Listeria monocytogenes infection differentially affects expression of ligands for NK cells and NK cell responses, depending on the cell type infected

The pivotal role of NK cells in viral infection is extensively studied, whereas the role of NK cells in bacterial infection has been poorly investigated. Here, we have examined how Listeria monocytogenes (LM) affects expression of ligands for NK cell receptors and subsequent NK cell responses, depending on the type of cell infected. LM infected rat cell lines derived from different tissues were coincubated with splenic NK cells, and NK cell proliferation and IFN-γ production were measured. In addition, expression of ligands for the NK cell receptors Ly49 and NK cell receptor protein 1 (NKR-P1), MHC class I and C-type lectin-related molecules, respectively, was assessed. Infected pleural R2 cells, but not epithelium-derived colon carcinoma cell line CC531 cells, induced proliferation of NK cells. Reporter cells expressing the inhibitory NKR-P1G receptor or the activating NKR-P1F receptor were less stimulated under incubation with infected CC531 cells versus uninfected CC531 controls, suggesting that the ligand(s) in question were down-regulated by infection. Conversely, LM infection of R2 cells did not affect reporter cell stimulation compared with uninfected R2 controls. We characterized a rat monocyte cell line, termed RmW cells. In contrast to LM infected R2 cells that up-regulate MHC class I molecules, RmW cells displayed unchanged MHC class I expression following infection. In line with MHC class I expression, more NK cells produced a higher amount of IFN-γ against infected R2 cells compared with RmW cells. Together, L. monocytogenes infection may variously regulate cellular ligands for NK cells, depending on the cell type infected, affecting the outcome of NK cell responses.

Rat acute GvHD is Th1 driven and characterized by predominant donor CD4+ T-cell infiltration of skin and gut

Acute graft-versus-host disease (aGvHD) remains a significant hurdle to successful treatment of many hematological disorders. The disease is caused by infiltration of alloactivated donor T cells primarily into the gastrointestinal tract and skin. Although cytotoxic T cells mediate direct cellular damage, T helper (Th) cells differentially secrete immunoregulatory cytokines. aGvHD is thought to be initiated primarily by Th1 cells but a consensus is still lacking regarding the role of Th2 and Th17 cells. The aim of this study was to determine the contribution of distinct T-cell subsets to aGvHD in the rat. aGvHD was induced by transplanting irradiated rats with T-cell-depleted major histocompatibility complex-mismatched bone marrow, followed 2 weeks later by donor lymphocyte infusion. Near complete donor T-cell chimerism was achieved in the blood and lymphatic tissues, in contrast to mixed chimerism in the skin and gut. Skin and gut donor T cells were predominantly CD4⁺, in contrast to T cells in the blood and lymphatic tissues. Genes associated with Th1 cells were upregulated in gut, liver, lung, and skin tissues affected by aGvHD. Increased serum levels of CXCL10 and IL-18 preceded symptoms of aGvHD, accompanied by increased responsiveness to CXCL10 by blood CD4⁺ T cells. No changes in the expression of Th2- or Th17-associated genes were observed, indicating that aGvHD in this rat model is mainly Th1 driven. The rat model of aGvHD could be instrumental for further investigations of donor T-cell subsets in the skin and gut and for exploring therapeutic options to ameliorate symptoms of aGvHD.

Irradiation and bone marrow reconstitution affect the functional Ly49 natural killer cell repertoire in rats

Total body irradiation (TBI) is part of the preconditioning regimen for allogeneic bone marrow transplantation (alloBMT) and the procedure is associated with treatment-related toxicity and delayed immune reconstitution. Natural killer (NK) cells develop and acquire functional competence in close interaction with stromal bone marrow cells that are considered relatively radioresistant compared to the hematopoietic compartment. We thus undertook a study to assess the effect of TBI on the reconstitution of class I MHC-specific Ly49 NK cell receptors in a rat model of alloBMT. In rats subjected to TBI alone or followed by MHC-matched BMT, the irradiation conditioning induced a skewing of the Ly49 repertoire. Specifically, the activating Ly49s3^bright subset exhibited increased frequency and receptor density which correlated with augmented alloreactivity relative to untreated control rats. Our results highlight the plasticity of NK cells and indicate that ionizing radiation (IR) affects the stromal compartment and as a consequence the maturation and functional properties of bone marrow-derived NK cells. These changes lasted throughout the 6 months observation period, showing that irradiation induces long term effects on the generation of the NK cell receptor repertoire.

The early days of NK cells: an example of how a phenomenon led to detection of a novel immune receptor system - lessons from a rat model

In this review, I summarize some of the early research on NK cell biology and function that led to the discovery of a totally new receptor system for polymorphic MHC class I molecules. That NK cells both could recognize and kill tumor cells but also normal hematopoietic cells through expression of MHC class I molecules found a unifying explanation in the “missing self” hypothesis. This initiated a whole new area of leukocyte receptor research. The common underlying mechanism was that NK cells expressed receptors that were inhibited by recognition of unmodified “self” MHC-I molecules. This could explain both the killing of tumor cells with poor expression of MHC-I molecules and hybrid resistance, i.e., that F1 hybrid mice sometimes could reject parental bone marrow cells. However, a contrasting phenomenon termed allogeneic lymphocyte cytotoxicity in rats gave strong evidence that some of these receptors were activated rather than inhibited by recognition of polymorphic MHC-I. This was soon followed by molecular identification of both inhibitory and stimulatory Ly49 receptors in mice and rats and killer cell immunoglobulin-like receptors in humans that could be either inhibited or activated when recognizing their cognate MHC-I ligand. Since most of these receptors now have been molecularly characterized, their ligands and the intracellular pathways leading to activation or inhibition identified, we still lack a more complete understanding of how the repertoire of activating and inhibitory receptors is formed and how interactions between these receptors for MHC-I molecules on a single NK cell are integrated to generate a productive immune response. Although several NK receptor systems have been characterized that recognize MHC-I or MHC-like molecules, I here concentrate on the repertoires of NK receptors encoded by the natural killer cell gene complex and designed to recognize polymorphic MHC-I molecules in rodents, i.e., Ly49 (KLRA) receptors.

Abstract 474: Complete regression and long-term specific protective immune responses obtained in rodent tumor models after intratumoral treatment with LTX-315

LTX-315 is a de novo designed peptide derived from a naturally occurring host defence peptide. LTX-315 has the potential to induce long-term specific protective immune responses by stimulating immune cells, inducing tumor cell lysis with subsequent release of danger signals (e.g. HMGB1) and tumor associated antigens (TAA`s). A complete tumor regression has been obtained in several syngenic rodent tumor models by intratumoral (i.t.) injection with LTX-315. The effect was T- cell- dependent since the intervention was inefficient in immune-deficient animals. Studies on treated tumor tissue confirmed infiltration of immune cells and a switch in the cytokine profile towards a Th1 response. Successfully treated animals were protected against re-challenge with the tumor cell type treated, but not against other types of tumor cells. Moreover, tumor resistance could be adoptively transferred by spleen cells from LTX-315-treated animals. The resistance was abrogated by depletion of T- lymphocytes. Additional studies also indicate that LTX-315`s potential to locally activate the innate immune system by the immunogenic stressing of cells, in addition to the subsequent release of endogenous adjuvants and natural danger signals, provides a strong rationale for using LTX-315 as an adjuvant for vaccines based on tumor-associated antigens (TAA) and for combination with other types of immune–modulatory therapies. LTX-315 is currently being tested in a Phase I dose escalation clinical study and may represents a novel strategy for personalized in situ vaccination against cancer.

