Quantification by flow cytofluorimetry of HLA class I molecules at the surface of murine cells transformed by cloned HLA genes

A method allowing quantitative analysis of the expression of foreign antigens at the surface of transformed cells is described. Aminoethyl-Sephadex G-25 beads labelled with different amounts of fluorescein isothiocyanate (FITC) were used to calibrate an Epics V cell sorter. The quantity of FITC molecules bound per bead was found to be a linear function of relative fluorescence intensity expressed by channel number for intermediate and high levels of fluorescence and a non-linear function for low levels. The lowest limit of detectable fluorescence was 3400 FITC molecules bound per bead. Using FITC-conjugated monoclonal antibodies (m.Ab.) the number of HLA class I molecules expressed at the surface of murine LMTK- (H-2Kk) cells transfected by cloned HLA class I genes was estimated and compared with the amount expressed on human B (Raji) and T (MOLT 4) lymphoblastoid reference cells. Murine transformed cells expressed approximately 3 times less HLA class I determinants per surface unit (micrometer 2) than Raji cells and 1.4 times less than MOLT 4 cells. Similar results were obtained by Scatchard analysis of the same populations using radiolabelled m.Ab. A detailed analysis of fluorescent cells showed that 5-20% of transformed cells expressed less than 33,000 HLA class I molecules/cell whereas most MOLT 4 cells exhibited at least 280,000 molecules/cell and the majority of Raji cells more than 700,000 molecules/cell. The expression of foreign antigen did not reduce the amount of H-2Kk molecules at the surface of transformed cells (mean of 350,000 molecules/cell).     

Differences in the interleukin-2 (IL-2) receptor system in human and mouse: α chain is required for formation of the functional mouse IL-2 receptor

Reconstitution with mouse interleukin-2 (IL-2) receptor subunits demonstrated that the mouse IL-2 receptor complex was different from the human complex in the α chain requirement for the functional mouse receptor complex. The heterotrimeric complex of the mouse exogenous α and β chains and the endogenous γ chain on mouse lymphoid BW5147 cells showed the ability to bind IL-2 with high affinity, resulting in IL-2-induced tyrosine phosphorylation of a cytosolic tyrosine kinase, JAK3, which is involved in IL-2-dependent signals. Exogenous introduction of the β chain with the endogenous γ chain, however, could neither confer appreciable IL-2 binding nor IL-2-induced signal transduction on BW5147 cells, unlike the human βγ heterodimer. Mouse spleen CD8⁺ cells, not having the α chain initially, showed IL-2-dependent cell proliferation only when expression of the α chain was induced. Collectively, these results illustrate that the functional mouse IL-2 receptor complex necessarily includes the α chain, and that the regulation of CD8⁺ T cell growth during immune reaction depends upon α chain expression.     

Role of IL-18 in the secretion of Il-1beta, sIL-1RII, and IL-1Ra by human neutrophils

In the present study we investigated the effect of IL-18 on the production of IL-1β, IL-1Ra and sIL-1RII by human neutrophils. Our observations indicate that rhIL-18 induces IL-1β and, to a lesser extend, IL-1Ra and sIL-1RII production by human neutrophils isolated form peripheral blood. However, this effect was less important in comparison with LPS-stimulation. Moreover, the results obtained suggest that IL-18 can induce priming of neutrophils for IL-1β and, to a lesser extend, IL-1Ra and sIL-1RII production by LPS-stimulated cells. The capacity of IL-18 to serve as an effective modulator for IL-1β and its regulatory proteins may have significance in the inflammatory and immune reactions mediated by IL-1β.     

Differential effects of interleukin-10 on the expression of HLA class II and CD1 molecules induced by granulocyte/macrophage colony-stimulating factor/interleukin-4

Interleukin (IL)-10 down-regulates HLA class II molecules, whether constitutively expressed or up-regulated by interferon-γ or IL-4 on monocytes but not on B lymphocytes. In this study we show that IL-10 does not inhibit HLA class II expression induced by the combination granulocyte/macrophage colony-stimulating factor and IL-4 on monocytes, although it simultaneously abrogates the expression of CD1 molecules induced by the same combination of cytokines. CD1 molecules can act as element of genetic restriction for CD4^? CD8^? T lymphocytes, and the suppression of CD1 expression by IL-10 abolished antigen presentation to CD1-restricted CD4^? CD8^? T cell receptor-positive T cells. Although HLA class II expression was not down-regulated by IL-10, the antigen specific proliferative response of CD4⁺ T cells was nevertheless decreased. This was not caused by down-regulation of known co-stimulatory molecules such as B7.1, B7.2 and ICAM-1. IL-10 decreased the antigen specific proliferative response further by directly influencing the T lymphocytes. Our results indicate that IL-10 exerts some of its immunoregulatory functions by differential modulation of antigen presenting molecules, induced by the same combination of cytokines.     

