Parapneumonic empyema in children before the era of pneumococcal vaccination.

BACKGROUND: During the past two decades, the incidence of paediatric empyema has increased in many countries. PURPOSE: The aim of this retrospective hospital chart review was to evaluate the incidence, aetiology and clinical and laboratory characteristics of parapneumonic empyema in children. SUBJECTS AND METHODS: Twenty-one patients were admitted to a university hospital from the area with a population of 84,000 children in 1991-2009. RESULTS: The annual incidence of parapneumonic empyema was 1.6/100,000 children in 1991-1998, 0.2/100,000 children in 1999-2005 and 2.7/100,000 children in 2006-2009. Bacterial aetiology was identified in 52% of the cases, and pneumococcus caused 45% of the cases with bacterial aetiology detected. The clinical and laboratory findings in children with and without pleural effusion on admission were surprisingly similar. The development of empyema in hospital during antibiotic therapy was associated with persistent fever and serum C-reactive protein (CRP) >200 mg/L for 48 h after admission. CONCLUSION: The incidence of parapneumonic empyema in children fluctuated but in the long run, increased in 1991-2009. Pneumococcus caused half of the cases with bacterial diagnosis available. Since 2010, pneumococcal vaccination has belonged to the general vaccination programme, and the effect on the incidence of empyema remains to be seen.

The OX-44 molecule couples to signaling pathways and is associated with CD2 on rat T lymphocytes and a natural killer cell line.

The MRC OX-44 molecule, which is expressed on all peripheral leukocytes, identifies the subset of thymocytes capable of proliferating in response to alloantigens and lectins (Paterson, D.J., J.R. Green, W.A. Jefferies, M. Puklavec, and A.F. Williams. 1987. J. Exp. Med. 165:1). When we isolated monoclonal antibodies (mAbs) on the basis of their ability to activate the phosphatidylinositol signaling pathway in RNK-16 cells (a rat leukemia line with natural killer activity), three of the resulting mAbs recognized the OX-44 molecule. Addition of these mAbs to RNK-16 elicits protein tyrosine phosphorylation, generates inositol phosphates, and increases the concentration of cytoplasmic free calcium. These responses require the addition of intact mAb and are not observed with F(ab')2 fragments. One of these mAbs (7D2) is mitogenic for freshly isolated rat splenic T cells and synergizes with a mAb to the T cell antigen receptor in this activation. A 50-60-kD glycoprotein coprecipitates with the OX-44 molecule from RNK-16 cells and rat splenic T cells. Peptide mapping and reprecipitation studies indicate that the coprecipitating molecule is CD2. Thus, the OX-44 molecule can couple to multiple signaling pathways and associates with CD2 on both RNK-16 and rat T cells.

Activating Ly-49D and inhibitory Ly-49A natural killer cell receptors demonstrate distinct requirements for interaction with H2-D(d).

The activating Ly-49D receptor and the inhibitory Ly-49A receptor mediate opposing effects on natural killer (NK) cell cytotoxicity after interaction with the same major histocompatibility complex ligand, H2-D(d). To compare Ly-49D and Ly-49A interactions with H2-D(d), we created mutations in H2-D(d) and examined the functional ability of these mutants to activate lysis through Ly-49D or to inhibit lysis through Ly-49A. Specific single amino acid changes in either the H2-D(d) alpha(1) helix or the alpha(2) helix abrogated Ly-49D-mediated cytotoxicity, but these changes had no significant effect on Ly-49A-dependent inhibition. Each of three alpha(2) domain mutations in the floor of the peptide binding groove reduced functional recognition by either Ly-49D or Ly-49A, but all three were required to fully abrogate inhibition by Ly-49A. Our studies indicate that Ly-49D/H2-D(d) interactions require distinct determinants compared with Ly-49A/H2-D(d) interactions. These differences have important implications for the integration of activating and inhibitory signals in NK cells.

NKR-P1A is a target-specific receptor that activates natural killer cell cytotoxicity.

