Knockdowns of CD3zeta chain in Primary NK Cells Illustrate Modulation of Antibody-Dependent Cellular Cytotoxicity against Human Immunodeficiency Virus-1.

Multifaceted natural killer (NK) cell activities are indispensable for controlling human immunodeficiency virus (HIV)-1 transmission and pathogenesis. Among the diverse functions of NK cells, antibody-dependent cellular cytotoxicity (ADCC) has been shown to predict better HIV-1 protection (1-3). ADCC is initiated by the engagement of an Fcg receptor CD16 with an Fc portion of the antibody, leading to phosphorylation of the CD3z chain (CD3) and Fc receptor g chain (FcRg) as well as downstream signaling activation. Though CD3 and FcR were thought to have overlapping roles in NK cell ADCC, several groups have reported that CD3-mediated signals trigger a more robust ADCC (4-8). However, few studies have illustrated the direct contribution of CD3z in HIV-1-specific ADCC. To further understand the roles played by CD3 in HIV-1-specific ADCC, we developed a CD3z knockdown system in primary human NK cells. We observed that HIV-1-specific ADCC was inhibited by CD3z perturbation. In summary, we demonstrated that CD3 is important for eliciting HIV-1-specific ADCC, and this dynamic can be utilized for NK cell immunotherapeutics against HIV-1 infection and other diseases.

Environment rather than breed or body site shapes the skin bacterial community of healthy sheep as revealed by metabarcoding.

BACKGROUND: The skin is inhabited by a variety of micro-organisms, with bacteria representing the predominant taxon of the skin microbiome. In sheep, the skin bacterial community of healthy animals has been addressed in few studies, only with culture-based methods or sequencing of cloned amplicons. OBJECTIVES: The objectives of this study were to determine the sheep skin bacterial community composition by using metabarcoding for a detailed characterisation and to determine the effect of body part, breed and environment. MATERIALS AND METHODS: Overall, 267 samples were taken from 89 adult female sheep, belonging to three different breeds and kept on nine different farms in Switzerland. From every individual, one sample each was taken from belly, left ear and left leg and metabarcoding of the 16S rRNA V3-V4 hypervariable region was performed. RESULTS: The main phyla identified were Actinobacteriota, Firmicutes, Proteobacteria and Bacteriodota. The alpha diversity as determined by Shannon's diversity index was significantly different between sheep from different farms. Beta diversity analysis by principal coordinate analysis (PCoA) showed clustering of the samples by farm and body site, while breed had only a marginal influence. A sparse partial least squares discriminant analysis (sPLS-DA) revealed seven main groups of operational taxonomic units (OTUs) of which groups of OTUs were specific for some farms. CONCLUSIONS AND CLINICAL RELEVANCE: These findings indicate that environment has a larger influence on skin microbial variability than breed, although the sampled breeds, the most abundant ones in Switzerland, are phenotypically similar. Future studies on the sheep skin microbiome may lead to novel insights in skin diseases and prevention.

Antigen-specific memory NK cell responses against HIV and influenza use the NKG2/HLA-E axis.

Multiple studies have broadened the roles of natural killer (NK) cells functioning as purely innate lymphocytes by demonstrating that they are capable of putative antigen-specific immunological memory against multiple infectious agents including HIV-1 and influenza. However, the mechanisms underlying antigen specificity remain unknown. Here, we demonstrate that antigen-specific human NK cell memory develops upon exposure to both HIV and influenza, unified by a conserved and epitope-specific targetable mechanism largely dependent on the activating CD94/NKG2C receptor and its ligand HLA-E. We validated the permanent acquisition of antigen specificity by individual memory NK cells by single-cell cloning. We identified elevated expression of KLRG1, α4ß7, and NKG2C as biomarkers of antigen-specific NK cell memory through complex immunophenotyping. Last, we uncovered individual HLA-E-restricted peptides that may constitute the dominant NK cell response in HIV-1- and influenza-infected persons in vivo. Our findings clarify the mechanisms contributing to antigen-specific memory NK cell responses and suggest that they could be potentially targeted therapeutically for vaccines or other therapeutic interventions.

Multiplex interrogation of the NK cell signalome reveals global downregulation of CD16 signaling during lentivirus infection through an IL-18/ADAM17-dependent mechanism.

