NK Cell Receptors in Immune Recognition and Regulation

This article describes the diverse receptors expressed by natural killer (NK) cells and their crucial role in regulating NK-cell recognition, activation, and cytotoxic responses. Natural killer (NK) cells are essential components of the innate immune system that provide rapid protection against infected and abnormal cells. Their ability to distinguish healthy cells from potential targets depends largely on a diverse array of cell-surface receptors. These receptors integrate activating and inhibitory signals to regulate NK-cell responses. Understanding NK-cell receptors therefore provides important insight into how NK cells recognize targets and control their cytotoxic activity.

Table of Contents

NK Cell Receptors and Their Signaling Function

Natural killer (NK) cells rely on a diverse repertoire of surface receptors to distinguish healthy cells from infected, stressed, or transformed cells. These receptors can broadly be divided according to their functional effects into inhibitory and activating NK-cell receptors. Inhibitory receptors predominantly recognize MHC class I molecules on target cells and transmit signals that restrain NK-cell cytotoxicity. In contrast, activating receptors detect signals associated with cellular stress or abnormality and promote the cytotoxic response when activating signals outweigh the inhibitory input.

Despite these distinct functions, NK-cell receptors are not classified solely based on their extracellular structure. Both activating and inhibitory receptors belong to receptor families characterized by either lectin-like or immunoglobulin-like extracellular domains. Thus, receptors with similar external architecture can produce opposing cellular responses because of differences in their intracellular signaling mechanisms.

The functional outcome of receptor engagement is largely determined by the signaling motifs associated with the receptor or its adaptor proteins. Activating NK-cell receptors commonly signal through intracellular immunoreceptor tyrosine-based activation motifs (ITAMs), whereas inhibitory receptors characteristically contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs). The integration of these opposing signals ultimately determines whether an NK cell remains restrained or proceeds to eliminate the target cell.

Inhibitory Receptors and MHC Class I Recognition

Inhibitory receptors play a central role in determining whether an NK cell will initiate a cytotoxic response against a potential target. Their recognition of MHC class I molecules on healthy cells provides an important inhibitory signal that helps prevent the destruction of normal tissues. In humans, this function is mediated largely by the killer-cell immunoglobulin-like receptor (KIR) family, a structurally diverse group of receptors characterized by immunoglobulin-like extracellular domains.

The receptor families responsible for this inhibitory mechanism differ between species. Functional KIRs are characteristic of primates and are absent in rodents. Instead, mice employ the Ly49 receptor family, whose members possess lectin-like extracellular domains and perform a comparable role by recognizing MHC class I molecules and suppressing NK-cell cytotoxicity toward healthy cells. Conversely, functional Ly49 receptors of this type are not present in humans. Thus, although humans and rodents use different receptor families, both systems employ MHC class I recognition as a major mechanism for regulating NK-cell activity.

Diversity of Inhibitory Receptors and HLA-E Recognition

Both KIR and Ly49 receptors exhibit substantial genetic diversity and polymorphism, allowing individual receptor variants to recognize different forms of MHC class I molecules. Their ligand specificity is frequently determined by polymorphic regions of these molecules. In mice, the principal ligands include the H-2K and H-2D molecules, whereas human KIRs recognize particular allelic forms of the HLA-A, HLA-B, and HLA-C molecules. This extensive receptor–ligand diversity contributes to the ability of NK cells to monitor changes in MHC class I expression across different cells.

In humans, inhibitory NK-cell receptors are not restricted to the KIR family. An important additional receptor is CD94–NKG2A, a disulfide-linked heterodimer composed of the glycoproteins CD94 and NKG2A. NKG2A belongs to the NKG2 receptor family, whose other members predominantly function as activating receptors. Unlike KIRs, which commonly detect polymorphic HLA-B and HLA-C molecules directly, CD94–NKG2A recognizes HLA-E, a non-classical MHC class I molecule. Surface expression of HLA-E depends on its association with peptides derived from the leader sequences of classical HLA class I molecules, including HLA-A, HLA-B, and HLA-C. Consequently, the presence of HLA-E at the cell surface provides an indirect indication of the cell’s overall MHC class I production and processing.

When CD94–NKG2A engages HLA-E on a potential target cell, it delivers an inhibitory signal through its intracellular signaling machinery. This interaction suppresses NK-cell cytotoxicity when cells maintain adequate MHC class I expression. Thus, the CD94–NKG2A–HLA-E pathway helps NK cells distinguish cells with normal MHC class I expression from those with reduced protective signaling.