Citation Format: Øystein Rekdal, Gunnar Kvalheim, Pål-Dag Line, Bent Rolstad, Ketil Camilio, Gerd Berge, Janne Nestvold, Mengyu Wang, Jihua Shi, Ali Areffard, Baldur Sveinbjørnsson. Complete regression and long-term specific protective immune responses obtained in rodent tumor models after intratumoral treatment with LTX-315 . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 474. doi:10.1158/1538-7445.AM2013-474

Immune reconstitution and graft-versus-host reactions in rat models of allogeneic hematopoietic cell transplantation

Allogeneic hematopoietic cell transplantation (alloHCT) extends the lives of thousands of patients who would otherwise succumb to hematopoietic malignancies such as leukemias and lymphomas, aplastic anemia, and disorders of the immune system. In alloHCT, different immune cell types mediate beneficial graft-versus-tumor (GvT) effects, regulate detrimental graft-versus-host disease (GvHD), and are required for protection against infections. Today, the “good” (GvT effector cells and memory cells conferring protection) cannot be easily separated from the “bad” (GvHD-causing cells), and alloHCT remains a hazardous medical modality. The transplantation of hematopoietic stem cells into an immunosuppressed patient creates a delicate environment for the reconstitution of donor blood and immune cells in co-existence with host cells. Immunological reconstitution determines to a large extent the immune status of the allo-transplanted host against infections and the recurrence of cancer, and is critical for long-term protection and survival after clinical alloHCT. Animal models continue to be extremely valuable experimental tools that widen our understanding of, for example, the dynamics of post-transplant hematopoiesis and the complexity of immune reconstitution with multiple ways of interaction between host and donor cells. In this review, we discuss the rat as an experimental model of HCT between allogeneic individuals. We summarize our findings on lymphocyte reconstitution in transplanted rats and illustrate the disease pathology of this particular model. We also introduce the rat skin explant assay, a feasible alternative to in vivo transplantation studies. The skin explant assay can be used to elucidate the biology of graft-versus-host reactions, which are known to have a major impact on immune reconstitution, and to perform genome-wide gene expression studies using controlled combinations of minor and major histocompatibility between the donor and the recipient.

Identification of an MHC class I ligand for the single member of a killer cell lectin-like receptor family, KLRH1

Natural killer cells are able to recognize and kill target cells according to differences in MHC class I expression. In rodents, the Ly49 receptors are primarily responsible for this MHC differentiation. We previously described the cloning of a novel C-type lectin-like receptor, KLRH1, encoded in the NK complex adjacent to the Ly49 genes and expressed by subsets of NK and NKT cells. MHC influence on selection of KLRH1(+) NK cells in congenic strains suggested that KLRH1 may have an MHC ligand, although we were unable to identify any such ligand. In this study, we have used a sensitive reporter system and Fc fusion protein to demonstrate that KLRH1 binds specifically to the classical MHC class I molecule RT1-A2 of the RT1(n) haplotype. Cytolytic activity of KLRH1-transfected RNK-16 cells was also inhibited by target cells expressing RT1-A2(n). Thus, KLRH1 represents a novel family of MHC allele-specific inhibitory receptors expressed by NK cells.

Abstract LB-161: A complete regression and therapeutic vaccination against aggressive malignant rat mesenchymal tumors with “stemness” characteristics obtained by intratumoral injection of LTX-315 (Oncopore®)

Background: In order to develop more effective anticancer agents there is a shift from not only treating the bulk of cells in a tumor but also to target the self renewing cancer stem cells (CSCs). Standard treatment with chemotherapy and radiotherapy is unable to eradicate CSCs and therefore other treatment modalites against CSCs are needed. LTX-315 (Oncopore®) is a chemically modified cytolytic peptide which is equally active against drug-sensitive and drug-resistant cancer cells. Earlier animal studies have demonstrated that treatment of syngenic murine A20 B-lymphomas and CT26WT carcinomas with intratumoral (i.t.) injection with LTX-315 resulted in complete tumor regression. The present study was undertaken to investigate whether LTX-315 induced an antitumor response in vivo in a novel malignant rat mesenchymal tumor model with “stemness” properties. Methods: Following long-term culturing of bone marrow-derived rat mesenchymal cells, a transformed mesenchymal cell line (rTMSC) has been been developed. rTMSC has, as the non-malignant rMSC, maintained osteogenic and adipose differentation properties and side population cells from rTMSC are highly clonogenic both in vitro and in vivo. Previously we have shown that s.c. as well as i.v. or i.p. injection of rTMSCs form immature solid sarcomas both in nude rats as well as in PVG rats. In this study the rTMSCs were marked with a dual reporter gene GFP and Luciferase and a CCD camera was used for dynamic living imaging. rTMSCs (5×105 cells) were subcutaneously inoculated into PVG rats and established tumors treated i.t. with LTX-315. Results: Intratumoral treatment with LTX-315 resulted in a complete regression in the majority of the treated animals. Successfully treated mice were protected against s.c. or i.p. re-challenge with rTMSCs. Conclusions: Taken together, our results suggest that LTX-315 treatment induced complete regression and long-term, specific cellular immunity against malignant rat mesenchymal tumor cells with “stemness” properties”. Thus, intratumoral administration of a cytolytic peptide might, in addition to providing local tumor control, confer a novel strategy for therapeutic vaccination against established cancers containing CSCs

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-161. doi:10.1158/1538-7445.AM2011-LB-161

Kinetics of lymphocyte reconstitution after allogeneic bone marrow transplantation: markers of graft-versus-host disease

GVHD causes extensive morbidity and mortality in patients who receive alloHCT. Predictive and reliable markers for GVHD are currently lacking but required to improve the safety and accessibility of alloHCT. We present an experimental rat model of myeloablative total body irradiation and fully mismatched major and minor histoincompatible, T cell-depleted BMT, followed by delayed infusion of donor lymphocytes. This treatment, in contrast to marrow transplantation alone, resulted in severe aGVHD and 100% lethality within 2-6 weeks. We investigated the reconstitution kinetics and phenotypes of donor leukocyte subpopulations as well as the histopathology of selected organs that may correlate with GVHD, with the goal to find potential disease-related markers. We observed histological changes mainly confined to the skin, with degenerative changes in the basal layer. LNs and spleen showed deranged architecture with markedly increased accumulation of lymphocytes, whereas the gut, liver, and lungs appeared normal. Of the lymphocyte markers tested, donor-derived CD62L(+) T cells were markedly decreased in animals suffering from GVHD. Furthermore, we observed peripheral depletion of CD4(+)CD25(hi)FoxP3(+) T(reg), which was in contrast to controls. The relative frequency of these lymphocyte subpopulations in blood may therefore serve as accessible cellular markers of aGVHD. We propose that the animal model presented is instructive for the identification of clinically relevant markers of GVHD, which could improve disease diagnosis and management in alloHCT.

