Abundance Of Natural Killer Cells Research Articles

Identifying genetic variants that influence the abundance of cell states in single-cell data.

Disease risk alleles influence the composition of cells present in the body, but modeling genetic effects on the cell states revealed by single-cell profiling is difficult because variant-associated states may reflect diverse combinations of the profiled cell features that are challenging to predefine. We introduce Genotype-Neighborhood Associations (GeNA), a statistical tool to identify cell-state abundance quantitative trait loci (csaQTLs) in high-dimensional single-cell datasets. Instead of testing associations to predefined cell states, GeNA flexibly identifies the cell states whose abundance is most associated with genetic variants. In a genome-wide survey of single-cell RNA sequencing peripheral blood profiling from 969 individuals, GeNA identifies five independent loci associated with shifts in the relative abundance of immune cell states. For example, rs3003-T (P = 1.96 × 10-11) associates with increased abundance of natural killer cells expressing tumor necrosis factor response programs. This csaQTL colocalizes with increased risk for psoriasis, an autoimmune disease that responds to anti-tumor necrosis factor treatments. Flexibly characterizing csaQTLs for granular cell states may help illuminate how genetic background alters cellular composition to confer disease risk.

Natural Killer Cell Presence in Antibody-Mediated Rejection.

Transcript analyses highlight an important contribution of natural killer (NK) cells to microvascular inflammation (MVI) in antibody-mediated rejection (ABMR), but only few immunohistologic studies have quantified their spatial distribution within graft tissue. This study included 86 kidney transplant recipients who underwent allograft biopsies for a positive donor-specific antibody (DSA) result. NK cells were visualized and quantified within glomeruli and peritubular capillaries (PTC), using immunohistochemistry for CD34 alongside CD16/T-bet double-staining. Staining results were analyzed in relation to histomorphology, microarray analysis utilizing the Molecular Microscope Diagnostic System, functional NK cell genetics, and clinical outcomes. The number of NK cells in glomeruli per mm2 glomerular area (NKglom) and PTC per mm2 cortical area (NKPTC) was substantially higher in biopsies with ABMR compared to those without rejection, and correlated with MVI scores (NKglom Spearman's correlation coefficient [SCC] = 0.55, p < 0.001, NKPTC 0.69, p < 0.001). In parallel, NK cell counts correlated with molecular classifiers reflecting ABMR activity (ABMRprob: NKglom 0.59, NKPTC 0.75) and showed a trend towards higher levels in association with high functional FCGR3A and KLRC2 gene variants. Only NKPTC showed a marginally significant association with allograft function and survival. Our immunohistochemical results support the abundance of NK cells in DSA-positive ABMR.

Association Between Diverse Cell Death Patterns Related Gene Signature and Prognosis, Drug Sensitivity, and Immune Microenvironment in Glioblastoma

Glioblastoma (GBM) is the most invasive type of glioma and is difficult to treat. Diverse programmed cell death (PCD) patterns have a significant association with tumor initiation and progression. A novel prognostic model based on PCD genes may serve as an effective tool to predict the prognosis of GBM. The study incorporated 11 PCD patterns, namely apoptosis, necroptosis, pyroptosis, ferroptosis, cuproptosis, entotic cell death, netotic cell death, parthanatos, lysosome-dependent cell death, autophagy-dependent cell death, alkaliptosis, and oxeiptosis, to develop the model. To construct and validate the model, both bulk and single-cell transcriptome data, along with corresponding clinical data from GBM cases, were obtained from the TCGA-GBM, REMBRANDT, CGGA, and GSE162631 datasets. A cell death-related signature containing 14 genes was constructed with the TCGA-GBM cohort and validated in the REMBRANDT and CGGA datasets. GBM patients with a higher cell death index (CDI) were significantly associated with poorer survival outcomes. Two separate clusters associated with clinical outcomes emerged from unsupervised analysis. A multivariate Cox regression analysis was conducted to examine the association of CDI with clinical characteristics, and a prognostic nomogram was developed. Drug sensitivity analysis revealed high-CDI GBM patients might be resistant to carmustine while sensitive to 5-fluorouracil. Less abundance of natural killer cells was found in GBM cases with high CDI and bulk transcriptome data. A cell death-related prognostic model that could predict the prognosis of GBM patients with good performance was established, which could discriminate between the prognosis and drug sensitivity of GBM.

Single-cell RNA Sequencing Identifies Natural Kill Cell-Related Transcription Factors Associated With Age-Related Macular Degeneration.

