Location-specific regulation of transgenic Ly49A receptors by major histocompatibility complex class I molecules

Chronic lymphocytosis of functionally immature natural killer cells

BACKGROUND

The development of natural killer (NK) cells in the bone marrow is not well characterized. We recently described a mouse (referred to as an NK cell-deficient [NKD] mouse) with a selective deficiency in NK cells caused by the insertion of a transgene construct into the genetic locus for the basic leucine zipper transcription factor ATF-2. NK cells in this mouse were both phenotypically and functionally immature and accumulated in the bone marrow at a stage at which constitutive NK cell proliferation occurs in wild-type mice.

OBJECTIVE

We hypothesized that excess IL-15 could potentially overcome this developmental block, allowing normal emigration of mature NK cells from the bone marrow to the periphery.

METHODS

Double-transgenic mice were generated by crossing the NKD mice with transgenic mice overexpressing IL-15.

RESULTS

The double-transgenic mice had a dramatic accumulation of phenotypically immature NK cells in the bone marrow and subsequently in the blood, liver, and spleen. NK cells from these double-transgenic mice manifested functional deficits similar to those observed in NK cells from NKD mice, as assessed by decreased cytokine production and cytotoxicity.

CONCLUSION

Rather than bypass the observed developmental defect in NKD mice, excess IL-15 drove a massive accumulation of phenotypically and functionally immature NK cells in the bone marrow and periphery.

CLINICAL IMPLICATIONS

We propose that these double-transgenic mice will serve as a murine model of chronic NK cell lymphocytosis in human patients.

Quantifying the reduction in accessibility of the inhibitory NK cell receptor Ly49A caused by binding MHC class I proteins in cis

Murine natural killer (NK) cells are inhibited by target cell MHC class I molecules via Ly49 receptors. However, Ly49 receptors can be made inaccessible to target cell MHC class I by a cis interaction with its MHC class I ligand within the NK cell membrane. It has recently been demonstrated that MHC class I proteins transfer from the target cells to the NK cell. Here, we establish that the number of transferred MHC class I proteins is proportional to the number of Ly49A receptors at the NK cell surface. Ly49A+ NK cells from mice expressing the Ly49A ligand H-2D(d) showed a 90% reduction in Ly49A accessibility compared to Ly49A+ NK cells from H-2D(d)-negative mice. The reduction was caused both by lower expression of Ly49A and interactions in cis between Ly49A and H-2D(d) at the NK cell surface. Approximately 75% of the Ly49A receptors on H-2D(d)-expressing NK cells were occupied in cis with endogenous H-2D(d) and only 25% were free to interact with H-2D(d) molecules in trans. Thus, H-2D(d) ligands control Ly49A receptor accessibility through interactions both in cis and in trans.

GFP transgenic mice show dynamics of lung macrophages

The dynamics of tissue macrophages are poorly understood. We have developed a model where only lung macrophages express high levels of enhanced green fluorescent protein (EGFP) and are easily identified and followed by confocal microscopy. The EGFP+ cells had the morphology of macrophages and express CD11c, CD11b, and F4/80, but not NK1.1 or CD3. The F4/80+ EGFP+ cells were found exclusively in the lung and not in lymph nodes, spleen, blood, liver, intestine, or uterus. These EGFP+ cells are phagocytic and can be activated to migrate within the lung in response to LPS stimulation. In this study, we describe a new model system that allows the specific study of macrophages in the lung.

Arrested natural killer cell development associated with transgene insertion into the Atf2 locus

Natural killer (NK) cell development in the bone marrow is not fully understood. Following lineage commitment, these cells appear to advance through a series of developmental stages that are beginning to be characterized. We previously reported a selective deficiency of NK cells in a C57BL/6 mouse with a transgenic construct consisting of the cDNA for the Ly49A major histocompatibility complex (MHC) class 1-specific inhibitory receptor driven by the granzyme A gene. This mouse has few NK cells in peripheral tissues with relative preservation of other immune cells, including T and B cells. Herein we demonstrate that these mice have an accumulation of NK cells with an immature phenotype in the bone marrow, consistent with a block at a previously proposed stage in normal NK-cell development. The phenotype is associated with transgenic insertion into Atf2, the gene for the basic leucine zipper (bZIP) transcription factor family member ATF-2. Although analysis of Atf2-null NK cells shows no defect, the transgenic mice express abnormal truncated Atf2 transcripts that may mediate a repressor effect because ATF2 can heterodimerize with other bZIP molecules. The defect is cell intrinsic, suggesting that certain bZIP molecules play significant roles in NK-cell development.

