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Ronald N Germain

Publications and source records attributed to Ronald N Germain.

32 records · Page 2Linked to original sources

An innately interesting decade of research in immunology.

"Nature has provided, in the white corpuscles as you call them-in the phagocytes as we call them-a natural means of devouring and destroying all disease germs. There is at bottom only one genuinely scientific treatment for all diseases, and that is to stimulate the phagocytes." So opined B.B. in G.B. Shaw's The Doctor's Dilemma in a dramatic restatement of a key portion of Ilya Metchnikoff's Nobel Prize address: "Whenever the organism enjoys immunity, the introduction of infectious microbes is followed by the accumulation of mobile cells, of white corpuscles of the blood in particular which absorb the microbes and destroy them. The white corpuscles and the other cells capable of doing this have been designated 'phagocytes,' (i.e., devouring cells) and the whole function that ensures immunity has been given the name of 'phagocytosis'". Based on these insights into the foundation of resistance to infectious disease, Metchnikoff was awarded the 1908 Nobel Prize in Physiology or Medicine together with Paul Ehrlich (Fig. 1). Although both were cited for discoveries in immunity, the contributions of the two men seem worlds apart. Ehrlich's studies did not deal with generic responses to infection, but rather with the highly specific nature of antibodies and their relationship to the cells producing them: "As the cell receptor is obviously preformed, and the artificially produced antitoxin only the consequence, i.e. secondary, one can hardly fail to assume that the antitoxin is nothing else but discharged components of the cell, namely receptors discharged in excess". But biological systems are just that-systems-and the parts need to work together. And so we arrive, a century later, at an appreciation for just how intimately related these two seemingly disparate aspects of host defense really are.

Adjuvants, Immunologic↗

T-cell activation: the power of one.

Adaptive immunity depends on antigen-specific activation of resting lymphocytes. Using high-resolution live-cell imaging, a single ligand has been found to trigger a biochemical response in T cells. On the basis of this and other recent findings, a 'pseudodimer' with one foreign- and one self-antigen-engaged receptor linked via a CD4 molecule has been proposed as the fundamental unit of effective T-cell signaling.

Adaptation, Physiological↗

TCR ligand discrimination is enforced by competing ERK positive and SHP-1 negative feedback pathways.

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

Animals↗

Distinct contributions of different CD40 TRAF binding sites to CD154-induced dendritic cell maturation and IL-12 secretion.

The mechanisms by which CD40 controls the maturation and antigen presentation functions of dendritic cells (DC) remains largely undefined in this critical cell type. To examine this question, we have employed retroviral transduction of primary bone marrow-derived mouse DC. Mutation of the distinct binding sites for TNF receptor-associated factor 6 (TRAF6) and for TRAF 2, 3, and 5 in the CD40 cytoplasmic domain revealed their independent contributions to DC maturation and activation of NF-kappaB. In contrast, disruption of the TRAF6 but not the TRAF 2,3,5 binding site markedly decreased IL-12 p40 secretion along with p38 and JNK activation in response to CD154 stimulation. These data document a clear bifurcation of the CD40 signaling cascade in primary DC at the level of the receptor's two distinct and autonomous TRAF binding sites, and reveal the predominant role of the TRAF6 binding site in CD40-induced pro-inflammatory cytokine production by these cells.

Amino Acid Sequence↗

On the role of self-recognition in T cell responses to foreign antigen.

The key role of the thymus in shaping the peripheral T cell receptor (TCR) repertoire has been appreciated for nearly a quarter of a century. For most of that time, a single model has dominated thinking about the physiological role of the positive selection process mediated by TCR recognition of self-peptides and major histocompatibility complex (MHC) molecules. This developmental filter was believed to populate secondary lymphoid tissues with T cells bearing receptors best able to recognize unknown foreign peptides associated with the particular allelic forms of the MHC molecules present in an individual. More recently, self-recognition has been suggested to regulate the viability of naïve T cells. Here we focus on new results indicating that a critical contribution of positive selection to host defense is insuring that each peripheral T cell can use self-recognition to (i) enhance TCR signaling sensitivity upon foreign antigen recognition and (ii) augment the clonal expansion that accompanies limiting foreign antigen display at early points in an infectious process. We also detail new insights into the intracellular signaling circuitry that underlies the effective discrimination between low- and high-quality ligands of the TCR and speculate on how this design might facilitate an additional contribution of self-recognition to T cell activation in the presence of foreign stimuli.

Animals↗

Ligand-dependent regulation of T cell development and activation.

Mature CD4+ and CD8 + T lymphocytes develop in the thymus from precursors with diverse clonally distributed receptors, possessing binding sites with negligible, intermediate, or high affinity for selfpeptide major histocompatibility complex (MHC) ligands. Positive- and negative-selection processes acting on this precursor pool yield a peripheral T cell population comprised of cells with receptors (TCR) capable of self-peptide MHC ligand recognition, but largely depleted of those able to mediate overt self-responsiveness. The Lymphocyte Biology Section of the Laboratory of Immunology studies how self-ligand recognition guides T cell development in the thymus and influences the functionality of naive and activated T cells in the periphery. It also seeks to define the molecular basis for the discrimination between self-ligands and foreign antigens that controls T cell activation to effector function. Finally, it uses a combination of conventional cellular immunological methods, biochemical and biophysical studies, and advanced imaging techniques to visualize, quantitate, and model the various steps in the development of primary and memory T cell immune responses.

Animals↗

Self-recognition promotes the foreign antigen sensitivity of naive T lymphocytes.

