Jennifer Poursine-Laurent

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.

STAT-3 Activation Is Required for Normal G-CSF-Dependent Proliferation and Granulocytic Differentiation

To investigate the role of signal transducer and activator of transcription (STAT) proteins in granulocyte colony-stimulating factor (G-CSF)-regulated biological responses, we generated transgenic mice with a targeted mutation of their G-CSF receptor (termed d715F) that abolishes G-CSF-dependent STAT-3 activation and attenuates STAT-5 activation. Homozygous mutant mice are severely neutropenic with an accumulation of immature myeloid precursors in their bone marrow. G-CSF-induced proliferation and granulocytic differentiation of hematopoietic progenitors is severely impaired. Expression of a constitutively active form of STAT-3 in d715F progenitors nearly completely rescued these defects. Conversely, expression of a dominant-negative form of STAT-3 in wild-type progenitors results in impaired G-CSF-induced proliferation and differentiation. These data suggest that STAT-3 activation by the G-CSFR is critical for the transduction of normal proliferative signals and contributes to differentiative signals.

Costimulation of multiple NK cell activation receptors by NKG2D

The activation of NK cells is mediated through specific interactions between activation receptors and their respective ligands. Little is known, however, about whether costimulation, which has been well characterized for T cell activation, occurs in NK cells. To study the function of NKG2D, a potential NK costimulatory receptor, we have generated two novel hamster mAbs that recognize mouse NKG2D. FACS analyses demonstrate that mouse NKG2D is expressed on all C57BL/6 IL-2-activated NK (lymphokine-activated killer (LAK)) cells, all splenic and liver NK cells, and ∼50% of splenic NKT cells. Consistent with limited polymorphism of NKG2D, its sequence is highly conserved, and the anti-NKG2D mAbs react with NK cells from a large number of different mouse strains. In chromium release assays, we show that stimulation of NK cells with anti-NKG2D mAb can redirect lysis. Also, enhanced lysis of transfected tumor targets expressing NKG2D ligand could be inhibited by addition of anti-NKG2D mAb. Interestingly, stimulation of LAK cells via NKG2D alone does not lead to cytokine release. However, stimulation of LAK via both an NK activation receptor (e.g., CD16, NK1.1, or Ly-49D) and NKG2D leads to augmentation of cytokine release compared with stimulation through the activation receptor alone. These results demonstrate that NKG2D has the ability to costimulate multiple NK activation receptors.

Coordinate expression of cytokines and chemokines by NK cells during murine cytomegalovirus infection.

Cytokines and chemokines activate and direct effector cells during infection. We previously identified a functional group of five cytokines and chemokines, namely, IFN-γ, activation-induced T cell-derived and chemokine-related cytokine/lymphotactin, macrophage-inflammatory protein 1α, macrophage-inflammatory protein 1β, and RANTES, coexpressed in individual activated NK cells, CD8+ T cells, and CD4+ Th1 cells in vitro and during in vivo infections. However, the stimuli during infection were not known. In murine CMV (MCMV) infection, the DAP12/KARAP-associated Ly49H NK cell activation receptor is crucial for resistance through recognition of MCMV-encoded m157 but NK cells also undergo in vivo nonspecific responses to uncharacterized stimuli. In this study, we show that Ly49H ligation by m157 resulted in a coordinated release of all five cytokines/chemokines from Ly49H+ NK cells. Whereas other cytokines also triggered the release of these cytokines/chemokines, stimulation was not confined to the Ly49H+ population. At the single-cell level, the production of the five mediators showed strong positive correlation with each other. Interestingly, NK cells were a major source of these five cytokines/chemokines in vitro and in vivo, whereas infected macrophages produced only limited amounts of macrophage-inflammatory protein 1α, macrophage-inflammatory protein1β, and RANTES. These findings suggest that both virus-specific and nonspecific NK cells play crucial roles in activating and directing other inflammatory cells during MCMV infection.

Increased granulocyte colony-stimulating factor responsiveness but normal resting granulopoiesis in mice carrying a targeted granulocyte colony-stimulating factor receptor mutation derived from a patient with severe congenital neutropenia.

The role of mutations of the granulocyte colony-stimulating factor receptor (G-CSFR) in the pathogenesis of severe congenital neutropenia (SCN) and the subsequent development of acute myeloid leukemia (AML) is controversial. Mice carrying a targeted mutation of their G-CSFR that reproduces the mutation found in a patient with SCN and AML have been generated. The mutant G-CSFR allele is expressed in a myeloid-specific fashion at levels comparable to the wild-type allele. Mice heterozygous or homozygous for this mutation have normal levels of circulating neutrophils and no evidence for a block in myeloid maturation, indicating that resting granulopoiesis is normal. However, in response to G-CSF treatment, these mice demonstrate a significantly greater fold increase in the level of circulating neutrophils. This effect appears to be due to increased neutrophil production as the absolute number of G-CSF-responsive progenitors in the bone marrow and their proliferation in response to G-CSF is increased. Furthermore, the in vitro survival and G-CSF-dependent suppression of apoptosis of mutant neutrophils are normal. Despite this evidence for a hyperproliferative response to G-CSF, no cases of AML have been detected to date. These data demonstrate that the G-CSFR mutation found in patients with SCN is not sufficient to induce an SCN phenotype or AML in mice.