The role of natural killer cells in the defense against Listeria monocytogenes lessons from a rat model

Ly49 receptors in rodents, like killer cell immunoglobulin-like receptors in humans, regulate natural killer (NK) cell activity. Although inhibitory Ly49 receptors clearly recognize classical major histocompatibility complex class I (MHC-I) molecules, the role for the activating Ly49 receptors has been less well understood. Here, we discuss recent data from a rat model for listeriosis. Rats depleted of NK cells, or more specifically the Ly49 receptor-bearing cells, showed increased bacterial loads in their spleen. Athymic nude rats with no functional T cells but increased numbers of Ly49-expressing NK cells were more resistant to infection, indicating a central role of NK cells in early immune defense against Listeria in this species. Listeria infection of macrophages or enteric epithelial cells led to upregulation of MHC-I, including nonclassical (Ib) molecules not regularly recognized by T cells. We have shown that activating Ly49 receptors are more efficiently stimulated when binding to upregulated class Ib antigens on infected cells. From this we postulate that activating Ly49 receptors may have a sentinel function in the early immune response against Listeria in detecting diseased cells 'flagged' by increased MHC-Ib expression.

Expression profiling of major histocompatibility and natural killer complex genes reveals candidates for controlling risk of graft versus host disease

Background

The major histocompatibility complex (MHC) is the most important genomic region that contributes to the risk of graft versus host disease (GVHD) after haematopoietic stem cell transplantation. Matching of MHC class I and II genes is essential for the success of transplantation. However, the MHC contains additional genes that also contribute to the risk of developing acute GVHD. It is difficult to identify these genes by genetic association studies alone due to linkage disequilibrium in this region. Therefore, we aimed to identify MHC genes and other genes involved in the pathophysiology of GVHD by mRNA expression profiling.

Methodology/Principal Findings

To reduce the complexity of the task, we used genetically well-defined rat inbred strains and a rat skin explant assay, an in-vitro-model of the graft versus host reaction (GVHR), to analyze the expression of MHC, natural killer complex (NKC), and other genes in cutaneous GVHR. We observed a statistically significant and strong up or down regulation of 11 MHC, 6 NKC, and 168 genes encoded in other genomic regions, i.e. 4.9%, 14.0%, and 2.6% of the tested genes respectively. The regulation of 7 selected MHC and 3 NKC genes was confirmed by quantitative real-time PCR and in independent skin explant assays. In addition, similar regulations of most of the selected genes were observed in GVHD-affected skin lesions of transplanted rats and in human skin explant assays.

Conclusions/Significance

We identified rat and human MHC and NKC genes that are regulated during GVHR in skin explant assays and could therefore serve as biomarkers for GVHD. Several of the respective human genes, including HLA-DMB, C2, AIF1, SPR1, UBD, and OLR1, are polymorphic. These candidates may therefore contribute to the genetic risk of GVHD in patients.

The rat NK cell receptors Ly49s4 and Ly49i4 recognize nonclassical MHC-I molecules on Listeria monocytogenes-infected macrophages

Ly49 receptors in rodents, like KIRs in humans, regulate NK cell activity. Although inhibitory Ly49 receptors clearly recognize MHC-I molecules, ligands for the activating Ly49 receptors are less well defined. Here, we show that the activating Ly49s4 and the inhibitory Ly49i4 receptors recognize nonclassical MHC-I molecules on the rat macrophage cell line R2 (RT1(d)). Listeria infection of R2 macrophages led to increased expression of classical and nonclassical MHC-I molecules. Coincubation of these infected cells with reporter cells expressing Ly49i4 or Ly49s4 increased the reporter cell responses. These responses were blocked by mAb OX18 (anti-MHC-I) and AAS1 (anti-nonclassical MHC-I). IFN-γ treatment of normal R2 cells also increased the MHC-I expression and enhanced the reporter cell responses. These results suggest that activating and inhibitory Ly49 receptors monitor MHC-I expression on Listeria-infected cells.

Mycoplasma contamination revisited: mesenchymal stromal cells harboring Mycoplasma hyorhinis potently inhibit lymphocyte proliferation in vitro

Background

Mesenchymal stromal cells (MSC) have important immunomodulatory effects that can be exploited in the clinical setting, e.g. in patients suffering from graft-versus-host disease after allogeneic stem cell transplantation. In an experimental animal model, cultures of rat T lymphocytes were stimulated in vitro either with the mitogen Concanavalin A or with irradiated allogeneic cells in mixed lymphocyte reactions, the latter to simulate allo-immunogenic activation of transplanted T cells in vivo. This study investigated the inhibitory effects of rat bone marrow-derived MSC subsequently found to be infected with a common mycoplasma species (Mycoplasma hyorhinis) on T cell activation in vitro and experimental graft-versus-host disease in vivo.

Principal Findings

We found that M. hyorhinis infection increased the anti-proliferative effect of MSC dramatically, as measured by both radiometric and fluorimetric methods. Inhibition could not be explained solely by the well-known ability of mycoplasmas to degrade tritiated thymidine, but likely was the result of rapid dissemination of M. hyorhinis in the lymphocyte culture.

Conclusions

This study demonstrates the potent inhibitory effect exerted by M. hyorhinis in standard lymphocyte proliferation assays in vitro. MSC are efficient vectors of mycoplasma infection, emphasizing the importance of monitoring cell cultures for contamination.

Listeria monocytogenes infection affects a subset of Ly49-expressing NK cells in the rat

NK cells are protective against certain bacterial and viral infections, and their production of IFN-γ is important for the early innate immune defence against L. monocytogenes. We have previously shown that depletion of NK cells in rats leads to increased bacterial burden upon L. monocytogenes infection, and that a subset of NK cells encompassing the majority of Ly49 receptors (Ly49s3⁺ NK cells) contributed to this effect. In this study, we have further investigated how the Ly49s3⁺ NK cell subset is affected by L. monocytogenes infection. We observed an increased percentage of Ly49s3⁺ NK cells in the spleen and a reduction in the bone marrow within the first 48 hrs of L. monocytogenes infection. Concomitantly, we observed increased expression levels of the inflammatory chemokine receptors CCR5 and CXCR3 by Ly49s3⁺ bone marrow NK cells, as compared to Ly49s3⁻ NK cells, suggesting involvement of Ly49s3⁺ NK cells in the early phase of infection. However, NK cell production of IFN-γ was independent of Ly49 receptor expression. Furthermore, we observed increased expression levels of MHC class I molecules on both macrophages and NK cells during the first 48 hrs of infection, paralleled by a reduction in the surface expression of Ly49s3 on NK cells. In conclusion, L. monocytogenes infection modulates the tissue distribution of Ly49s3⁺ NK cells, and induces increased MHC class I expression and hence reduced surface expression of Ly49 receptors on NK cells. These changes indicate that L. monocytogenes infection may have multiple effects on NK cells in vivo, and suggests the involvement of Ly49-expressing NK cells in the immune responses towards L. monocytogenes.

