Page 937 - Veterinary Immunology, 10th Edition

IL-25 stimulates the production of type 2 cytokines by ILC2 cells
VetBooks.ir and facilitates differentiation of Th2 cells. IL-33 acts on Th2 cells,

ILC2 cells, basophils, and mast cells to drive production of IL-4, IL-
5, and IL-13. ILC2s are an important early source of IL-4, IL-13, and

IL-5 (Box 28.2).

Box 28.1

Parasites, Microbiota and Host

Immunity

Intestinal helminth parasites are members of the microbiota. Their
presence alters the composition of the microbiota because the
worms release metabolites and excretory/secretory products, and
they stimulate their host's immune system. The composition of the

bacterial microbiota also influences helminth colonization. Both
helminths and the bacterial microbiota influence the Th17-Treg cell
balance as well as Th2 responses. Alterations in the Th17-Treg cell
balance, especially those that reduce intestinal inflammation,

promote helminth survival. For example, premature initiation of
Treg responses in Trichuris muris infections can inhibit protective
immunity. Conversely, Treg depletion reduces the worm burden
and enhances intestinal Th1 inflammatory responses. Tregs appear

to limit Th2 cell expansion, especially early in infections.

Box 28.2

Tuft Cells

Among the less common cells that line the small intestine are “tuft

cells”, so-called because they have a small tuft of microvilli that
extends into the intestinal lumen. Until recently, their function was
unknown. Now it has been shown that these cells play a key role in

initiating type 2 immune responses to helminths. Tuft cells
apparently can sense the “taste” of intestinal helminths. In
response they produce large amounts of IL-25, a cytokine that
recruits eosinophils, activates both Th2 cells and ILC2 cells, and
causes them to secrete more IL-13 and IL-4 (Fig. 28.10). IL-13 and

IL-4 from ILC2 cells stimulates further tuft cell proliferation and IL-

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