Despite their importance, natural killer (NK) cell responses are frequently dysfunctional during human immunodeficiency virus-1 (HIV-1) and simian immunodeficiency virus (SIV) infections, even irrespective of antiretroviral therapies, with poorly understood underlying mechanisms. NK cell surface receptor modulation in lentivirus infection has been extensively studied, but a deeper interrogation of complex cell signaling is mostly absent, largely due to the absence of any comprehensive NK cell signaling assay. To fill this knowledge gap, we developed a novel multiplex signaling analysis to broadly assess NK cell signaling. Using this assay, we elucidated that NK cells exhibit global signaling reduction from CD16 both in people living with HIV-1 (PLWH) and SIV-infected rhesus macaques. Intriguingly, antiretroviral treatment did not fully restore diminished CD16 signaling in NK cells from PLWH. As a putative mechanism, we demonstrated that NK cells increased surface ADAM17 expression via elevated plasma IL-18 levels during HIV-1 infection, which in turn reduced surface CD16 downregulation. We also illustrated that CD16 expression and signaling can be restored by ADAM17 perturbation. In summary, our multiplex NK cell signaling analysis delineated unique NK cell signaling perturbations specific to lentiviral infections, resulting in their dysfunction. Our analysis also provides mechanisms that will inform the restoration of dysregulated NK cell functions, offering potential insights for the development of new NK cell-based immunotherapeutics for HIV-1 disease.

Upregulation of the NKG2D ligand ULBP2 by JC polyomavirus infection promotes immune recognition by natural killer cells.

BACKGROUND: JC polyomavirus(JCPyV) causes progressive multifocal leukoencephalopathy(PML), a potentially fatal complication of severe immune suppression with no effective treatment. Natural killer (NK) cells play critical roles in defense against viral infections, yet NK cell response to JCPyV infection remains unexplored. METHODS: NK and T cell responses against the JCPyV VP1 were compared using intracellular cytokine staining (ICS) upon stimulation with peptide pools. A novel flow cytometry-based assay was developed to determine NK cell killing efficiency of JCPyV-infected astrocyte-derived SVG-A cells. Blocking antibodies were used to identify the specific NK cell receptors in immune recognition of JCPyV-infected cells. RESULTS: In about 40% of healthy donors, we detected robust CD107a upregulation and IFN-γ production by NK cells, extending beyond T cell responses. Next, using the NK cell-mediated killing assay, we showed that co-culture of NK cells and JCPyV-infected SVG-A cells leads to a 60% reduction in infection, on average. JCPyV-infected cells had enhanced expression of ULBP2 - a ligand for the activating NK cell receptor NKG2D and addition of NKG2D blocking antibodies decreased NK cell degranulation. CONCLUSION: NKG2D-mediated activation of NK cells plays a key role in controlling JCPyV replication and may be a promising immunotherapeutic target to boost NK cell anti-JCPyV activity.

NK cell education: Physiological and pathological influences.

Natural killer (NK) cells represent a critical defense against viral infections and cancers. NK cells require integration of activating and inhibitory NK cell receptors to detect target cells and the balance of these NK cell inputs defines the global NK cell response. The sensitivity of the response is largely defined by interactions between self-major histocompatibility complex class I (MHC-I) molecules and specific inhibitory NK cell receptors, so-called NK cell education. Thus, NK cell education is a crucial process to generate tuned effector NK cell responses in different diseases. In this review, we discuss the relationship between NK cell education and physiologic factors (type of self-MHC-I, self-MHC-I allelic variants, variant of the self-MHC-I-binding peptides, cytokine effects and inhibitory KIR expression) underlying NK cell education profiles (effector function or metabolism). Additionally, we describe the broad-spectrum of effector educated NK cell functions on different pathologies (such as HIV-1, CMV and tumors, among others).

Holding back: HLA-associated inhibition of NK cells gives HIV a leg up.

In this issue of Cell Host & Microbe, Li et al. unveil an association between HLA-B∗46:01 and HIV disease progression in Asian populations. The distinct natural killer (NK) cell signature identified in individuals carrying HLA-B∗46:01 strongly suggests that NK cell inhibition plays a role in loss of HIV control.

Learning to Be Elite: Lessons From HIV-1 Controllers and Animal Models on Trained Innate Immunity and Virus Suppression.