Partial NK cell tolerance induced by radioresistant host cells in rats transplanted with MHC-mismatched bone marrow

We have studied the effect of radioresistant host cells in inducing tolerance and adaptation of the MHC recognition repertoire of donor-derived NK cells in stem cell allotransplanted (allo-SCT) rats. Sub-lethally irradiated PVG.1AV1 rats (RT1(av1)) were transplanted with bone marrow from fully MHC-mismatched allotype-marked PVG.7B (RT1(c)) rats; MHC-identical PVG (RT1(c)) controls were transplanted in parallel. In the PVG.7B → PVG.1AV1 allogeneic chimeras, NK cells were donor derived and showed partial tolerance toward host cells. Allogeneic chimeras failed to efficiently reject PVG.1AV1 cells by an NK-mediated mechanism in vivo (allogeneic lymphocyte cytotoxicity), and IL-2-cultured NK cells derived from these chimeras showed diminished cytolytic activity against PVG.1AV1 cells in vitro. There were corresponding changes in the phenotype and function of the highly alloreactive Ly49i2(+) NK cells, which are specifically inhibited by a donor MHC class I ligand, RT1-A1(c). The ligand-negative host MHC haplotype apparently induced expression of a second uncharacterized inhibitory MHC receptor responsible for the partial tolerance toward host-derived cells, along with a modest increase in Ly49i2 receptor levels. The host MHC haplotype did not induce a general hyporesponsiveness in Ly49i2(+) NK cells, which showed normal activation responses in a panel of MHC congenic strains. The data suggest that the MHC constitution of radiation-resistant host cells can have permanent, albeit not fully tolerogenic, effects on the development of a functional NK repertoire following allo-SCT.

The activating rat Ly49s5 receptor responds to increased levels of MHC class Ib molecules on Listeria monocytogenes-infected enteric epithelial cells

We have investigated whether rat Ly49 receptors can monitor Listeria-infected intestinal epithelial cells through altered expression of MHC class I molecules. The rat colon carcinoma epithelial cell line CC531 infected with Listeria expressed higher levels of both classical and nonclassical MHC-I molecules. Reporter cells expressing the activating Ly49s5 receptor displayed increased stimulatory responses when incubated with Listeria-infected CC531 cells in vitro, which could be blocked with mAb 8G10 specific for nonclassical MHC-I molecules of the RT1(u) haplotype, but not with mAb OX18 reacting with classical MHC-I molecules in this haplotype. Similar responses were observed against IFN-γ-treated cells that also upregulated their expression of MHC-I molecules. Thus, the Ly49s5 receptor can respond to increased levels of nonclassical MHC-I molecules induced on target cells by either bacterial infection or cytokine stimulation. We furthermore found that splenic NK and NKT cells produced IFN-γ in response to Listeria-infected CC531 cells, and that this was not limited to Ly49-expressing cells, since similar levels of IFN-γ production were observed in Ly49(+) and Ly49(-) NK cell subsets. Therefore, NK cells may recognize Listeria-infected cells through both MHC-I-dependent and -independent innate immune receptor systems.

Two complementary rat NK cell subsets, Ly49s3+ and NKR-P1B+, differ in phenotypic characteristics and responsiveness to cytokines

Two major subsets of rat NK cells can be distinguished based on their expression of the Ly49s3 or the NKR-P1B lectin-like receptor. Ly49s3(+) NK cells, but not NKR-P1B(+) NK cells, express a wide range of Ly49 receptors. Here, we have examined differences between these two subsets in their expression of certain NK cell-associated molecules as well as their responses to cytokines. A microarray analysis suggested several differentially expressed genes, including preferential expression of NKG2A/C receptors by NKR-P1B(+) NK cells. This was confirmed by staining with tetramers of RT.BM1, the putative ligand of CD94/NKG2, indicating that Ly49 and CD94/NKG2 receptors separate into distinct NK cell compartments. Further, expression of CD25 by Ly49s3(+) NK cells was associated with more rapid proliferation in response to IL-2 as compared with NKR-P1B(+) NK cells. Thus, certain inflammatory situations may preferentially expand the Ly49s3(+) NK cells. Moreover, freshly isolated Ly49s3(+) and NKR-P1B(+) NK cells produce similar amounts of cytokines, and a minor Ly49s3(-)NKR-P1B(-) double-negative NK subset appears to be hyporesponsive based on its significantly lower IFN-gamma production. Collectively, our data demonstrate divergent profiles of NKR-P1B(+) and Ly49s3(+) NK cells, indicating distinct tasks in vivo.

The role of natural killer cells in resistance to the intracellular bacterium Listeria monocytogenes in rats

We have investigated the influence of early innate immune resistance mechanisms on infection with the intracellular bacterium Listeria monocytogenes in rats. Rats were injected i.v. with various amounts of Listeria and the number of bacterial colonies in the spleen was determined at different time points after infection. A bacterial dose as low as 2 x 10(4) cells gave reproducible infection within the spleen. Athymic nude rats lacking normal T cells but with a robust NK cell repertoire for MHC antigens were more resistant to bacterial replication within the spleen than were normal littermate rats and eliminated the infection within 3 days. In vivo depletion of NK cells, or NK subpopulations expressing Ly49 receptors, increased the bacterial load in the spleen, indicating that these cells were important in the initial control of Listeria infection. An increased frequency of Ly49 expressing NK cells in Listeria-infected rats further supported this notion. As several rat strains, unlike mice, display a large repertoire of MHC-recognizing activating Ly49 receptors, these observations raise the interesting possibility that NK cells may recognize alterations in the MHC-I molecules on Listeria-infected cells leading to their elimination before the adaptive immune system comes into play.

Pectic polysaccharides from Biophytum petersianum Klotzsch, and their activation of macrophages and dendritic cells

The Malian medicinal plant Biophytum petersianum Klotzsch (Oxalidaceae) is used as a treatment against various types of illnesses related to the immune system, such as joint pains, inflammations, fever, malaria, and wounds. A pectic polysaccharide obtained from a hot water extract of the aerial parts of B. petersianum has previously been reported to consist of arabinogalactans types I and II (AG-I and AG-II), probably linked to a rhamnogalacturonan backbone. We describe here further structural characteristics of the main polysaccharide fraction (BP1002) and fractions obtained by enzymatic degradations using endo-alpha-d-(1-->4)-polygalacturonase (BP1002-I to IV). The results indicate that in addition to previously reported structures, rhamnogalacturan type II and xylogalacturonan areas appear to be present in the pectic polymer isolated from the plant. Atomic force microscopy confirmed the presence of branched structures, as well as a polydisperse nature. We further tested whether the BP1002 main fraction or the enzymatically degraded products could induce immunomodulating activity through stimulation of subsets of leukocytes. We found that macrophages and dendritic cells were activated by BP1002 fractions, while there was little response of T cells, B cells, and NK cells. The enzymatic treatment of the BP1002 main fraction gave important information on the structure-activity relations. It seems that the presence of rhamnogalacturonan type I is important for the bioactivity, as the bioactivity decreases with the decreased amounts of rhamnose, galactose, and arabinose. The demonstration of bioactivity by the plant extracts might indicate the mechanisms behind the traditional medical use of the plant.

