Natural killer (NK) cells are innate lymphocytes with spontaneous antitumor activity, and they produce interferon-gamma (IFN-gamma) that primes immune responses. Whereas T helper cell subsets differentiate from naive T cells via specific transcription factors, evidence for NK cell diversification is limited. In this report, we characterized intestinal lymphocytes expressing the NK cell natural cytotoxicity receptor NKp46. Gut NKp46+ cells were distinguished from classical NK cells by limited IFN-gamma production and absence of perforin, whereas several subsets expressed the nuclear hormone receptor retinoic acid receptor-related orphan receptor t (RORgammat) and interleukin-22 (IL-22). Intestinal NKp46+IL-22+ cells were generated via a local process that was conditioned by commensal bacteria and required RORgammat. Mice lacking IL-22-producing NKp46+ cells showed heightened susceptibility to the pathogen Citrobacter rodentium, consistent with a role for intestinal NKp46+ cells in immune protection. RORgammat-driven diversification of intestinal NKp46+ cells thereby specifies an innate cellular defense mechanism that operates at mucosal surfaces.

A fundamental question in hematopoietic stem cell (HSC) biology is how self-renewal is controlled. Here we show that the molecular regulation of two critical elements of self-renewal, inhibition of differentiation and induction of proliferation, can be uncoupled, and we identify Notch signaling as a key factor in inhibiting differentiation. Using transgenic Notch reporter mice, we found that Notch signaling was active in HSCs in vivo and downregulated as HSCs differentiated. Inhibition of Notch signaling led to accelerated differentiation of HSCs in vitro and depletion of HSCs in vivo. Finally, intact Notch signaling was required for Wnt-mediated maintenance of undifferentiated HSCs but not for survival or entry into the cell cycle in vitro. These data suggest that Notch signaling has a dominant function in inhibiting differentiation and provide a model for how HSCs may integrate multiple signals to maintain the stem cell state.

T cell responsiveness to an epitope is affected both by its affinity for the presenting MHC molecule and the affinity of the MHC-peptide complex for TCR. One limitation of cancer immunotherapy is that natural tumor antigens elicit relatively weak T cell responses, in part because high-affinity T cells are rendered tolerant to these antigens. We report here that amino acid substitutions in a natural MHC class I-restricted tumor antigen that increase the stability of the MHC-peptide-TCR complex are significantly more potent as tumor vaccines. The improved immunity results from enhanced in vivo expansion of T cells specific for the natural tumor epitope. These results indicate peptides that stabilize the MHC-peptide-TCR complex may provide superior antitumor immunity through enhanced stimulation of specific T cells.

Stem cells are thought to balance self-renewal and differentiation through asymmetric and symmetric divisions, but whether such divisions occur during hematopoietic development remains unknown. Using a Notch reporter mouse, in which GFP acts as a sensor for differentiation, we image hematopoietic precursors and show that they undergo both symmetric and asymmetric divisions. In addition we show that the balance between these divisions is not hardwired but responsive to extrinsic and intrinsic cues. Precursors in a prodifferentiation environment preferentially divide asymmetrically, whereas those in a prorenewal environment primarily divide symmetrically. Oncoproteins can also influence division pattern: although BCR-ABL predominantly alters the rate of division and death, NUP98-HOXA9 promotes symmetric division, suggesting that distinct oncogenes subvert different aspects of cellular function. These studies establish a system for tracking division of hematopoietic precursors and show that the balance of symmetric and asymmetric division can be influenced by the microenvironment and subverted by oncogenes.

Tumor antigen-specific T-cell tolerance limits the efficacy of therapeutic cancer vaccines. Antigen-presenting cells mediate the induction of T-cell tolerance to self-antigens. We therefore assessed the fate of tumor-specific CD4+ T cells in tumor-bearing recipients after in vivo activation of antigen-presenting cells with antibodies against CD40. Such treatment not only preserved the responsiveness of this population, but resulted in their endogenous activation. Established tumors regressed in vaccinated mice treated with antibody against CD40 at a time when no response was achieved with vaccination alone. These results indicate that modulation of antigen-presenting cells may be a useful strategy for enhancing responsiveness to immunization.

