T-cell activation: the power of one

miR-369-3p modulates inducible nitric oxide synthase and is involved in regulation of chronic inflammatory response

Dendritic cells are the most important antigen-presenting cells that link the innate and acquired immune system. In our previous study, we identified that the upregulation of miR-369-3p suppresses the LPS-induced inflammatory response, reducing C/EBP-β, TNFα and IL-6 production. With the aim of gaining further insight into the biological function of miR-369-3p during acute inflammatory response, in the present study we identified novel gene targets of miR-369-3p and demonstrated the suppressive ability of these genes on the inflammatory dendritic cells. Bioinformatic analyses revealed that iNOS is a potential target of miR-369-3p. We demonstrated that the ectopic induction of miR-369-3p markedly reduced iNOS mRNA and protein as well as NO production. Moreover, we found that the upregulation of miR-369-3p decreased the release of TNFα, IL-6, IL-12, IL-1α, IL-1β in response to LPS, and increased the production of anti-inflammatory cytokines such as IL-10 and IL-1RA. In addition, LPS-induced nuclear translocation of NF-kB was inhibited by miR-369-3p. Levels of miR-369-3p were decreased in human inflamed regions of human intestine obtained from IBD patients. Our results provide novel additional information on miR-369-3p as a potential core of the signaling regulating the inflammatory response. These findings suggest that miR-369-3p should be considered as a potential target for the future development of new molecular therapeutic approaches.

T-cell receptor binding affinities and kinetics: impact on T-cell activity and specificity

The interaction between the T-cell receptor (TCR) and its peptide-major histocompatibility complex (pepMHC) ligand plays a critical role in determining the activity and specificity of the T cell. The binding properties associated with these interactions have now been studied in many systems, providing a framework for a mechanistic understanding of the initial events that govern T-cell function. There have been various other reviews that have described the structural and biochemical features of TCR : pepMHC interactions. Here we provide an overview of four areas that directly impact our understanding of T-cell function, as viewed from the perspective of the TCR : pepMHC interaction: (1) relationships between T-cell activity and TCR : pepMHC binding parameters, (2) TCR affinity, avidity and clustering, (3) influence of coreceptors on pepMHC binding by TCRs and T-cell activity, and (4) impact of TCR binding affinity on antigenic peptide specificity.

TCR triggering by the pMHC complex: valency, affinity, and dynamics

The interaction between the T cell receptor (TCR) and a peptide-loaded major histocompatibility complex (pMHC) is one of the most-studied interactions in immunology, and yet the precise mechanism by which this system operates is still not fully understood. One key issue is whether TCR triggering minimally requires monomeric pMHC complexes or higher-order multimers (two or more pMHCs). Any model of TCR triggering must explain the high sensitivity, specificity, and dynamic range of ligand responsiveness that this receptor system exhibits. Most models of TCR triggering have not fully appreciated the dynamic aspects of TCR triggering. TCR triggering happens very quickly, and the properties of sensitivity and specificity can be explained by a model that accounts for the interaction dynamics of such a receptor system. In this paper, it is proposed that the important parameter in TCR triggering is the immobilization of the TCR-pMHC complex in the plasma membrane. Whether this involves monomeric or multimeric pMHCs may depend on the affinity of the TCR for the pMHC.

Measuring rotational diffusion of MHC class I on live cells by polarized FPR

Clustering of membrane proteins is a dynamic process which can regulate cellular function and signaling. The size of receptor and other membrane protein clusters can in principle be measured in terms of their rotational diffusion. However, in practice, measuring rotation of membrane proteins of live cells has been difficult, largely because of the difficulty of rigidly attaching reporter groups to the molecules of interest. Here we show that polarized photobleaching recovery can detect rotation of membrane proteins genetically tagged with yellow fluorescent protein, YFP. MHC class I molecules were engineered with a rigid, in-sequence, YFP tag followed at the C-terminus by a pair of crosslinkable domains. When crosslinker was added we could detect changes in rotational anisotropy decay consistent with clustering of the MHC molecules. This result points the way to use of engineered fluorescent fusion proteins to measure rotational diffusion in native cell membranes.

Basic science for the clinician 30: The immunologic synapse

Interactions between discrete and independent cells, immune or otherwise, present a variety of potential problems. How do you make sure the cells can communicate effectively? How do you preclude neighboring cells from "listening in on a conversation" that may be meant only for one set of "ears"? How can you sharpen the hearing of those ears so they will be capable of detecting small signals but not get distracted by random noise in the system? How can you selectively enhance the sense of hearing in times of great need or urgency and then diminish the "gain" of the system when it is not immediately required? How can you assure that the call will be terminated at the end of the conversation? Passage of communication molecules and/or interaction of cell surface markers require close and stable apposition of the cell delivering the message with the receiving cell. In the nervous system, these problems were successfully addressed in the nerve-nerve or nerve-myocyte (neuromuscular junction) synapses. Not surprisingly, given the parsimony of nature, the immune system uses some of the same design features, even some of the same molecules, to achieve an effective communication strategy. The term "immunologic synapse" was coined only 2 decades ago, but the structure it describes has become a very hot topic in immunology and cell biology. The immunologic synapse allows the activation of a unique T cell, with an antigen receptor recognizing its antigen in the grasp of the antigen-presenting cell's (APC's) major histocompatibility complex (MHC). A better understanding of this transient immune cell-cell interactive structure allows one to weave the functions of T cell antigen receptors, lipid rafts, adaptor molecules, and nuclear signaling molecules together into one cohesive, flowing communication supersystem. Appreciation of the intricacies of the synapse also identifies targets that one day may be used to interfere with antigen-specific immune responses, eg, autoimmunity.