Impact of MHC mismatch and freezing on bone graft incorporation: an experimental study in rats

Cortical bone graft failure develops for poorly defined reasons, and the effects of the immune responses on the incorporation of an allograft are less clear. In a rat model of tibial allotransplantation, we have studied biometric and histological changes of the graft and the humoral immune response against it. We have also compared fresh with prefrozen grafts to study putative effects of freezing on the healing of the graft and the immune response against it. Fresh and frozen cortical bone grafts matched or mismatched for major histocompatibility complex antigens (syngeneic and allogeneic grafts) were implanted in an 8-mm segmental defect in the tibia. The construct was stabilized with intramedullary nailing. Incorporation of the graft was assessed with use of conventional radiography, micro computed tomography (CT(, biomechanical testing and histological examination. The immune response was evaluated by monitoring distribution of leukocytes in the blood and by measuring antibodies in a tailor-made fluorescence activating cell scanning (FACS( analysis. We found that the fresh syngeneic grafts were well integrated after 8 weeks with intact bone cells. In the fresh allogeneic grafts, all cells were dead with radiological signs of resorption, and mechanical testing indicated failure of incorporation. The frozen grafts showed poorer overall reconstruction than the fresh syngeneic grafts, but the incorporation was better than the fresh allogeneic grafts. A measurable alloantibody response was only detected after fresh allografting. The combined results suggest that freezing of bone allograft impedes the antibody response against major histocompatibility complex (MHC( antigens and improves incorporation, but frozen allografts still perform poorer than do frozen syngeneic grafts.

Polysaccharides with complement fixing and macrophage stimulation activity from Opilia celtidifolia, isolation and partial characterisation

AIM OF THE STUDY

The present study is aimed to determine the bioactivity and structure of polysaccharides present in the leaves from the Malian medicinal plant Opilia celtidifolia [Guill. & Perr. Endl. ex Walp (Opiliaceae)].

MATERIALS AND METHODS

The polysaccharides from the leaves of Opilia celtidifolia were isolated from water extracts of the leaves using gelfiltration and anion exchange chromatography giving the fractions Oc50A1 and Oc50A2. Monosaccharide composition was determined by gas chromatography of the derived TMS-derivatives of the methyl-glycosides. Linkages were determined of the partly methylated, partly acetylated alditol acetates obtained after a process including reduction, methylation, hydrolysis, reduction and acetylation followed by GC-MS. Effects on the complement system and the macrophages were determined using specific methods aimed for studying those activities.

RESULTS

The polysaccharide fractions isolated from the leaves of Opilia celtidifolia has high complement fixing activity and induce nitrite oxide release from macrophages in a dose dependent manner. The fractions had an ICH50 of 0.5 and 0.9 microg/ml respectively in the complement fixing assay. They induced the release of 7.2 and 7.3 microM of nitrite oxide from macrophages respectively at a dose of 100 microg/ml. The monosaccharide composition in Oc50A1 and Oc50A2, analysed, showed the presence of arabinose (26.7 and 13.2%), galactose (31.5 and 28%) and galacturonic acid (5.3 and 7.8%) respectively. The Yariv test confirmed the presence of arabinogalactan type II in both fractions. Structural analyses did also show the presence of terminal and 1-4 linked galacturonic acid and terminal and 1-2 linked rhamnose. Endo-polygalacturonanase treatment was performed to isolate the heavily substituted parts of the polysaccharides. These parts contained the same monosaccharides in similar proportion, and showed stronger dose dependent complement fixing activity and also stimulated macrophages to release nitrite oxide.

CONCLUSIONS

The leaves of Opilia celtidifola contains polysaccharides of pectic type that have both complement fixing and macrophage stimulating activity.

Strain-dependent expression of four structurally related rat Ly49 receptors; correlation with NK gene complex haplotype and NK alloreactivity

Natural killer (NK) cells from certain rat strains promptly kill MHC allogeneic lymphocytes in vivo, a rejection phenomenon termed allogeneic lymphocyte cytotoxicity (ALC). ALC can be reproduced in vitro, and is preferentially mediated by a subset of NK cells expressing the Ly49 stimulatory receptor 3 (Ly49s3) in PVG strain rats. Functional studies have suggested that Ly49s3 triggers NK cell alloreactivity, but its importance relative to other Ly49 receptors has not been investigated. In this study, we have characterized three rat Ly49 receptors with close sequence similarity to Ly49s3 in the extracellular region, i.e., Ly49s4, Ly49 inhibitory receptor 3 (Ly49i3), and Ly49i4. Similar to Ly49s3, Ly49s4 mediated cellular activation while Ly49i4 inhibited NK cytolytic function. Ly49s4, -i3, and -i4 all reacted with a previously described anti-Ly49s3 monoclonal antibody (mAb) (DAR13), but not a novel mAb (STOK6), which was shown to be specific for Ly49s3. Expression of these Ly49 receptors varied markedly between inbred strains, in patterns related to their NK gene complex (NKC) haplotype, and ability to mediate ALC. Three major groups of NKC haplotypes could be discerned by restriction fragment length polymorphism analysis. Ly49s3 was present in strains from one of the groups, which corresponded with the "high" ALC responders. Ly49s3 surface expression was also markedly reduced in the presence of its putative MHC class Ib ligand(s) in MHC congenic strains. These data support the notion that Ly49s3 functions as a triggering MHC receptor both in vitro and in vivo. MHC ligands for the other three Ly49 receptors remain to be determined.

The Novel Inhibitory NKR-P1C Receptor and Ly49s3 Identify Two Complementary, Functionally Distinct NK Cell Subsets in Rats

The proximal region of the NK gene complex encodes the NKR-P1 family of killer cell lectin-like receptors which in mice bind members of the genetically linked C-type lectin-related family, while the distal region encodes Ly49 receptors for polymorphic MHC class I molecules. Although certain members of the NKR-P1 family are expressed by all NK cells, we have identified a novel inhibitory rat NKR-P1 molecule termed NKR-P1C that is selectively expressed by a Ly49-negative NK subset with unique functional characteristics. NKR-P1C(+) NK cells efficiently lyse certain tumor target cells, secrete cytokines upon stimulation, and functionally recognize a nonpolymorphic ligand on Con A-activated lymphoblasts. However, they specifically fail to kill MHC-mismatched lymphoblast target cells. The NKR-P1C(+) NK cell subset also appears earlier during development and shows a tissue distribution distinct from its complementary Ly49s3(+) subset, which expresses a wide range of Ly49 receptors. These data suggest the existence of two major, functionally distinct populations of rat NK cells possessing very different killer cell lectin-like receptor repertoires.

Bioactive pectic polysaccharides from Glinus oppositifolius (L.) Aug. DC., a Malian medicinal plant, isolation and partial characterization

Glinus oppositifolius (L.) Aug. DC. (Aizoaceae) is a Malian medicinal plant used against various types of illnesses related to the immune response, like joint pains, inflammations, fever, malaria and wounds. Two pectin type polysaccharides, GOA1 and GOA2, being isolated from a 50 degrees C water extract from the aerial parts of Glinus oppositifolius were investigated for their activity towards the complement system and different leukocyte subsets because of the assumed effects on conditions related to the immune system. The polysaccharide polymer in GOA1 was shown to contain considerable amounts of the neutral sugars arabinose (26.4 mol%) and galactose (42.9 mol%), and methylation analysis indicated the presence of arabinogalactans type I (AG-I) and type II (AG-II). GOA2 was rich in galacturonic acid (68.3 mol%), along with rhamnose, arabinose and galactose. Structural studies indicated that rhamnose and galacturonic acid might constitute a rhamnogalacturonan backbone, often found in pectic substances, with side chains consisting of arabinose and galactose. Both GOA1 and GOA2 were shown to exhibit potent dose-dependent complement fixating activities, and induced chemotaxis of macrophages, T cells and NK cells.

