Natural cytotoxicity in rats: strain distribution and genetics

Recognition of Listeria monocytogenes infection by natural killer cells: Towards a complete picture by experimental studies in rats

The study of cellular immune responses in animal disease models demands detailed knowledge of development, function, and regulation of immune cells, including natural killer (NK) cells. Listeria monocytogenes (LM) bacterium has been explored in a large area of research fields, including the host pathogen interaction. Although the importance role of NK cells in controlling the first phase of LM burden has been investigated, the interaction between NK cells and infected cells in details are far from being comprehended. From in vivo and in vitro experiments, we can drive several important pieces of knowledge that hopefully contribute to illuminating the intercommunication between LM-infected cells and NK cells. Experimental studies performed in rats revealed that certain NK cell ligands are influenced in LM-infected cells. These ligands include both classical- and non-classical MHC class I molecules and C-type lectin related (Clr) molecules that are ligands for Ly49- and NKR-P1 receptors respectively. Interaction between these receptors:ligands during LM infection, demonstrated stimulation of rat NK cells. Hence, these studies provided additional knowledge to the mechanisms NK cells utilise to recognise and respond to LM infection outlined in the current review.

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.

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.

Neonatally tolerant rats actively eliminate donor-specific lymphocytes despite persistent chimerism

Rats from the allotype-marked PVG-RT7b and PVG-RT1u-RT7b strains were injected at birth with semi-allogenic F1 bone marrow (BM) cells from athymic nude rats (PVG-rnu/rnu x PVG-RT1u-rnu/rnu) to induce neonatal tolerance. As adults, 97% of the animals accepted donor-specific allogeneic skin grafts and a majority (65%) of rats were chimeric, expressing the major histocompatibility complex class I and allotype marker of the donor strain. Similar results were obtained when PVG-RT1u-RT7b rats were injected at birth with fully allogeneic PVG-rnu/rnu nude BM cells: as adults, 94% accepted donor-specific skin allografts and 76% of recipients were chimeric. Donor derived CD4 T cells, CD8 T cells and B cells were found in low numbers (less than 2%) in peripheral blood of rats made tolerant by F1 BM cells. A large proportion of T cells bore the phenotype of recent thymic emigrants, suggesting that they were newly produced. All the evidence was consistent with clonal deletion tolerance, induced centrally within the thymus. The thymus was chimeric and thymocytes failed to respond in vitro to alloantigens of the donor-specific haplotype; donor-specific skin allografts survived indefinitely on athymic nude recipients reconstituted with CD4+CD8- thymocytes or peripheral CD4 T cells from tolerant animals. The chimeric state was interesting, since the PVG and PVG-RT1u rat strains contain a natural killer (NK) cell system that rapidly eliminates (within 24 h) intravenously injected allogeneic or semi-allogeneic lymphocytes--a phenomenon known as allogeneic lymphocyte cytotoxicity or ALC. When neonatal tolerant rats were tested, the ALC index (a measure of cell killing) was unchanged in nonchimeric tolerant rats and significantly altered (reduced killing), but not abolished in chimeric animals. Hence, the injection of allogeneic BM cells which induced specific tolerance in the T cell population failed to tolerize the NK cell system, allowing the constant killing of newly produced donor-derived lymphocytes and putting at risk the very survival of the allogenic BM cells. This has interesting implications for clinical transplantation.

Control of rat natural killer cell-mediated allorecognition by a major histocompatibility complex region encoding nonclassical class I antigens