Agent-based modeling of the context dependency in T cell recognition

Antigen recognition by T cells is a key event in the adaptive immune response. T cells scan the surface of antigen-presenting cells (APCs) or target cells for specific peptides bound to MHC molecules. In the physiological setting, a typical APC presents tens of thousands of diverse endogenous self-derived peptides complexed to MHC (pMHC complexes). When 'foreign' peptides are presented, they constitute a small fraction of the total surface peptide repertoire. As T cells seem to be capable of discerning minute amounts of 'foreign' peptides among a complex background of self-peptides, endogenous peptides are generally assumed to play no role in recognition. However, recent results suggest that these background peptides may alter the sensitivity of T cells to foreign peptides. Current experimental limitations preclude analysis of peptide mixtures approaching physiological complexity, making it difficult to further address the role of complex background peptides. In this paper, we present a computational model to test how complex, varied peptide populations on an APC could potentially modulate a T cell's ability to detect the presence of small numbers of agonist peptides among a diverse population. We use the model to investigate the notion that under physiological conditions, T cell recognition of foreign peptides is context dependent, that is, T cells process signals gathered from all pMHC interactions, not just from a few agonist peptides while ignoring all others.

Comparative immunophenotyping of monocytes from symptomatic and asymptomatic atopic individuals

BACKGROUND

Allergy has at least two components - a genetic predisposition referred to as atopy and the progress from an atopic state to clinically apparent disease. Peripheral blood monocytes are circulating myeloid precursors of antigen-presenting cells. The expression of cell surface proteins on monocytes may therefore witness the disease status and affect the development of allergic disease.

METHODS

Monocytes were isolated from atopic individuals with seasonal allergic rhinitis (n = 10), from atopic individuals sensitized to aeroallergens but without any signs of acute disease (n = 11), and from healthy nonatopic donors (n = 21). Detailed comparative phenotypic analysis of CD14(+) and FcepsilonRI(+)CD14(+) monocytes was performed by flow cytometry.

RESULTS

CD14(+) monocytes from symptomatic atopic donors showed a significant increase in the cell surface intensity of the integrin adhesion molecule CD11c over monocytes from asymptomatic atopic and nonatopic donors. Asymptomatic atopic individuals showed significantly enhanced expression of FcepsilonRI on the CD14(high)CD16(dim) monocyte subset compared with this subset from symptomatic atopic and nonatopic donors.

CONCLUSION

The increase in monocyte surface intensity of the adhesion molecule CD11c may be involved in the manifestation of allergic disease. FcepsilonRI on CD14(high)CD16(dim) monocytes of asymptomatic atopic donors may be of functional importance for the maintenance of clinical unresponsiveness toward allergens.

CD4 enhances T cell sensitivity to antigen by coordinating Lck accumulation at the immunological synapse

How T cells respond with extraordinary sensitivity to minute amounts of agonist peptide and major histocompatibility complex (pMHC) molecules on the surface of antigen-presenting cells bearing large numbers of endogenous pMHC molecules is not understood. Here we present evidence that CD4 affects the responsiveness of T helper cells by controlling spatial localization of the tyrosine kinase Lck in the synapse. This finding, as well as further in silico and in vitro experiments, led us to develop a molecular model in which endogenous and agonist pMHC molecules act cooperatively to amplify T cell receptor signaling. At the same time, activation due to endogenous pMHC molecules alone is inhibited. A key feature is that the binding of agonist pMHC molecules to the T cell receptor results in CD4-mediated spatial localization of Lck, which in turn enables endogenous pMHC molecules to trigger many T cell receptors. We also discuss broader implications for T cell biology, including thymic selection, diversity of the repertoire of self pMHC molecules and serial triggering.

B Cell Positive Selection: Road Map to the Primary Repertoire?

Accumulating evidence indicates that positive selection events mediate differentiation, lineage commitment, and longevity of B lymphocytes. The BCR plays a central role, dictating the likelihood that newly formed cells will complete maturation, as well as whether cells persist within mature pools. Competition among B cells for limited, life span-promoting resources, which include self-ligands, lineage-specific cytokines, and innate receptor ligands, underlie these selective processes. Together, these observations suggest that positive selection is a critical feature in the establishment and maintenance of all lymphocyte pools, prompting re-evaluation of the underlying biological rationale for this process.