Isolated Pneumocystis carinii Cell Wall Glucan Provokes Lower Respiratory Tract Inflammatory Responses

Abstract

Macrophage-induced lung inflammation contributes substantially to respiratory failure during Pneumocystis carinii pneumonia. We isolated a P. carinii cell wall fraction rich in glucan carbohydrate, which potently induces TNF-α and macrophage-inflammatory protein-2 generation from alveolar macrophages. Instillation of this purified P. carinii carbohydrate cell wall fraction into healthy rodents is accompanied by substantial increases in whole lung TNF-α generation and is associated with neutrophilic infiltration of the lungs. Digestion of the P. carinii cell wall isolate with zymolyase, a preparation containing predominantly β-1,3 glucanase, substantially reduces the ability of this P. carinii cell wall fraction to activate alveolar macrophages, thus suggesting that β-glucan components of the P. carinii cell wall largely mediate TNF-α release. Furthermore, the soluble carbohydrate β-glucan receptor antagonists laminariheptaose and laminarin also substantially reduce the ability of the P. carinii cell wall isolate to stimulate macrophage-inflammatory activation. In contrast, soluble α-mannan, a preparation that antagonizes macrophage mannose receptors, had minimal effect on TNF-α release induced by the P. carinii cell wall fraction. P. carinii β-glucan-induced TNF-α release from alveolar macrophages was also inhibited by both dexamethasone and pentoxifylline, two pharmacological agents with potential activity in controlling P. carinii-induced lung inflammation. These data demonstrate that P. carinii β-glucan cell wall components can directly stimulate alveolar macrophages to release proinflammatory cytokines mainly through interaction with cognate β-glucan receptors on the phagocyte.

Pneumocystis carinii Cell Wall β-Glucan Induces Release of Macrophage Inflammatory Protein-2 from Alveolar Epithelial Cells via a Lactosylceramide-mediated Mechanism

“…In fact, neutrophil infiltration in the lungs of patients with PC pneumonia is a stronger predictor of respiratory failure and death than the organism burden itself (2). Additional investigations indicate that P. carinii cell wall ␤-glucans strongly stimulate neutrophilic lung inflammation (3,11). Nonspecific anti-inflammatory therapies, such as adjunctive corticosteroids, have significantly decreased mortality during severe PC pneumonia (31).…”

“…The following antibodies were used: anti-CD11b (M-19, Santa Cruz Biotechnology Inc., Santa Cruz, CA), FITC anti-CD45 (OX-1, BD Pharmingen, San Diego, CA), anti-CDw17 (Huly-M13, Ancell, Bayport, MN), anti-asialoGM1 2 (Matreya, State College, PA). We recently described the purification and characterization of a ␤-glucan-rich cell wall isolate from P. carinii (3). This isolate was found to contain 95.7% carbohydrate and 4.3% protein by weight using the orcinol-sulfuric acid method and BCA protein determinations, respectively.…”

“…The final wash was assayed for endotoxin using a modified Limulus amebocyte lysate assay (Whittaker M.A. Bioproducts, Walkersville, MD) and consistently found to contain Ͻ0.125 unit/ml endotoxin (3).…”

“…Neutrophilic lung inflammation promotes diffuse alveolar damage and contributes significantly to the respiratory failure, which is characteristic of severe PC pneumonia (2). The exact mechanisms by which PC induce this lung inflammation are not fully understood, although recent studies indicate that ␤-glucan components of the organism cell wall drive neutrophil accumulation during this pneumonia (3).…”

“…Previous studies demonstrate that the PC cyst cell wall contains complex carbohydrates rich in ␤-glucans, chitin, and a mannose-rich major surface glycoprotein complex termed gpA (5,7). We have recently purified particulate ␤-glucan from P. carinii and have demonstrated that alveolar macrophages produce tumor necrosis factor-␣ (TNF␣) and macrophage inflammatory protein-2 (MIP-2) in response to this PC cell wall component (3,11).…”