Selective regulation of ICAM-1 and major histocompatibility complex class I and II molecule expression on epidermal Langerhans cells by some of the cytokines released by keratinocytes and T cells

Epidermal Langerhans cells (LC) are major histocompatibility complex (MHC) class II (Ia)-positive dendritic cells that act as potent antigen-presenting or accessory cells for primary and secondary T cell-dependent immune responses. Recent studies have disclosed that the morphological, functional, and phenotypic characteristics of LC are variably and drastically modulated by external stimuli both in vivo and in vitro. However, little is known of the biological significance of diverse cytokines in regulating the surface molecules of LC. To determine the regulatory properties of ICAM-1, Ia, and MHC class I (H-2K) molecules in LC, we have examined the effects of interleukin (IL)-1α, IL-1β, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-10, interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), and granulocyte-macrophage colony-stimulating factor (GM-CSF) on the expression of these molecules. Among the cytokines examined, IFN-γ markedly and reproducibly up-regulates the expression of H-2K, but not ICAM-1, in Ia⁺ LC in a time- and dose-dependent manner. TNF-α consistently up-regulates the expression of ICAM-1, but not H-2K, in a time- and dose-dependent manner. IL-10 slightly but reproducibly inhibits the expression of ICAM-1, but not H-2K, in a time- and dose-dependent manner. IL-10 potently inhibits the TNF-α-induced ICAM-1 up-regulation, but not the IFN-γ-induced H-2K up-regulation. Moreover, no cytokine consistently affects the Ia expression of LC. In addition, slight enhancing effects have been observed on H-2K expression by IL-4, and on ICAM-1 expression by IL-1α, IL-1β, or GM-CSF. The present data suggest that the selective regulation is operative in a certain cell surface moiety of LC by various cytokines. These results further facilitate our understanding of immunobiology of LC.     

血清ICAM-1、SIL-2R、IL-2水平对重症急性胰腺炎患者病情及预后评估的临床价值

目的探讨血清细胞间黏附分子-1(ICAM-1)、可溶性白介素2受体(SIL-2R)、白细胞介素-2(IL-2)水平对重症急性胰腺炎(AP)患者病情及预后评估的价值。方法回顾性收集2016年2月至2019年2月收治的重症AP 51例作为重症组,收集同期收治的轻症AP 40例作为轻症组,选取同期来医院体检且留存完整血样标本的30例作为对照组,酶联免疫吸附法(ELISA)测定血ICAM-1、SIL-2R、IL-2水平,并进行病情Ranson评分,分析三者与AP病情进展及重症AP预后的关系。结果轻症组、重症组入院时血清ICAM-1、SIL-2R水平高于对照组,IL-2水平低于对照组(P<0.05),重症组血清ICAM-1、SIL-2R水平高于轻症组,IL-2水平低于轻症组P<0.05);重症组入院时Ranson评分高于轻症组(P<0.05);死亡组血清ICAM-1、SIL-2R及Ranson评分均高于生存组,IL-2水平低于生存组(P<0.05);ICAM-1与SIL-2R、Ranson评分均呈正相关(r=0.784、0.560,P均<0.05),与IL-2呈负相关(r=-0.486,P<0.05),SIL-2R与Ranson评分呈正相关(r=0.688,P<0.05),与IL-2呈负相关(r=-0.567,P<0.05),IL-2与Ranson评分呈负相关(r=-0.523,P<0.05);ICAM-1>183.83ng/mL时,预测重症AP灵敏度、特异性分别为86.27%、92.50%;SIL-2R>45.75 pg/mL时预测重症AP灵敏度、特异性分别为88.24%、90.00%;IL-2<3.41 pg/mL时预测重症灵敏度、特异性分别为84.31%、90.00%。结论AP伴明显ICAM-1、SIL-2R表达上调,IL-2表达降低,三者均与AP病情进展有关,可作为预测重症AP发病及评估AP预后的依据。     

HIV-1 proteins in infected cells determine the presentation of viral peptides by HLA class I and class II molecules and the nature of the cellular and humoral antiviral immune responses—A review

