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

Abstract

Infection with Giardia duodenalis is one of the most common causes of diarrheal disease in the world. While numerous studies have identified important contributions of adaptive immune responses to parasite control, much less work has examined innate immunity and its connections to the adaptive response during this infection. We explored the role of complement in immunity to Giardia using mice deficient in mannose-binding lectin (Mbl2) or complement factor 3a receptor (C3aR). Both strains exhibited delayed clearance of parasites and a reduced ability to recruit mast cells in the intestinal submucosa. C3aR-deficient mice had normal production of antiparasite IgA, but ex vivo T cell recall responses were impaired. These data suggest that complement is a key factor in the innate recognition of Giardia and that recruitment of mast cells and activation of T cell immunity through C3a are important for parasite control. Giardia duodenalis is one of the most common protozoan infections of humans as well as other mammals throughout the world and is a leading cause of diarrheal disease in these species (1-3). Symptomatic infections occur in ϳ20 to 80% of humans with positive stool samples and are characterized by nausea, vomiting, epigastric pains, and diarrhea (1, 2, 4, 5). These symptoms are associated with nutrient malabsorption and can lead to weight loss and malnutrition in children, exposing this vulnerable group to failure to thrive and developmental problems (6, 7). Disease resolves spontaneously in Ͼ85% of cases. In certain cases, in spite of a healthy and fully developed immune system, the acute phase of the disease develops into chronic disease. In these cases, symptoms of the disease will reappear for short and recurrent periods (3,4,8). The mechanisms explaining interactions between the host and the parasite leading to parasite clearance and disease pathogenesis are poorly understood.Studies of immune responses against Giardia have demonstrated important roles for both innate and adaptive immunity (7, 9). Antibody production following infection is robust, and IgA is very effective at eliminating parasites (9). Antibody-independent roles of T cells can also eliminate infections (10). Early studies indicated that Giardia trophozoites are susceptible to killing by factors in nonimmune human serum, milk, and intestinal fluid (11-13). Recently, killing by normal serum was demonstrated to involve the lectin pathway (14), consistent with the expression of N-acetylglucosamine (GlcNAc) on the surface of trophozoites (15,16). Studies of cells of the innate immune system indicate that macrophages, dendritic cells, mast cells, and intestinal epithelial cells are all involved. 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 interle...

Monitoring C3aR Expression Using a Floxed tdTomato-C3aR Reporter Knock-in Mouse

“…Our data confirm a previous report (83) demonstrating that PE MCs do not express C3aR at steady-state, although functional studies suggest that C3a activates MCs from skin and the small intestine (84,85). In line with this notion, variable and complex C3aR expression has been shown in human MCs.…”

The Intersection of Immune Responses, Microbiota, and Pathogenesis in Giardiasis

“…[58] reported a delay in the clearance of Giardia parasites and recruitment of mast cells to the small intestine in mice lacking MBL or complement component C3a receptor (C3aR). Furthermore, mice lacking C3aR also exhibited significantly reduced T-cell responses against parasite antigens following infection but IgA levels within the small intestine were unaffected [58]. Taken together, these results indicate a role for the complement system in not only activating but also mediating downstream immune responses during infection.…”

“…Additionally, it would be interesting to understand if mast cell-derived IL-6 is critical to Th17 cell development. Li et al [88] found a reduction in IL-17 responses in the absence of signaling via C3aR, which suggest that a decrease in mast cell recruitment also correlates with decreased IL-17 production. In human patients that have recovered from Giardia , IL-17 has been found to be upregulated when effector memory CD4 + T-cell are restimulated with Giardia antigen [89]; in addition, several studies [25, 87, 90, 91] reported the upregulation of IL-17 during murine or bovine infections, and delayed parasite clearance in IL-17 knockout mice has also been reported [87, 90].…”

The Chymase Mouse Mast Cell Protease-4 Regulates Intestinal Cytokine Expression in Mature Adult Mice Infected with Giardia intestinalis

“…Mast cell deficient c-kit w/wv (c-kit dependent deficiency in the mast cell compartment) mice and c-kit depleted mice both failed to control a Giardia infection [26], suggesting that mast cells and c-kit dependent mechanisms are necessary for elimination of a G. intestinalis infection. In addition, the complement factor 3a receptor was found to be important for recruitment of mast cells to the mucosa during Giardia-infections in mice [27]. There was also an effect on the adaptive immune system since mast cell-deficient mice failed to produce parasite-specific IgA [26].…”