A Role for G-CSF Receptor Signaling in the Regulation of Hematopoietic Cell Function but Not Lineage Commitment or Differentiation

To investigate the specificity of cytokine signals in hematopoietic differentiation, we generated mice with a targeted mutation of their G-CSF receptor (G-CSFR) such that the cytoplasmic (signaling) domain of the G-CSFR is replaced with the cytoplasmic domain of the erythropoietin receptor. In homozygous mutant mice, expression of this chimeric receptor had no apparent affect on lineage commitment and was able to support the production of morphologically mature neutrophils. However, mutant neutrophils displayed reduced chemotaxis, and G-CSF-stimulated mobilization of neutrophils and hematopoietic progenitors from the bone marrow to blood was markedly impaired. Thus, the G-CSFR is generating unique signals that are required for certain specialized hematopoietic cell functions but are not required for granulocytic differentiation or lineage commitment.

Ischemic neurons recruit natural killer cells that accelerate brain infarction

Significance Stroke is a devastating illness second only to cardiac ischemia as a cause of death worldwide. Long-time attempts to salvage dying neurons and preserve neurological functions via various neuroprotective agents have failed, owing at least in part to medical science’s limited knowledge of ischemia-induced elements that participate in irreversible neurovascular damage. The present study was performed to understand the role of natural killer (NK) cells, a key member of the innate immune system, in stroke. We discovered that NK cells infiltrated the brains of stroke patients and mice with induced stroke. Multiple pathways by which NK cells exacerbate brain infarction are discovered. This study revealed the role of NK cells in the pathogenesis of stroke. Brain ischemia and reperfusion activate the immune system. The abrupt development of brain ischemic lesions suggests that innate immune cells may shape the outcome of stroke. Natural killer (NK) cells are innate lymphocytes that can be swiftly mobilized during the earliest phases of immune responses, but their role during stroke remains unknown. Herein, we found that NK cells infiltrated the ischemic lesions of the human brain. In a mouse model of cerebral ischemia, ischemic neuron-derived fractalkine recruited NK cells, which subsequently determined the size of brain lesions in a T and B cell-independent manner. NK cell-mediated exacerbation of brain infarction occurred rapidly after ischemia via the disruption of NK cell tolerance, augmenting local inflammation and neuronal hyperactivity. Therefore, NK cells catalyzed neuronal death in the ischemic brain.

A Ligand for the Murine NK Activation Receptor Ly-49D: Activation of Tolerized NK Cells from β2-Microglobulin- Deficient Mice

Mouse NK cells express inhibitory NK receptors that recognize target cell MHC class I molecules and activation receptors that are less well defined. The Ly-49D activation receptor on C57BL/6 NK cells recognizes Chinese hamster ovary cells and triggers natural killing. In this study, we demonstrate that a Chinese hamster classical MHC class I molecule is the ligand for Ly-49D in a reporter gene assay system as well as in NK cell killing assays. Ly-49D recognizes the Chinese hamster class I molecule better when it is expressed with Chinese hamster β2-microglobulin (β2m) than murine β2m. However, it is still controversial that Ly-49D recognizes H-2Dd, as we were unable to demonstrate the specificity previously reported. Using this one ligand-one receptor recognition system, function of an NK activation receptor was, for the first time, investigated in NK cells that are tolerized in β2m-deficient mice. Surprisingly, Ly-49D-killing activity against ligand-expressing targets was observed with β2m-deficient mouse NK cells, albeit reduced, even though “tolerized” function of Ly-49D was expected. These results indicate that Ly-49D specifically recognizes the Chinese hamster MHC class I molecule associated with Chinese hamster β2m, and indicate that the Ly-49D NK cell activation receptor is not tolerized in β2m deficiency.

Development of thymic NK cells from double negative 1 thymocyte precursors

The differentiation of natural killer (NK) cells and a subpopulation of NK cells which requires an intact thymus, that is, thymic NK cells, is poorly understood. Previous in vitro studies indicate that double negative (CD4⁻CD8⁻, DN) thymocytes can develop into cells with NK cell markers, but these cells have not been well characterized. Herein, we generated and characterized NK cells differentiating from thymic DN precursors. Sorted DN1 (CD44⁺CD25⁻) CD122⁻NK1.1⁻ thymocytes from Rag1(⁻/⁻) mice were adoptively transferred into Rag1(⁻/⁻)Ly5.1 congenic mice. After intrathymic injection, donor-derived cells phenotypically resembling thymic NK cells were found. To further study their differentiation, we seeded sorted DN1 CD122⁻)NK1.1⁻ thymocytes on irradiated OP9 bone marrow stromal cells with IL-15, IL-7, Flt3L, and stem cell factor. NK1.1⁺ cells emerged after 7 days. In vitro differentiated NK cells acquired markers associated with immature bone marrow-derived NK cells, but also expressed CD127, which is typically found on thymic NK cells. Furthermore, we found that in vitro cells generated from thymic precursors secreted cytokines when stimulated and degranulated on target exposure. Together, these data indicate that functional thymic NK cells can develop from a DN1 progenitor cell population.

Structural variation of the mouse natural killer gene complex

The natural killer gene complex (NKC) on chromosome 6 contains clusters of genes that encode both activation and inhibitory receptors expressed on mouse natural killer (NK) cells. NKC genes, particularly belonging to the Nkrp1 and Ly49 gene families, display haplotype differences between different mouse strains and allelic polymorphisms of individual genes, as previously revealed by conventional analysis in a small number of inbred mouse strains. Herein we used array-based comparative genomic hybridization (aCGH) to efficiently compare the NKC in 21 mouse strains to the reference C57BL/6 strain. By using unsupervised clustering methods, we could sort these variations into the same groups as determined by previous RFLP analyses of Nkrp1 and Ly49 genes. Prospective analyses of aCGH and RFLP data validated these relationships. Moreover, aCGH data predicted monoclonal antibody reactivity with an allospecific determinant on molecules expressed by NK cells. Taken together, these data demonstrate the structural variation in the NKC between mouse strains as well as the usefulness of aCGH in analysis of complex, polymorphic gene clusters.