The goals of molecular virology and immunology during the second half of the 20th century have been to provide the conceptual approaches and the tools for the development of safe and efficient virus vaccines for the human population. The success of the vaccination approach to prevent virus epidemics was attributed to the ability of inactivated and live virus vaccines to induce a humoral immune response and to produce antiviral neutralizing antibodies in the vaccinees. The successful development of antiviral vaccines and their application to most of the human population led to a marked decrease in virus epidemics around the globe. Despite this remarkable achievement, the developing epidemics of HIV-caused AIDS (accompanied by activation of latent herpesviruses in AIDS patients), epidemics of Dengue fever, and infections with respiratory syncytial virus may indicate that conventional approaches to the development of virus vaccines that induce antiviral humoral responses may not suffice. This may indicate that virus vaccines that induce a cellular immune response, leading to the destruction of virus-infected cells by CD8⁺ cytotoxic T cells (CTLs), may be needed. Antiviral CD8⁺ CTLs are induced by viral peptides presented within the peptide binding grooves of HLA class I molecules present on the surface of infected cells. Studies in the last decade provided an insight into the presentation of viral peptides by HLA class I molecules to CD8⁺ T cells. These studies are here reviewed, together with a review of the molecular events of virus replication, to obtain an overview of how viral peptides associate with the HLA class I molecules. A similar review is provided on the molecular pathway by which viral proteins, used as subunit vaccines or inactivated virus particles, are taken up by endosomes in the endosome pathway and are processed by proteolytic enzymes into peptides that interact with HLA class II molecules during their transport to the plasma membrane of antigen-presenting cells. Such peptides are identified by T-cell receptors present on the plasma membrane of CD4⁺ T helper cells. The need to develop viral synthetic peptides that will have the correct amino acid motifs for binding to HLA class I A, B, and C haplotypes is reviewed.The development of HIV vaccines that will stimulate, in an uninfected individual, the humoral (antibody) and cellular (CTL) immune defenses against HIV and HIV-infected cells, respectively, and may lead to protection from primary HIV infection are discussed. The need to eliminate the release of HIV virions from infected cells introduced by an infected donor to an uninfected recipient may require both the humoral and cellular immune responses. However, such CTLs may fail to identify HIV-infected cells with integrated HIV proviral DNA that do not express viral genes and proteins. Based on reported results on the immunization of monkeys with uninfected cells, which prevented infection with SIV grown in the same type of cells, it may be possible to consider immunization of specific human populations against HLA haplotypes prevalent in HIV-infected donors. Since HIV virions may carry the HLA class I molecules present in the infected donors' cells, synthesis of CTLs to the mutated amino acid sequence in peptide binding grooves of the foreign HLA haplotypes may induce anti-HLA CTLs in the immunized individual, which may destroy HIV-infected, virus synthesizing donor cells, as well as donor cells containing latent proviral DNA. Such anti-foreign HLA CTLs may prevent the release of virions from the infecting donor's cells. The importance of HLA haplotypes for protection against HIV will be discussed.Dedicated to the memory of Dr. Albert Sabin (1906–1993).     

Substitutions at the Glu62 residue of human interleukin-2 differentially affect its binding to the α chain and the βγ complex of the interleukin-2 receptor

Human interleukin-2 (IL-2) α helix B is more conserved than the whole molecule, but has been less studied than other α helices of IL-2. Using site-directed mutagenesis, several IL-2 mutants in this helix were obtained. We found that the IL-2 mutant containing Leu at position 62 (Leu⁶²-IL-2) loses its ability to bind IL-2 receptor subunit α (IL-2Rα), but retains binding affinity to IL-2R subunit βγ as well as some bioactivity; nevertheless, another substitution at the same residue, Arg⁶² IL-2, loses its binding ability to both IL-2Rα and IL-2Rβγ, and can no longer stimulate IL-2-dependent cell growth, showing that Glu⁶² not only takes part in IL-2Rα binding, but can also affect IL-2 binding to IL-2Rβγ. In this regard, Glu⁶² may be a key site in the IL-2/IL-2Rα interaction, and can facilitate IL-2R ternary-complex formation, leading to IL-2Rα-mediated, IL-2-stimulated signal transduction.     

