Transcriptomic Analysis of the Host Response to Giardia duodenalis Infection Reveals Redundant Mechanisms for Parasite Control

The immune system has numerous mechanisms that it can use to combat pathogens and eliminate infections. Nevertheless, studies of immune responses often focus on single pathways required for protective responses. We applied microarray analysis of RNA in order to investigate the types of immune responses produced against infection with the intestinal pathogen Giardia duodenalis. Infection with G. duodenalis is one of the most common causes of diarrheal disease in the world. While several potential antiparasitic effector mechanisms, including complement lysis, nitric oxide (NO), and α-defensin peptides, have been shown to inhibit parasite growth or kill Giardia in vitro, studies in vivo have thus far shown clear roles only for antibody and mast cell responses in parasite control. A total of 96 transcripts were identified as being upregulated or repressed more than 2-fold in the small intestine 10 days following infection. Microarray data were validated using quantitative PCR. The most abundant category of transcripts was antibody genes, while the most highly induced transcripts were all mast cell proteases. Among the other induced transcripts was matrix metalloprotease 7 (Mmp7), the protease responsible for production of mature α-defensins in mice. While infections in Mmp7-deficient mice showed only a small increase in parasite numbers, combined genetic deletion of Mmp7 and inducible nitric oxide synthase (iNOS, Nos2) or pharmacological blockade of iNOS in Mmp7-deficient mice resulted in significant increases in parasite loads following infection. Thus, α-defensins and NO are redundant mechanisms for control of Giardia infections in vivo.