and D.B., performed the experiments and analysed the data. patches (PP) in treated and infected animals increase the expression of interleukins such as IL-25, IL-21, and IL-9, augmenting mucus production in the crypts, and improving chemokines, such as OX40 and CCL20 in the mucosa. Our results suggest that the Th17/Th2 regulatory mechanism provoked by the administration of the anti-IL-23 antibody prevents the implantation of the intestinal nematode in mice. The diminished inflammatory IL-17 levels alter the Th9 environment perhaps as a Proc consequence of IL-17 inhibiting IL-9 expression. These Th9 conditions may explain the successful treatment against Inflammatory Bowel Disease (IBD) both with antibodies against IL-23 or through parasitization MZ1 with nematodes. == Introduction == Parasitism by intestinal nematodes triggers a strong Th2 response in the host, with an increase in interleukin markers of this response, such as IL-4, IL-5, IL-10, IL-151,2. The activation of Th2 by helminth contamination diminishes the inflammatory intestinal processes3, and therefore helminth infections have been used to treat inflammatory intestinal diseases as an alternative to treatment with antibodies against IL-2348. This Th2 activation has recently been confirmed to be the result of the induction of IL-25 MZ1 by tuft cells9,10. This interleukin, also called IL-17E, induces IL-13, which stimulates the production of interleukins involved in the Th2 response11,12. IL-25 production also precedes the rise in IL-9 levels MZ1 in the parasite-expulsion process, with IL-9 being ultimately responsible for eliminating the nematodes13,14by inducing variations in the intestinal mucosal niche, such as increased mucus by goblet-cell activation15, greater intestinal contractibility16, and altered intestinal MZ1 permeability17. In addition, IL-9 induces mast-cell expansion and secretion of specific proteases, such as mouse mast-cell protease 1 (mMCPT-1)18,19, as well as higher levels of CCL11 or eotaxin, the chemokine that specifically attracts eosinophils20. Elliottet al.21, previously described that the levels of IL-17 are suppressed during nematode parasitism. Among other functions, IL-23 is involved mainly in promoting Th17 differentiation and proliferation22, helping to sustain IL-17 production by CD4+T cells23as well as repressing IL-9 production24, which as mentioned above is largely responsible for nematode expulsion by creating a hostile environment in the intestine. Thus, it is expected that decreasing the amount of circulating IL-23 should boost IL-25, IL-13 and IL-9 production and consequently modifies the physiological conditions suitable for the establishment of gastrointestinal nematodes through the stimulation of the hostile conditions described above. For this study, we have employed the mouse pinwormAspiculuris tetraptera, a nematode with a direct life cycle that induces little damage to the host epithelium and does not trigger a host inflammatory response25. Using this model, we MZ1 injected an anti-IL23 monoclonal antibody in mice prior to a challenge infection with embryonated eggs ofA. tetrapterato determine whether the reduction of circulating IL-23 could prevent the successful implantation of parasite in the intestine. The parasitological data both of nematode establishment as well as development rates have been studied, comparing the results to control (untreated mice). Similarly, the different interleukin expression levels in spleen, mesenteric lymph nodes (MSN), and Peyers patches (PP) were analyzed, together with other parameters, including chemokines in the intestinal mucosa in the different mice groups (treated with the anti-IL-23 antibodies and infected, treated but uninfected and control infected mice). The data compiled corroborate that Th17 and specifically IL-17 are needed to maintain the immunological and physiological environment conducive to nematode establishment in the intestine. The IL-23, has inhibitory effects on IL-9 production21,25. The decrease in the normal levels of this interleukin provide a Th9 environment that impedes the nematode establishment in the intestine and may account for the treatment against inflammatory bowel diseases (IBD) both with antibodies against IL-23 as well as by nematode parasitism. This suggests that a higher level of intestinal IL-9 would permit treatment against these chronic inflammatory bowel diseases. == Results == In order to investigate whether decreasing circulating IL-23.