NKR-P1A is a lectinlike surface molecule expressed on rat natural killer (NK) cells. NKR-P1A has structural and functional features of an activating NK cell receptor, but a requirement for NKR-P1A in target cell lysis has not been determined. To define the role of NKR-P1A in natural killing, we have generated a mutant of the rat NK cell line, RNK-16, lacking expression of all members of the NKR-P1 receptor family. Although these NKR-P1-deficient NK cells were able to kill many standard tumor targets, including YAC-1, they were selectively deficient in the lysis of IC-21 macrophage, B-16 melanoma, and C1498 lymphoma targets. Reexpression of a single member of the NKR-P1 family, NKR-P1A, on mutant cells restored lysis of IC-21, and killing of IC-21 targets through rat NKR-P1A was completely blocked by F(ab')2 anti-NKR-P1A. Reexpression of NKR-P1A also restored transmembrane signaling to IC-21, as assessed by the generation of inositol-1,4,5-trisphosphate. The generation of inositol-1,4,5-trisphosphate was also restored in response to B-16 targets, but both B-16 and C1498 cells remained resistant to lysis, indicating that other NK cell molecules, perhaps within the NKR-P1 family, are required for the efficient killing of these tumors. These results are the first to demonstrate that NKR-P1A is a target-specific receptor that activates natural killing.

Mouse Ly-49A interrupts early signaling events in natural killer cell cytotoxicity and functionally associates with the SHP-1 tyrosine phosphatase.

The lytic activity of natural killer (NK) cells is inhibited by the expression of class I major histocompatibility complex (MHC) antigens on target cells. In murine NK cells, Ly-49A mediates inhibition of cytotoxicity in response to the class I MHC antigen H-2Dd. In this report, we studied the function of mouse Ly-49A in both the rat NK cell tumor line, RNK-16, transfected with Ly-49A cDNA, and in primary NK cells. We show that ligation of Ly-49A by H-2Dd inhibits early signaling events during target cell stimulation, including polyphosphoinositide turnover and tyrosine phosphorylation. We also show that Ly-49A directly associates with the cytoplasmic tyrosine phosphatase SHP-1, and that Ly-49A function is impaired in NK cells from SHP-1 mutant viable motheaten mice and from SHP-1-deficient motheaten mice. Finally, we demonstrate that mutational substitution of the tyrosine within the proposed SHP-1 binding motif in Ly-49A completely abrogates inhibition of NK cell cytotoxicity through this receptor. These results demonstrate that Ly-49A interrupts early activating signals in NK cells, and that SHP-1 is an important mediator of Ly-49A function.

Molecular cloning of gp42, a cell-surface molecule that is selectively induced on rat natural killer cells by interleukin 2: glycolipid membrane anchoring and capacity for transmembrane signaling.

We have previously shown that in vitro culture of rat natural killer (NK) cells in high concentrations of recombinant interleukin 2 (rIL-2) leads to the expression of a surface glycoprotein with a molecular mass of approximately 42 kD. This glycoprotein, gp42, is not induced on other lymphocytes and thus provides a lineage-specific marker for rIL-2-activated NK cells. We here present the nucleotide sequence for gp42 cDNA. The open reading frame encodes 233 amino acids with three potential sites for N-linked glycosylation. The deduced amino acid sequence lacks an apparent transmembrane domain and instead contains a hydrophobic COOH terminus that is characteristic of glycosylphosphatidylinositol (GPI)-anchored surface proteins. Consistent with this, gp42 is cleaved from the NK-like cell line, RNK-16, by phosphatidylinositol-specific phospholipase C (PI-PLC), as is gp42 expressed on CHO cells that have been transformed with gp42 cDNA. On rIL-2-activated NK cells, gp42 is resistant to PI-PLC, though our studies suggest that gp42 on these cells is still expressed as a GPI-anchored molecule. Antibody to gp42 stimulates in RNK-16 cells an increase in inositol phosphates and in intracellular calciu, signals that are associated with the activation of lymphocytes, including NK cells. rIL-2-activated NK cells, however, lack this response to gp42 as well as to other stimuli. Thus, gp42, the only NK-specific activation antigen, is a GPI-anchored surface molecule with the capacity to stimulate transmembrane signaling.

Mouse Ly-49D recognizes H-2Dd and activates natural killer cell cytotoxicity.