The ability of natural killer (NK) cells to eliminate normal allogeneic hemic cells is well established in several species including mice, rats, and humans. The controlling elements for NK susceptibility in these species map to the major histocompatibility complex (MHC), but in contrast to findings in mice and humans, the mode of inheritance is not always recessive in rats. This finding is not easily explained by the missing self and hemopoietic histocompatibility (Hh) models for NK recognition, and has led to the idea that certain alloantigens may trigger NK cell reactivity. In our in vitro system for assessing rat NK alloreactivity, we have employed target and inhibitor cells from a large panel of MHC congenic, intra-MHC recombinant and MHC mutant rat strains, as well as appropriate F1 hybrids between them, and we show the following: (a) The nonclassical class I (RT1.C) region was most important in determining the susceptibility of target cells to alloreactive NK cells in vitro. Lymphocyte susceptibility to lysis in vivo also mapped to the C region, which supports the concept that the in vivo and in vitro alloreactivity assays reflect the same recognition process. (b) Four different RT1-controlled NK allospecificities (represented by the u, l, a, and n haplotypes) could be discerned when we used polyclonal NK cells from the PVG (RT1c) strain as effector cells. Three of the target specificities recognized were controlled mainly by the RT1.C region. (c) The expression of RT1.C region-controlled parental strain NK allodeterminants could be demonstrated in F1 hybrids heterozygous for the C region alone and were therefore inherited nonrecessively. (d) Loss of an RT1.C region-controlled NK allospecificity could be shown with the MHC mutant LEW.1LM1 rat strain characterized by a genomic deletion of about 100 kb of the C region. Taken together, these observations have demonstrated a major importance of the nonclassical class I region, i.e., RT1.C, in controlling rat NK allorecognition, and have thereby assigned a hitherto undescribed immunological property to this region. Furthermore, some of the present data are consistent with the existence of polymorphic NK-triggering alloantigens that are coded for by the RT1.C region.

Allospecific recognition of hemic cells in vitro by natural killer cells from athymic rats: evidence that allodeterminants coded for by single major histocompatibility complex haplotypes are recognized

We have previously shown that large granular lymphocyte (LGL)-enriched cell populations have the capacity to spontaneously recognize and kill allogeneic small lymphocytes and bone marrow cells (BMC) in vitro in certain strain combinations of rats. Here, we have studied the alloreactivity of natural killer (NK) cells from PVG nude (RT1c) rats against a panel of major histocompatibility complex (MHC) incompatible hemic cells. Both lymphocytes and BMC from the AO (RT1u), DA (RT1a), BN (RT1n) as well as the MHC-congenic PVG-RT1u (RT1u) rat strains were efficiently killed in vitro, whereas cells from syngeneic PVG rats were spared. The structures recognized on lymphocytes and BMC were probably similar since the two cell populations inhibited each other in cross-competition experiments. A number of features aligned the alloreactive effector cells with NK cells and not T cells. (a) Only about 5% of the effector cells from nude spleens expressed the T cell antigens CD3, CD5 or T cell receptor (TcR) alpha/beta whereas greater than 50% of the cells expressed markers present on NK cells (CD2, CD8, OX52 and the rat NK cell-specific marker NKR-P1 recognized by the monoclonal antibody 3.2.3). (b) The alloreactive cells were granular since pretreatment of nude spleen cells with the lysosomotropic agent L-leucine methyl ester which eliminated LGL, simultaneously abolished the cytolysis of both allogeneic lymphocytes and YAC-1 tumor cells. (c) Nude spleen cells stimulated with human recombinant interleukin 2 for 1 week in vitro generated large granular proliferating cells which were CD3-, CD5-, TcR alpha/beta-, but greater than 95% 3.2.3+. These cells efficiently killed allogeneic hemic cells from the same rat strains as did freshly isolated effector cells. (d) The cytolysis of allogeneic hemic cells could effectively be inhibited with unlabelled NK-sensitive (YAC-1 and K-562), but not NK-resistant (Roser leukemia) tumor cells. Cross-competition studies showed that PVG nude NK cells discriminated between AO, BN and DA BMC, suggesting that different alloantigens were positively recognized by subsets of NK cells. The mode of inheritance of the allodeterminant specifically recognized on AO BMC was investigated in crosses and backcrosses between AO and BN or DA rats. A gene dosage effect was observed in that this determinant was expressed at a slightly reduced level in F1 hybrids.(ABSTRACT TRUNCATED AT 250 WORDS)

Fidelity of the repertoire in T cell reconstituted athymic nude rats. Preservation of a deficit in alloresponsiveness over one year