Although antiretroviral therapy (ART) has drastically changed the lives of people living with human immunodeficiency virus-1 (HIV-1), long-term treatment has been associated with a vast array of comorbidities. Therefore, a cure for HIV-1 remains the best option to globally eradicate HIV-1/acquired immunodeficiency syndrome (AIDS). However, development of strategies to achieve complete eradication of HIV-1 has been extremely challenging. Thus, the control of HIV-1 replication by the host immune system, namely functional cure, has long been studied as an alternative approach for HIV-1 cure. HIV-1 elite controllers (ECs) are rare individuals who naturally maintain undetectable HIV-1 replication levels in the absence of ART and whose immune repertoire might be a desirable blueprint for a functional cure. While the role(s) played by distinct human leukocyte antigen (HLA) expression and CD8+ T cell responses expressing cognate ligands in controlling HIV-1 has been widely characterized in ECs, the innate immune phenotype has been decidedly understudied. Comparably, in animal models such as HIV-1-infected humanized mice and simian Immunodeficiency Virus (SIV)-infected non-human primates (NHP), viremic control is known to be associated with specific major histocompatibility complex (MHC) alleles and CD8+ T cell activity, but the innate immune response remains incompletely characterized. Notably, recent work demonstrating the existence of trained innate immunity may provide new complementary approaches to achieve an HIV-1 cure. Herein, we review the known characteristics of innate immune responses in ECs and available animal models, identify gaps of knowledge regarding responses by adaptive or trained innate immune cells, and speculate on potential strategies to induce EC-like responses in HIV-1 non-controllers.

A Genome-Wide CRISPR/Cas9-Based Screen Identifies Heparan Sulfate Proteoglycans as Ligands of Killer-Cell Immunoglobulin-Like Receptors.

While human leukocyte antigen (HLA) and HLA-like proteins comprise an overwhelming majority of known ligands for NK-cell receptors, the interactions of NK-cell receptors with non-conventional ligands, particularly carbohydrate antigens, is less well described. We previously found through a bead-based HLA screen that KIR3DS1, a formerly orphan member of the killer-cell immunoglobulin-like receptor (KIR) family, binds to HLA-F. In this study, we assessed the ligand binding profile of KIR3DS1 to cell lines using Fc fusion constructs, and discovered that KIR3DS1-Fc exhibited binding to several human cell lines including ones devoid of HLA. To identify these non-HLA ligands, we developed a magnetic enrichment-based genome-wide CRISPR/Cas9 knock-out screen approach, and identified enzymes involved in the biosynthesis of heparan sulfate as crucial for the binding of KIR3DS1-Fc to K562 cells. This interaction between KIR3DS1 and heparan sulfate was confirmed via surface plasmon resonance, and removal of heparan sulfate proteoglycans from cell surfaces abolished KIR3DS1-Fc binding. Testing of additional KIR-Fc constructs demonstrated that KIR family members containing a D0 domain (KIR3DS1, KIR3DL1, KIR3DL2, KIR2DL4, and KIR2DL5) bound to heparan sulfate, while those without a D0 domain (KIR2DL1, KIR2DL2, KIR2DL3, and KIR2DS4) did not. Overall, this study demonstrates the use of a genome-wide CRISPR/Cas9 knock-out strategy to unbiasedly identify unconventional ligands of NK-cell receptors. Furthermore, we uncover a previously underrecognized binding of various activating and inhibitory KIRs to heparan sulfate proteoglycans that may play a role in NK-cell receptor signaling and target-cell recognition.

Prevalence of coat colour traits and congenital disorders of South American camelids in Austria, Germany and Switzerland.

BACKGROUND: The increasing popularity of alpacas and llamas outside of South America is undeniable. The associated limited genetic diversity raises questions about health and other genetically determined traits like coat colour. Therefore, a survey studying the prevalence of congenital disorders and coat colours and patterns in South American camelids was performed in Austria, Germany and Switzerland. Moreover, the motivation for keeping these animals, the herd size and breeds was assessed. RESULTS: A total of 146 questionnaires were returned corresponding to 16 farms from Austria, 69 farms from Germany, and 61 farms from Switzerland. In total, the returned surveys reported data on 2770 animals including ~ 85% alpacas and ~ 15% llamas. The most common alpaca breed was Huacaya (87.7%), the most common llama breed was Wooly (15.6%). Breeding (69.4%), wool production (63.3%) and keeping them as pets (53.7%) were the most common motivations to keep these animals, although this varied among countries. The three coat colour groups, solid white (24.8%), brown and black (64.8%) and grey (10.4%), occurred at different frequencies. About 7% of the South American camelids with solid white coat showed blue-pigmented eyes, corresponding to the known blue-eyed white phenotype, of which more than every second animal was apparently deaf. Uniform solid coloured animals occurred predominantly (81.4%), whereas pinto (8.8%), speckled (6.4%) and spotted (3.4%), also known as appaloosa, were comparably less prevalent. In total 161 observations of congenital disorders occurring during a 5-year-period were reported. The most prevalent disorders were in the group of musculoskeletal disorders such as spiral toe growth (16.4%), hyperextension of the fetlock joint (12.3%), angular limb deformities (11.0%) and axial rotation of the limbs (8.2%). CONCLUSIONS: This survey revealed first insights into the occurrence of different traits and disorders in the current South American camelid population of Austria, Germany, and Switzerland. The identification of the most common musculoskeletal disorders might encourage the breeders to eliminate affected animals from their breeding program to decrease the incidence although traits such as spiral toe growth might also represent phenocopies.

