Natural killer cell responses during viral infections: flexibility and conditioning of innate immunity by experience

Abstract

Natural killer (NK) cells mediate innate defense against viral infections, but the mechanisms in place to access their functions as needed during diverse challenges while limiting collateral damage are poorly understood. Recent molecular characterization of effects mediated through infection-induced inhibitory/activating NK receptor-ligand pairs and cytokines are providing new insights into pathways regulating their responses and revealing unexpected consequences for NK cell subset effects, maintenance, proliferation and function through times overlapping with adaptive and long-lived immunity. The observations define flexible pathways for experience-induced 'conditioning' and challenge narrowly defined roles for NK cells and innate immunity as first responders with prescribed functions. They suggest that individual experiences as well as genes influence the innate immune resources available to fight off an infection.

Figure 1. NK cell receptor genes and their products

Only receptors mentioned in the text are represented. (A) NK cell receptor genes reside principally in two genomic regions named the natural killer gene complex or the lymphoctye receptor complex. The natural killer gene complex (NKC) on human chromosome 12p12-p13 and mouse chromosome 6 encodes receptor molecules of the the C-type lectin-like family. Within the Ly49 gene family presents the most extensive inter-specific and inter-individual variation in terms of gene content. Whereas in rodents Ly49 define haplotypes with different gene number and extensive allelic polymorphism, in humans, there is a single Ly49 gene which corresponds to the pseudogene, LY49L. Other C-type receptor genes (including the single CD94 gene as well as the NKG2, NKRP1 and CLR gene families are more conserved. The leukocyte receptor complex, on human chromosome 19q13.4 and mouse chromosome 7, encodes immunoglobin-like receptors. Within this complex, the KIR gene family is highly variable between species and individuals. Humans carry polymorphic KIR haplotypes that also vary in gene number. Rodents lack KIR genes but carry ILT (known as paired-iimunoglobin-receptor (Pir) genes in mice) and Nkp46 gene homologues. (B) Natural killer receptor products of the genes mediate inhibitory or activating signals, the balance of which determines NK cell activity. Irrespective of their family, inhibitory NK cell receptors (in red) bear an immunotyrosin inhibitory motif (ITIM, red disk) in the cytoplasmic domain. Activating receptors (other colors) associate with adaptor proteins that carry immunotyrosine activating motifs (ITAM, green disks). NKG2D forms homodimers expressed in virtually all NK cells. The preferred adaptor protein is DAP10 but in mice NKG2D can also associate with DAP12. Activating NKG2C/CD94, Ly49 and KIR receptors associate to the signaling adaptor DAP12 whereas NKP46 uses CD3ζ.

Figure 2. Receptor regulation of NK cells during viral infections

The inhibitory receptors are mostly appreciated for their ability to recognize classical and non-classical major histocompatibility class 1 (MHC1) molecules in balancing towards dampened NK cell stimulation under normal conditions, but there are a few examples of viral products accessing these receptors to avoid NK cell-mediated defense. There is a growing list of examples of activating receptors, from both highly diverse and highly conserved gene famlies in human and mouse, recognizing ligands expressed on virus-infected cells. A few of these directly recognize viral products. Most indirectly recognize either MHC1 or non-classical MHC1 along with virus-induced changes on infected cells or stress molecules induced in virus-infected cells. (SeeTable 1 for detailed examples.)

Figure 3. Two potential mechanisms for losing inhibitory signals from KIR receptors

NK cells are regulated by a balance between activating and inhibitory receptors. In health this balance favors inhibition of NK cells, because the inhibitory signal dominates the activating signal. During viral infections the inhibitory signal derived from MHC class I may be lost by down-regulation of cell surface MHC class I, thus favoring NK cell activation: “Missing-self model”. However multiple mechanisms exist to change the peptide repertoire of cell surface MHC class I. If this change favors presentation of weak inhibitory “antagonist” peptides then these peptides can efficiently disrupt the inhibitory signal due to strong and intermediate inhibitory peptides, and thus also favor NK cell activation: “Altered-self model”.

Figure 4. Receptor-mediated regulation of NK cell subset proportions and numbers during viral infection

The importance of stimulating NK cells through activating receptors to modify the proportions of subsets expressing particular activating receptors and/or sustain their numbers under conditions of extended viral infection is becoming clear. It requires the induction of a ligand by the viral infection and the presence of an activating receptor for the ligand on NK cells. Because the absence of stimulation through activating receptors can result in profound decreases in total NK cell numbers or subsets (red triangles, pointing down) (A) whereas the stimulation through activating receptors can sustain the cells but change the proportions expression particular activating receptors (green triangles, pointing up) (B), the responses have consequences for availability of innate resources in fighting off infection. The example depicted is based on expression of a highly polymorphic and polygenic activating receptor in mouse and human, i.e. Ly49 or KIR. Any requirement for these receptors would allow opportunities for viruses to pass through “holes” in the genetic repertoire of subgroups in a diverse population while others mount effective defense. Use of conserved activating receptors, such as CD94/NKG2C, might act to prime for responses to several viruses and enhance defense against subsequent primary viral infections. Thus, the mechanism of changing the proportions or numbers of NK cells during conditions of extended viral infections is an example of how NK cells are conditioned by experience.

Figure 5. Cytokine regulation of NK cell responses to viral infection

Many studies of NK cells in isolation and under immunodeficient conditions have demonstrated the roles for different cytokines, including the type 1 IFNs (IFNαβ), IL-12, and IL-15, as well as activating receptors in stimulating NK cell IFNγ production, elevated cytotoxic function, and proliferation. (A) At times of peak innate cytokine responses to infections in immunocompetent mice, however, the factors have key non-overlapping effects. Under these conditions, type 1 IFN is required for induction of systemic elevated cytotoxicity, IL-12 for IFNγ, and IL-15 for proliferation. At the very earliest times after infection, local type 1 IFN induction of NK cell IFNγ can be observed, but it is tightly regulated. The observations suggest that the conditions of infections are limiting NK cell responses to individual cytokines. (B) Studies characterizing NK cell expression of the signal transducers of transcription, STAT1 and STAT4, show that the cytokine effects are regulated in part by high-level basal expression of STAT4 and dynamic regulation of STAT1 levels, with type 1 IFN induction of STAT1 acting to limit type 1 IFN access to STAT4. The balance is important in allowing early type 1 IFN, while protecting from the detrimental consequences of unregulated, induction of IFNγ. Availability of IL-12 provides an alternative pathway to IFNγ. The mechanism shaping the effects of type 1 IFNs is an example of how NK cells are rapidly conditioned by experience during viral infection.