Spontaneous mutations in the human CMV HLA class I homologue UL18 affect its binding to the inhibitory receptor LIR-1/ILT2/CD85j

Human cytomegalovirus (HCMV) down-regulates cell surface expression of HLA class I molecules (HLA-I). UL18, an HCMV-encoded HLA-I homologue, has been proposed to protect virus-infected cells against NK cell recognition by engaging the inhibitory receptor leukocyte Ig-like receptor (LIR)-1, which also binds a broad spectrum of HLA-I alleles, including HLA-G1. Because genetic and biological differences exist among HCMV strains, we characterized laboratory (AD169) and clinical (4636, 13B, 109B) strain-derived UL18 proteins. Compared to the known AD169-derived UL18, mutations were found in clinical strain-derived UL18. They were clustered in the alpha3 domain (13B), previously shown to be critical for LIR-1 binding, or in the alpha1 domain (4636). Iotan cytotoxicity assays, pretreatment of LIR-1+ NKL with soluble 4636-UL18 completely abolished LIR-1-dependent protection from NK lysis, conferred by the expression of HLA-G1 on target cells (721.221-HLA-G1+). Similarly, flow cytometry, Biacore and ELISA experiments showed 4636-UL18 and 13B-UL18 to have the strongest binding capacity to LIR-1. Our results suggest the importance of two independent UL18 regions for LIR-1 binding, one localized on the tip of the alpha3 domain, and another composed of two loops that emerge from the alpha1 domain. Strain variations in these domains may result in different UL18-mediated effects on LIR-1+ cells during the course of HCMV infection.     

Identification and cellular distribution of the rat interleukin-2 receptor β chain: induction of the IL-2Rα−β+ phenotype by major histocompatibility complex class I recognition during T cell development in vivo and by T cell receptor stimulation of CD4+8+ immature thymocytes in vitro

A mouse monoclonal antibody (mAb) to the rat interleukin-2 receptor β (IL-2Rβ) chain was generated using IL-2Rβ cDNA-transfected mouse L929 cells for immunization and differential screening. This antibody, called L316, detects a cell surface protein with an apparent molecular mass of about 80 kDa. In peripheral lymphoid organs of young adult rats, IL-2Rβ expression is restricted to T and natural killer (NK) cells, and less than 10% of IL-2Rβ⁺ cells co-express the IL-2Rα chain. IL-2Rβ was detected on all NKRP-1^hi (NK⁻) and NKRP-1^lo cells (T-lineage cells of unknown function), most peripheral γδ T cells and on 30–40% of CD8 and 10% of CD4 αβ T cells. In the adult rat thymus, mAb L316 detects a small subset (about 1%) of predominantly IL-2Rα⁻ cells which express cell surface markers characteristic of mature T lymphocytes and contain a high proportion of CD4⁻8⁻ and CD4⁻8⁺ αβ T cell receptor (TCR)⁺ thymocytes. TCR-V usage suggests that major histocompatibility complex (MHC) class I plays a more important role than MHC class II in the selection of these cells. On immature CD4⁺8⁺ rat thymocytes, IL-2Rβ cell surface expression is readily induced by TCR stimulation in vitro, supporting the idea that in vivo, the IL-2Rβ⁺ phenotype is the result of TCR engagement during thymic selection.     

Interleukin-2 receptor β and γ chain dysregulation during the inhibition of CD4 T cell activation by human immunodeficiency virus-1 gp120

We have observed that CD4 T lymphocytes from human immunodeficiency virus (HIV)-infected patients marginally express interleukin-2 receptor (IL-2R)β and IL-2Rγ chains which are essential for IL-2 signal transduction. To analyze this observation further, we studied the influence of gp120 on the cell surface expression of IL-2Rβ and IL-2Rγ by purified CD4 lymphocytes in vitro. Cross-linking of the T cell receptors of these lymphocytes initiates entry into the cell cycle as measured by CD69 and CD71 cell surface expression and [³H]thymidine incorporation. It also induces the cell surface expression of IL-2Rβ and IL-2Rγ. We have shown that treatment of the CD4 T lymphocytes with HIV-1 gp120 before anti-CD3 stimulation impedes cell cycle progression as measured by reduced CD71 expression and inhibition of [³H]thymidine incorporation. Furthermore, cell surface expression of IL-2Rβ and IL-2Rγ subunits, which form the functional intermediate-affinity IL-2R, are significantly inhibited. More importantly, addition of exogenous IL-2 does not restore the proliferation of the CD4 T cells treated with gp120, suggesting that cells are anergic and/or that the remaining IL-2R are not functional. This is the first study of IL-2Rβ and IL-2Rγ dysregulation in the context of HIV infection and shows that CD4 is also involved in IL-2R expression.     