Although activation of natural killer (NK) cytotoxicity is generally inhibited by target major histocompatibility complex (MHC) class I expression, subtle features of NK allorecognition suggest that NK cells possess receptors that are activated by target MHC I. The mouse Ly-49D receptor has been shown to activate NK cytotoxicity, although recognition of MHC class I has not been demonstrated previously. To define Ly-49D-ligand interactions, we transfected the mouse Ly-49D receptor into the rat NK line, RNK-16 (RNK.mLy-49D). As expected, anti- Ly-49D monoclonal antibody 12A8 specifically stimulated redirected lysis of the Fc receptor- bearing rat target YB2/0 by RNK.mLy-49D transfectants. RNK.mLy-49D effectors were tested against YB2/0 targets transfected with the mouse MHC I alleles H-2Dd, Db, Kk, or Kb. RNK.mLy-49D cells lysed YB2/0.Dd targets more efficiently than untransfected YB2/0 or YB2/0 transfected with Db, Kk, or Kb. This augmented lysis of H-2Dd targets was specifically inhibited by F(ab')2 anti-Ly-49D (12A8) and F(ab')2 anti-H-2Dd (34-5-8S). RNK.mLy-49D effectors were also able to specifically lyse Concanavalin A blasts isolated from H-2(d) mice (BALB/c, B10.D2, and DBA/2) but not from H-2(b) or H-2(k) mice. These experiments show that the activating receptor Ly-49D specifically interacts with the MHC I antigen, H-2Dd, demonstrating the existence of alloactivating receptors on murine NK cells.

Monitoring the quality of frozen-thawed venous segments using bioimpedance spectroscopy.

OBJECTIVE: Storage at temperatures as low as -80 °C and below (cryopreservation) is considered a method for long-term preservation of cells and tissues, and especially blood vessel segments, which are to be used for clinical operations such as transplantation. However, the freezing and thawing processes themselves can induce injuries to the cells and tissue by damaging the structure and consequently functionality of the cryopreserved tissue. In addition, the level of damage is dependent on the rate of cooling and warming used during the freezing-thawing process. Current methods for monitoring the viability and integrity of cells and tissues after going through the freezing-thawing cycle are usually invasive and destructive to the cells and tissues. Therefore, employing monitoring methods which are not destructive to the cryopreserved tissues, such as bioimpedance measurement techniques, is necessary. In this study we aimed to design a bioimpedance measurement setup to detect changes in venous segments after freezing-thawing cycles in a noninvasive manner. APPROACH: A bioimpedance spectroscopy measurement technique with a two-electrode setup was employed to monitor ovine jugular vein segments after each cycle during a process of seven freezing-thawing cycles. MAIN RESULTS: The results demonstrated changes in the impedance spectra of the measured venous segments after each freezing-thawing cycle. SIGNIFICANCE: This indicates that bioimpedance spectroscopy has the potential to be developed into a novel method for non-invasive and non-destructive monitoring of the viability of complex tissue after cryopreservation.

Matrix metalloproteinases in mild and severe temporomandibular joint internal derangement synovial fluid.

OBJECTIVES: The first objective of this study was to verify the presence of and identify the molecular forms of matrix metalloproteinases (MMPs), including collagenases (MMP-1, MMP-8, and MMP-13) and gelatinases (MMP-2 and MMP-9), in the synovial fluid (SF) of mild and severe temporomandibular joint internal derangement (TMJ-ID). Another objective was to evaluate whether the SF MMPs are potential diagnostic markers that reflect the stage of intra-articular inflammation in the TMJ. STUDY DESIGN: The subjects were 44 patients with mild (n = 16) or severe (n = 28) TMJ-ID; they were classified on the basis of subjective symptoms, clinical and radiographic findings, and surgical observations. The patients were surgically treated, and SF samples were collected immediately before the operation. The collagenase activity of SF samples was analyzed by means of a type I collagen degradation assay. The levels and molecular forms of the SF MMPs as well as the tissue inhibitors of MMPs (TIMP-1 and TIMP-2) were analyzed with Western immunoblotting and gelatin zymography. RESULTS: The SF of both the mild and the severe TMJ-ID patients exhibited free collagenase activity and activity capable of further degrading the (3/4)(alphaA) fragments. Ninety-two-kilodalton proMMP-9 and its 121-kD complex form, as well as 72-kD proMMP-2 were significantly increased in the mild TMJ-ID group (P <.05 in all cases). Both 70- to 80-kD neutrophil type and 45- to 55-kD mesenchymal cell-type MMP-8 (corresponding to the latent and active forms) were observed in mild and severe TMJ-ID SF, but they predominated in mild TMJ-ID. Both MMP-1 and MMP-13 were observed in both groups, and in mild TMJ-ID SF the low-molecular weight forms of MMP-1 indicated activation of the enzyme. CONCLUSIONS: The degradation of type I collagen in the TMJ is evidently due to the collective action of many collagenolytic MMPs present in the SF of patients with mild and severe TMJ-ID. The elevated levels of MMP-2, MMP-9, and MMP-8 in the SF of patients with mild TMJ-ID eventually reflect the active phase of TMJ destruction. These observations may have considerable diagnostic and therapeutic significance in the management of TMJ disorders.

