Transcriptional control of natural killer cell differentiation and function.

Gene expression can be modulated depending on physiological and developmental requirements. A multitude of regulatory genes, which are organized in interdependent networks, guide development and eventually generate specific phenotypes. Transcription factors (TF) are a key element in the regulatory cascade controlling cell fate and effector functions. In this review, we discuss recent data on the diversity of TF that determine natural killer (NK) cell fate and NK cell function.

Monocytes control natural killer cell differentiation to effector phenotypes.

Natural killer (NK) cells play a major role in immunologic surveillance of cancer. Whether NK-cell subsets have specific roles during antitumor responses and what the signals are that drive their terminal maturation remain unclear. Using an in vivo model of tumor immunity, we show here that CD11b(hi)CD27(low) NK cells migrate to the tumor site to reject major histocompatibility complex class I negative tumors, a response that is severely impaired in Txb21(-/-) mice. The phenotypical analysis of Txb21-deficient mice shows that, in the absence of Txb21, NK-cell differentiation is arrested specifically at the CD11b(hi)CD27(hi) stage, resulting in the complete absence of terminally differentiated CD11b(hi)CD27(low) NK cells. Adoptive transfer experiments and radiation bone marrow chimera reveal that a Txb21(+/+) environment rescues the CD11b(hi)CD27(hi) to CD11b(hi)CD27(low) transition of Txb21(-/-) NK cells. Furthermore, in vivo depletion of myeloid cells and in vitro coculture experiments demonstrate that spleen monocytes mediate the terminal differentiation of peripheral NK cells in a Txb21- and IL-15Rα-dependent manner. Together, these data reveal a novel, unrecognized role for Txb21 expression in monocytes in promoting NK-cell development and help appreciate how various NK-cell subsets are generated and participate in antitumor immunity.

Cutting edge: CD8+ T cell priming in the absence of NK cells leads to enhanced memory responses.

It is uncertain whether NK cells modulate T cell memory differentiation. By using a genetic model that allows the selective depletion of NK cells, we show in this study that NK cells shape CD8(+) T cell fate by killing recently activated CD8(+) T cells in an NKG2D- and perforin-dependent manner. In the absence of NK cells, the differentiation of CD8(+) T cells is strongly biased toward a central memory T cell phenotype. Although, on a per-cell basis, memory CD8(+) T cells generated in the presence or the absence of NK cells have similar functional features and recall capabilities, NK cell deletion resulted in a significantly higher number of memory Ag-specific CD8(+) T cells, leading to more effective control of tumors carrying model Ags. The enhanced memory responses induced by the transient deletion of NK cells may provide a rational basis for the design of new vaccination strategies.

Superantigen-activated regulatory T cells inhibit the migration of innate immune cells and the differentiation of naive T cells.

Regulatory T cells can be used as tools to suppress pathogenic T cells in autoimmunity, graft-vs-host-disease, and transplantation. But even when high numbers of Ag-specific regulatory T cells are available, it is still possible under certain in vivo and in vitro conditions for effector T cells to escape effective control. Current reports suggest that the degree of suppression is modulated by the inflammatory milieu, which can induce resistance to suppression in effector T cells or subvert the inhibitory function of the regulatory T cells. Cells of the innate immune system integrate early signals of injury and infection and have a major impact on the ensuing inflammation. Hence, the modification of these initial events can be key to allowing suppression to dominate. The approach we took here was to test whether the in vivo preactivation of endogenous regulatory T cells with a superantigen could enhance their suppressive potency. We provide evidence that this not only proved effective in expanding the pool of preactivated regulatory T cells but also in preventing the migration of NK cells and granulocytes upon sensitization with matured dendritic cells. The attenuation of innate immune activation was accompanied by linked suppression of adoptively transferred OVA-specific T cells when APC coexpressing OVA and the superantigen were injected. These data suggest that the preactivation of regulatory T cells is a promising approach to increase their potency.

p110gamma and p110delta isoforms of phosphoinositide 3-kinase differentially regulate natural killer cell migration in health and disease.