NK Cells Contribute to the Immune Risk Profile in Kidney Transplant Candidates.

Background: A previously proposed immune risk profile (IRP), based on T cell phenotype and CMV serotype, is associated with mortality in the elderly and increased infections post-kidney transplant. To evaluate if NK cells contribute to the IRP and if the IRP can be predicted by a clinical T cell functional assays, we conducted a cross sectional study in renal transplant candidates to determine the incidence of IRP and its association with specific NK cell characteristics and ImmuKnow® value. Material and Methods: Sixty five subjects were enrolled in 5 cohorts designated by age and dialysis status. We determined T and NK cell phenotypes by flow cytometry and analyzed multiple factors contributing to IRP. Results: We identified 14 IRP+ [CMV seropositivity and CD4/CD8 ratio < 1 or being in the highest quintile of CD8+ senescent (28CD-/CD57+) T cells] individuals equally divided amongst the cohorts. Multivariable linear regression revealed a distinct IRP+ group. Age and dialysis status did not predict immune senescence in kidney transplant candidates. NK cell features alone could discriminate IRP- and IRP+ patients, suggesting that NK cells significantly contribute to the overall immune status in kidney transplant candidates and that a combined T and NK cell phenotyping can provide a more detailed IRP definition. ImmuKnow® value was negatively correlated to age and significantly lower in IRP+ patients and predicts IRP when used alone or in combination with NK cell features. Conclusion: NK cells contribute to overall immune senescence in kidney transplant candidates.

Brief Report: Decreased JC Virus-Specific Antibody-Dependent Cellular Cytotoxicity in HIV-Seropositive PML Survivors.

BACKGROUND: Progressive multifocal leukoencephalopathy (PML) is an often fatal disease caused by JC virus (JCV) in severely immunocompromised patients, including HIV patients. Development of therapeutics to prevent or treat PML is an urgent medical need. While JCV-specific T cells are crucial to control JCV and recover from PML, the role played by antibodies remains unclear. Anti-JCV antibodies, including potent neutralizing antibodies, can be detected in most infected adults, yet in PML patients, JCV seems to escape from neutralization. Whether antibodies can contribute to JCV control by eliciting Fc-mediated effector functions activity has not been evaluated. METHODS: We measured the capacity of plasma anti-JCV VP1 antibodies to recruit Fc receptor (FcR)-bearing effector cell functions in 28 HIV patients, comparing subjects without PML with PML survivors (PML S) who were alive 1 year after disease onset or PML progressors (PML P) who succumbed within the first year. Antibody titers against JCV VP1 and HIV gp140 trimer were determined by end-point titer dilution ELISA. FcR-mediated natural killer cell degranulation and IFN-γ production were measured as surrogate for in vitro antibody-dependent cellular cytotoxicity (ADCC). RESULTS: PML S had higher JCV antibody titers than PML P and patients without PML. However, anti-JCV antibodies had a higher ability to functionally engage FcR in PML P than PML S. Antibody titers and ADCC activity did not vary over time in PML S. Anti-HIV antibody titers and ADCC activity were similar among groups. CONCLUSIONS: The ability of anti-JCV antibodies to stimulate FcR-bearing effector cell activity might contribute to the outcome of PML. Further studies are warranted to define Fc-mediated functions of anti-JCV antibodies and evaluate whether ADCC can contain JCV replication.

A Natural Impact: NK Cells at the Intersection of Cancer and HIV Disease.