HLA class II molecule-mediated signal transduction mechanism responsible for the expression of interleukin-1β and tumor necrosis factor-α genes induced by a staphylococcal superantigen

Superantigens including staphylococcal enterotoxins (SE) bind to major histocompatibility complex class II molecules and interact with T cells bearing particular vβ chains. SEB was shown to induce the expression of interleukin (IL)-1β and tumor necrosis factor (TNF)-α genes in human peripheral blood monocytes bearing HLA class II molecules. Monoclonal antibodies directed against HLA-DR and -DQ abolished the SEB-induced expression of both the EL-1β and TNF-α genes, suggesting that the HLA class II molecules mediated the gene expression. Therefore, we investigated the signal transduction mechanism responsible for the expression of IL-1β and TNF-α genes induced by binding of SEB to the HLA class II molecules. Three protein tyrosine kinase (PTK) inhibitors, genistein, herbimycin A., and tyrphostin, each of which has a different mechanism of action, strongly inhibited the expression of the monokine mRNA induced by SEB. Analyses of PTK activity revealed that SEB induced a rapid increase of membrane-associated PTK activity and this was blocked by tyrphostin. Furthermore, H-7 inhibited the expression of the monokine mRNA induced by SEB, suggesting the involvement of protein kinase C (PKC) in the signaling pathway. The involvement of PKC was confirmed by the observations that phorbol 12-myristate 13-acetate (PMA), a direct activator of PKC, induced the expression of the monokine mRNA and that SEB evoked the activation of membrane-associated PKC. Both activation of PKC and expression of the monokine mRNA induced by SEB appeared to be inhibited by tyrphostin, but those induced by PMA were not. Taken together, these findings indicate that both PTK and PKC play essential roles in HLA class II molecule-mediated signal transduction elicited by SEB and that PTK activation may precede PKC activation in the signaling pathway.     

KIR specificity and avidity of standard and unusual C1, C2, Bw4, Bw6 and A3/11 amino acid motifs at entire HLA:KIR interface between NK and target cells,the functional and evolutionary classification of HLA class I molecules

Natural killer (NK) cells make vital contributions to the immune system and the reproductive system. Notably, NK cells of donor origin can recognize and kill residual leukaemic cells and cure malignant patients in hematopoietic stem cell (HSC) transplant setting. NK cell function is regulated by KIRs that recognize cognate HLA class I molecules on target cells, depending on their amino acid residues. In review, we addressed the question of binding capacity and avidity of HLA class I molecules to different killer cell immunoglobulin‐like receptors (KIRs) depending on all interacting amino acid residues both on HLA and KIR side. We searched PubMed database and analysed available HLA:KIR crystallographic data for amino acid residues in HLA molecules, those physically involved in binding KIRs (termed here the “entire KIR interface”). Within entire KIR interface, we selected five functional sequence motifs (14–19, 66–76, 77–84, 88–92 and 142–151) and classified them according to the conservation of their amino acid sequences among 8,942 HLA class I molecules. Although some conserved amino acid motifs were shared by different groups of KIR ligands, the HLA motif combinations were exclusive for the ligand groups. In 135 common HLA class I molecules with known HLA:KIR recognition, we found 54 combinations of five motifs in each of the KIR‐binding interfaces (C1, C2, Bw4, A3/11) and conserved non‐KIR‐binding interfaces. Based on the entire KIR interface, this analysis allowed to classify 8,942 HLA class I molecules into KIR specificity groups. This functional and evolutionary classification of entire KIR interfaces provides a tool for unambiguously predicting HLA:KIR interactions for common and those HLA molecules that have not yet been functionally tested. Considering the entire KIR interface in HLA class I molecules, functional interactions of HLA and KIR can be predicted in immune responses, reproduction and allotransplantation. Further functional studies are needed on the HLA:KIR interaction variations caused by the repertoires of peptides presented by HLA molecules and KIR polymorphisms at allelic level.     

