mAbs to NK Antigens: Mouse NKG2D, Ly49 Antibody and More...

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Introduction

NK cells, morphologically classified as large granular lymphocytes (LGL), are important effector lymphocytes of innate immunity. Functionally, they exhibit cytolytic activity against a variety of allogeneic targets in a non-specific, contact-dependent, non-phagocytotic process which does not require prior sensitization to an antigen. These cells also have a regulatory role in the immune system through the release of cytokines which in turn stimulate other immune functions.

NK cells do share several properties with conventional cytotoxic T cells (CTL); CTL, NK and LAK cells* appear to possess similar mechanisms for cytolysis including secretion of pore-forming proteins, serine proteases and other proteins. The cytotoxic activity is positively regulated by IL-2 and interferons and is diminished by prostaglandins and TGF-beta. Furthermore, T cells and NK cells similarly express CD2 and utilize LFA-1 surface antigen to enhance effector cell adhesion to target cells.

However, NK cells can be distinguished from T lymphocytes by the expression of distinct phenotypic markers such as CD16⁺, CD56⁺ (human NK cells only) and lack of rearranged T cell receptor gene products. Once considered relatively homogeneous, it is now known that NK cells are highly diverse. Within an individual, expression of different combinations of receptors creates a diverse NK cell repertoire, which exhibits specificity in the immune response.

Figure 1: TRAIL, as detected by staining with TRAIL specific antibody PE N2B2, is expressed on C57Bl/6 liver NK cells.

A number of surface antigens can be utilized to define NK cells and to identify functionally distinct subsets within this heterogeneous population. So far, in the human, phenotypic distinction of NK cells is mainly achieved by expression of CD56 (or CD57), while in the mouse, expression of DX5/CD49b and NK1.1 (only in NK1.1⁺ mouse strains) is considered as a best phenotypic marker for NK cells. Recent development of specific antibodies to the human and mouse NKG2D suggest that this marker is also expressed by all NK cells.

NKG2D, as detected by staining with NKG2D specific antibody CX5, is expressed by all NK cells from mouse spleen.

It has been possible to associate specific functional activities with several antigens expressed on NK cells. NK cells use specific receptors to mediate killing through the recognition of distinct ligands expressed on target cells. These receptors fall into two functional types, inhibitory and stimulatory. The inhibitory receptors are further subdivided as Killer cell Ig-like receptors (KIR family), a family of type I membrane proteins and lectin-like type II membrane proteins. All known ligands for these inhibitory receptors are MHC class I or molecules of host or pathogen origin that are homologous to MHC class I. Ligation of such molecules by MHC I on target cells results in inhibition of the NK or T cell cytotoxic activity through the Immunoreceptor Tyrosine-based Inhibitory Motif (ITIM). Where defined, several members of the Ly-49 family in the mouse bind to mouse MHC class I and similar to KIR proteins serve as the inhibitory receptors for mouse NK cells.

While less is known about the NK stimulatory receptors, it is known that these receptors have charged amino acid residues in their transmembrane portions allowing them to interact with other signaling proteins containing Immunoreceptor Tyrosine-based Activation Motifs (ITAMs). Similar to T cell signaling pathways, when these NK stimulatory receptors are engaged, ITAMs recruit downstream signaling molecules including kinases such as ZAP-70 resulting in proliferation and activation of cytotoxicity.

Experimental evidence from several laboratories on human and/or mouse NK cells suggests collaboration between cell surface protein such as CD94 and members of the NKG2 family of proteins to play a role in NK function.

NKG2AB6, as detected by two-color staining with FITC anti-mouse CD3 and NKG2AB6 specific antibody PE 16a11, is expressed by NK and NKT cells from C57Bl/6 mouse spleen.

Studies of the origin and ontogeny of NK lineage have elucidated an NK/T cell-restricted progenitor cell in blood of mouse fetus. This population of cells is NK1.1⁺ and CD117⁺ (c-kit) and similar to the NK lineage progenitor cells from mouse bone marrow responds to IL-15 for NK cell differentiation. IL-15 also stimulates expression of some of the MHC-specific inhibitory receptor families expressed by NK cells. The data reported so far suggests mechanism such as cellular anergy, differential expression of the members of the inhibitory NK receptors, and the regulation of these receptors on the NK cell surface by cytokines and other immune signaling pathways as possible ways to provide tolerance to self for NK cell lineage.

eBioscience provides a collection of antibodies for mouse and human NK and NKT cell research (see chart below). Please visit this site regularly, as we add new antibodies relevant for this area of research every month.

*LAK cells constitute an additional killer cell population which arise from lymphatic cells in the presence of IL-2. They appear to represent a functional unique cytotoxic effector cell system with an exceptionally wide target cell spectrum including normal and malignant cells of different origin. LAK cells, however, show a profound heterogeneity concerning the expression of phenotype surface markers and it remains to be determined if they are a unique cell lineage.

References

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10. Cerwenka A, Baron JL, Lanier LL. 2001. Ectopic expression of retinoic acid early inducible-1 gene (RAE-1) permits natural killer cell-mediated rejection of a MHC class I-bearing tumor in vivo. Proc Natl Acad Sci U S A. 98(20):11521-6.