Age-related Macular Degeneration (AMD) poses a growing global health concern as the leading cause of central vision loss in elderly people. This study focuses on unraveling the intricate involvement of Natural Killer (NK) cells in AMD, shedding light on their immune responses and cytokine regulatory roles. Transcriptomic data from the Gene Expression Omnibus database were utilized, employing single-cell RNA-seq analysis. High-dimensional weighted gene co-expression network analysis (hdWGCNA) and single-cell regulatory network inference and clustering (SCENIC) analysis were applied to reveal the regulatory mechanisms of NK cells in early-stage AMD patients. Machine learning models, such as random forests and decision trees, were employed to screen hub genes and key transcription factors (TFs) associated with AMD. Distinct cell clusters were identified in the present study, especially the T/NK cluster, with a notable increase in NK cell abundance observed in AMD. Cell-cell communication analyses revealed altered interactions, particularly in NK cells, indicating their potential role in AMD pathogenesis. HdWGCNA highlighted the turquoise module, enriched in inflammation-related pathways, as significantly associated with AMD in NK cells. The SCENIC analysis identified key TFs in NK cell regulatory networks. The integration of hub genes and TFs identified CREM, FOXP1, IRF1, NFKB2, and USF2 as potential predictors for AMD through machine learning. This comprehensive approach enhances our understanding of NK cell dynamics, signaling alterations, and potential predictive models for AMD. The identified TFs provide new avenues for molecular interventions and highlight the intricate relationship between NK cells and AMD pathogenesis. Overall, this study contributes valuable insights for advancing our understanding and management of AMD.

Multiomic profiling of transplant glomerulopathy reveals a novel T-cell dominant subclass

Kidney transplant (KTx) biopsies showing transplant glomerulopathy (TG) (glomerular basement membrane double contours (cg) > 0)) and microvascular inflammation (MVI) in the absence of C4d staining and donor-specific antibodies (DSA)s do not fulfill the criteria for chronic active antibody-mediated rejection (CA-AMR) diagnosis and do not fit into any other Banff category. To investigate this, we initiated a multicenter intercontinental study encompassing 36 cases, comparing the immunomic and transcriptomic profiles of 14 KTx biopsies classified as cg+MVI DSA-/C4d- with 22 classified as CA-AMR DSA+/C4d+ through novel transcriptomic analysis using the NanoString Banff-Human Organ Transplant (B-HOT) panel and subsequent orthogonal subset analysis using two innovative 5-marker multiplex immunofluorescent panels. Nineteen genes were differentially expressed between the two study groups. Samples diagnosed with CA-AMR DSA+/C4d+ showed a higher glomerular abundance of natural killer cells and higher transcriptomic cell type scores for macrophages in an environment characterized by increased expression of complement-related genes (i.e., C5AR1) and higher activity of angiogenesis, interstitial fibrosis tubular atrophy, CA-AMR, and DSA-related pathways when compared to samples diagnosed with cg+MVI DSA-/C4d-. Samples diagnosed with cg+MVI DSA-/C4d- displayed a higher glomerular abundance and activity of T-cells (CD3+, CD3+CD8+, and CD3+CD8-). Thus, we show that using novel multiomic techniques, KTx biopsies with cg+MVI DSA-/C4d- have a prominent T-cell presence and activity, putting forward the possibility that these represent a more T-cell-dominant phenotype.

Time-course transcriptome analysis of lungs from mice infected with inhaled aerosolized Stenotrophomonas maltophilia.

Stenotrophomonas maltophilia (SMA) has emerged as an important pathogen capable of causing an opportunistic and nosocomial infection. We performed RNA sequencing (RNA-seq) of lung tissues from mice with pulmonary SMA infection over time via aerosolized intratracheal inhalation to investigate transcription profile changes in SMA-infected lungs. A mouse model of acute lethal SMA pneumonia was established in this study using aerosolized intratracheal inhalation, laying the groundwork for future SMA research. RNA-seq was then used to create a transcriptional profile of the lungs of the model mice at 0, 4, 12, 24, 48, and 72 hours post-infection (hpi). Mfuzz time clustering, weighted gene coexpression network analysis (WGCNA), and Immune Cell Abundance Identifier for mouse (ImmuCellAI-mouse) were used to analyze RNA-seq data. A gradual change in the lung transcriptional profile was observed, which was consistent with the expected disease progression. At 4 hpi, the expression of genes related to the acute phase inflammatory response increased, as predicted abundance of innate immune cells. At this stage, an increased demand for energy was also observed, including an increase in the expression of genes involved in circulation, muscle function and mitochondrial respiratory chain function. The expression of genes associated with endoplasmic reticulum stress (ERS) and autophagy increased at 24 hpi. Unlike the number of natural killer (NK) cells following most bacterial lung infections, the abundance of NK cells decreased following infection with SMA. The expression levels of Cxcl10, Cd14, Gbp5, Cxcr2, Tnip1, Zc3h12a, Egr1, Sell and Gbp2 were high and previously unreported in SMA pneumonia, and they may be important targets for future studies. To our knowledge, this is the first study to investigate the pulmonary transcriptional response to SMA infection. The findings shed light on the molecular mechanisms underlying the pathogenesis of SMA pneumonia, which may aid in the development of therapies to reduce the occurrence of SMA pulmonary infection.