The mechanisms that regulate NK cell trafficking are unclear. Phosphoinositide-3 kinases (PI3K) control cell motility and the p110gamma and p110delta isoforms are mostly expressed in leukocytes, where they transduce signals downstream of G protein coupled receptors (GPCR) or tyrosine kinase receptors, respectively. Here, we set out to determine the relative contribution of p110gamma and p110delta to NK cell migration in mice. Using a combination of single-cell imaging analysis of transgenic cells reporting on PI3K activity in real time and small molecule inhibitors of p110gamma and p110delta, we show here that the tyrosine-kinase coupled p110delta is linked to GPCR signaling and, depending on the GPCR, may even be preferentially activated over p110gamma. Using gene-targeted mice, we showed that both isoforms were essential for NK cell chemotaxis to CXCL12 and to CCL3 and, in vivo, for normal NK cell migration during pregnancy and to the inflamed peritoneum. By contrast, only p110delta was indispensable for chemotaxis to S1P and CXCL10 and for NK cell distribution throughout lymphoid and nonlymphoid tissues and for extravasation to tumors. These results implicate p110delta downstream of GPCRs in NK cells and highlight its nonredundant role as a key regulator of NK cell trafficking in health and disease.

Dendritic cell migration to peripheral lymph nodes.

Dendritic cells are potent antigen-presenting cells endowed with the unique ability to prime T-cell responses. To present foreign antigens to na ive T cells, dendritic cells must migrate from inflamed or injured peripheral tissues to the closest draining lymph nodes through afferent lymphatic vessels. In addition, conventional dendritic cells, plasmacytoid dendritic cells and monocytes enter lymph nodes from blood crossing high endothelial venules. The selective migration of dendritic cells and their residence in non lymphoid as well as in lymphoid organs are tightly regulated events, whose molecular control is being unraveled rapidly. In this chapter, we review key aspects of what is known about dendritic cell traffic to peripheral nodes from tissues, in particular skin, and from blood. A better understanding of the regulation of dendritic cell migration for optimal priming of T-cell responses is essential for future advances in manipulating dendritic cell traffic as a means to improve immune responses in clinical settings.

CD40L+ CD4+ memory T cells migrate in a CD62P-dependent fashion into reactive lymph nodes and license dendritic cells for T cell priming.

There is growing evidence that the maturation state of dendritic cells (DCs) is a critical parameter determining the balance between tolerance and immunity. We report that mouse CD4(+) effector memory T (T(EM)) cells, but not naive or central memory T cells, constitutively expressed CD40L at levels sufficient to induce DC maturation in vitro and in vivo in the absence of antigenic stimulation. CD4(+) T(EM) cells were excluded from resting lymph nodes but migrated in a CD62P-dependent fashion into reactive lymph nodes that were induced to express CD62P, in a transient or sustained fashion, on high endothelial venules. Trafficking of CD4(+) T(EM) cells into chronic reactive lymph nodes maintained resident DCs in a mature state and promoted naive T cell responses and experimental autoimmune encephalomyelitis (EAE) to antigens administered in the absence of adjuvants. Antibodies to CD62P, which blocked CD4(+) T(EM) cell migration into reactive lymph nodes, inhibited DC maturation, T cell priming, and induction of EAE. These results show that T(EM) cells can behave as endogenous adjuvants and suggest a mechanistic link between lymphocyte traffic in lymph nodes and induction of autoimmunity.

L-selectin-negative CCR7- effector and memory CD8+ T cells enter reactive lymph nodes and kill dendritic cells.