The regulatory functions of Ly-49A, Ly-49D and Ly-49G2 on NK cells in the recognition and rejection of the alloantigen in vivo

Ly-49A is an inhibitory receptor that binds H-2Dd and H-2Dk. The downregulation of Ly-49A is thought to mediate NK self tolerance in vivo. In this study, we analyzed the regulation of Ly-49A, D and G2 on NK cells in an in vivo rejection model. After injection with 1 x 10(8) B10.D2 spleen cells (SC) into B 10 mice, we found Ly-49A downregulated within 3 h on NK cells of B10 mice, whereas expressions of Ly-49D and G2 were augmented. To investigate effects of different expression patterns of Ly-49 receptors on NK cells, Ly-49A, D or G2-depleted B10 mice were inoculated with B10.D2 SC. NK cells from SC of Ly-49A-depleted and B10.D2 SC-injected B10 mice showed enhanced cytotoxicity to Dd-positive targets in vitro. Furthermore, reduced numbers of B10.D2 SC were observed in Ly-49A or G2-depleted B10 mice, whereas increased numbers of B10.D2 SC were observed in Ly-49D-depleted B10 mice after inoculation with B10.D2 SC in vivo. These findings indicated that the downregulation of Ly-49A and the augmentation of Ly-49D expression may mediate NK cells to recognize and kill Dd antigen efficiently. In conclusion, each Ly-49 isoform may play independent roles in the regulation of activation or inhibition on NK cells.

Computational modeling of human natural killer cell development suggests a selection process regulating coexpression of KIR with CD94/NKG2A

Natural killer cells fail to lyse target cells expressing sufficient levels of self MHC class I molecules, providing one mechanism to secure self tolerance. Inhibition of lysis is mediated by inhibitory receptors expressed by NK cells, such as the murine Ly49 receptors, human KIR receptors and CD94/NKG2A, expressed by both species. To ensure that most, if not all, NK cells express at least one inhibitory receptor for self MHC class I, selection processes have been postulated for murine NK cells regulating the number and identity of inhibitory receptors expressed by each cell. The presence of similar selection processes in human NK cells has not been demonstrated. In previous studies using mathematical modeling we have shown that, in the Ly49 system, the sequential model (in which gene expression and selection operate simultaneously) is most likely to explain the observed expression frequencies. We also predicted the parameters (such as receptor-ligand binding affinity levels) under which the models fit with the observed frequencies. This study aims to evaluate whether these models may be valid in the human system. Our data suggest that if selection operates during human NK cell development, it affects the co-expression of CD94/NKG2A and KIR rather than KIR expression alone, and is more likely to be governed by the two-step selection model.

Receptor/ligand avidity determines the capacity of Ly49 inhibitory receptors to interfere with T-cell receptor-mediated activation

The specificity and the relative affinity of many Ly49 receptors for major histocompatibility complex class I ligands have been studied in detail in various adhesion and binding assays. However, how the level of cell surface expression of a given Ly49 receptor and its ligand affinity influence the strength of the inhibition signal is not well documented. To address this issue, we developed a series of human Jurkat T-cell transfectants expressing the whole range of Ly49A and Ly49C levels found in vivo on natural killer and T cells and evaluated their capacity to alter superantigen-induced NF-AT activation and interleukin-2 production. We show that the strength of the inhibition induced by Ly49A/H-2Dd interaction correlates with Ly49A density up to a certain level after which increasing expression does not further inhibit significantly the T-cell receptor-induced activation. This system also represents a valuable tool for the determination of the relative strength of the inhibitory signals of Ly49 receptors following their interactions with different ligands. Even at high levels of expression there was no evidence that engagement of Ly49A with H-2b class I molecules provided an inhibitory signal. Moreover, we showed that functional inhibitory interactions of Ly49C with H-2b class I molecules were only the result of H-2Kb and that H-2d represent lower affinity ligands for Ly49C than H-2b. Therefore, depending on the relative affinity of Ly49 receptors for their ligands, the modulation of their expression level will be determinant for the functional outcome of activated T cells.

