Page 420 - Veterinary Immunology, 10th Edition

functions: they mediate inflammation and they are potent B-cell
VetBooks.ir helpers (see Fig. 20.14). Cytokines of the IL-17 family play a key role

in protective type 1 responses to extracellular bacteria and assist in
the clearance of fungi. Under some circumstances, Th17 cells may

convert into IFN-γ–producing Th1 cells. Likewise, they can
differentiate into regulatory T cells (Tregs) when inflammation is
resolved. The balance between Th17 and Treg cells is critical to
maintaining homeostasis during immune responses and

inflammation. Excessive Th17 activity can lead to the development
of chronic inflammatory diseases.

Box 14.2

Interleukin-17

The interleukin-17 family contains multiple members (IL-17A
through F). IL-17F is also called IL-25. They have no sequence
similarity to any other cytokines. The most important is IL-17A
since it induces inflammation. IL-17 is produced by Th17 cells

under the influence of IL-23. IL-17 binds to a family of cell surface
receptors (IL-17RA through RE) that signal through NF-κB.
IL-17 plays a key role in immunity to extracellular bacteria and

fungi since it recruits granulocytes through its actions on stem
cells. It stimulates the production GM-CSF leading to a
neutrophilia (Chapter 5). It promotes the recruitment and survival
of macrophages, and it stimulates the production of
proinflammatory cytokines and antibacterial peptides from many

cell types. It attracts neutrophils and macrophages (but not
eosinophils) to inflammatory sites. It triggers production of many
other cytokines such as IL-1, IL-6, GM-CSF, and TNF-α, as well as

chemokines and prostaglandins. IL-17 contributes significantly to
immunologic diseases such as asthma, lupus, and rheumatoid
arthritis.

Regulatory T Cells (Tregs)

Regulatory T cells are typical lymphocytes that express CD4 and
CD25 (the α chain of the IL-2 receptor, which is discussed in detail
in Chapter 20). Their most characteristic feature, however, is their

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