A single intravenous injection of a relatively small number of T cells contained in the population of rat thoracic duct lymphocytes (TDL) is sufficient to restore to normal the peripheral T cell pool of athymic PVG.rnu/rnu nude rats. The donor T cells expand greater than 10-15-fold, self-renew, and restore immunocompetency to nude recipients permanently (greater than 2 yr). We asked whether the T cell repertoire was affected by the expansion and self-renewal process. Nude recipients were injected with syngeneic PVG TDL that had been allospecifically depleted (negatively selected) by consecutive passage from blood to thoracic duct lymph through two irradiated (DAxPVG)F1 intermediate rats. Negatively selected TDL were tested before transfer by the P----F1 popliteal LN GVH assay and showed a greater than 90% depletion of specific reactivity to DA alloantigens. Surviving cells or their progeny were recovered from LN or TDL of nude recipients 8 and 12 mo after transfer. The deficit in GVH reactivity to the DA haplotype persisted, but normal GVH activity was demonstrated against a third party (AOxPVG)F1 alloantigen. The "hole" in the repertoire could not be attributed to tolerance induced by the co-transfer of contaminating irradiated F1 TDL. PVG TDL passaged consecutively through (AOxPVG)F1 and (DAxPVG)F1 intermediates and devoid of (AOxPVG)F1 cells remained specifically depleted to both AO and DA haplotypes when recovered from nude recipients 4 and 13 mo later, but displayed GVH activity to a third-party (BNxPVG)F1 alloantigen. Thus the exact specificity of the T cell repertoire of the original inoculum was faithfully maintained in nude recipients throughout the initial phase of rapid expansion and the continued self-renewal of the mature peripheral T cell pool.

Specific inhibition of natural killer (NK) activity against different alloantigens

Allogeneic lymphocyte cytotoxicity (ALC), i.e., rapid rejection of i.v. injected allogeneic lymphocytes in unprimed hosts, is an example of NK activity. Apparently anomalous rejection patterns, such as acceptance of F1 hybrid cells by parental hosts and rejection of parental cells by F1 hybrid hosts in many strain combinations, would fit the hypothesis that the effector cells in ALC recognize the absence of certain self-molecules (passwords) rather than the presence of nonself determinants. However, cold target inhibition studies showed that ALC displays allospecificity: when a mixture of radiolabeled AO and DA cells were injected i.v. into euthymic or athymic PVG rats, adding a surplus of cold DA cells reduced killing only of labeled DA cells and vice versa. Furthermore, semiallogeneic cold target cells were ineffective in inhibiting elimination of fully allogeneic cells, which supports the argument against a modification of the hypothesis that self-determinants inhibit a postbinding stage of lysis. Finally, (DA x AO)F1 cells injected into (DA x PVG)F1 hosts were rapidly rejected, despite the fact that donor and host shared expressed DA determinants. In sum, our results show that a hypothesis based on inhibition of killing by self-determinants can only be sustained with extensive modifications, and favor the alternative mechanism that the effector cells positively recognize the presence of allospecific determinants on the target cell surface.

The rapid elimination of allogeneic lymphocytes: relationship to established mechanisms of immunity and to lymphocyte traffic

