Suzanne Lemieux

Licensing of natural killer cells by host major histocompatibility complex class I molecules

Self versus non-self discrimination is a central theme in biology from plants to vertebrates, and is particularly relevant for lymphocytes that express receptors capable of recognizing self-tissues and foreign invaders. Comprising the third largest lymphocyte population, natural killer (NK) cells recognize and kill cellular targets and produce pro-inflammatory cytokines. These potentially self-destructive effector functions can be controlled by inhibitory receptors for the polymorphic major histocompatibility complex (MHC) class I molecules that are ubiquitously expressed on target cells. However, inhibitory receptors are not uniformly expressed on NK cells, and are germline-encoded by a set of polymorphic genes that segregate independently from MHC genes. Therefore, how NK-cell self-tolerance arises in vivo is poorly understood. Here we demonstrate that NK cells acquire functional competence through ‘licensing’ by self-MHC molecules. Licensing involves a positive role for MHC-specific inhibitory receptors and requires the cytoplasmic inhibitory motif originally identified in effector responses. This process results in two types of self-tolerant NK cells—licensed or unlicensed—and may provide new insights for exploiting NK cells in immunotherapy. This self-tolerance mechanism may be more broadly applicable within the vertebrate immune system because related germline-encoded inhibitory receptors are widely expressed on other immune cells.

Epistasis between mouse Klra and major histocompatibility complex class I loci is associated with a new mechanism of natural killer cell-mediated innate resistance to cytomegalovirus infection

Experimental infection with mouse cytomegalovirus (MCMV) has been used to elucidate the intricate host-pathogen mechanisms that determine innate resistance to infection. Linkage analyses in F2 progeny from MCMV-resistant MA/My (H2 k) and MCMV-susceptible BALB/c (H2 d) and BALB.K (H2 k) mouse strains indicated that only the combination of alleles encoded by a gene in the Klra (also called Ly49) cluster on chromosome 6, and one in the major histocompatibility complex (H2) on chromosome 17, is associated with virus resistance. We found that natural killer cell–activating receptor Ly49P specifically recognized MCMV-infected cells, dependent on the presence of the H2 k haplotype. This binding was blocked using antibodies to H-2Dk but not antibodies to H-2Kk. These results are suggestive of a new natural killer cell mechanism implicated in MCMV resistance, which depends on the functional interaction of the Ly49P receptor and the major histocompatibility complex class I molecule H-2Dk on MCMV-infected cells.

Allorecognition by murine natural killer cells: lysis of T-lymphoblasts and rejection of bone-marrow grafts

Summary: Natural killer (NK) cells of inbred mice reject allogeneic bone‐marrow cells, and NK cells of F1 hybrid mice can reject parental bone‐marrow cells (hybrid resistance). In some cases these patterns of rejection can be mimicked in vitro by utilizing IL‐2 cultured NK effector cells and allogeneic or parental T‐lymphoblasts as target cells. Lysis of allogeneic parental targets in vitro can be explained on the basis of the missing self hypothesis. Subsets of NK cells that bear non‐overlapping MHC class I inhibitory receptors belonging to the Ly49 family lyse allogeneic targets because they do not express self class I molectiles of the NK cell donor. Parental strain targets are lysed because they do not express all of the self class I antigens of the Fl hybrid, and hence fail to deliver inhibitory signals to all subsets of Fl NK cells. The expression of Ly49 receptors on NK cells is regulated by liost MHC to ensure maximal sensitivity to alterations in self class I molecules and to prevent autoreactivity. In many instances, however, the rejection of allogeneic bone marrow cells in vivo cannot be readily explained by the missing self hypothesis. In these instances, it appears that rejection is initiated by class 1 MHC receptors on NK ceils Out recognize allogeneic class I molecules as non‐self, and activate rather than inhibit NK cell function.

Fine tuning of natural killer cell specificity and maintenance of self tolerance in MHC class I-deficient mice

TAP1 −/−, β2‐microglobulin (β2m) −/− and TAP1/β2m −/− mice all express low but quantitatively different levels of MHC class I molecules. Using these mice, we have addressed questions relating to the fine tuning of natural killer (NK) cell specificity and maintenance of self tolerance in the NK cell system. NK cells from B6 wild‐type mice killed target cells from TAP1 −/−, β2m −/− and TAP1/β2m −/− mice in vivo and rejected bone marrow grafts from the same mice in vivo at equivalent levels. NK cells from TAP1 −/−, β2m −/− mice did not kill target cells or reject bone marrow grafts from TAP1/β2m −/− mice. NK cells in all MHC class I‐deficient mice were tolerant to autologous MHC class I‐deficient cells, as revealed by in vitro cytotoxicity assays using NK cell effectors activated with the interferon‐inducing agent Tilorone, or by in vivo bone marrow graft experiments. However, the self‐tolerant state of MHC class I‐deficient NK cells was broken by in vitro stimulation with IL‐2 for 4 days. Under these conditions, NK cells from the MHC class I‐deficient mice killed autologous MHC class I‐deficient cells while MHC class I‐positive targets were spared. The C‐type lectin inhibitory receptor Ly49C has a specificity for H‐2Kb and is expressed on a subset of NK1.1+ cells in B6 mice. Wild‐type and all MHC class I‐deficient mice had similar numbers of Ly49C‐positive NK1.1+ cells. However, Ly49C expression was markedly down‐regulated on NK1.1+ cells from B6 mice, as compared to TAP1 −/−, β2m −/− and TAP1/β2m −/− mice. In vitro stimulation of NK cells with IL‐2 for 4 days did not significantly change this pattern. The present results are discussed in relation to the role of MHC class I molecules and Ly49 receptors in shaping the NK cell repertoire and raise new questions about maintenance of self tolerance in the NK cell system.