Despite efficient suppression of plasma viremia in people living with HIV (PLWH) on cART, evidence of HIV-induced immunosuppression remains, and normally benign and opportunistic pathogens become major sources of co-morbidities, including virus-induced cancers. In fact, cancer remains a primary cause of death even in virally suppressed PLWH. Natural killer (NK) cells provide rapid early responses to HIV infection, contribute substantially to disease modulation and vaccine protection, and are also major therapeutic targets for cancer immunotherapy. However, much like other lymphocyte populations, recent burgeoning evidence suggests that in chronic conditions like HIV, NK cells can become functionally exhausted with impaired cytotoxic function, altered cytokine production and impaired antibody-dependent cell-mediated cytotoxicity. Recent work suggests functional anergy is likely due to low-level ongoing virus replication, increased inflammatory cytokines, or increased presence of MHClow target cells. Indeed, HIV-induced loss of NK cell-mediated control of lytic EBV infection has been specifically shown to cause lymphoma and also increases replication of CMV. In this review, we will discuss current understanding of NK cell modulation of HIV disease, reciprocal exhaustion of NK cells, and how this may impact increased cancer incidences and prospects for NK cell-targeted immunotherapies. Finally, we will review the most recent evidence supporting adaptive functions of NK cells and highlight the potential of adaptive NK cells for cancer immunotherapy.

Semaphorin 7A modulates cytokine-induced memory-like responses by human natural killer cells.

Cytokine-induced memory-like (CIML) NK cells are endowed with the capacity to mediate enhanced effector functions upon cytokine or activating receptor restimulation for several weeks following short-term preactivation with IL-12, IL-15, and IL-18. Promising results from a first-in-human clinical trial highlighted the clinical potential of CIML NK cells as adoptive immunotherapy for patients with hematologic malignancies. However, the mechanisms underlying CIML NK cell differentiation and increased functionality remain incompletely understood. Semaphorin 7A (SEMA7A) is a potent immunomodulator expressed in activated lymphocytes and myeloid cells. In this study, we show that SEMA7A is substantially upregulated on NK cells stimulated with cytokines, and specifically marks activated NK cells with a strong potential to release IFN-γ. In particular, preactivation of NK cells with IL-12+IL-15+IL-18 resulted in greater than tenfold upregulation of SEMA7A and enhanced expression of the ligand for SEMA7A, integrin-ß1, on CIML NK cells. Strikingly, preactivation in the presence of antibodies targeting SEMA7A lead to significantly decreased IFN-γ production following restimulation. These results imply a novel mechanism by which cytokine-enhanced SEMA7A/integrin-ß1 interaction promotes CIML NK cell differentiation and maintenance of increased functionality. Our data suggest that targeting SEMA7A/integrin-ß1 signaling might provide a novel immunotherapeutic approach to potentiate antitumor activity of CIML NK cells.

Progressive lentivirus infection induces natural killer cell receptor-expressing B cells in the gastrointestinal tract.

OBJECTIVE: Recently, a seemingly novel innate immune cell subset bearing features of natural killer and B cells was identified in mice. So-called NKB cells appear as first responders to infections, but whether this cell population is truly novel or is in fact a subpopulation of B cells and exists in higher primates remains unclear. The objective of this study was to identify NKB cells in primates and study the impact of HIV/SIV infections. DESIGN AND METHODS: NKB cells were quantified in both naive and lentivirus infected rhesus macaques and humans by excluding lineage markers (CD3, CD127) and positive Boolean gating for CD20, NKG2A/C and/or NKp46. Additional phenotypic measures were conducted by RNA-probe and traditional flow cytometry. RESULTS: Circulating cytotoxic NKB cells were found at similar frequencies in humans and rhesus macaques (range, 0.01-0.2% of total lymphocytes). NKB cells were notably enriched in spleen (median, 0.4% of lymphocytes), but were otherwise systemically distributed in tonsil, lymph nodes, colon, and jejunum. Expression of immunoglobulin was highly variable, but heavily favoured IgM and IgA rather than IgG. Interestingly, NKB cell frequencies expanded in PBMC and colon during SIV infection, as did IgG expression, but were generally unaltered in HIV-infected humans. CONCLUSION: These results suggest a cell type expressing both natural killer and B-cell features exists in rhesus macaques and humans and are perturbed by HIV/SIV infection. The full functional niche remains unknown, but the unique phenotype and systemic distribution could make NKB cells unique targets for immunotherapeutics or vaccine strategies.

Human Herpes Virus 8 in HIV-1 infected individuals receiving cancer chemotherapy and stem cell transplantation.

