Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 150

NK cells require priming by various factors, such as IL15 transmitted by dendritic
cells or macrophages, IL12, or IL18, to achieve their full effector potential [ 276 ].
Cytokine receptors IL2R, IL15R, and IL21R are involved in NK-cell development
and maturation.
Natural killer cells express NK-cell markers and possibly a T-cell receptor,
but not the B-cell receptor. The NK complex of the genome encodes for NK-
cell receptors and type-2 C-type lectin-like molecules that are involved in cell
recognition.

3.11.5.1 NK-Cell Stimulatory and Inhibitory Receptors

Detection of NK-cell targets relies on a set of plasmalemmal stimulatory and
inhibitory receptors. Natural killer cells express inhibitory receptors (tolerance to
self) for both MHC class-1 or non-MHC ligands.
The body's immunological tolerance to its own cells relies on cells of both innate
and adaptive immunity. Natural killer cells express various inhibitory receptors
that recognize self MHC molecules that prevent cytolysis. Autoreactive T cells
do not experience the negative selection in the thymus that cause deletion of
most developing T cells with specificity for self-antigens. NK cells use inhibitory
receptors to gauge the absence of constitutively expressed self molecules on
susceptible target cells.

NK-Cell Inhibitory Receptors

Killer inhibitory receptors comprise killer cell lectin-like receptors, such as

KLRa3 143 that detects the MHC class-1 molecule H2 K b ,KLRc1, 144

as well

as killer cell (KIR; CD158a, CD158b1-b2, CD158c-CD158d, CD158e1-e2,

CD158f-CD158k, CD158z) and leukocyte (LIR or LILR; CD85a-CD85m)

immunoglobulin-like receptors [ 277 , 278 ].
Proteins of the KIR and LIR families are structurally and functionally similar.
Killer cell immunoglobulin-like receptors are expressed not only by NK cells,
but also a subpopulation of T cells. 145 Receptors of the LIR family have a more
widespread cellular distribution. They are expressed on various hematopoietic cell
types, such as NK, T, and B lymphocytes, as well as macrophages, mastocytes, and
dendritic cells.
Members of the KIR and LIR families possess isoforms with different cyto-
plasmic signaling functions. Receptors of the KIR and LIR families with long

143 A.k.a. lymphocyte antigen Ly49c.
144 A.k.a. member A of group 2 of NK receptors (NKG2a).
145 Genes that encode the KIR receptors that preclude the innate cytotoxic activity of NK lym-
phocytes constitute a genetic region on chromosome 19, the so-called human leukocyte receptor
complex (LCR).