NATURAL KILLER CELLS
Compared to T- and B-cell development, much less is known about the biological and bio-chemical events that drive NK-cell development. The observation that T and NK cells share many surface markers as well as functional activities led to the identification of a common T/NK precursor. This precursor population is believed to be the source of a subpopulation of dendritic cells, which expresses other lymphoid phenotypic markers, in contrast with the predominant population of dendritic cells that are of myeloid lineage.
Unlike T and B cells, no marker is exclusively expressed by NK cells. Phenotypically mature NK cells are defined as CD3- , CD16 +/- , CD56+ lymphocytes. NK cells can ac-count for 5–27% of circulating lymphocytes. Also in contrast to T and B lymphocytes, NK cells do not rearrange their TcR or Ig genes; they develop normally in scid mice or RAG-1-/- or RAG-2-/- knockout mice. Although committed NK progenitors can be found in the thymus, NK cells can develop in athymic nude mice; therefore, the thymus is not required for NK development. It is believed that NK cells can develop in a variety of organs; how-ever, the major site of NK-cell development has yet to be discerned. Functionally active NK cells can be isolated from the fetal thymus at 6 weeks of gestation, prior to formation of the thymic rudiment. Thus NK cells are probably the first functionally active lymphocyte in the developing fetus.
NK cells do not express an antigen-specific receptor homologous to the TcR and BcR and thus are considered part of the innate immune system as opposed to the adaptive im-mune system. NK cells are active in the naive or nonimmunized host and are considered oneof the first lines of defense against infection. They are important early producers of IFN-γ . NK cells secrete other cytokines and chemokines such as TNF, GM-CSF, MIP-1α , MIP-1β , and RANTES. Traditionally, NK cells have been functionally defined by the ability to mediate spontaneous or “natural” killing of virus-infected, tumor, and allogeneic target cells. This cell-mediated lysis is perforin-dependent. The CD16+NK cells also mediate an-tibody-dependent cellular cytotoxicity (ADCC).
The study of NK-cell receptors has made significant strides in the past few years. Signals generated from activation and inhibitory receptors on the surface of the NK cells compete to influence the cellular response. Positive stimuli are required to activate NK-cell effector function, while inhibitory receptors and cytokines control specificity and regulation. NK-cell inhibitory signals are mediated by receptor for MHC class I antigens. However, the sig-nals that activate NK cells are less well defined.
Traditionally NK cells were defined as mediators of MHC nonrestricted cytolysis because of their ability to lyse autologous and allogeneic tumor or virally infected targets. The ob-servation that NK cells preferentially lysed target cells that expressed low levels or no MHC class I antigens precipitated the study of MCH class I receptors on NK cells. Three distinct MHC class I receptor families have recently been identified: LY49, CD94/NKG2, and KIR. Although structurally distinct, each of these receptor families uses a similar mechanism to mediate inhibitory signaling—the inhibitory sequences (ITIM) in the cytoplasmic domains of the receptors. Tyrosine phosphorylation of these sequences is induced by interaction with a MHC-I molecule. The subsequent recruitment of the tyrosine phos-phatases SHP-1 and, possibly, SHP-2 results in NK-cell inhibition. NK-cell proliferation, cytotoxicity, and cytokine production are all downregulated as a consequence of MHC-I–NK receptor interaction.
In humans, the best characterized receptors are those of the KIR (killer cell–in-hibitory receptor) family that are expressed on overlapping subsets of human NK cells and a subset of T cells. The 12 members of the KIR family are structurally different from each other. Some of the members of this family have short cytoplasmic tails that do not contain ITIM sequences and therefore may not be functionally identical to the members of the fam-ily with long intracytoplasmic tails that include ITIM sequences.
Human NK cells also express CD94/NKG2 receptors, which are also expressed on the surface of a subset of T cells. The LY49 receptors, in contrast, are only expressed in murine NK cells.
NK cells can be activated by a variety of ligands, involving a variety of different receptors. NKR-P1 receptors are membrane receptors of the C-type lectin superfamily, which are expressed on most NK and a subset of T cells. These receptors interact with glyco- proteins, such as those expressed in mammalian cells, and are believed to deliver acti-vating signals to the NK cells. However, the response to NKR-P1 triggering in human NK cells is variable. This may be due to the presence of additional, functionally distinct NKR-P1 isoforms. Rodent NKR-P1 molecules contain the CXCP motif also found in CD4 and CD8 that interacts with phosphorylated p56lck, and this is strong evidence of their ability to be involved in cell stimulation. This sequence is not found in human NKR-P1.
CD16 is a low-affinity IgG Fc receptor (FcγRIII) expressed on most NK cells, acti-vated monocytes, and a subset of T cells. The transmembrane-anchored CD16 isoform is complexed with the CD3ζ chain or the FceRγ chain. Receptor ligation results in the phos-phorylation of tyrosine residues within the ITAM motif in the cytoplasmic domains of the CD3ζ chain or the FceRγ chain. This is followed by the recruitment and phosphorylation of ZAP70, activation of phospholipase C, stimulation of PI 3-kinase and MAP kinase, p21 ras activation, and translocation of NFAT. CD16 engagement on NK cells results in ADCC and cytokine secretion.
CD2 is expressed on NK and T cells. CD2 activation by monoclonal antibody or its counterreceptor, CD58 (LFA-3), induces as similar cascade of intracellular signaling events as CD16 ligation and induces NK cell–mediated cytotoxicity. CD2 is thought not to be a primary activation receptor on NK cells, but rather a costimulatory receptor that aug-ments NK-cell activation.