Estimates on the intake of food additives in Finland.

An estimation of the intakes of 30 food additives in Finland was conducted combining analytical data, food balance sheets, import and export statistics. The results indicated that most calculated average food additive intakes were well below the ADI values and internationally at an acceptable level. The intakes of nitrates, nitrites, saccharin and cyclamates were above or close to the respective ADI values. More studies are planned on these substances in order to establish possible special risk groups.

Import control of irradiated foods by the thermoluminescence method.

A thermoluminescence (TL) method was applied for the import control of irradiated foods. The method is based on the determination of the TL of mineral contaminants in foods. Detection of irradiation was incorporated in official Finnish control procedures in spring 1990. For foodstuffs with a reduced microbe content and in which no fumigant residues are found, possible irradiation is investigated by the TL method. The minerals are separated from the foods in different ways: picking is used for spices; water rinsing for herbs, spices, berries and mushrooms; high-density liquid to separate the organic material from the mineral fraction in seafood; and carbon tetrachloride for foods forming gels with water. To date about 140 food samples have been analysed for control purposes: 50 samples of herbs and spices, 25 samples of berries and mushrooms and 65 samples of seafood. Of these, 14 samples of herbs and spices and 5 samples of seafood were shown to have been irradiated. Differences in TL intensity between irradiated and unirradiated samples were at least 1 and usually 3-4 orders of magnitude.

Biocompatibility of nickel-titanium shape memory metal and its corrosion behavior in human cell cultures.

Nickel-titanium alloy (Nitinol) is a metallic biomaterial that has a unique thermal shape memory, superelasticity, and high damping properties. Nitinol is potentially very useful in orthopedic surgery, for example. At present, there are not enough confirmative biocompatibility data available on Nitinol. The aim of our study was to clarify the primary cytotoxicity and corrosion rate of Nitinol in human cell cultures. Comparisons were made with stainless steel (Stst), titanium (Ti), composite material (C), and control cultures with no test discs. Human osteoblasts (OB) and fibroblasts (FB) were incubated for 10 days with test discs of equal size, 6 x 7 mm. The cultures were photographed and the cells counted. Samples from culture media were collected on days 2, 4, 6, and 8, and the analysis of metals in the media was done using flameless atomic absorption spectrophotometry. The proliferation of FB was 108% (Nitinol), 134% (Ti) (p < 0.02), 107% (Stst), and 48% (C)(p < 0.0001) compared to the control cultures. The proliferation of OB was 101% (Nitinol), 100% (Ti), 105% (Stst), and 54% (C) (p < 0.025) compared to the controls. Initially, Nitinol released more nickel (129-87 micrograms/L) into the cell culture media than Stst (7 micrograms/L), but after 2 days the concentrations were about equal (23-5 micrograms/L versus 11-1 micrograms/L). The titanium concentrations from both Nitinol and Ti samples were all < 20 micrograms/L. We conclude that Nitinol has good in vitro biocompatibility with human osteoblasts and fibroblasts. Despite the higher initial nickel dissolution, Nitinol induced no toxic effects, decrease in cell proliferation, or inhibition on the growth of cells in contact with the metal surface.

Molecular cloning of the NK1.1 antigen, a member of the NKR-P1 family of natural killer cell activation molecules.

In mice, the NK1.1 alloantigen is expressed on all NK cells and it initiates transmembrane signals that activate cytotoxicity. NK1.1 has been mapped to a chromosomal region that encodes two families of receptor-like molecules, the NKR-P1 family and the Ly-49 family. Both of these gene families encode type II integral membrane proteins whose extracellular domains are homologous with known C-type lectins. We have isolated and expressed a member of the NKR-P1 gene family (mNKR-P1.9) and have demonstrated both by immunofluorescence and by immunoprecipitation that this cDNA encodes NK1.1. These findings, which demonstrate the receptor-like structure of NK1.1, will facilitate studies regarding the role of NK1.1 in natural killing and regarding the identification of possible NK1.1 ligands.