Regulation of antiviral CD8+ T cells by inhibitory natural killer cell receptors

Recent evidence indicates that CD8(+) T cells express natural killer cell receptors that constrain the range and magnitude of their activities. For virus-specific CD8(+) T cells, upregulation of these receptors serves to control infection, while concurrently minimizing bystander pathology. Dysregulated expression of these receptors, however, may foster the establishment of persistent virus infection.

B6 Strain Ly49I Inhibitory Gene Expression on T Cells in FVB.Ly49IB6 Transgenic Mice Fails to Prevent Normal T Cell Functions

Inhibitory Ly49 receptors expressed on NK cells provide a mechanism for tolerance to normal self tissues. The immunoregulatory tyrosine-based inhibitory motifs present in some Ly49s are able to transmit an inhibitory signal upon ligation by MHC class I ligands. In our system, as well as others, mice transgenic for inhibitory Ly49 receptors express these receptors on both NK and T cells. FVB (H2(q)) mice transgenic for the B6 strain Ly49I (Ly49I(B6)) express the inhibitory Ly49 receptor on the surface of both T and NK cells. Although Ly49I functions to prevent NK-mediated rejection of H2(b) donor bone marrow cells in this transgenic mouse strain, the T cells do not appear to be affected by the expression of the Ly49I transgene. FVB.Ly49I T cells have normal proliferative capabilities both in vitro and in vivo in response to the Ly49I ligand, H2(b). In vivo functional T cell assays were also done, showing that transgenic T cells were not functionally affected. T cells in these mice also appear to undergo normal T cell development and activation. Only upon stimulation with suboptimal doses of anti-CD3 in the presence of anti-Ly49I is T cell proliferation inhibited. These data are in contrast with findings in Ly49A, and Ly49G2 receptor transgenic models. Perhaps Ly49I-H2(b) interactions are weaker or of lower avidity than Ly49A-H-2D(d) interactions, especially in T cells.

NK cell inhibitory receptor Ly-49C residues involved in MHC class I binding

Mouse NK cells express Ly-49 receptors specific for classical MHC class I molecules. Several of the Ly-49 receptors have been characterized in terms of function and ligand specificity. However, the only Ly-49 receptor-ligand interaction previously described in detail is that between Ly-49A and H-2D(d), as studied by point mutations in the ligand and the crystal structure of the co-complex of these molecules. It is not known whether other Ly-49 receptors bind MHC class I in a similar manner as Ly-49A. Here we have studied the effect of mutations in Ly-49C on binding to the MHC class I molecules H-2K(b), H-2D(b), and H-2D(d). The MHC class I molecules were used as soluble tetramers to stain transiently transfected 293T cells expressing the mutated Ly-49C receptors. Three of nine mutations in Ly-49C led to loss of MHC class I binding. The three Ly-49C mutations that affected MHC binding correspond to Ly-49A residues that are in contact or close to H-2D(d) in the co-crystal, demonstrating that MHC class I binding by Ly-49C is dependent on residues in the same area as that used by Ly-49A for ligand contacts.

Regulation of the natural killer cell receptor repertoire

Natural killer cells express inhibitory receptors specific for MHC class I proteins and stimulatory receptors with diverse specificities. The MHC-specific receptors discriminate among different MHC class I alleles and are expressed in a variegated, overlapping fashion, such that each NK cell expresses several inhibitory and stimulatory receptors. Evidence suggests that individual developing NK cells initiate expression of inhibitory receptor genes in a sequential, cumulative, and stochastic fashion. Superimposed on the receptor acquisition process are multiple education mechanisms, which act to coordinate the stimulatory and inhibitory specificities of developing NK cells. One process influences the complement of receptors expressed by individual NK cells. Other mechanisms may prevent NK cell autoaggression even when the developing NK cell fails to express self-MHC-specific inhibitory receptors. Together, these mechanisms ensure a self-tolerant and maximally discriminating NK cell population. Like NK cells, a fraction of memory phenotype CD8(+) T cells, as well as other T cell subsets, express inhibitory class I--specific receptors in a variegated, overlapping fashion. The characteristics of these cells suggest that inhibitory receptor expression may be a response to prior antigenic stimulation as well as to poorly defined additional signals. A unifying hypothesis is that both NK cells and certain T cell subsets initiate expression of inhibitory receptors in response to stimulation.

