Transcriptome analysis of host responses to Giardia lamblia infections and identification of novel effector mechanisms in parasite control

As many as one billion cases of infection with Giardia lamblia are thought to occur each year. T cells help control the initial phase of the infection, although the exact mechanisms by which they act are still unknown. Similarly, while B cell-dependent mechanisms contribute to prevention of chronic infections, their role during the early part of the infection remains controversial. In particular, studies suggest Immunoglobulin (Ig) A may not be as crucial in the initial elimination of G. lamblia as they are for G. muris. Antibody-independent mechanisms controlling this infection have not been well-defined. While mast cells participate in the elimination of Giardia, how they are recruited to the site of infection and how they contribute in the elimination of the parasite are not known. We therefore applied microarray technology to profile intestinal gene expression in C57BL/6 mice in response to infection with G. lamblia trophozoites (GS (M)-H7 strain). A total of 96 transcripts were identified as being significantly regulated. Induced transcripts were categorized as coming primarily from B cells and mast cells, consistent with prior knowledge of this infection. Other regulated transcripts suggested activation of Paneth cells and changes in enterocyte function. We further explored the role of two induced transcripts with potential for a role in mucosal immunity: mannose-binding lectin (Mbl2) and matrix metalloproteinase 7 (Mmp7). Wild type mice expressed higher levels of Mmp7 and Mbl2 upon infection compared with uninfected mice. Furthermore, neither transcript was induced following infection of TNF deficient mice, which have a defect in the clearance of Giardia. Mbl-deficient mice showed an inability to recruit mast cells in the intestinal submucosa and were delayed in their elimination of Giardia infection. Mmp7 deficient mice also showed a slightly reduced ability to clear infections. Since nitric oxide has been shown to inhibit parasite growth in vitro but inducible nitric oxide synthase (Nos2) deletion had no effect in vivo, we combined the Mmp7 and Nos2 mutations in double knockout mice. When Mmp7-deficiency was compounded by the absence of Nos2, a significant delay in parasite clearance was observed. These data have identified new mechanisms which contribute to control of Giardia infection in the absence of antibodies. Mbl contributes to parasite control and may aid in the recruitment of mast cells. Mmp7 contributes to production of cryptdin peptides and, along with nitric oxide produced by Nos2, likely represent important factors directly controlling this intestinal pathogen.