Cutting Edge: Ly49A Inhibits TCR/CD3-Induced Apoptosis and IL-2 Secretion

To evaluate the importance of Ly49A on TCR-induced cellular events, we established clones of the 1F2 T cell hybridoma expressing either Ly49A or a chimeric version, Ly49A/H, where the Ly49A cytoplasmic domain has been replaced by the Ly49H cytoplasmic domain. Ligation of Ly49A, but not Ly49A/H, with its ligand H-2Dd or anti-Ly49A mAbs caused a specific inhibition of TCR/CD3-induced IL-2 secretion. Moreover, flow cytometry analysis of hypodiploid DNA and annexin V binding revealed that ligation of Ly49A protected cells from apoptosis induced by anti-CD3 mAbs or Ag. In contrast, ligation of the Ly49A/H chimeric receptor had no antiapoptotic effect. In addition, engagement of Ly49A selectively inhibited TCR-induced Fas ligand expression whereas TCR-induced Fas expression was not significantly affected. Expression of Ly49 inhibitory receptors on T cells may represent an important mechanism for the regulation of T cell survival in vivo by inhibiting TCR-induced apoptosis and IL-2 secretion.

Identification of murine natural killer cell subsets with monoclonal antibodies derived from 129 anti-C57BL/6 immune spleen cells☆

Two hybridomas producing monoclonal antibodies reactive with natural killer cells were selected after fusion of 129 anti-C57BL/6 immune spleen cells with P3X63-Ag8.653 myeloma cells. Treatment of normal or stimulated cells with the 4LO3311 or the 4LO439 mAb and rabbit complement inhibited natural killer and antibody-dependent cellular cytotoxicities, whereas cell lysis mediated by natural cytotoxic cells, cytotoxic T lymphocytes, or activated macrophages was unaffected. Lymphokine-activated killer activity was reduced after complement-mediated treatment of interleukin-2-stimulated spleen cells with the 4LO3311 mAb but not after treatment with the 4LO439 mAb. Similar treatment of spleen cells with either mAb had no effect on the mitogen-induced proliferation of T and B lymphocytes and did not alter the frequency of antibody plaque-forming cells in immune spleen cell suspensions. The 4LO3311 and 4LO439 mAbs thus appear to be specific for NK cells and their progeny. Flow cytometry analysis confirmed that 4LO3311+ and 4LO439+ cells are phenotypically identical to NK-1.1+ cells. The epitope recognized by the 4LO3311 mAb has the same strain distribution as the NK-2.1 alloantigen previously detected with NZB anti-BALB/c antiserum, whereas the 4LO439 mAb appears to identify a new NK cell marker exclusively expressed in mice of C57BL lineage. The relationship of the molecules detected with either the 4LO3311 or the 4LO439 mAb to polymorphic antigens of the Ly series is discussed.

Heterogeneity of natural killer cell subsets in NK-1.1+ and NK-1.1− inbred mouse strains and their progeny

The 4LO3311 monoclonal antibody, a new NK-specific reagent recently produced in our laboratory, reacts with spleen cells of 11 mouse strains, most of which do not express the NK-1.1 alloantigen recognized by the PK136 mAb. Among positive strains, the susceptibility of spleen cells to the complement-dependent NK-inhibiting activity of the 4LO3311 mAb was variable but independent of the initial NK cell activity level of cells tested. This property was furthermore not modified after poly(I:C) stimulation. The susceptibility of spleen cells to the in vitro 4LO3311 mAb plus complement treatment is however influenced by the absolute number of 4LO3311+ cells as well as by the density of the corresponding alloantigen at the cell surface. Moreover, it was established that the strain-related variations observed also depended upon the relative size of the 4LO3311 cell subset within the lytic NK cell population. Indeed, when C3H (NK-1.1-4LO3311+) mice were inoculated with the 4LO3311 mAb, the lytic activity of their spleen cells was almost unaltered but 4LO3311-reactive cells were no longer detected in the spleen of treated animals and remaining NK cells were totally resistant to the in vitro 4LO3311 mAb plus complement treatment. These findings indicate that the 4LO3311 mAb identifies a subset rather than all NK cells, even in a NK-1.1- strain. Since a NK-1.1-unreactive cell subset was identified in NZB (NK-1.1+4LO3311-) mice inoculated with the PK136 mAb, the NK-1.1+ cell population is not necessarily responsible for all the splenic NK cell activity in all NK-1.1+ strains. In B6C3F1 hybrid mice, a relatively large subset of NK-1.1-4LO3311- cells was found in addition to those expressing the NK-1.1, the 4LO3311 alloantigen, or both. According to these results, NK cell heterogeneity should thus be taken as an evolving concept whose resolution appears more and more complex with the identification of new NK-specific reagents.