CD8+ T cells rapidly acquire NK1.1 and NK cell-associated molecules upon stimulation in vitro and in vivo

NKT cells express both NK cell-associated markers and TCR. Classically, these NK1.1+TCRalphabeta+ cells have been described as being either CD4+CD8- or CD4-CD8-. Most NKT cells interact with the nonclassical MHC class I molecule CD1 through a largely invariant Valpha14-Jalpha281 TCR chain in conjunction with either a Vbeta2, -7, or -8 TCR chain. In the present study, we describe the presence of significant numbers of NK1.1+TCRalphabeta+ cells within lymphokine-activated killer cell cultures from wild-type C57BL/6, CD1d1-/-, and Jalpha281-/- mice that lack classical NKT cells. Unlike classical NKT cells, 50-60% of these NK1.1+TCRalphabeta+ cells express CD8 and have a diverse TCR Vbeta repertoire. Purified NK1.1-CD8alpha+ T cells from the spleens of B6 mice, upon stimulation with IL-2, IL-4, or IL-15 in vitro, rapidly acquire surface expression of NK1.1. Many NK1.1+CD8+ T cells had also acquired expression of Ly-49 receptors and other NK cell-associated molecules. The acquisition of NK1.1 expression on CD8+ T cells was a particular property of the IL-2Rbeta+ subpopulation of the CD8+ T cells. Efficient NK1.1 expression on CD8+ T cells required Lck but not Fyn. The induction of NK1.1 on CD8+ T cells was not just an in vitro phenomenon as we observed a 5-fold increase of NK1.1+CD8+ T cells in the lungs of influenza virus-infected mice. These data suggest that CD8+ T cells can acquire NK1.1 and other NK cell-associated molecules upon appropriate stimulation in vitro and in vivo.

Expression of Ly49A on T cells alters the threshold for T cell responses

In this study we investigated the balance between activating and inhibitory signals during T cell activation. We have used transgenic mice in which CD8+ T cells expressed an inhibitory receptor, Ly49A, and a specific activating alphabeta TCR. This TCR recognizes an lymphocytic choriomeningitis virus peptide in combination with H-2Db. We observed a quantitative influence on cellular responses that depended upon the activating signals received through the TCR and the inhibitory signals received through Ly49A. By varying the peptide concentration given to stimulating cells or target cells, we could adjust the amount of ligand available to trigger the TCR. At low doses of peptide, Ly49A-expressing T cells were unresponsive on target cells that expressed H-2Dd, but responded against target cells without H-2Dd. However, this inhibition could be overcome by increasing the peptide concentration or by addition of anti-Ly49A F(ab')2 fragments. Thus, rather than behaving as simple "off" switches, our data indicate that Ly49 receptors modulate T cell signaling so that higher amounts of activating signals are required for effector-cell responses.

In vivo natural killer cell activities revealed by natural killer cell-deficient mice

Studies of natural killer (NK) cell function in vivo have been challenging primarily due to the lack of animal models in which NK cells are genetically and selectively deficient. Here, we describe a transgenic mouse with defective natural killing and selective deficiency in NK1.1(+) CD3(-) cells. Despite functionally normal B, T, and NK/T cells, transgenic mice displayed impaired acute in vivo rejection of tumor cells. Adoptive transfer experiments confirmed that NK1.1(+) CD3(-) cells were responsible for acute tumor rejection, establishing the relationship of NK1.1(+) CD3(-) cells to NK cells. Additional studies provided evidence that (i) NK cells play an important role in suppressing tumor metastasis and outgrowth; (ii) NK cells are major producers of IFNgamma in response to bacterial endotoxin but not to interleukin-12, and; (iii) NK cells are not essential for humoral responses to T cell-independent type 2 antigen or the generalized Shwartzman reaction, both of which were previously proposed to involve NK cells.

Ly49I NK cell receptor transgene inhibition of rejection of H2b mouse bone marrow transplants

