the yeast.

The complement system plays a key role in resistance to cryptococcosis. In the present study, we examined several factors that influence the binding of C3 cleavage fragments to Cryptococcus neoformans. Binding of C3 was determined by using normal human serum supplemented with 125I-labeled C3. Incubation of encapsulated cryptococci in 20% serum led to the binding of approximately 3.2 X 10(6) molecules of C3 to each cell. The binding of C3 was markedly inhibited by heating the serum at 56 degrees C for 30 min or by chelation of the serum with EDTA. Chelation of the serum with EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] reduced binding of C3 by 37%. These results indicated that activation of C3 cleavage fragments and their binding to C. neoformans was primarily dependent upon the alternative pathway. Bound C3 could be removed by incubation with 1.0 M hydroxylamine (pH 10) but not by incubation with 3.5 M NaSCN or with phosphate-buffered saline containing 0.1% sodium dodecyl sulfate. These results suggested that C3 fragments were bound to C. neoformans by ester bonds. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of C3 fragments eluted from the yeast showed the presence of protein bands consistent with the presence of iC3b. C3b was not detected on the yeast after incubation with serum for time intervals as short as 2.5 min, indicating a rapid conversion of cell-bound C3b to iC3b. These results indicate that iC3b is the ligand which most likely interacts with the phagocyte C3 receptors involved in the phagocytosis of C. neoformans.