Matthew F. Krummel

Enhancement of Antitumor Immunity by CTLA-4 Blockade

One reason for the poor immunogenicity of many tumors may be that they cannot provide signals for CD28-mediated costimulation necessary to fully activate T cells. It has recently become apparent that CTLA-4, a second counterreceptor for the B7 family of costimulatory molecules, is a negative regulator of T cell activation. Here, in vivo administration of antibodies to CTLA-4 resulted in the rejection of tumors, including preestablished tumors. Furthermore, this rejection resulted in immunity to a secondary exposure to tumor cells. These results suggest that blockade of the inhibitory effects of CTLA-4 can allow for, and potentiate, effective immune responses against tumor cells.

Visualizing regulatory T cell control of autoimmune responses in nonobese diabetic mice

The in vivo mechanism of regulatory T cell (Treg cell) function in controlling autoimmunity remains controversial. Here we have used two-photon laser-scanning microscopy to analyze lymph node priming of diabetogenic T cells and to delineate the mechanisms of Treg cell control of autoimmunity in vivo. Islet antigen–specific CD4+CD25− T helper cells (TH cells) and Treg cells swarmed and arrested in the presence of autoantigens. These TH cell activities were progressively inhibited in the presence of increasing numbers of Treg cells. There were no detectable stable associations between Treg and TH cells during active suppression. In contrast, Treg cells directly interacted with dendritic cells bearing islet antigen. Such persistent Treg cell–dendritic cell contacts preceded the inhibition of TH cell activation by dendritic cells, supporting the idea that dendritic cells are central to Treg cell function in vivo.

Antigen-engaged B cells undergo chemotaxis toward the T zone and form motile conjugates with helper T cells.

Interactions between B and T cells are essential for most antibody responses, but the dynamics of these interactions are poorly understood. By two-photon microscopy of intact lymph nodes, we show that upon exposure to antigen, B cells migrate with directional preference toward the B-zone–T-zone boundary in a CCR7-dependent manner, through a region that exhibits a CCR7-ligand gradient. Initially the B cells show reduced motility, but after 1 d, motility is increased to approximately 9 μm/min. Antigen-engaged B cells pair with antigen-specific helper T cells for 10 to more than 60 min, whereas non-antigen-specific interactions last less than 10 min. B cell–T cell conjugates are highly dynamic and migrate extensively, being led by B cells. B cells occasionally contact more than one T cell, whereas T cells are strictly monogamous in their interactions. These findings provide evidence of lymphocyte chemotaxis in vivo, and they begin to define the spatiotemporal cellular dynamics associated with T cell–dependent antibody responses.

Interactions between PD-1 and PD-L1 promote tolerance by blocking the TCR–induced stop signal

Programmed death 1 (PD-1) is an inhibitory molecule expressed on activated T cells; however, the biological context in which PD-1 controls T cell tolerance remains unclear. Using two-photon laser-scanning microscopy, we show here that unlike naive or activated islet antigen–specific T cells, tolerized islet antigen–specific T cells moved freely and did not swarm around antigen-bearing dendritic cells (DCs) in pancreatic lymph nodes. Inhibition of T cell antigen receptor (TCR)-driven stop signals depended on continued interactions between PD-1 and its ligand, PD-L1, as antibody blockade of PD-1 or PD-L1 resulted in lower T cell motility, enhanced T cell–DC contacts and caused autoimmune diabetes. Blockade of the immunomodulatory receptor CTLA-4 did not alter T cell motility or abrogate tolerance. Thus, PD-1–PD-L1 interactions maintain peripheral tolerance by mechanisms fundamentally distinct from those of CTLA-4.Programmed death-1 (PD-1) is an inhibitory molecule expressed on activated T cells, however, the biological context in which PD-1 controls T cell tolerance remains unclear. Using two-photon laser-scanning microscopy, we showed that unlike naïve or activated islet antigen-specific T cells, tolerized islet antigen-specific T cells moved freely and did not swarm around antigen-bearing dendritic cells (DC) in pancreatic lymph nodes. Inhibition of T cell receptor (TCR)-driven stop signals depended on continued PD-1-PD-L1 interactions, as antibody blockade of PD-1 or PD-L1 decreased T cell motility, enhanced T cell-DC contacts, and caused autoimmune diabetes. CTLA-4 blockade did not alter T cell motility or abrogate tolerance. Thus, PD-1-PD-L1 interactions maintain peripheral tolerance by mechanisms fundamentally distinct from those of CTLA-4.

