Page 1154 - Veterinary Immunology, 10th Edition

VetBooks.ir Cellular Defenses

Natural Killer Cells

The cytotoxic innate lymphoid cells known as natural killer (NK)
cells are discussed in Chapter 19. NK cells can detect and kill cancer
cells and infected or damaged cells without prior priming. They

have two major types of receptors: inhibitory receptors that can
recognize the absence of MHC class I molecules on a cell surface
and in so doing are triggered to kill abnormal cells; and activating
receptors that can recognize the presence of stress-induced proteins
on cell surfaces and as a result will kill stressed cells. Thus NK cells

effectively kill two types of cellular targets: cells that fail to express
MHC class I molecules and cells that express certain stress-related
proteins. Both conditions commonly apply to cancer cells. As a

result, NK cells play a significant role in the destruction of tumors.

Cytotoxic T Cells

T cells have the potential to destroy cancer cells. They possess
receptors that can recognize antigen-MHC complexes expressed on

all nucleated cells. The immense size of the T cell antigen receptor
repertoire enables it to identify and respond to new antigens made
by cancer cells.
Until recently, evidence that T cells could control tumor growth

came only from mice and human melanomas. For example, T cell
stimulation by IL-2 benefited a subset of melanoma patients.
Subsequent studies with the checkpoint inhibitor ipilimumab that
interferes with T cell CTLA-4 also showed great benefit to

melanoma patients. Infusion of tumor-infiltrating lymphocytes,
some of which are cytotoxic, may also benefit some melanoma
patients.
Tumor-infiltrating CD8+ cytotoxic T cells are found in many

cancers. Their presence tends to be associated with a better
prognosis. Some success has been achieved in humans by isolating
these cells from excised tumors and growing them in tissue culture.
These cultured cells are then transfused into the patient and may

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