Coexpression of Multiple NK Cell Receptors

Unlike B- and T-cell antigen receptors, NK-cell receptors do not undergo allelic exclusion. Consequently, a single NK cell can simultaneously display several different inhibitory receptors, including multiple KIRs or Ly49 receptors, with each receptor capable of recognizing a particular MHC class I molecule or a closely related group of MHC variants. Human NK cells can therefore exhibit considerable receptor diversity at the individual-cell level. Some cells have been identified that express CD94–NKG2A together with multiple KIRs, with as many as six distinct KIRs detected on a single cell.

The simultaneous expression of several receptors broadens the range of MHC class I molecules that an individual NK cell can monitor. This provides a more comprehensive system for detecting the MHC class I variants normally present on the body’s cells. When these inhibitory receptors encounter their appropriate MHC class I ligands, the resulting inhibitory signals help maintain NK-cell tolerance toward healthy tissues and reduce the likelihood of inappropriate cytotoxic activity.

Activating NK Cell Receptors and Recognition of Cellular Stress

Activating receptors on human and murine NK cells share several structural characteristics, with many belonging to the C-type lectin-like receptor family. Although these receptors contain domains related to calcium-dependent carbohydrate-recognition structures, NK cells primarily use them to recognize specific protein determinants rather than carbohydrates. The NKG2 receptor family represents one of the major groups of activating receptors found in both humans and mice.

Among these receptors, NKG2D is a particularly important mediator of NK-cell activation. Its intracellular signaling pathway shares functional similarities with signaling initiated through CD28 in T lymphocytes. NKG2D recognizes a group of non-polymorphic, MHC class I-like molecules that do not associate with β2-microglobulin. Cells generally increase the expression of these ligands when they experience cellular stress caused by DNA damage, infection, malignant transformation, or tissue injury. Their recognition by NKG2D therefore provides NK cells with a mechanism for detecting abnormal or stressed cells and promotes responses such as cytotoxicity and cytokine production.

Mice possess an additional activating receptor family known as Ly49 receptors, which has no functional counterpart of this type in humans. One notable member is Ly49H, an activating C-type lectin-like receptor that recognizes m157, an MHC class I-like protein encoded by murine cytomegalovirus (MCMV). Unlike receptors that primarily detect changes in host-cell physiology, Ly49H can directly recognize a virus-derived ligand. Engagement of Ly49H activates NK-cell responses against MCMV-infected cells and is therefore an important component of antiviral protection in mice. Animals lacking functional Ly49H-mediated recognition are substantially more susceptible to MCMV infection, demonstrating how activating NK-cell receptors can provide direct immune surveillance against pathogens.

CD16 and Antibody-Dependent NK Cell Activation

Several receptors shared with other immune-cell populations also activate NK cells and enhance their effector functions. NK-cell activation therefore depends not only on receptors specifically associated with NK cells but also on these broadly expressed immune receptors. Among these, FcγRIII (CD16) is particularly important because it enables NK cells to recognize antibody-coated target cells. CD16 binds to the Fc region of IgG antibodies that have attached to antigens present on the surface of a target cell. Engagement of this receptor generates activating signals that promote NK-cell cytotoxicity and forms the basis of antibody-dependent cellular cytotoxicity (ADCC).

ADCC provides an important mechanism for eliminating cells associated with infection or malignant transformation. For example, virus-infected cells may display viral proteins on their plasma membrane. Antibodies generated by B cells against these viral antigens can bind to the infected cell, thereby coating its surface with IgG. NK cells can then recognize the Fc portions of these antibodies through CD16. This interaction activates the NK cell and promotes the destruction of the antibody-coated target, thereby contributing to the elimination of infected cells.

Other receptors can also contribute to NK-cell activation, including CD2, which interacts with the adhesion molecule LFA-3, as well as receptors that respond to inflammatory cytokines. The ability of NK cells to integrate signals from these diverse receptors allows them to respond effectively to abnormal cellular conditions and participate in the immune-mediated elimination of infected and transformed cells.

Conclusion

NK-cell receptors form a diverse and highly coordinated system that enables NK cells to distinguish healthy cells from infected, stressed, or transformed cells. The integration of signals generated by inhibitory and activating receptors determines whether an NK cell remains quiescent or initiates an immune response. Inhibitory receptors, particularly those that monitor MHC class I expression, provide essential protection against the inappropriate destruction of healthy cells, whereas activating receptors enable the recognition and elimination of abnormal cells. Additional receptors such as CD16 allow NK cells to cooperate with antibodies through ADCC, further expanding their defensive capabilities. Together, these receptor-mediated mechanisms make NK cells effective components of innate immunity, capable of rapidly responding to infection, cellular stress, and malignancy while maintaining appropriate tolerance toward healthy tissues.