T lymphocytes lacking the lymph node-homing receptors L-selectin and CCR7 do not migrate to lymph nodes in the steady state. Instead, we found here that lymph nodes draining sites of mature dendritic cells or adjuvant inoculation recruited L-selectin-negative CCR7- effector and memory CD8+ T cells. This recruitment required CXCR3 expression on T cells and occurred through high endothelial venules in concert with lumenal expression of the CXCR3 ligand CXCL9. In reactive lymph nodes, recruited T cells established stable interactions with and killed antigen-bearing dendritic cells, limiting the ability of these dendritic cells to activate naive CD4+ and CD8+ T cells. The inducible recruitment of blood-borne effector and memory T cells to lymph nodes may represent a mechanism for terminating primary and limiting secondary immune responses.

Induced recruitment of NK cells to lymph nodes provides IFN-gamma for T(H)1 priming.

Naive T cells are stimulated by antigen-presenting dendritic cells (DCs) in secondary lymphoid organs, but whether other types of cell participate in T cell priming is unclear. Here we show in mice that natural killer (NK) cells, which are normally excluded from lymph nodes, are rapidly recruited in a CCR7-independent, CXCR3-dependent manner to lymph nodes on stimulation by the injection of mature DCs. Recruitment of NK cells is also induced by some, but not all, adjuvants and correlates with the induction of T helper cell type 1 (T(H)1) responses. NK cell depletion and reconstitution experiments show that NK cells provide an early source of interferon-gamma (IFN-gamma) that is necessary for T(H)1 polarization. Taken together, our results identify an induced pathway of NK cell migration in antigen-stimulated lymph nodes and a mechanism by which some adjuvants may facilitate T(H)1 responses.

Regulation of dendritic cell migration to the draining lymph node: impact on T lymphocyte traffic and priming.

Antigen-pulsed dendritic cells (DCs) are used as natural adjuvants for vaccination, but the factors that influence the efficacy of this treatment are poorly understood. We investigated the parameters that affect the migration of subcutaneously injected mouse-mature DCs to the draining lymph node. We found that the efficiency of DC migration varied with the number of injected DCs and that CCR7+/+ DCs migrating to the draining lymph node, but not CCR7-/- DCs that failed to do so, efficiently induced a rapid increase in lymph node cellularity, which was observed before the onset of T cell proliferation. We also report that DC migration could be increased up to 10-fold by preinjection of inflammatory cytokines that increased the expression of the CCR7 ligand CCL21 in lymphatic endothelial cells. The magnitude and quality of CD4+ T cell response was proportional to the number of antigen-carrying DCs that reached the lymph node and could be boosted up to 40-fold by preinjection of tumor necrosis factor that conditioned the tissue for increased DC migration. These results indicate that DC number and tissue inflammation are critical parameters for DC-based vaccination.

Apoptosis-dependent subversion of the T-lymphocyte epitope hierarchy in lymphoma cells.

Tumor cells undergoing programmed death are an attractive source of tumor-associated antigens, and evidences are available for their therapeutic efficacy in vivo when used either alone or in association with dendritic cells. However, little is known about the specificity of the immune response induced by such antigen formulation. Indeed, activation of specific proteases during apoptosis may influence the cytoplasmic degradation of proteins and the generation of CTL epitopes. We show here that on injection of C57BL/6 mice either with RMA lymphoma cells induced to apoptosis or bone marrow-derived dendritic cells pulsed with apoptotic RMA cells, a specific and protective CTL response is induced, which, however, is not directed against the immunodominant CTL epitope gag(85-93). Lack of in vivo expansion of gag(85-93)-specific CTL in vaccinated mice is attributable to the apoptosis-dependent loss of gag(85-93) in dying tumor cells. Indeed, we found loss of gag(85-93) in RMA, MBL-2, and EL-4G+ lymphoma cells, which share gag(85-93) as an immunodominant CTL epitope, induced to apoptosis by UV irradiation, mitomycin C, doxorubicin, or daunorubicin. This phenomenon appears to be caspase-dependent, because caspase inhibition by N-benzyloxycarbonyl-Val-Ala-asp-fluoromethylketone prevents apoptosis of lymphoma cells and loss of gag(85-93). Therefore, subversion of the epitope hierarchy in apoptotic tumor cells might be relevant in the induction of tumor-specific T-lymphocyte responses.