Expression of regulator of G protein signalling proteins in natural killer cells, and their modulation by Ly49A and Ly49D

The small GTPase accelerators regulator of G protein signalling (RGS) proteins are important regulators of proximal signalling from G protein coupled receptors. Although natural killer (NK) cells express a number of G-protein coupled receptors, expression of RGS proteins has not been investigated. We analysed the expression of RGS proteins in rat NK cells, and detected mRNA for RGS1, RGS2, RGS5, RGS8, RGS16, and RGS18. Interestingly, when we included a panel of different leucocyte subsets, we found that RGS8 was selectively expressed by NK cells. NK cells are under control of both activating and inhibitory receptors and, utilizing a xenogeneic system where the mouse activating Ly49D or inhibitory Ly49A receptors were transfected into the rat RNK-16 cell line, the potential regulation of RGS proteins by single NK cell receptors was studied. We found that ligation of Ly49D led to a rapid and transient increase in message for RGS2, while Ly49A ligation up-regulated RGS2, RGS16, and RGS18 mRNA. Both receptors also induced a prolonged increase in RGS2 endogenous protein levels. These findings suggest that RGS proteins may be influenced by or involved in NK cell receptor events, suggesting a crosstalk between G-protein coupled receptors and NK cell receptors.

Two structurally related rat Ly49 receptors with opposing functions (Ly49 stimulatory receptor 5 and Ly49 inhibitory receptor 5) recognize nonclassical MHC class Ib-encoded target ligands

The Ly49 family of lectin-like receptors in rodents includes both stimulatory and inhibitory members. Although NK alloreactivity in mice is regulated primarily by inhibitory Ly49 receptors, in rats activating Ly49 receptors are equally important. Previous studies have suggested that activating rat Ly49 receptors are triggered by polymorphic ligands encoded within the nonclassical class Ib region of the rat MHC, RT1-CE/N/M, while inhibitory Ly49 receptors bind to widely expressed classical class Ia molecules encoded from the RT1-A region. To further investigate rat Ly49-mediated regulation of NK alloreactivity, we report in this study the identification and characterization of two novel paired Ly49 receptors that we have termed Ly49 inhibitory receptor 5 (Ly49i5) and Ly49 stimulatory receptor 5 (Ly49s5). Using a new mAb (mAb Fly5), we showed that Ly49i5 is an inhibitory receptor that recognizes ligands encoded within the class Ib region of the u and l haplotypes, while the structurally related Ly49s5 is an activating receptor that recognizes class Ib ligands of the u haplotype. Ly49s5 is functionally expressed in the high NK-alloresponder PVG strain, but not in the low alloresponder BN strain, in which it is a pseudogene. Ly49s5 is hence not responsible for the striking anti-u NK alloresponse previously described in BN rats (haplotype n), which results from repeated alloimmunizations with u haplotype cells. The present studies support the notion of a complex regulation of rat NK alloreactivity by activating and inhibitory Ly49 members, which may be highly homologous in the extracellular region and bind similar class Ib-encoded target ligands.

Phenotype and natural killer cell sensitivity of a radiation-induced acute T-cell leukaemia (Roser leukaemia) in PVG rats

A radiation-induced T-cell leukaemia [Roser leukaemia (RL)] in the rat was conditioned for growth in vitro by repeated in vivo-in vitro passages. This in vitro cell line, termed RL-T, maintained its leukaemia-inducing property when transferred to syngeneic PVG rats. It expresses several T-cell markers and the T-cell alpha/beta receptor-CD3 complex. RL-T, furthermore, expresses major histocompatibility complex (MHC) I antigens, both classical (RT1.A) and nonclassical (RT1.C), which makes it susceptible to killing by alloreactive natural killer cells in vitro.

cDNA cloning of a rat orthologue of SH2D2A encoding T-cell-specific adaptor protein (TSAd): expression in T and NK cells

The T-cell-specific adapter protein (TSAd), encoded by the SH2D2A gene, has been implicated in modulation of proximal signaling events as well as in transcriptional regulation in human T cells. We have isolated its rat homologue ( rSH2D2A) from an NK cell cDNA library and mapped the corresponding gene to chromosome 2 with a hamster-rat radiation hybrid cell panel. rSH2D2A encodes a 376 amino acid protein (rTSAd) which shows greater homology to mouse than human TSAd. In rats, rTSAd was specifically expressed by NK cells and T cells but not by other leukocytes tested. Similarly, in humans we observed abundant transcripts for TSAd in NK cells and T cells. The data suggest that TSAd may have a regulatory role in cellular activation of T and NK cells.

Activating and inhibitory Ly49 receptors modulate NK cell chemotaxis to CXC chemokine ligand (CXCL) 10 and CXCL12

NK cells can migrate into sites of inflammatory responses or malignancies in response to chemokines. Target killing by rodent NK cells is restricted by opposing signals from inhibitory and activating Ly49 receptors. The rat NK leukemic cell line RNK16 constitutively expresses functional receptors for the inflammatory chemokine CXC chemokine ligand (CXCL)10 (CXCR3) and the homeostatic chemokine CXCL12 (CXCR4). RNK-16 cells transfected with either the activating Ly49D receptor or the inhibitory Ly49A receptor were used to examine the effects of NK receptor ligation on CXCL10- and CXCL12-mediated chemotaxis. Ligation of Ly49A, either with Abs or its MHC class I ligand H2-D(d), led to a decrease in chemotactic responses to either CXCL10 or CXCL12. In contrast, Ly49D ligation with Abs or H2-D(d) led to an increase in migration toward CXCL10, but a decrease in chemotaxis toward CXCL12. Ly49-dependent effects on RNK-16 chemotaxis were not the result of surface modulation of CXCR3 or CXCR4 as demonstrated by flow cytometry. A mutation of the Src homology phosphatase-1 binding motif in Ly49A completely abrogated Ly49-dependent effects on both CXCL10 and CXCL12 chemotaxis, suggesting a role for Src homology phosphatase-1 in Ly49A/chemokine receptor cross-talk. Ly49D-transfected cells were pretreated with the Syk kinase inhibitor Piceatannol before ligation, which abrogated the previously observed changes in migration toward CXCL10 and CXCL12. Piceatannol also abrogated Ly49A-dependent inhibition of chemotaxis toward CXCL10, but not CXCL12. Collectively, these data suggest that Ly49 receptors can influence NK cell chemotaxis within sites of inflammation or tumor growth upon interaction with target cells.

Sphingosine 1 phosphate induces the chemotaxis of human natural killer cells. Role for heterotrimeric G proteins and phosphoinositide 3 kinases

We have examined the effect of sphingolipids on the chemotaxis of human natural killer (NK) cells. Messenger RNA for Edg-1, Edg-6 and Edg-8 but not Edg-3, are expressed in these cells. Sphingosine 1 phosphate (SPP), dihydro SPP (DHSPP) or the CC chemokine RANTES (CCL5), but not sphingosine induces the chemotaxis of these cells. Pertussis toxin inhibits the chemotaxis induced by these ligands. Permeabilization of NK cells with streptolysin O (SLO) and introduction of blocking antibodies to the heterotrimeric G proteins, showed that Galpha(i2), Galpha(s), Galpha(q/11) or Galpha(13) mediate the chemotaxis of SPP, whereas Galpha(i2), Galpha(o) or Galpha(q/11) mediate the chemotaxis of DHSPP. Galpha(i2), Galpha(o), Galpha(s), Galpha(q/11), Galpha(z) or Galpha(12 )mediates RANTES-induced NK cell chemotaxis. Further analysis showed that phosphoinositide 3 kinase (PI3K) inhibitors wortmannin and LY294002 inhibit NK cell chemotaxis induced by SPP, DHSPP or RANTES. Blocking antibody to PI3Kgamma inhibits the chemotaxis induced by the three ligands, whereas anti-PI3Kbeta was without effect. In contrast, SPP and DHSPP recruit PI3Kbeta isozyme into NK cell membranes, suggesting that although this isoform is not involved in chemotaxis, it is activated by these phospholipids.