Genetic control of anti-idiotypic vaccination against coronavirus infection

The idiotypic network can be experimentally altered to induce protective immune responses against microbial pathogens. Both internal image and noninternal image anti‐idiotypic (anti‐Id) antibodies have been shown to trigger antigen (Ag)‐specific immune responses. Therefore, mechanisms of anti‐Id vaccination appear to go beyond structural mimicry of Ag, but remain undefined. Using the neurotropic murine coronavirus animal model, we have previously shown that a polyclonal noninternal image anti‐Id (Ab2) could vaccinate BALB/c mice. To characterize its mode of action, we have examined the immune modulating capability of this Ab2 in vivo in strains of mice with different H‐2 haplotypes. Even though only internal image anti‐Id are expected to induce non‐genetically restricted immunity, this noninternal image Ab2 induced protective immunity in four of eight genetically different strains of mice susceptible to coronavirus infection. These were BALB/c (H‐2d), DBA/1 (H‐2d), DBA/1 (H‐2q), and SWR (H‐2q) mice. Protection was generally correlated with the induction of specific antiviral Ab (Ab3) that showed biological properties, such as virus neutralization in vitro, similar to the initial Ab1. To evaluate the genetic implication of the H‐2 haplotypes in protection, congenic mice were also tested. Vaccination profiles suggest that cooperation between background gene(s) of the BALB/c mouse with H‐2d and H‐2q loci is necessary for an optimal protective immune response, although the main genetic element(s) regulating the antiviral response to Ab2 inoculation appeared to be located outside the major histocompatibility complex. These results are consistent with the ability of Ab2 to induce protective antiviral antibodies in genetically different animals by biological mimicry.

Enrichment of murine splenic natural killer (NK) cells by the sequential elimination of non-NK cells

A simple and reliable three-step procedure to enrich for murine endogenous splenic NK cells is described. The method is based on the sequential elimination of non-NK cell subsets by standard and inexpensive techniques executed in a specific order. First, macrophages and other adherent cells are eliminated by incubation on plastic surface. Secondly, the T cells are excluded from the multicellular aggregates formed by agglutination of the remaining cells with wheat germ lectin. Thirdly, after dissociation of the aggregates with N-acetyl-D-glucosamine and osmotic lysis of erythrocytes, NK cells are separated from other nucleated cells by nylon wool filtration. C57BL/6 spleen cells were used to establish the enrichment procedure. Usually their NK cell activity is intermediate but occasionally either low or high NK cell activity was observed in input cell suspensions. The NK cell activity recovery and the degree of enrichment varied inversely with the initial NK cell activity level of the input cell suspension. When initial NK cell activity was intermediate, it was enriched 10-30-fold. Experiments were done to establish if suppressor cells, and nylon wool-adherent, naturally activated NK cells, putatively present in input cells, could have been responsible for the abnormal initial NK cell activity detected in some C57BL/6 spleen cell suspensions and for the variations in the degree of enrichment achieved by the method here described. Either no or negligeable suppressor cell activity was noted in the cell fractions normally discarded at each step of the procedure. On the other hand, nylon wool-adherent NK cells were eliminated during the fractionation of spleen cells with higher than average initial NK cell activity and would account for the lower NK cell enrichment obtained in these conditions.

The murine NK2.1 antigen : a 130 kD glycoprotein dimer expressed by a natural killer cell subset of the spleen, thymus and lymph nodes

Murine natural killer (NK) cells express a few antigens not found on other leukocyte subsets. The NK1.1 antigen, that is present in only a few mouse strains, has been extensively characterized whereas our knowledge of the NK2.1 antigen, which is more commonly expressed, remains, as yet, limited. Our laboratory has previously reported the production of a mAb (4LO3311) recognizing a murine NK cell-specific molecule with a similar strain distribution as the NK2.1 antigen formerly defined with an NZB anti-BALB/c antiserum. In this study, we demonstrate by sequential immunoprecipitation that 4LO3311 represents the first NK2.1 antigen-specific mAb. This reagent was used to immunoprecipitate the NK2.1 antigen from 125I-labeled lysates of fresh NK-enriched spleen cells. SDS-PAGE analyses revealed that the NK2.1 antigen is expressed at the cell surface as a N-glycosylated disulfide-linked protein dimer with approximately 65 kD subunits. The NK2.1 antigen is likely to be anchored in the plasma membrane by a peptide moiety since its expression on NK cells was not affected by treatment with phosphatidylinositol-specific phospholipase C. In addition to be present on a splenic NK cell subset, the NK2.1 antigen is shown to be expressed by a small number of CD4-CD8-thymocytes and by a subset of CD4-CD8-IgG- lymph node cells. Finally, it is shown here that unlike the NKR-P1, the rat homologue of the murine NK1.1 antigen, neither the NK2.1 nor the NK1.1 antigen is expressed by polymorphonuclear leukocytes.