The Ly49 family of genes encode NK cell receptors that bind class I MHC Ags and transmit negative signals if the cytoplasmic domains have immunoregulatory tyrosine-based inhibitory motifs (ITIMs). 5E6 mAbs recognize Ly49C and Ly49I receptors and depletion of 5E6+ NK cells prevents rejection of allogeneic or parental-strain H2d bone marrow cell (BMC) grafts. To determine the function of the Ly49I gene in the rejection of BMC grafts, we transfected fertilized eggs of FVB mice with a vector containing DNA for B6 strain Ly49I (Ly49IB6). Ly49IB6 is ITIM+ and is recognized by 5E6 as well as Ly49I-specific 8H7 mAbs. Normal FVB H2q mice reject H2b but not H2d BMC allografts, and the rejection of H2b BMC was inhibited partially by anti-NK1.1 and completely by anti-asialo GM1, but not by anti-CD8, Abs. In FVB mice, NK1.1 is expressed on only 60% NK cells. FVB. Ly49IB6 hosts failed to reject H2d or H2b BMC, but did reject class I-deficient TAP-1-/- BMC, indicating that NK cells were functional. Nondepleting doses of anti-Ly49I Abs reversed the acceptance of H2b BMC by FVB.Ly49IB6 mice. FVB.Ly49IB6+/- mice were crossed and back-crossed with 129 mice-H2b, 5E6-, poor responders to H2d BMC grafts. While transgene-negative H2b/q F1 or first-generation back-crossed mice rejected H2b marrow grafts (hybrid resistance), transgene-positive mice did not. Thus B6 strain Ly49I receptors transmit inhibitory signals from H2b MHC class I molecules. Moreover, Ly49IB6 has no positive influence on the rejection of H2d allografts.

Ly49A expression on T cells alters T cell selection

Ly49 receptors are inhibitory receptors expressed on subsets of both NK cells and NK1.1(+) T cells. The function of these receptors on NK cells is believed to be important in maintaining self-tolerance, yet their role on T cells is unclear. In this report we investigated how an Ly49A transgene alters T and NK cell development in an in vivo environment, where a ligand for Ly49A is expressed. Ly49A transgenic mice that co-expressed an MHC ligand for Ly49A, H-2D(d), developed a severe inflammatory disorder that resulted in death within the first weeks of age. T cells expressing forbidden TCR V(beta) chains were found both in the thymus and periphery of transgenic mice, while non-transgenic littermates had successfully deleted these T cell subsets. These data indicate that the expression of Ly49A on T cells could alter T cell selection and allow survival of potentially self-reactive T cells.

Transgenic expression of Ly-49A in thymocytes alters repertoire selection

A T cell-specific Ly-49A transgene inhibits TCR-mediated activation in the presence of H-2Dd. Expression of this transgene by developing thymocytes impairs negative selection evidenced by a failure to delete potentially autoreactive T cells and development of a graft-vs-host-disease-like syndrome. In mice carrying both the Ly-49A and a class II-restricted TCR transgene, positive selection was lost, but only when H-2Dd was present on thymic epithelium. These results are consistent with models suggesting that thymic selection is dependent on the perceived intensity of TCR signaling. More interestingly, these results show that Ly-49A does not simply provide a strict on/off switch for T cell responses. Since Ly-49A may shift the signaling threshold of TCR-induced triggering, inducible expression of Ly-49A may regulate peripheral memory/activated T cells by raising the threshold for T cell reactivation.

MHC Class I Mosaic Mice Reveal Insights into Control of Ly49C Inhibitory Receptor Expression in NK Cells

We have analyzed lymphocyte development in natural MHC class I chimeric mice, generated through a transgenic approach in β2-microglobulin (β2m)−/− mice. In these mice, MHC class I+ cells coexist with an equal proportion of MHC class I-deficient cells. These MHC class I mosaic mice had normal numbers of CD8+ T cells, which had a target cell specificity similar to that of wild-type mice. Consequently, the mice did not develop any signs of autoimmunity. They also had normal numbers of NK cells. This allowed an examination of the MHC class I influence on the expression of the Ly49C inhibitory receptor on NK cells. This receptor binds to H-2Kb. It is expressed at low levels on NK cells in wild-type mice of the H-2b haplotype, but at markedly higher levels on NK cells in β2m−/− mice and other strains of mice lacking expression of H-2Kb. Relatively little is known about how MHC class I molecules affect expression of the Ly49 receptors. Through the analysis of the present MHC class I mosaic mice, we demonstrate that the expression levels of Ly49C on NK cells is a consequence not only of MHC class I expression in the environment, but also of the expression of MHC class I molecules by the NK cells themselves. These findings are discussed in relation to the biological role of the calibration of the Ly49 inhibitory receptor expression in relation to self-MHC class I.

