Jackson G. Egen

CTLA-4: new insights into its biological function and use in tumor immunotherapy

The discovery of multiple costimulatory cell surface molecules that influence the course of T cell activation has increased our appreciation of the complexity of the T cell response. It remains clear, however, that CD28 and cytotoxic T lymphocyte antigen 4 (CTLA-4) are the critical costimulatory receptors that determine the early outcome of stimulation through the T cell antigen receptor (TCR). Details of how the T cell integrates TCR stimulation with the costimulatory signals of CD28 and the inhibitory signals of CTLA-4 remain to be established, but unique features of the cell biology of CTLA-4 provide important insights into its function. We summarize here recent findings that suggest a previously unrecognized role for CTLA-4 in the regulation of T cell responses. We also describe preclinical and clinical results that indicate manipulation of CTLA-4 has considerable promise as a strategy for the immunotherapy of cancer.

Cytotoxic T lymphocyte antigen-4 accumulation in the immunological synapse is regulated by TCR signal strength

CD28 and CTLA-4 engagement with B7 expressed by APCs generates critical regulatory signals for T cell activation. CD28 is expressed on the T cell surface and enhances T cell expansion, while CTLA-4 localizes primarily to an intracellular compartment and inhibits T cell proliferation. We demonstrate that CTLA-4 has several unique trafficking properties that may regulate its ability to attenuate a T cell response. Importantly, accumulation of CTLA-4 at the immunological synapse is proportional to the strength of the TCR signal, suggesting that cells receiving stronger stimuli are more susceptible to CTLA-4-mediated inhibition. This may represent a novel feedback control mechanism in which a stimulatory signal regulates the recruitment of an inhibitory receptor to a functionally relevant site on the cell surface.

IgE⁺ memory B cells and plasma cells generated through a germinal-center pathway.

Immunoglobulin E (IgE) antibodies are pathogenic in asthma and allergic diseases, but the in vivo biology of IgE-producing (IgE+) cells is poorly understood. A model of the differentiation of IgE+ B cells proposes that IgE+ cells develop through a germinal-center IgG1+ intermediate and that IgE memory resides in the compartment of IgG1+ memory B cells. Here we have used a reporter mouse expressing green fluorescent protein associated with membrane IgE transcripts (IgE-GFP) to assess in vivo IgE responses. In contrast to the IgG1-centered model of IgE switching and memory, we found that IgE+ cells developed through a germinal-center IgE+ intermediate to form IgE+ memory B cells and plasma cells. Our studies delineate a new model for the in vivo biology of IgE switching and memory.

Therapeutic antibodies reveal Notch control of transdifferentiation in the adult lung

Prevailing dogma holds that cell–cell communication through Notch ligands and receptors determines binary cell fate decisions during progenitor cell divisions, with differentiated lineages remaining fixed. Mucociliary clearance in mammalian respiratory airways depends on secretory cells (club and goblet) and ciliated cells to produce and transport mucus. During development or repair, the closely related Jagged ligands (JAG1 and JAG2) induce Notch signalling to determine the fate of these lineages as they descend from a common proliferating progenitor. In contrast to such situations in which cell fate decisions are made in rapidly dividing populations, cells of the homeostatic adult airway epithelium are long-lived, and little is known about the role of active Notch signalling under such conditions. To disrupt Jagged signalling acutely in adult mammals, here we generate antibody antagonists that selectively target each Jagged paralogue, and determine a crystal structure that explains selectivity. We show that acute Jagged blockade induces a rapid and near-complete loss of club cells, with a concomitant gain in ciliated cells, under homeostatic conditions without increased cell death or division. Fate analyses demonstrate a direct conversion of club cells to ciliated cells without proliferation, meeting a conservative definition of direct transdifferentiation. Jagged inhibition also reversed goblet cell metaplasia in a preclinical asthma model, providing a therapeutic foundation. Our discovery that Jagged antagonism relieves a blockade of cell-to-cell conversion unveils unexpected plasticity, and establishes a model for Notch regulation of transdifferentiation.

Addendum: IgE + memory B cells and plasma cells generated through a germinal-center pathway

Addendum: IgE + memory B cells and plasma cells generated through a germinal-center pathway

Protein Localization in Negative Signaling

Cellular activation is regulated by the integration of both positive and negative signals. While positive signaling can be simplistically thought of as the initiation of a biochemical pathway leading to activation of a cellular process, negative signaling can have multiple meanings. From the standpoint of a cellular process such as proliferation or protein secretion, the initiation of any pathway that functions to downregulate these events can be defined as negative signaling. For instance, transforming growth factor β (TGFβ) receptor can transduce a negative signal by upregulating cyclin-dependent kinase inhibitory proteins (CKIs) leading to cell cycle arrest. Negative signaling can also refer to the activation of specific pathways that inhibit cellular stimulation, such as the inhibitory function of the glycine and γ-aminobutyric acid (GABA) receptors in neurons. Upon binding to their respective ligand, these receptors undergo a conformational change that allows the entry of chloride ions into the cell, thereby generating a hyperpolarizing inhibitory signal. The signals generated in the above examples have inhibitory consequences for cellular activation; however, the intricacies of these processes can be thought of as being similar to those of positive signaling. In both cases, specific signaling pathways are activated but with differing downstream effects. This chapter focuses on a distinct form of negative signaling in which extrinsic cell signals serve to directly antagonize positive signals.