Ly49i2 is an inhibitory rat natural killer cell receptor for an MHC class Ia molecule (RT1-A1c)

We have exploited strain-specific differences in the NK allorecognition repertoires to generate rat monoclonal antibodies against receptors involved in the control of allogeneic responses by rat NK cells. The monoclonal antibody STOK2 binds to a homodimeric glycoprotein that has been implicated as an inhibitory receptor for an MHC molecule in the PVG strain. In the present study, we haveidentified this glycoprotein as a novel rat Ly49 receptor (Ly49i2) containing an immunoreceptor tyrosine-based inhibitory motif. Ligation of the Ly49i2 receptor induces inhibitory signals, and Ly49i2 coprecipitates with the inhibitory tyrosine phosphatase SHP-1 in stably transfected RNK-16 cells. Moreover, it inhibits natural killing of lymphoblast targets and transfected fibroblast targets expressingthe classical MHC class Ia allele RT1-A1(c). Ly49i2, therefore, is an inhibitory receptor for specific MHC class Ia molecules, similar to inhibitory members of the mouse Ly49 family.

Characterization of a novel killer cell lectin-like receptor (KLRH1) expressed by alloreactive rat NK cells

NK cells have the ability to recognize and kill MHC-mismatched hemopoietic cells. In the present study, strain-specific differences in the rat NK allorecognition repertoire were exploited to generate Abs against receptors that may be involved in allogeneic responses. A mAb termed STOK9 was selected, and it reacted with subsets of NK cells and NKR-P1(+) T cells from certain rat strains possessing highly alloreactive NK cells. The STOK9(+) NK subset was broadly alloreactive and lysed Con A lymphoblast targets from a range of MHC-mismatched strains. The mAb STOK9 precipitated a 75-kDa dimeric glycoprotein from NK lysates. Expression cloning revealed that each monomer consisted of 231 aa with limited homology to other previously characterized killer cell lectin-like receptors (KLRs). This glycoprotein therefore constitutes a novel KLR branch, and it has been termed KLRH1. A gene in the central region of the natural killer gene complex on rat chromosome 4 encodes KLRH1. A mouse homolog appears to be present as deduced from analyses of genomic trace sequences. The function of KLRH1 is unknown, but it contains an immunoreceptor tyrosine-based inhibitory motif, suggesting an inhibitory function. The MHC haplotype of the host appears to influence KLRH1 expression, suggesting that it may function as an MHC-binding receptor on subsets of NK cells and T lymphocytes.

The athymic nude rat: an animal experimental model to reveal novel aspects of innate immune responses?

Athymic nude rats resemble nude mice in their lack of a normal thymus and functionally mature T cells. They have been useful in the study of mechanisms of tumor growth or graft rejection in immunocompromised hosts since they can accept major histocompatibility complex (MHC) mismatched organ allografts or xenografts for several months and because a number of tumor cell lines of human and rodent origin grow well in these rats. Injection of a few helper T (Th) cells from euthymic littermate rats partly restores the pool of mature T cells as well as full immunocompetence to reject organ allografts and has helped to reveal some of the cell interactions necessary for rejection to occur In contrast, immunologically naive athymic nude rats of certain strains, acutely reject allografts consisting of lymphocytes or bone marrow cells, which is due to the presence of alloreactive natural killer cells. These cells can recognize and kill MHC incompatible hematopoietic cells through the recognition of both mismatches within the classical (RT1.A) and nonclassical (RT1.C/E) MHC class I regions with a repertoire of inhibitory and activating killer lectin-like receptors (KLR) for MHC-I molecules, encoded by the Ly-49 portion of the rat natural killer cell gene complex (NKC). Some of these receptors have been identified and molecularly cloned and show similarities with NK receptors identified in the mouse. Other leukocytes in nude rats, such as dendritic cells, may also contribute to specific innate immune responses in the absence of mature T cells. Nude rats develop T-like cells expressing CD3 and T-cell receptor (TCR) with increasing age. Though their phenotype in peripheral lymphatic tissues resembles that of normal T cells, consisting mainly of CD4+ or CD8+ cells, they lack alloreactivity in vivo and their TCR repertoire is more of an oligoclonal nature. Their contribution to allograft rejection in T-cell-reconstituted rats is therefore questionable, and their role in innate immune response in these rats still enigmatic.

The effect of in vivo depletion of NKR-P1+ or CD8+ lymphocytes on the acute rejection of allogeneic lymphocytes (ALC) in the rat

We have depleted lymphocyte subsets in PVG and AO rats with MoAbs 3.2.3 (against NKR-P1 on NK and NK/T cells) and OX-8 (against CD8 on CTL and NK cells), and examined the effect on the killing of YAC-1 target cells in vitro and the effect on the acute rejection of small allogeneic lymphocytes in vivo (allogeneic lymphocyte cytotoxicity, ALC). While 3.2.3 treatment led to only a partial depletion of 3.2.3-positive cells in PVG rats, this treatment drastically reduced the number of NKR-P1+ cells in AO rats, abolished splenic NK activity against the NK-sensitive tumour target YAC-1, and markedly diminished the ALC response. Rats treated with OX-8 for 1 day showed a similar loss of NK cell function in vivo and in vitro. However, in rats treated with OX-8 for 3 days a 3.2.3+ and OX-8- population consisting of NK cells appeared, restoring ALC. The results demonstrate that NK cell responses can be greatly diminished after in vivo treatment with these MoAbs. Furthermore, they demonstrate that ALC is not necessarily linked to expression of the CD8 molecule.

Release from tonic inhibition of T cell activation through transient displacement of C-terminal Src kinase (Csk) from lipid rafts

In resting peripheral T cells, Csk is constitutively present in lipid rafts through an interaction with the Csk SH2-binding protein, PAG, also known as Cbp. Upon triggering of the T cell antigen receptor (TCR), PAG/Cbp is rapidly dephosphorylated leading to dissociation of Csk from lipid rafts. However, tyrosine phosphorylation of PAG/Cbp resumes after 3--5 min, at which time Csk reassociates with the rafts. Cells overexpressing a mutant Csk that lacks the catalytic domain, but displaces endogenous Csk from lipid rafts, have elevated basal levels of TCR-zeta-chain phosphorylation and spontaneous activation of an NFAT-AP1 reporter from the proximal interleukin-2 promoter as well as stronger and more sustained responses to TCR triggering than controls. We suggest that a transient release from Csk-mediated inhibition by displacement of Csk from lipid rafts is important for normal T cell activation.

