Controlling natural killer cell responses: integration of signals for activation and inhibition

Abstract

Understanding how signals are integrated to control natural killer (NK) cell responsiveness in the absence of antigen-specific receptors has been a challenge, but recent work has revealed some underlying principles that govern NK cell responses. NK cells use an array of innate receptors to sense their environment and respond to alterations caused by infections, cellular stress, and transformation. No single activation receptor dominates; instead, synergistic signals from combinations of receptors are integrated to activate natural cytotoxicity and cytokine production. Inhibitory receptors for major histocompatibility complex class I (MHC-I) have a critical role in controlling NK cell responses and, paradoxically, in maintaining NK cells in a state of responsiveness to subsequent activation events, a process referred to as licensing. MHC-I-specific inhibitory receptors both block activation signals and trigger signals to phosphorylate and inactivate the small adaptor Crk. These different facets of inhibitory signaling are incorporated into a revocable license model for the reversible tuning of NK cell responsiveness.

Figure 1

Multiple signals control NK cell responses. An array of NK cell activation receptors binds to ligands on other cells. Target cell killing requires signals for lytic granule polarization and degranulation. Combinations of coactivation receptors synergize to induce killing and a secretory response. Some of the secretion response is independent of transcription. Inhibitory receptors (Red) for MHC-I exert dominant inhibition of cytotoxicity and secretion, as seen in the NK--DC interaction. DC also receive signals from ligands on NK cells. In the absence of inhibition through MHC-I, target cells that express ligands for activation receptors are killed. NK cells respond to various soluble activators such as cytokines and chemokines that deliver signals for functions such as migration and survival. The prosurvival cytokine IL-15 bound to the IL-15R α chain is trans-presented by other cells to the IL-15R βγ_c chains on NK cells. In response to soluble factors, NK cells secrete IFN-γ, TNF-α, other cytokines, and chemokines. Signals received through receptors that bind soluble ligands are not subject to inhibition through MHC-I. Soluble HLA-G binds to CD158d and induces a transcriptional response for a unique set of proinflammatory, proangiogenic molecules.

Figure 2

A molecular basis for synergistic activation through pairs of coactivation receptors on NK cells. Cytotoxicity and secretion by primary resting NK cells requires synergistic signals from two coactivation receptors, such as 2B4 and NKG2D or 2B4 and DNAM-1. This synergy is required to overcome inhibition by the ubiquitin ligase c-Cbl of a Vav1-dependent activation pathway. Stimulation through 2B4 results in a Fyn-dependent, Syk-independent, selective phosphorylation of Tyr113, but not Tyr128 in the adaptor protein SLP-76. Conversely, stimulation through NKG2D or DNAM-1 results in selective, Syk-independent phosphorylation of Tyr128, but not Tyr113 in SLP-76. In contrast, stimulation by the Fcγ receptor CD16 results in Syk-dependent phosphorylation of both Tyr113 and Tyr128, as is the case after TCR signaling in T cells.

Figure 3

Two components of negative signaling by inhibitory receptors for MHC-I. Signaling by activation receptors (Green) requires the adaptor molecule Crk, actin polymerization, and phosphorylation of the guanine exchange factor Vav1. Recruitment of tyrosine phosphatase SHP-1 by phosphorylated ITIMs in the cytoplasmic tail of inhibitory receptors (Red) results in Vav1 dephosphorylation. In addition, inhibitory receptors induce Crk phosphorylation by the tyrosine kinase c-Abl. Phosphorylated Crk dissociates from cytoskeletal signaling complexes that include the scaffold proteins c-Cbl, paxillin, and p130CAS.

Figure 4

NK cells detect the loss but not the absence of MHC-I. In the absence of self MHC-I, inhibitory receptors on NK cells are not engaged and NK cells remain unresponsive. In the presence of MHC-I, inhibitory receptors such as KIR deliver an inhibitory signal that prevents disarming and/or a signal that renders NK cells responsive (i.e., arming signal). The two options are not mutually exclusive and have the same outcome: NK cells become licensed by either maintaining or acquiring responsiveness, and they can kill target cells. NK cells that lack inhibitory receptors for self MHC-I are not licensed. Licensing is a calibrated process that provides quantitative tuning of NK cell responsiveness. For clarity, receptor--ligand combinations for activation are not shown.