BACKGROUND: Human Herpes Virus 8 (HHV8) can cause Kaposi's Sarcoma (KS) in immunosuppressed individuals. However, little is known about the association between chemotherapy or hematopoietic stem cell transplantation (HSCT), circulating HHV8 DNA levels, and clinical KS in HIV-1-infected individuals with various malignancies. Therefore, we examined the associations between various malignancies, systemic cancer chemotherapy, T cell phenotypes, and circulating HHV8 DNA in 29 HIV-1-infected participants with concomitant KS or other cancer diagnoses. METHODS: We quantified HHV8 plasma viral loads and cell-associated HHV8 DNA and determined the relationship between circulating HHV8 DNA and lymphocyte counts, and markers of early and late lymphocyte activation, proliferation and exhaustion. RESULTS: There were no significant differences in plasma HHV8 DNA levels between baseline and post-chemotherapy time points or with the presence or absence of clinical KS. However, in two participants circulating HHV8 DNA increased following treatment for KS or HSCT for lymphoma,. We observed an approximately 2-log10 reduction in plasma HHV8 DNA in an individual with KS and multicentric Castleman disease following rituximab monotherapy. Although individuals with clinical KS had lower mean CD4+ T cell counts and percentages as expected, there were no significant associations with these factors and plasma HHV8 levels. We identified increased proportions of CD8+ and CD4+ T cells expressing CD69 (P = 0.01 & P = 0.04 respectively), and increased CD57 expression on CD4+ T cells (P = 0.003) in participants with detectable HHV8. CONCLUSION: These results suggest there is a complex relationship between circulating HHV8 DNA and tissue-based disease in HIV-1 and HHV8 co-infected individuals with various malignancies.

Human Immunodeficiency Virus Type 1 Persistence Following Systemic Chemotherapy for Malignancy.

Background: Systemic chemotherapies for various malignancies have been shown to significantly, yet transiently, decrease numbers of CD4+ T lymphocytes, a major reservoir for human immunodeficiency virus type 1 (HIV-1) infection. However, little is known about the impact of cytoreductive chemotherapy on HIV-1 reservoir dynamics, persistence, and immune responses. Methods: We investigated the changes in peripheral CD4+ T-cell-associated HIV-1 DNA and RNA levels, lymphocyte activation, viral population structure, and virus-specific immune responses in a longitudinal cohort of 15 HIV-1-infected individuals receiving systemic chemotherapy or subsequent autologous stem cell transplantation for treatment of hematological malignancies and solid tumors. Results: Despite a transient reduction in CD4+ T cells capable of harboring HIV-1, a 1.7- and 3.3-fold increase in mean CD4+ T-cell-associated HIV-1 RNA and DNA, respectively, were observed months following completion of chemotherapy in individuals on antiretroviral therapy. We also observed changes in CD4+ T-cell population diversity and clonal viral sequence expansion during CD4+ T-cell reconstitution following chemotherapy cessation. Finally, HIV-1 DNA was preferentially, and in some cases exclusively, detected in cytomegalovirus (CMV)- and Epstein-Barr virus (EBV)-responsive CD4+ T cells following chemotherapy. Conclusions: Expansion of HIV-infected CMV/EBV-specific CD4 + T cells may contribute to maintenance of the HIV DNA reservoir following chemotherapy.

HIV-1-Mediated Downmodulation of HLA-C Impacts Target Cell Recognition and Antiviral Activity of NK Cells.

It was widely accepted that HIV-1 downregulates HLA-A/B to avoid CTL recognition while leaving HLA-C unaltered in order to prevent NK cell activation by engaging inhibitory NK cell receptors, but it was recently observed that most primary isolates of HIV-1 can mediate HLA-C downmodulation. Now we report that HIV-1-mediated downmodulation of HLA-C was associated with reduced binding to its respective inhibitory receptors. Despite this, HLA-C-licensed NK cells displayed reduced antiviral activity compared to their unlicensed counterparts, potentially due to residual binding to the respective inhibitory receptors. Nevertheless, NK cells were able to sense alterations of HLA-C expression demonstrated by increased antiviral activity when exposed to viral strains with differential abilities to downmodulate HLA-C. These results suggest that the capability of HLA-C-licensed NK cells to control HIV-1 replication is determined by the strength of KIR/HLA-C interactions and is thus dependent on both host genetics and the extent of virus-mediated HLA-C downregulation.