Alpha fetoprotein levels in neonatal hyperbilirubinaemia.

Serum alpha fetoprotein (AFP) levels were studied in 15 neonatally hyperbilirubinaemic children and 15 controls matched for sex and gestational age. All children were born between 38 and 40 weeks of gestation. During the first seven weeks of postnatal life hyperbilirubinaemic children had serum AFP concentrations over twice as high as controls. At the age of 5-7 days the mean (+/- S.E.M.) serum AFP values were 52.4 +/- 5.8 mg/l for hyperbilirubinaemic children and 24.8 +/- 4.3 mg/l for controls (p less than 0.001). At 20-25 days of age they were 7.28 +/- 1.10 and 2.75 +/- 0.45 mg/l, respectively (p less than 0.001), and at 40-49 days 1.39 +/- 0.21 and 0.46 +/- 0.07 mg/l (p less than 0.001). However, no correlation was found between serum bilirubin and AFP concentrations in hyperbilirubinaemic children.

Immune rejection of a large sarcoma following cyclophosphamide and IL-12 treatment requires both NK and NK T cells and is associated with the induction of a novel NK T cell population.

Combined immunotherapy with cyclophosphamide (Cy) and IL-12, but not IL-12 alone, stimulates eradication of a large established solid tumor (20 mm), MCA207, a methylcholanthrene-induced murine sarcoma. In these studies we demonstrate that NK1.1(+) cells and CD1d-dependent NK T cells each play important yet distinct roles in regression of a large tumor in response to Cy and IL-12, and we define a novel NK T cell subset, selectively increased by this treatment. Mice depleted of NK1.1(+) cells demonstrated more rapid initial tumor growth and prolonged tumor regression following treatment, but tumors were eventually eradicated. In contrast, initial tumor regression following therapy was unimpaired in CD1d(-/-) mice, which are deficient in most NK T cells, but tumors recurred. No tumor regression occurred following Cy and IL-12 therapy in CD1d(-/-) mice that were depleted of NK1.1(+) cells. We found that Cy and IL-12 induced the selective increase in liver and spleen lymphocytes of a unique NK T subpopulation (DX5(+)NK1.1(-)CD3(+)). These cells were not induced by treatment in CD1d(-/-) mice. Our studies demonstrate a contribution of both NK and NK T cells to the Cy- and IL-12-stimulated anti-tumor response. We describe the selective induction of a distinct NK T cell subset by Cy and IL-12 therapy, not seen following IL-12 therapy alone, which we suggest may contribute to the successful anti-tumor response induced by this immunotherapeutic regimen.

Natural killer cell cytolytic activity is inhibited by NKG2-A and activated by NKG2-C.

NKG2 is a small family of type II transmembrane proteins possessing extracellular C-type lectin domains expressed primarily on NK cells. The function of these proteins is unknown. We have developed mAbs that recognize NKG2 family members on Western blots. Examination of cell extracts from NK cell clones demonstrates that individual NKG2 family members are expressed on subsets of NK cells. Because the anti-NKG2 mAb do not react with the cell surface Ag, signaling function was studied by generating cell lines that express a chimeric receptor consisting of a cytoplasmic and transmembrane domain from NKG2-A or NKG2-C fused to the extracellular segment of mouse NKR-P1C (NK1.1 Ag). Transfectants of the rat NK cell line, RNK-16, were tested for the effect of anti-NK1.1 mAb on cytolytic activity against the FcR+ target cell lines, P388D1 and P815. The anti-NK1.1 mAb stimulated lytic activity and calcium mobilization in the NK cell line expressing the NKG2-C/NKR-P1C chimeric receptor. By contrast, anti-NK1.1 inhibited the lytic activity and failed to stimulate a calcium response in cells expressing the NKG2-A/NKR-P1C chimeric receptor. Immunoprecipitation experiments demonstrated that the inhibitory cytoplasmic tyrosine phosphatase SHP-1 selectively associates with the NKG2-A/NKR-P1C chimeric receptor, but not with the stimulatory NKG2-C/NKR-P1C receptor. These data indicate that NKG2-A and NKG2-C deliver, respectively, inhibitory and activating transmembrane signals in NK cells.

The alpha2 domain of H-2Dd restricts the allelic specificity of the murine NK cell inhibitory receptor Ly-49A.