Pan-cancer analysis of natural killer (NK) cell infiltration in human malignancies: Molecular features and clinical implications.

2563 Background: NK cells are part of the innate immune system that are not antigen-specific, but can be redirected to targets of interest using multiple strategies. Advantages of NK cells over T cells include the use of allogeneic off-the-shelf products and lower risk of cytokine release syndrome. Numerous NK-specific immunotherapies are under development for the treatment of cancer, although none are yet FDA-approved. Here, we conducted a pan-cancer analysis of NK cell abundance in &gt;90,000 tumor samples across 45 cancer types using the Caris Precision Oncology Alliance (POA) database. Features of prostate cancer (PCa, n=3365) and renal cell carcinoma (RCC, n=1106) were explored in depth. Methods: DNA (targeted/whole-exome) and RNA (whole-transcriptome) sequencing was performed for patient tumors (N=90,916) submitted to Caris Life Sciences (Phoenix, AZ). NK cell fractions were inferred from RNA-seq data using quanTIseq (Finotello, 2019). Real-world overall survival (OS) was determined from insurance claims, and Kaplan-Meier estimates were calculated. Statistical significance was determined using X2 and Mann-Whitney U tests with corrections for multiple hypothesis testing where appropriate. Results: Median NK cell fractions ranged from 7-9% (medulloblastoma and gliomas) to 2% (thyroid and thymic cancers), with intermediate levels observed for PCa (4.6%) and RCC (3.1%). High (&gt; median) NK cell fractions were associated with improved OS (hazard ratios, 0.28–0.84, p&lt;0.05) for 28 of 45 cancer types, while 16 of 45 including low-grade glioma, GIST, and thyroid had HR &lt;1.0 (0.264-0.997) with 95% CI crossing 1.0. Improved OS was notable in PCa (HR 0.46, 95% CI 0.38–0.56, p&lt;0.0001) and RCC (HR 0.52; 95% CI 0.39–0.70, p&lt;0.0001). Unexpectedly, tumors with higher NK infiltrates were less frequently PD-L1+ (SP142) by IHC in PCa (2.8% vs 5.4%, p=0.08) and RCC (12.4% vs 30.8%, p&lt;0.0001) ( i.e. an inverse relationship), despite a lack of correlation of NK infiltration with dMMR/MSI status or tumor mutation burden (TMB) in both cancers. Conversely, increased NK infiltration was associated with a 1.4- to 2.1–fold increased mRNA expression of immunomodulatory receptors LAG3 and TIGIT in both PCa (p&lt;0.0001) and RCC (p&lt;0.0001), but a 1.4–fold decrease in B7-H3 in RCC (p&lt;0.0001). Conclusions: High NK cell infiltration is associated with improved OS in numerous cancer types, including many with inadequate therapeutic options. Our findings suggest broad deployment of NK engagers and CAR-NK products in a range of malignancies. The positive correlation between NK cells and LAG3/TIGIT suggest that combination approaches may be warranted. The inverse relationship between NK infiltrates and PD-L1, and lack of association with standard immune therapy markers (MSI status, TMB), indicates the potential use of NK cell approaches in situations where other immunotherapies are ineffective.

NK cells with tissue-resident traits shape response to immunotherapy by inducing adaptive antitumor immunity.

