Glycosphingolipids Mediate Pneumocystis Cell Wall β-Glucan Activation of the IL-23/IL-17 Axis in Human Dendritic Cells

Abstract

Pneumocystis species are opportunistic fungal organisms that cause severe pneumonia in immune-compromised hosts, with resultant high morbidity and mortality. Recent work indicates that IL-17 responses are important components of host defense against fungal pathogens. In the present study, we demonstrate that cellsurface b-glucan components of Pneumocystis (PCBG) stimulate human dendritic cells (DCs) to secrete IL-23 and IL-6. These cytokines are well established to stimulate a T helper-17 (Th17) phenotype. Accordingly, we further observe that PCBG-stimulated human DCs interact with lymphocytes to drive the secretion of IL-17 and IL-22, both Th17-produced cytokines. The activation of DCs was shown to involve the dectin-1 receptor with a downstream activation of the Syk kinase and subsequent translocation of both the canonical and noncanonical components of the NF-kB transcription factor family. Finally, we demonstrate that glycosphingolipid-rich microdomains of the plasma membrane participate in the activation of DCs by PCBG through the accumulation of lactosylceramide at the cell surface during stimulation with PCBG. These data strongly support the idea that the b-glucan surface components of Pneumocystis drive the activation of the IL-23/IL-17 axis during this infection, through a glycosphingolipid-initiated mechanism.Keywords: Pneumocystis; b-glucan; dendritic cells; Pneumocystis infection remains an all too common cause of pneumonia in immune-compromised hosts. Before the early 1990s, this infection was largely related to cases of childhood leukemia and infants with severe malnourishment. However, with the onset of the HIV pandemic, Pneumocystis pneumonia (PcP) has emerged as one of most serious causes of pneumonia among this patient population (1-3). In more recent years, an increasing number of PcP cases have been attributed to immunosuppressed individuals with malignancy, or as a result of immunosuppressive agents administered for organ transplantation or autoimmune diseases (4-10).Earlier studies established the central importance of T cells in the host defense against Pneumocystis infection, where CD4 cell counts of less than 200 cells/mm 3 were shown to place individuals at increased risk for this infection (11). More recently, CD4-independent mechanisms were also demonstrated to be important in clearing this infection (12). This is also supported by the fact that patients receiving B cell-suppressive therapy, and animal models of B-cell deficiency, also reveal a higher risk for developing PcP (13,14). Thus, inadequate immune response across multiple components of host defense can render an individual susceptible to this infection.In addition, exaggerated innate inflammatory responses in patients with PcP appear to be associated with a higher risk of developing respiratory failure, as indicated by early work from our laboratory showing that the degree of respiratory failure in immunosuppressed patients with PcP correlated best with the degree of inflammation, and not with the organism burden itself (15...

Dectin Immunoadhesins and Pneumocystis Pneumonia

“…These data indicate that ␤-glucan-and Dectin-1-mediated signals contribute to hypoxemia during Pneumocystis IRIS, consistent with recent observations implicating the ␤-glucan-rich ascus in producing a proinflammatory response (56). Possible mechanisms of inflammatory exacerbation include induction of IL-8 secretion and activation of the IL-23/IL-17 axis by Pneumocystis ␤-glucan (59,60), as well as the effects of Dectin-1 signaling on Th1 and Th17 differentiation of CD4 ϩ T cells (33,56,61,62), which have been shown to contribute to the risk for IRIS (63,64). Although intracellular staining of PMA-and ionomycin-stimulated lung cells did not reveal a strong Th17 signature (see Fig.…”

Characterization of a Distinct Host Response Profile to Pneumocystismurina Asci during Clearance of Pneumocystis Pneumonia

“…els of albumin in the alveolar environment. The results of these studies also suggest that the presence of ␤-glucan associated with the asci in the untreated mice also may activate the IL-23/IL-17 axis as IL-17 levels were also higher in the untreated mice at day 7, as has been recently described (14).…”

“…6). This finding may be related to the host response to the ␤-glucan associated with the asci as a recent report indicates that Pneumocystis ␤-glucan may drive the activation of the IL-23/IL-17 axis (14). Levels of these cytokines mostly declined within each of the treated or untreated mouse groups over time (significance indicated by gray dashed brackets).…”

“…Previous studies have interrogated the immune responses to the Pneumocystis ␤-glucans in vitro by direct challenge of Pneumocystis-derived ␤-glucans to rat-derived alveolar epithelial cells and human-derived dendritic cells and lymphocytes (14,56). Here, we eliminated most of the organism-associated ␤-glucans in an in vivo setting and contrasted these responses with an ascus-replete pneumonia containing high levels of ␤-glucans.…”

“…Numerous studies have demonstrated that purified Pneumocystis ␤-glucan stimulates expression of inflammatory cytokines and chemokines in vitro (8)(9)(10)(11)(12)(13)(14)(15)(16). In addition, ␤-glucan in the cell wall of Pneumocystis has been shown to stimulate pulmonary inflammation (17,18).…”