Chitinase activity has been demonstrated in certain populations of AAMØ in both mice and humans [52, 56, 57] . Chitinolytic activity by macrophages has also been implicated in host defense against chitin-containing fungal pathogens [56, 58] . To test whether or not chitinase activity is induced by Toxoplasma infection, a chitinase assay was performed on whole brain lysates from naïve and infected animals. Three chitin substrates labeled with 4-methylumbelliferone (4MU) were used to assess the type of chitinase activity present. Upon hydrolysis, 4MU is released and can be measured fluorometrically to determine chitolytic activity. Our data reveal that chitinase activity is significantly increased in the brains of infected mice as compared to the naïve group in only one of the three substrates ( Figure 2B ). This substrate, 4MU Nacetyl-b-D-glucosaminide, is suitable for detection of exochitinase activity where the enzyme degrades the non-reducing end of the chitin [59] . Several studies have linked chitinases and chitinase-like proteins to inflammation [58] [59] [60] . This family of 18 glycosyl hydrolases is typically induced during Th2 type immune responses and plays a role in tissue remodeling, fibrosis, and the modulation of both the innate and adaptive immune response [60] . Acidic mammalian chitinase (AMCase) and chitotriosidase (CHIT1) are unique members of this family in that they possess an enzymatically active domain that hydrolyzes the b 1-4 linkages that exist in chitin [56, 58] . Analysis using qRT-PCR demonstrated a significant upregulation of AMCase but not CHIT1. In addition, the chitinase-like protein, Ym-1 (Chi313) was also upregulated following infection ( Figure S4 ). This molecule is known to inhibit IL-12 production and induce alternative activation in macrophages [25, 61, 62] . In contrast to Ym-1, that has been associated with AMCase production by macrophages in the lung and airway [63] , Ym-2 and BRP-39 were not upregulated in infected brains ( Figure 2C , S4). Since chitin has been shown to activate and recruit AAMØ, it is possible that the cyst wall may serve as the stimulus for chitinase activity in this population of cells. To test this further, BMDM were co-cultured with different forms of the parasite. The addition of tachyzoites, bradyzoites or sTAg did not lead to chitinase production ( Figure 2D ). In contrast, live cysts and cyst antigen led to a significant increase in chitinase activity that was abolished following chitinase treatment of cysts ( Figure 2D , S4). Furthermore, treatment with IL-4 to induce alternative activation in macrophages did not lead to increased chitinase activity. Indeed measurement of IL-4, IL-4Ra and the IL-4dependent RELM-a [64] [65] [66] in the brains of chronically infected mice showed no significant increase over naïve mice ( Figure S4 ). These data suggest that the presence of chitin in the cyst wall induces a phenotype of macrophage characterized by the production of the enzymatically active chitinase, AMCase and is distinct from IL-4 induced activation.