Phosphoinositide 3-Kinase-Dependent Inhibition of Dendritic Cell Interleukin-12 Production byGiardia lamblia

Abstract

Dendritic cell interactions with pathogenic microbes initiate and direct the development of subsequent adaptive responses. The protozoan pathogen Giardia lamblia infects the mammalian small intestine, leading to nutrient malabsorption and diarrhea but rarely causing inflammation. In order to begin to understand how the innate immune system responds to this parasite and shapes the eventual adaptive response, we examined the interaction between parasites and murine bone marrow-derived dendritic cells (DCs). DCs incubated with live parasites or parasite extracts displayed enhanced levels of CD40. The expression of CD80 and CD86 also increased, but less than was seen with lipopolysaccharide-activated DCs. Small amounts of interleukin-6 (IL-6) and tumor necrosis factor alpha were secreted by these DCs, whereas no IL-10 or IL-12 could be detected. Coincubation of DCs with parasite extracts along with known Toll-like receptor (TLR) ligands resulted in enhanced secretion of IL-10 and reduced secretion of IL-12. The levels of major histocompatibility complex class II, CD80, and CD86 were also reduced compared to DCs stimulated with TLR ligands alone. Finally, studies with an extracellular signal-regulated kinase 1/2 pathway inhibitor, a phosphoinositide 3-kinase (PI3K) inhibitor, and anti-IL-10 receptor antibody revealed that the PI3K pathway is the dominant mechanism of inhibition in DCs incubated with both lipopolysaccharide and Giardia. These data suggest that this parasite actively interferes with host innate immunity, resulting in an immune response able to control the infection but devoid of strong inflammatory signals.

Persistent G. lamblia impairs growth in a murine malnutrition model

“…The mechanisms driving this response are poorly understood, and the clinical studies did not exclude the presence of coinfecting helminths, which is associated with both mucosal eosinophils and susceptibility to G. lamblia (46). Acute Giardia infection with axenic trophozoites in mice does not demonstrate a robust inflammatory response (46), consistent with in vitro findings of suppressed DC cytokine production following exposure to Giardia or Giardia products in vitro (47). G. lamblia contains a biologically unique β-1,3 GalNAc homopolymer in its cyst wall (48), which in our cyst challenge model could partially explain why a different immune response was seen.…”

Complement Activation by Giardia duodenalis Parasites through the Lectin Pathway Contributes to Mast Cell Responses and Parasite Control

“…Our laboratory previously showed that dendritic cells responded to parasite extracts by upregulating the expression of surface molecules, including CD40, CD80, and CD86, but that very little cytokine production was induced. Indeed, parasite extracts actually blocked IL-12 production in response to several TLR agonists, although IL-10 production was enhanced (19). A recent report showed that the endoplasmic reticulum (ER) chaperone protein BiP from Giardia could signal through TLRs, but like our own studies with dendritic cells, extremely high levels of protein were required to detect a response (39).…”

“…In vitro macrophages have been shown to be capable of ingesting Giardia, and we recently showed that macrophages accumulate in the lamina propria following infection (17,18). Bone marrow-derived dendritic cells mature in response to Giardia extracts, but cytokine production in response to Toll-like receptor (TLR) agonists is modulated toward interleukin-10 (IL-10) and away from IL-12 (19). Mast cells are also recruited following infection and are required for the efficient control of infection (20,21).…”

Giardia duodenalis Cathepsin B Proteases Degrade Intestinal Epithelial Interleukin-8 and Attenuate Interleukin-8-Induced Neutrophil Chemotaxis

“…Previous studies have demonstrated that mast cell hyperplasia occurs in the late stages of a Giardia infection or following parasite clearance (20,21) and that eosinophil accumulation may occur in vivo in an isolate-dependent manner (22). Furthermore, G. duodenalis parasite products have been shown to modulate dendritic cell responses to lipopolysaccharide (23,24), while separate studies have demonstrated that intestinal epithelial cells (IECs) exposed to Giardia trophozoites produce a unique chemokine profile (25). The need to investigate G. duodenalis's ability to modulate its host's proinflammatory responses is now apparent due to data collected from in vivo animal and human studies.…”