Mouse NK lymphocytes express Ly-49 receptors, which inhibit cytotoxicity upon ligation by specific MHC I molecules on targets. Different members of the lectin-like mouse Ly-49 receptor family recognize distinct subsets of murine H-2 molecules, but the molecular basis for the allelic specificity of Ly-49 has not been defined. We analyzed inhibition of natural killing by chimeric MHC I molecules in which the alpha1, alpha2, or alpha3 domains of the Ly-49A-binding allele H-2Dd were exchanged for the corresponding domains of the nonbinding allele H-2Db. Using the Ly-49A-transfected rat NK cell line, RNK-mLy-49A.9, we demonstrated that the H-2Dd alpha2 domain alone accounts for allelic specificity in protection of rat YB2/0 targets in vitro. We also showed that the H-2Dd alpha2 domain is sufficient to account for the allele-specific in vivo protection of H-2b mouse RBL-5 tumors from NK cell-mediated rejection in D8 mice. Thus, in striking contrast to the alpha1 specificity of Ig-like killer inhibitory receptors for human HLA, the lectin-like mouse Ly-49A receptor is predominantly restricted by the H-2Dd alpha2 domain in vitro and in vivo.

NKR-P1, an activating molecule on rat natural killer cells, stimulates phosphoinositide turnover and a rise in intracellular calcium.

NKR-P1 is a 60-kDa homodimer expressed on all rat NK cells. Previous studies by others suggest that NKR-P1 may play a role in NK cell activation because antibody to NKR-P1 stimulates the release of granules from NK cells, and anti-NKR-P1 causes redirected lysis by activated NK cells against targets that express FcR. To examine the mechanism of transmembrane signaling by NKR-P1, we studied the rat NK cell line, RNK-16. We here demonstrate that F(ab')2 antibody to NKR-P1 stimulates phosphoinositide turnover and a rise in intracellular calcium within RNK-16 cells. The response is augmented by cross-linking the F(ab')2 antibody. The phosphoinositide/calcium pathway is also stimulated by NKR-P1 in activated rat NK cells, although no response is detectable in polymorphonuclear cells, which also express NKR-P1. We also demonstrate that RNK-16 cells kill the anti-NKR-P1 (3.2.3) hybridoma and that exposure to the hybridoma target cells stimulates phosphoinositide turnover in RNK-16 cells. Both killing and phosphoinositide turnover are inhibited by F(ab')2 anti-NKR-P1, implicating NKR-P1 in both responses. In contrast, neither cytotoxicity nor phosphoinositide turnover is appreciably blocked by F(ab')2 anti-NKR-P1 in response to YAC-1 targets. Thus, with either target, killing is linked to phosphoinositide turnover, but killing of YAC-1 involves pathways that differ from those that direct killing of the anti-NKR-P1 hybridoma. Our studies support the hypothesis that NKR-P1 may serve as an activating cell-surface receptor on NK cells, and they clarify the mechanisms by which it activates NK cells.

Natural killing of xenogeneic cells mediated by the mouse Ly-49D receptor.

NK lymphocytes lyse certain xenogeneic cells without prior sensitization. The receptors by which NK cells recognize xenogeneic targets are largely uncharacterized but have been postulated to possess broad specificity against ubiquitous target ligands. However, previous studies suggest that mouse NK cells recognize xenogeneic targets in a strain-specific manner, implicating finely tuned, complex receptor systems in NK xenorecognition. We speculated that mouse Ly-49D, an activating NK receptor for the MHC I ligand, H2-Dd, might display public specificities for xenogeneic target structures. To test this hypothesis, we examined the lysis of xenogeneic targets by mouse Ly-49D transfectants of the rat NK cell line RNK-16 (RNK. Ly-49D). Of the xenogeneic tumor targets tested, RNK.Ly-49D, but not untransfected RNK-16, preferentially lysed tumor cells derived from Chinese hamsters and lymphoblast targets from rats. Ly-49D-dependent recognition of Chinese hamster cells was independent of target N-linked glycosylation. Mouse Ly-49D also specifically stimulated the natural killing of lymphoblast targets derived from wild-type and MHC-congenic rats of the RT1lv1 and RT1l haplotypes, but not of the RT1c, RT1u, RT1av1, or RT1n haplotypes. These studies demonstrate that Ly-49D can specifically mediate cytotoxicity against xenogeneic cells, and they suggest that Ly-49D may recognize xenogeneic MHC-encoded ligands.