Type 2 innate lymphoid cells control eosinophil homeostasis

Eosinophils are specialized myeloid cells associated with allergy and helminth infections. Blood eosinophils demonstrate circadian cycling, as described over 80 years ago, and are abundant in the healthy gastrointestinal tract. Although a cytokine, interleukin (IL)-5, and chemokines such as eotaxins mediate eosinophil development and survival, and tissue recruitment, respectively, the processes underlying the basal regulation of these signals remain unknown. Here we show that serum IL-5 levels are maintained by long-lived type 2 innate lymphoid cells (ILC2) resident in peripheral tissues. ILC2 cells secrete IL-5 constitutively and are induced to co-express IL-13 during type 2 inflammation, resulting in localized eotaxin production and eosinophil accumulation. In the small intestine where eosinophils and eotaxin are constitutive, ILC2 cells co-express IL-5 and IL-13; this co-expression is enhanced after caloric intake. The circadian synchronizer vasoactive intestinal peptide also stimulates ILC2 cells through the VPAC2 receptor to release IL-5, linking eosinophil levels with metabolic cycling. Tissue ILC2 cells regulate basal eosinophilopoiesis and tissue eosinophil accumulation through constitutive and stimulated cytokine expression, and this dissociated regulation can be tuned by nutrient intake and central circadian rhythms.

Deletional Tolerance Mediated by Extrathymic Aire-Expressing Cells

The prevention of autoimmunity requires the elimination of self-reactive T cells during their development and maturation. The expression of diverse self-antigens by stromal cells in the thymus is essential to this process and depends, in part, on the activity of the autoimmune regulator (Aire) gene. Here we report the identification of extrathymic Aire-expressing cells (eTACs) resident within the secondary lymphoid organs. These stromally derived eTACs express a diverse array of distinct self-antigens and are capable of interacting with and deleting naïve autoreactive T cells. Using two-photon microscopy, we observed stable antigen-specific interactions between eTACs and autoreactive T cells. We propose that such a secondary network of self-antigen–expressing stromal cells may help reinforce immune tolerance by preventing the maturation of autoreactive T cells that escape thymic negative selection.

Repair and regeneration of the respiratory system: complexity, plasticity, and mechanisms of lung stem cell function

Respiratory disease is the third leading cause of death in the industrialized world. Consequently, the trachea, lungs, and cardiopulmonary vasculature have been the focus of extensive investigations. Recent studies have provided new information about the mechanisms driving lung development and differentiation. However, there is still much to learn about the ability of the adult respiratory system to undergo repair and to replace cells lost in response to injury and disease. This Review highlights the multiple stem/progenitor populations in different regions of the adult lung, the plasticity of their behavior in injury models, and molecular pathways that support homeostasis and repair.

Dynamics of the immunological synapse: finding, establishing and solidifying a connection.

A coordinated series of molecular interactions leads to the establishment of an immunological synapse. Migrating lymphocytes scan antigen-processing cells and are made to stop upon recognition of their specific ligand. Microclusters of TCRs/CD4 form over a large contact site, then TCRs coalesce. Coalescence occurs in response to signals generated in the first encounters and in response to costimulatory signaling. The cytoskeleton rearranges and concentric rings of coreceptors and integrins surround the TCRs. This unexpected level of complexity of co-clustering and exclusion in the interface has generated much interest in the functional consequences of signaling and/or immune effector function.

Stabilized imaging of immune surveillance in the mouse lung

Real-time imaging of cellular and subcellular dynamics in vascularized organs requires image resolution and image registration to be simultaneously optimized without perturbing normal physiology. This problem is particularly pronounced in the lung, in which cells may transit at speeds >1 mm s−1 and in which normal respiration results in large-scale tissue movements that prevent image registration. Here we report video-rate, two-photon imaging of a physiologically intact preparation of the mouse lung that is stabilizing and nondisruptive. Using our method, we obtained evidence for differential trapping of T cells and neutrophils in mouse pulmonary capillaries, and observed neutrophil mobilization and dynamic vascular leak in response to stretch and inflammatory models of lung injury in mice. The system permits physiological measurement of motility rates of >1 mm s−1, observation of detailed cellular morphology and could be applied in the future to other organs and tissues while maintaining intact physiology.

The Yin and Yang of T Cell Costimulation

The Perspective by J. Allison and M. Krummel discusses a paper in this issue of Science by Waterhouse et al. (p. 985), which reports the lymphoproliferative phenotype of mice lacking the T cell antigen CTLA-4. The function of this molecule has been elusive, but the pronounced lymphadenopathy of these knockout mice provides strong evidence that it works in opposition to the positive signal generated by the costimulatory T cell antigen receptor-peptide and CD28-B7 interactions.