T cell-directed cancer immunotherapy often fails to generate lasting tumor control. Harnessing additional effectors of the immune response against tumors may strengthen the clinical benefit of immunotherapies. Here, we demonstrate that therapeutic targeting of the interferon-γ (IFN-γ)-interleukin-12 (IL-12) pathway relies on the ability of a population of natural killer (NK) cells with tissue-resident traits to orchestrate an antitumor microenvironment. In particular, we used an engineered adenoviral platform as a tool for intratumoral IL-12 immunotherapy (AdV5-IL-12) to generate adaptive antitumor immunity. Mechanistically, we demonstrate that AdV5-IL-12 is capable of inducing the expression of CC-chemokine ligand 5 (CCL5) in CD49a+ NK cells both in tumor mouse models and tumor specimens from patients with cancer. AdV5-IL-12 imposed CCL5-induced type I conventional dendritic cell (cDC1) infiltration and thus increased DC-CD8 T cell interactions. A similar observation was made for other IFN-γ-inducing therapies such as Programmed cell death 1 (PD-1) blockade. Conversely, failure to respond to IL-12 and PD-1 blockade in tumor models with low CD49a+ CXCR6+ NK cell infiltration could be overcome by intratumoral delivery of CCL5. Thus, therapeutic efficacy depends on the abundance of NK cells with tissue-resident traits and, specifically, their capacity to produce the DC chemoattractant CCL5. Our findings reveal a barrier for T cell-focused therapies and offer mechanistic insights into how T cell-NK cell-DC cross-talk can be enhanced to promote antitumor immunity and overcome resistance.

Identification of Genes Linking Natural Killer Cells to Apoptosis in Acute Myocardial Infarction and Ischemic Stroke

Natural killer (NK) cells are a type of innate lymphoid cell that are involved in the progression of acute myocardial infarction and ischemic stroke. Although multiple forms of programmed cell death are known to play important roles in these diseases, the correlation between NK cells and apoptosis-related genes during acute myocardial infarction and ischemic stroke remains unclear. In this study, we explored the distinct patterns of NK cell infiltration and apoptosis during the pathological progression of acute myocardial infarction and ischemic stroke using mRNA expression microarrays from the Gene Expression Omnibus database. Since the abundance of NK cells correlated positively with apoptosis in both diseases, we further examined the correlation between NK cell abundance and the expression of apoptosis-related genes. Interestingly, APAF1 and IRAK3 expression correlated negatively with NK cell abundance in both acute myocardial infarction and ischemic stroke, whereas ATM, CAPN1, IL1B, IL1R1, PRKACA, PRKACB, and TNFRSF1A correlated negatively with NK cell abundance in acute myocardial infarction. Together, these findings suggest that these apoptosis-related genes may play important roles in the mechanisms underlying the patterns of NK cell abundance and apoptosis in acute myocardial infarction and ischemic stroke. Our study, therefore, provides novel insights for the further elucidation of the pathogenic mechanism of ischemic injury in both the heart and the brain, as well as potential useful therapeutic targets.

Comprehensive characterization of tumor infiltrating natural killer cells and clinical significance in hepatocellular carcinoma based on gene expression profiles

Natural Killer (NK) cells are effector lymphocytes involved in tumor immunosurveillance, however, the specific mechanism in hepatocellular carcinoma (HCC) has not been well understood. In the present study, we estimated the relative abundances of NK cells in HCC using gene expression data, and found that NK cell abundance was lower in HCC tissues than in the adjacent normal tissues. With the common HCC subclasses, we also found that three HCC subclasses had distinct abundances of NK cells. Moreover, we also found strong association between NK cell abundances and genes encoding immune checkpoint proteins, such as KLRD1, CD96, TIGIT, CD86, HAVCR2, PDCD1 (PD-1), HLA-E, CD274 (PD-L1), and CTLA4, among which, KLRD1 vs. HLA-E, CD274 vs. PDCD1, and CTLA4 vs. CD86 were three pairs of receptors and ligands. Furthermore, we investigated the clinical significance of NK cell activities in HCC, and found that the NK cell abundances were highly associated with the response to sorafinib, and higher NK cell abundances may prolong both the recurrence-free and overall survival of HCC patients. In summary, the present study not only improved our understanding of the potential tumor immune evasion mechanism of NK cells in HCC, but also proposed the potential clinical application of NK activities in HCC treatment and risk assessment.