Allogeneic lymphocyte cytotoxicity (ALC) refers to the destruction of lymphocyte beginning within a few hours of intravenous injection into non-sensitized, allogeneic recipients. Usually this has been detected in rats and mice by comparing the localization of 51Cr-labelled lymphocytes in the tissues of allogeneic and syngeneic recipients. In a particular strain combination the existence of ALC is supported by deficient localization of allogeneic lymphocytes in the LN, lungs and blood mononuclear population and an excess of the label that had been associated with allogeneic cells in the lymph plasma, blood plasma and kidneys. As the destruction of the allogeneic cells occurs in the lymphatic tissues, especially the spleen, it is paradoxical that there is sometimes an excess of the label associated with allogeneic cells in the spleen but evidence is presented that most of the isotope is no longer associated with living cells in that organ at 24 h after transfer. The data cannot be explained by an altered distribution of allogeneic lymphocytes between different organs. Experiments on the early migration of lymphocytes from the blood of syngeneic and allogeneic recipients point unequivocally to the conclusion that the adhesion of lymphocytes to specialized vascular endothelium in LN and their consequent entry into LN does not require that the lymphocytes and the endothelial cells share MHC products. The characteristics of ALC stressed in this review include the following: 1) it is independent of T-cell activation, either of host T-cells or of donor T-cells: 2) B- and T-cells are about equally vulnerable to ALC; 3) it varies greatly between different strain combinations regardless of other indices of cellular and humoral immunity; 4) F1 hybrid donor cells are vulnerable but the effect is always less marked than with allogeneic cells; 5) ALC is less radio-sensitive than primary cellular or humoral immune responses; 6) adoptive transfer of ALC can be achieved with TDL from nude rats. The possible mechanism underlying ALC has been debated in terms of natural killer cells recognizing certain allo-antigens or alternatively pre-existing "natural" antibody with low affinity for allogeneic cells leading to their elimination by ADCC. The argument hinges on the necessity for antibody and cannot be resolved by current data.

Natural cytotoxicity: early killing of allogeneic lymphocytes in rats

In many strain combinations among inbred rats, intravenously injected 51Cr-labelled lymphocytes are destroyed in substantial numbers by unsensitized allogeneic hosts. Destruction of cells (referred to as natural cytotoxicity (NC)) occurs within a few hours of injection, and is characterised by a decreased accumulation of radioactivity in the lymph nodes and increased renal excretion of label by allogeneic hosts, as compared with the distribution of label in syngeneic recipients of the same cell suspension. An intact spleen is necessary for killing. The level of NC expressed is consistent for a given donor-host combination. Using arbitrary criteria to compare the levels of NC expressed by different donor-host combinations among inbred rats, 13 of 95 strain combinations have been shown to express high NC, 63 intermediate NC and 19 low NC. The level of NC expressed cannot be correlated with the extent to which donor and host differ in respect of known MHC genes. Segregation analysis has shown high NC to be controlled by at least 2 independently segregating genes, one of which is MHC-linked. It is possible to weaken or abrogate NC by the neonatal injection of bone-marrow cells from the donor strain, and to reverse this reduced reactivity by the injection of host strain lymphocytes. The substitution of either the donor (P1) or the host (P2) by the P1 X P2)F1 hybrid reduces or eliminates NC in strain combinations normally expressing high NC. It is currently uncertain whether NC can be augmented. In the single strain combination in which maturation has been studied, NC becomes evident during the 4th week of life and attains adult levels during the 6th-7th weeks. NC is at least partially radio-sensitive. Two groups of reactivities appear to be related to NC: (i) those which have been identified in the context of aberrant lymphocyte homing, and for which allogeneic lymphocytes are the targets; (ii) the group of natural resistance systems which includes NK cells, and whose reactivity is directed against a variety of other target cells.

Natural cytotoxicity in rats: radiation-induced changes in the early killing of allogeneic cells

In some strain combinations among inbred rats intravenously injected 51Cr-labeled lymphocytes are rapidly destroyed in substantial numbers by unsensitized allogeneic hosts. This phenomenon has been referred to as natural cytotoxicity (NC) and is characterized by decreased lymph node radioactivity, increased kidney and urine radioactivity, and to a lesser extent increased liver radioactivity in allogeneic hosts, when compared with the distribution of label in syngeneic recipients of the same cell suspension. A single exposure to 800 rad either 1 or 7 days before the injection of 51Cr-labeled lymphocytes effected a reduction in NC as defined by all the above parameters in a strain combination exhibiting high NC. The same dosage of radiation abolished NC in a strain combination exhibiting intermediate leads to low NC. Because NC was not always completely abolished, the phenomenon was held to be partially radiosensitive. An increased accumulation of 51Cr-labeled lymphocytes was seen in the lymph nodes of both syngeneic and allogeneic irradiated hosts when compared with nonirradiated controls, although the increase was greater in allogeneic than in syngeneic hosts. This increased colonization in the lymph nodes of irradiated hosts seemed unlikely to be due to an increase in the available "space' in the lymph nodes following irradiation.