External and Internal Calibration of the MHC Class I-Specific Receptor Ly49A on Murine Natural Killer Cells

Expression of the H-2Dd-specific inhibitory receptor Ly49A on murine NK cells is subject to MHC class I-dependent modulation in vivo. As a result, NK cells in H-2Dd-transgenic mice express low cell surface levels of Ly49A, whereas NK cells from nontransgenic C57BL/6 (B6) mice express high levels. The purpose of this study was to assess the role of MHC class I molecules on the NK cell itself vs those on surrounding cells in this calibration and to test whether the Ly49A levels are subject to regulation in mature NK cells also. Analysis of transgenic mice with mosaic expression of an H-2Dd/Ld transgene showed that MHC class I molecules on surrounding cells (external ligands) and on the NK cell itself (internal ligands) played distinct roles in the determination of Ly49A levels. External ligands were involved in down-regulation of Ly49A levels in vivo, whereas internal ligands kept the down-regulated levels of Ly49A low upon NK cell activation in vitro. Furthermore, in an experimental system based on adoptive transfer of spleen cells, receptor down-regulation of Ly49A occurred as a rapid adaptation process in mature NK cells after interaction with the H-2Dd ligand in vivo. This suggests that Ly49 levels are not fixed but can be changed in mature NK cells when they are exposed to a changed MHC class I environment.

Modulation of Ly49A receptors on mature cells to changes in major histocompatibility complex class I molecules

The expression of murine Ly49 receptors on natural killer (NK) cells and NK1.1+ T cells is believed to prevent these cells from responding against normal self-tissues. In this report we investigated whether the expression level of Ly49A was fixed on mature cells or if it could be adapted as the major histocompatibility complex (MHC) class I environment changed in vivo. By transferring peripheral T cells from Ly49A transgenic mice into BALB/c nude/nude and B6 nude/nude mice, we demonstrated that mature cells modulate their Ly49A receptor expression relative to the in vivo MHC class I environment. These results indicated that the expression of the inhibitory Ly49A receptor is not permanently fixed during a maturation and/or education process but rather is adapted to MHC class I changes on the surrounding cells.

Developmentally regulated extinction of Ly-49 receptor expression permits maturation and selection of NK1.1+ T cells

Clonally distributed inhibitory receptors negatively regulate natural killer (NK) cell function via specific interactions with allelic forms of major histocompatibility complex (MHC) class I molecules. In the mouse, the Ly-49 family of inhibitory receptors is found not only on NK cells but also on a minor (NK1.1+) T cell subset. Using Ly-49 transgenic mice, we show here that the development of NK1.1+ T cells, in contrast to NK or conventional T cells, is impaired when their Ly-49 receptors engage self-MHC class I molecules. Impaired NK1.1+ T cell development in transgenic mice is associated with a failure to select the appropriate CD1-reactive T cell receptor repertoire. In normal mice, NK1.1+ T cell maturation is accompanied by extinction of Ly-49 receptor expression. Collectively, our data imply that developmentally regulated extinction of inhibitory MHC-specific receptors is required for normal NK1.1+ T cell maturation and selection.

Natural killer cell receptors

In killing of cellular targets, natural killer cells employ receptors that activate them and receptors specific for MHC class I that inhibit their activation. Progress in understanding the inhibitory receptors has been rapid, and indications are that they fall into two distinct structural types that appear to utilize the same inhibitory signaling cascade; meanwhile, components of the activation cascade are being elucidated, permitting us to integrate the pathways involved.

Acquisition of Ly49 receptor expression by developing natural killer cells

The formation of the repertoire of mouse natural killer (NK) cell receptors for major histocompatibility complex (MHC) class I molecules was investigated by determining the developmental pattern of Ly49 receptor expression. During the first days after birth, few or no splenic NK cells express Ly49A, Ly49C, Ly49G2, or Ly49I receptors. The proportion of Ly49+ splenic NK cells gradually rises to adult levels during the first 6-8 wk of life. The appearance of appreciable numbers of splenic Ly49+ NK cells coincides with the appearance of NK activity at 3-4 wk. After in vivo transfer, NK cells not expressing specific Ly49 receptors can give rise to NK cells that do, and cells expressing one of these four Ly49 receptors can give rise to cells expressing others. Once initiated, expression of a Ly49 receptor is stable for at least 10 d after in vivo transfer. Hence, initiation of Ly49 receptor expression occurs successively. Interestingly, expression of one of the receptors tested, Ly49A, did not occur after in vivo transfer of Ly49A- cells. One possible explanation for these data is that the order of Ly49 receptor expression by NK cells is nonrandom. The results provide a framework for evaluating models of NK cell repertoire formation, and how the repertoire is molded by host class I MHC molecules.