1907P - Low NK cell abundance correlates with high expression of PD-1 in CD8+ T cells

BackgroundRenal cell carcinoma (RCC) is considered an immunogenic cancer with a high frequency of indel mutations, immuno-oncological (IO) sensitivity, and infiltration of T cells. Because natural killer (NK) cells are a lesser known population compared to their T cell counterparts, we aimed to investigate the intratumoral phenotype of the cells to assess the overall immune landscape of RCC. MethodsThe tumor, matching peripheral blood (PB) and healthy kidney tissue samples were obtained from 40 RCC patients that underwent radical nephrectomy and were phenotyped using multi-parameter flow cytometry and a comprehensive immunostaining panel containing 56 fundamental markers to cancer immunology. ResultsUsing hierarchical clustering and correlation analysis, we discovered that our cohort clustered into two distinct subgroups defined by the abundance of NK cells from the intratumoral lymphocyte population, defined as NKhigh (n=17; mean 24.3%) and NKlow (n=18; mean 7.5%). Consequently, an increased abundance of NK cells correlated with a lower expression of PD-1, particularly in the CD8+ T cells (r=-0.5722, p=0.0003), but not with LAG-3. Contrastingly, clinical parameters such as tumor grade, gender and age did not differ between the two NK subgroups. However, patients with high NK cell abundance significantly had more often necrosis present in the tumors (p=0.04). We also independently phenotyped two regions of the same tumor sample (n=10) to compare the periphery and core lymphocytes. Surprisingly, both spatially different regions displayed similar quantities of immune cell populations and immunophenotypic markers. Overall, the tumor immune landscape in RCC resulted in a proportion of tumors carrying a higher abundance of NK cells than their T cell-rich corresponding PB and healthy tissue samples, which supports our findings that a proportion of tumors accumulate NK cells. ConclusionsOur study has led to the discovery of two distinct RCC tumoral NK cell subgroups, NKhigh and NKlow. Next, we aim to further explore the transcriptional differences of the two subgroups with ongoing bulk RNA sequencing (RNAseq). Furthermore, prospective single-cell RNA sequencing (sc-RNAseq) will be carried out to investigate the immunological heterogeneities in higher resolution. Legal entity responsible for the studyThe authors. FundingUniversity of Helsinki, Hematology Research Unit Helsinki. DisclosureAll authors have declared no conflicts of interest.

Interleukin (IL)-18 Binding Protein Deficiency Disrupts Natural Killer Cell Maturation and Diminishes Circulating IL-18.

The cytokine interleukin (IL)-18 is a crucial amplifier of natural killer (NK) cell function. IL-18 signaling is regulated by the inhibitory effects of IL-18 binding protein (IL-18BP). Using mice deficient in IL-18BP (IL-18BPKO), we investigated the impact of mismanaged IL-18 signaling on NK cells. We found an overall reduced abundance of splenic NK cells in the absence of IL-18BP. Closer examination of NK cell subsets in spleen and bone marrow using CD27 and CD11b expression revealed that immature NK cells were increased in abundance, while the mature population of NK cells was reduced. Also, NK cells were polarized to greater production of TNF-α, while dedicated IFN-γ producers were reduced. A novel subset of IL-18 receptor α− NK cells contributed to the expansion of immature NK cells in IL-18BPKO mice. Splenocytes cultured with IL-18 resulted in alterations similar to those observed in IL-18BP deficiency. NK cell changes were associated with significantly reduced levels of circulating plasma IL-18. However, IL-18BPKO mice exhibited normal weight gain and responded to LPS challenge with a >10-fold increase in IFN-γ compared to wild type. Finally, we identified that the source of splenic IL-18BP was among dendritic cells/macrophage localized to the T cell-rich regions of the spleen. Our results demonstrate that IL-18BP is required for normal NK cell abundance and function and also contributes to maintaining steady-state levels of circulating IL-18. Thus, IL-18BP appears to have functions suggestive of a carrier protein, not just an inhibitor.

Natural killer-like signature observed post therapy in locally advanced rectal cancer is a determinant of pathological response and improved survival.