慢性乙肝患者外周血单个核细胞中CXCR1、CXCR2及IL-8 mRNA表达与干扰素治疗关系

目的:了解乙型肝炎病毒(HBV)慢性感染患者外周血单个核细胞(PBMC)中CXCR1、CXCR2及IL-8 mRNA表达水平及与α干扰素(IFN-α)治疗的关系。方法:采用实时定量PCR法动态观察30例慢性乙型肝炎患者接受IFN-α治疗前、治疗3个月、6个月后其外周血单个核细胞CXCR1、CXCR2及IL-8 mRNA表达水平。结果:治疗前慢性乙肝患者CXCR1、CXCR2及IL-8 mRNA表达水平分别为(0.44740.0386)、(0.4720 0.0458)、(1.1897 0.1028),均高于正常对照组(n=36),其中CXCR1及IL-8 mRNA水平升高显著,差异有统计学意义(P<0.01)。治疗过程中CXCR1、CXCR2及IL-8表达水平均显著下降。IFN-α治疗6个月后CXCR1、CXCR2及IL-8 mRNA表达水平分别为(0.41290.0395)、(0.4461 0.0477)、(0.8660 0.1307),与治疗前相比,差异有显著性(P<0.01或P<0.05)。治疗前的CX-CR1、CXCR2及IL-8的表达水平在HBV高复制组(HBV-DNA>106,n=22)明显高于HBV低复制组(HBV-DNA<106,n=8),差异有统计学意义(P<0.05)。结论:慢性乙肝患者外周血单个核细胞中CXCR1和IL-8表达水平显著升高,在干扰素治疗后,其表达水平下调,证明其可能与慢性乙肝炎症的发生机制相关。     

Effects of hypoxia on expression of transforming growth factor-beta1 and its receptors I and II in the amoeboid microglial cells and murine BV-2 cells

Transforming growth factor-beta1 (TGF-beta1) is widely recognized as a prototype of multifunctional growth factors and master switches in the regulation of key events of development, disease and repair. It is localized in neurons, astrocytes and brain macrophages in altered conditions but its localization in the amoeboid microglial cells (AMC), a nascent brain macrophage in the developing brain has remained unexplored. Here we report expression of TGF-beta1 and its receptors namely, transforming growth factor-beta receptor I (TbetaRI) and transforming growth factor-beta receptor II (TbetaRII) in AMC and BV-2 cells induced by hypoxia. Firstly, increase in TGF-beta1 mRNA expression and TGF-beta1 release was observed in the corpus callosum in postnatal rats subjected to a single hypoxic exposure. RT-PCR and Western blot analysis revealed a concomitant upregulation of TbetaRI and TbetaRII mRNA and protein. Secondly, immunofluorescence labeling showed that the preponderant AMC in the corpus callosum were immunoreactive for TGF-beta1 and its receptors. In rats subjected to hypoxia, immunoexpression of TGF-beta1 and both receptors was markedly enhanced. In longer surviving rats, the AMC transformed into ramified microglia but retained in them the immunoreactivity. In BV-2 cells exposed to hypoxia, TGF-beta1 mRNA expression and release of TGF-beta1 into the medium were significantly increased. It is noteworthy that expression of TbetaRI and TbetaRII mRNA and protein in hypoxic BV-2 cells was reduced indicating a differential response of AMC and BV-2 cells to hypoxia. Notwithstanding, it is unequivocal that AMC in the developing brain express and release TGF-beta1 into the ambient environment. We suggest that this may be a mechanism to help autoregulate microglial activation in adverse conditions via its receptors.     

2F1 antigen,the mouse homolog of the rat “mast cell function-associated antigen”, is a lectin-like type II transmembrane receptor expressed by natural killer cells

Inhibitory lectin-like receptors expressed on the surface of hematopoietic cells are critically involved in regulation of their effector functions. Here we report that a novel mAb specific for mouse NK cells, 2F1, recognizes the mouse homolog of the mast cell function-associated antigen (MAFA), an inhibitory lectin-like transmembrane receptor expressed on rat mast cells. The 2F1 antigen (2F1-Ag) and rat MAFA are structurally highly conserved and contain a cytoplasmic motif similar to the immunoreceptor tyrosine-based inhibitory motif that is presumably utilized for inhibitory signaling. We also identified a human homolog that is closely related to the rodent MAFA/2F1-Ag proteins. Like rat MAFA, 2F1-Ag is probably encoded by a single gene, which exhibits relatively little polymorphism. Strikingly, while rat MAFA is considered a mast cell antigen, we have been unable to detect cell surface expression of 2F1-Ag by mouse mast cell lines, bone marrow-derived mast cells, or peritoneal mast cells. Furthermore, mouse bone marrow-derived mast cells were devoid of 2F1-Ag mRNA. Instead, we find that approximately 40 % of mouse NK cells express 2F1-Ag. Thus, MAFA/2F1-Ag may modulate immunological responses on at least two different cell types bridging the specific and innate immune system.