IntroductionThe 2-signal model of T-cell activation 1,2 is frequently evoked in the field of tumor immunology to account for the failure of the immune system to successfully reject antigenic cancer cells in vivo. Cancer cells, it is argued, typically being the transformed counterparts of "nonprofessional" antigen-presenting cells (APCs), provide signal 1, but not signal 2, on their encounter with tumorspecific T cells, leading to the induction of T-cell tolerance. 3 Numerous studies, however, have shown that cells from at least one type of malignancy, that is, B-cell lymphoma, can efficiently activate T cells in vitro, including CD4 ϩ T cells specific for epitopes derived from their own unique immunoglobulin idiotype. 4,5 These tumor cells, being derived from APCs, constitutively express major histocompatibility complex (MHC) class I and II molecules as well as low but inducible levels of intercellular adhesion molecule 1, leukocyte function-associated antigen 3, and the costimulatory molecules B7-1 and B7-2 and therefore seem to be well equipped to provide the necessary signals required for T-cell activation. Indeed, cross-linking of CD40 on lymphoma cells has been shown to up-regulate their expression of adhesion and costimulatory molecules, resulting in a markedly enhanced T-cell response to B-cell tumors in vitro. 6 It is therefore paradoxical that malignant B cells, residing in the very compartment where T-cell responses are initiated, fail to be eliminated in immunocompetent hosts. In fact, it has been suggested that the consequence of antigen presentation by lymphoma cells in vivo may be T-cell tolerance rather than activation, due to the provision of partial but inadequate costimulatory signals by the lymphoma cells. 7 We have previously shown that a murine B-cell lymphoma (A20) transfected to express the model antigen influenza hemagglutinin (HA), is capable of activating HA-specific CD4 ϩ T cells from T-cell receptor (TCR) transgenic mice in vitro. 8 However, when these same transgenic T cells are transferred into mice bearing lymphoma expressing the model antigen (A20HA), the observed outcome is the induction of HA-specific CD4 ϩ T-cell anergy. 9 These results suggest either that (1) B-cell tumors are not efficient APCs in vivo, and their direct encounter with T cells is responsible for the induction of T-cell tolerance or (2) tumor antigens are captured and presented by host APCs to T cells in a context that favors the development of T-cell tolerance. In support of the first hypothesis, nonmalignant B cells have been shown to not only fail to activate naive T cells in vivo, 10 but to induce tolerance as well, even when the B cells had been activated with lipopolysaccharide (LPS). 11 An alternate explanation-supporting the second hypothesis-has been provided by the results of studies emphasizing the role of host bone marrow (BM)-derived cells in the induction and establishment of peripheral tolerance to self-antigens. [12][13][14] Therefore, to assess the relative contribution of antigen presentation ...

In this review, the authors evaluate the experimental evidence for a direct role of Wnt signaling HSCs as well as an indirect role through its influence on the HSC niche. Defining the mechanism of action of Wnt signaling in HSC maintenance in context of the surrounding microenvironment and determining how this signal may integrate with other niche derived signals represents the next challenge HSC biology.

For many cancers, autologous bone marrow transplantation (BMT) achieves a minimal residual disease state, yet relapse rates remain high. Using a syngeneic murine bone marrow transplant model, we demonstrate that vaccination with irradiated granulocyte-macrophage colony-stimulating factor (GM-CSF)–producing autologous tumor cells is effective in the post-BMT period and actually results in a greater tumor-free survival than vaccination in the nontransplant setting. Employing T cells specific for a model tumor-antigen, we find that transplantation of the tumor-bearing host results in a massive expansion and activation of tumor-specific T cells in the early posttransplant period, but this response rapidly declines in association with tumor progression. Immunization with irradiated GM-CSF tumor cells during the period of immune reconstitution results in the sustained amplification and activation of this response that closely correlates with freedom from relapse. These results demonstrate the feasibility of integrating GM-CSF vaccines in the postautologous BMT setting and suggest mechanisms that may contribute to the observed efficacy of immunization during the critical period of immune reconstitution.