Around 12-15% of patients with locally advanced rectal cancer undergo a pathologically complete response (tumor regression grade 4) to long-course preoperative chemoradiotherapy; the remainder exhibit a spectrum of tumor regression (tumor regression grade 1-3). Understanding therapy-related transcriptional alterations may enable better prediction of response as measured by progression-free and overall survival, in addition to aiding the development of improved strategies based on the underlying biology of the disease. To this end, we performed high-throughput gene expression profiling in 40 pairs of formalin-fixed paraffin-embedded rectal cancer biopsies and matched resections following long-course preoperative chemoradiotherapy (discovery cohort). Differential gene expression analysis was performed contrasting tumor regression grades in resections. Enumeration of the tumor microenvironment cell population was undertaken using in silico analysis of the transcriptional data, and real-time PCR validation of NCR1 undertaken. Immunohistochemistry and survival analysis was used to measure CD56+ cell populations in an independent cohort (n=150). Gene expression traits observed following long-course preoperative chemoradiotherapy in the discovery cohort suggested an increased abundance of natural killer cells in tumors that displayed a clinical response to CRT in a tumor regression grade-dependent manner. CD56+ natural killer-cell populations were measured by immunohistochemistry and found to be significantly higher in tumor regression grade 3 patients compared with tumor regression grade 1-2 in the validation cohort. Furthermore, it was observed that patients positive for CD56 cells after therapy had a better overall survival (HR=0.282, 95% CI=0.109-0.729, χ2=7.854, P=0.005). In conclusion, we have identified a novel post-therapeutic natural killer-like transcription signature in patients responding to long-course preoperative chemoradiotherapy. Furthermore, patients with a higher abundance of CD56-positive natural killer cells post long-course preoperative chemoradiotherapy had better overall survival. Therefore, harnessing a natural killer-like response after therapy may improve outcomes for locally advanced rectal cancer patients. Finally, we hypothesize that future assessment of this natural killer-like response in on-treatment biopsy material may inform clinical decision-making for treatment duration.

Genetic modification of primary natural killer cells overcomes inhibitory signals and induces specific killing of leukemic cells

Natural killer (NK) cells hold promise for improving the therapeutic potential of allogeneic hematopoietic transplantation, but their effectiveness is limited by inhibitory HLA types. We sought to overcome this intrinsic resistance by transducing CD56+CD3- NK cells with chimeric receptors directed against CD19, a molecule widely expressed by malignant B cells. An abundance of NK cells for transduction was secured by culturing peripheral blood mononuclear cells with K562 cells expressing the NK-stimulatory molecules 4-1BB ligand and interleukin 15, which yielded a median greater than 1000-fold expansion of CD56+CD3- cells at 3 weeks of culture, without T-lymphocyte expansion. Expression of anti-CD19 receptors linked to CD3zeta overcame NK resistance and markedly enhanced NK-cell-mediated killing of leukemic cells. This result was significantly improved by adding the 4-1BB costimulatory molecule to the chimeric anti-CD19-CD3zeta receptor; the cytotoxicity produced by NK cells expressing this construct uniformly exceeded that of NK cells whose signaling receptors lacked 4-1BB, even when natural cytotoxicity was apparent. Addition of 4-1BB was also associated with increased cell activation and production of interferon gamma and granulocyte-macrophage colony-stimulating factor. Our findings indicate that enforced expression of signaling receptors by NK cells might circumvent inhibitory signals, providing a novel means to enhance the effectiveness of allogeneic stem cell transplantation.

The effect of nickel compounds on immunophenotype and natural killer cell function of normal human lymphocytes

In order to elucidate effects of nickel on human lymphocytes in vitro, peripheral blood mononuclear cells from normal donors were initially tested for viability in the presence of increasing concentrations of two selected nickel salts, sparingly-soluble nickel subsulfide (Ni 3S 2), and promptly-soluble nickel sulfate (NiSO 4). After establishing the toxicity profile, the cells were cultured for 24 h with each compound at three nontoxic concentrations, 0.01 mM, 0.02 mM, and 0.04 mM, to determine its effect on lymphocyte immunophenotype and function. Cells were also cultured in the presence of 0.01–0.04 mM magnesium acetate, Mg(CH 3COO) 3, while still other cell samples were subjected to a mixture of Mg(CH 3COO) 2 plus either Ni 3S 2 or NiSO 4 at equimolar concentration. Following the culture, the immunophenotype of the cells was determined by indirect immunofluorescence, using monoclonal antibodies to major differentiation antigens of peripheral blood mononuclear cells, and their natural killer activity toward K562 target cells was measured. Both nickel salts were found to exert distinct effects on lymphocyte phenotype. Exposure of cells to Ni 3S 2 resulted in the decline of CD4 and natural killer cell populations. NiSO 4 diminished the abundance of natural killer cells and, to a limited extent, also of CD4 cells. The nickel salts tested suppressed natural cytotoxicity of peripheral blood mononuclear cells, with Ni 3S 2 acting more strongly than NiSO 4. The addition of Mg(CH 3COO) 2 to a nickel salt during in vitro culture abolished the above inhibitory effects. Nickel and magnesium salts did not affect CD3, CD8, CD20, and CD11a cell populations. The results indicate that nickel salts have deleterious effects on human peripheral blood mononuclear cells in short-term in vitro culture, but the magnitude of these effects varies, depending on the cell subsets.