Long-term donor chimerism after MHC (RT1) mismatched bone marrow transplantation in the rat: the role of host alloreactive NK cells

We have investigated the role of major histocompatibility complex (MHC) (RT1) disparities in the engraftment of bone marrow (BM) cells after whole body irradiation of rats. Mononuclear BM cells from PVG.RT7.2 (RT1c) rats were injected i.v. into sublethally (10Gy) whole body irradiated PVG (RT1c) rats and RT1 congenic and recombinant PVG rats. Repopulation of the BM, spleen, and blood with donor cells was assessed by FACS analysis of cells labelled with the fluorescein isothiocyanate (FITC)-labelled HIS41 monoclonal antibody (MoAb) against the RT7.2 marker. In RT1 matched (PVG.RT7.2 --> PVG) and RT1-mismatched combinations (PVG.RT7.2 --> PVG.1AV1), where radioresistant host natural killer (NK) cells could not recognize the BM inoculum as foreign, a donor chimerism close to 100% was observed after 6-8 weeks. However, in rat strain combinations where host NK cells could recognize an RT1 mismatch, almost no donor cells survived, and the rats were repopulated with leukocytes of host origin. In intra-MHC recombinant rat strains the element determining rejection or acceptance of the allograft mapped to the RT1-B/D-C/E/M region in PVG.R8 and PVG.R23 rats, in accordance with the patterns of NK alloreactivity in these strain combinations. NK cells may therefore be a primary obstacle to successful allogeneic BM engraftment in this model.

Rat natural killer cell receptor systems and recognition of MHC class I molecules

Rat natural killer (NK) cells recognize MHC-I molecules encoded by both the classical (RT1-A) and non-classical (RT1-C/E/M) MHC class I (MHC-I) regions. We have identified a receptor, the STOK2 antigen, which belongs to the Ly-49 family of killer cell lectin-like receptors, and we have localized the gene encoding it to the rat natural killer cell gene complex. We have also shown that it inhibits NK cytotoxicity when recognizing its cognate MHC-I ligand RT1-A1c on a target cell. This is the first inhibitory Ly-49-MHC-I interaction identified in the rat and highlights the great similarity between rat and mouse Ly-49 receptors and their MHC ligands. However, the mode of rat NK-cell recognition of target cells indicates that positive recognition of allo-MHC determinants, especially those encoded by the RT1-C/E/M region, is a prevalent feature. NK cells recruited to the peritoneum as a consequence of alloimmunization display positive recognition of allodeterminants. In one case, NK cells activated in this way have been shown to be specific for the immunizing, non-classical class I molecule RT1-Eu. These findings show that allospecific NK cells sometimes show features reminiscent of the adaptive immune response.

C1qRp elicits a Ca++ response in rat NK cells but does not influence NK-mediated cytotoxicity

The cell surface receptor C1qRp (receptor for C1q, regulating phagocytosis) present on macrophages and neutrophils, is presumed to stimulate phagocytosis in these cells. However, C1qRp is also present on natural killer (NK) cells, and in these cells its physiological function is not currently known. We have investigated putative functions of this cell surface molecule in rat NK cells with the aid of two novel monoclonal antibodies (MoAb) LOV3 and LOV8 against rat C1qRp. NK cells are known to be potent cytotoxic effector cells, both through specific recognition of ligands on a target cell and killing of antibody-coated target cells (antibody-dependent cellular cytotoxicity, ADCC). NK cells prestimulated with MoAbs LOV3 or LOV8 did not exhibit altered ADCC. Furthermore, the addition of MoAb LOV3 or LOV8 to cytotoxic cultures of NK cells and Fc-receptor positive tumour cells did not affect killing in a redirected killing assay, indicating that the receptor did not influence NK cytotoxicity. However, this is the first paper to show that an intracellular Ca++-response is induced in rat NK cells upon stimulation of C1qRp with LOV3 and LOV8. The response induced by the antibodies was only minimally reduced in the presence of EGTA, indicating that most of the response is owing to the Ca++ mobilization from intracellular calcium stores.

A soluble LAT deletion mutant inhibits T-cell activation: reduced recruitment of signalling molecules to glycolipid-enriched microdomains

The type III transmembrane adaptor protein linker for activation of T cells (LAT) is essential for membrane recruitment of signalling molecules following TCR activation. Here we show that although LAT deleted in the transmembrane domain is completely soluble, it can be tyrosine phosphorylated after anti-CD3 stimulation or pervanadate treatment. Overexpression of this deletion mutant in transiently transfected Jurkat TAg cells inhibits transcriptional activation of nuclear factor of activated T cells (NF-AT)/AP-1 reporter construct in a concentration-dependent manner. Furthermore, by selection of transiently transfected cells, a clear reduction of TCR-induced CD69 expression was observed in cells expressing the mutant. These dominant negative effects seemed to be dependent both on the ability of the membrane deletion mutant to reduce phosphorylation of endogenous LAT and to reduce interaction of endogenous LAT with PLC-gamma1 and Grb2. Consistent with this, the redistribution of PLC-gamma1 and Grb2 to glycolipid-enriched microdomains, called lipid rafts, after stimulation was inhibited when the soluble form of LAT was overexpressed. We suggest that the dominant negative effect is caused by the ability of the mutant to sequester signalling molecules in cytosol and thereby inhibit redistribution of signalling molecules to lipid rafts upon T-cell activation.

Characterization and molecular cloning of rat C1qRp, a receptor on NK cells

Here we report the generation of monoclonal antibodies (mAb) LOV3 and LOV8 to a 110-130-kDa membrane glycoprotein expressed by rat NK cells. This NK surface molecule was identified by eucaryotic expression cloning as the structural orthologue of the phagocytosis-stimulating receptor for complement factor C1q and mannose-binding lectin on human macrophages, C1qRp. Rat C1qRp is a monomeric type I integral membrane protein consisting of 643 amino acids with an N-terminal lectin-like domain, five epidermal growth factor-like domains, a transmembrane domain and a 45-residue cytoplasmic domain. It is encoded by a single gene on rat chromosome 3q41-q42 and is 67% and 87.5% identical at the amino acid level to human and mouse C1qRp, respectively. Rat C1qRp is expressed by resting and by activated NK cells, on subpopulations of NKR-P1(+) T cells (NK/T cells), dendritic cells, macrophages and granulocytes, but not by B cells or NKR-P1(-) T cells. Expression of this innate immune receptor is therefore not restricted to hematopoietic cells of the myeloid lineage, but is also expressed on subsets of cells of lymphoid origin. The mAb did not affect the cytotoxic function of NK cells, and C1qRp on NK cells may have functions not related to NK killing.

[Natural killer cells--important in the immunologic defense]

Natural Killer (NK) cells are a group of lymphocytes with a characteristic morphology and function. They are abundant in blood, spleen, liver and lungs. They are distinct from both T and B lymphocytes in their circulation patterns, profile of surface antigens, receptor repertoire and the way they discriminate between self and nonself. This latter NK function can partly be explained by an array of recently characterised NK receptors that can recognise and accurately discriminate between normal and altered MHC class I determinants. The basis for this discrimination is different from that of T cells and is discussed in this article. The role of NK cells in antimicrobial defense is well demonstrated, particularly that against viruses belonging to the herpesvirus group. A case report of a patient with a selective defect in NK cells and with recurrent viral infections is described. The role of NK cells in defense against malignant cells is more circumstantial, but NK cells do possess receptors which recognise tumour cells and kill them efficiently in vitro. A receptor which can recognise determinants unique for cancer cells has recently been described.

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.

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.