Major histocompatibility complex (MHC) class I and II molecules are highly polymorphic proteins that bind and present foreign peptides to the clonally distributed alphabeta receptors (TCR) of T lymphocytes. As a population, the immature T lymphocytes generated in the thymus express a very diverse set of TCR specificities. A process of positive selection filters this broad repertoire to optimize peripheral T cells for antigen recognition in the context of available MHC products. Only those precursor T cells whose TCRs generate an adequate but not excessive signalling response to self-peptides bound to the expressed MHC proteins undergo successful maturation. Here we show that post-thymic self-recognition facilitates the antigen reactivity of mature T cells. Both experimental and physiological interruption of T-cell contact with self-peptide MHC ligands leads to a rapid decline in signalling and response sensitivity to foreign stimuli. Because the adaptive immune system must be recruited early in an infectious process when antigen is limiting, these findings suggest that positive selection ensures predictable T-cell recognition of available self-ligands, which in turn promotes efficient responses to pathogens.

Animals↗

Constitutive presentation of a natural tissue autoantigen exclusively by dendritic cells in the draining lymph node.

The major histocompatibility complex (MHC)-dependent presentation of processed tissue-specific self-antigens can contribute to either peripheral (extrathymic) tolerance or the differentiation of autoreactive T cells. Here, we have studied the MHC class II molecule presentation of gastric parietal cell (PC)-specific H(+)/K(+)-ATPase, which induces a destructive autoimmune gastritis in BALB/c mice lacking CD4(+) CD25(+) regulatory T cells. Immunofluorescence microscopy showed physical association of CD11c(+) dendritic cells (DCs) with PCs in the gastric mucosa. H(+)/K(+)-ATPase protein was found within vesicular compartments of a few CD11c(+) DCs only in the draining gastric lymph node (LN) and these antigen-containing DCs increased markedly in number with the onset of tissue destruction in autoimmune animals. Both CD8alpha(hi) and CD8alpha(lo) gastric DCs, but not peripheral or mesenteric DCs, showed evidence of constitutive in vivo processing and presentation of H(+)/K(+)-ATPase. These data provide direct support for a widely held model of local tissue antigen uptake and trafficking by DCs in normal animals and demonstrate that DCs in the draining LN can present a tissue-specific self-antigen under noninflammatory conditions without fully deleting autoreactive T cells or inducing active autoimmunity.

Animals↗

Dynamic imaging of T cell-dendritic cell interactions in lymph nodes.

T cell immune responses begin within organized lymphoid tissues. The pace, topology, and outcomes of the cellular interactions that underlie these responses have, so far, been inferred from static imaging of sectioned tissue or from studies of cultured cells. Here we report dynamic visualization of antigen-specific T cells interacting with dendritic cells within intact explanted lymph nodes. We observed immunological synapse formation and prolonged interactions between these two cell types, followed by the activation, dissociation, and rapid migration of T cells away from the antigenic stimulus. This high-resolution spatiotemporal analysis provides insight into the nature of cell interactions critical to early immune responses within lymphoid structures.

Animals↗

Self-recognition and the regulation of CD4+ T cell survival.

CD4+ T cells differentiate in the thymus from committed precursors to mature naive cells ready for peripheral circulation. Successful maturation depends on adequate but not excessive signaling upon T cell receptor (TCR) engagement of self-peptide/MHC class II molecule ligands present in the thymic environment. Persistent TCR signaling throughout development from the CD4+CD8+ to the CD4+ state is required for completion of the developmental process. Recent work has suggested that a continuation of this signaling is essential for sustained survival of CD4+ T cells once they leave the thymus but our studies suggest otherwise. Although we found clear evidence for active TCR signaling involving recognition of self-ligands in peripheral lymphoid tissues, we did not see a substantial effect of loss of such signaling on the life-time of naive CD4+ T cells. Based on a careful review of the literature, we conclude that essentially all previous claims that MHC class II recognition plays a significant role in the survival of CD4+ T cells can be reinterpreted as an effect of self-recognition on proliferation in lymphopenic environments, maintaining population numbers without a marked effect on individual cell viability. We propose a possible explanation for why, in contrast, the viability of naive CD8+ T cells appears to show such self-MHC dependence and suggest that a primary function of self-recognition by T cells may be to enhance responses to foreign antigen.

Animals↗

MHC-dependent survival of naïve T cells? A complicated answer to a simple question.

The differentiation and survival of developing alpha beta thymocytes depends on effective T-cell receptor (TCR) signaling upon recognition of self peptide/major histocompatibility complex (MHC) molecule ligands. Although this concept is uniformly accepted with regard to immature thymocytes, there are conflicting reports as to whether or not MHC recognition is required for survival of mature peripheral naïve T cells. In this review, we assess these reports critically and conclude that in many cases, the differences observed in CD4(+) T-cell recovery between MHC-expressing and MHC-deficient animals can be attributed to proliferation occurring only in the MHC-expressing lymphopenic animals studied in these models systems, rather than to effects of MHC recognition on cell viability per se. Still other reports involve experimental manipulations that may have affected the intrathymic development of the T cells such that they receive a "poor" selecting signal, fail to fully mature, and thus behave more like thymocytes in their survival characteristics (i.e., show MHC dependence). With respect to CD8(+) T cells, we discuss data suggesting that some clones are more dependent upon the presence of MHC class I for survival than others. We propose that some CD8(+) T cells even in a wild-type host may behave like the manipulated CD4(+) T cells just described, and fail to mature completely with respect to their survival requirements. Although the proportion of CD8(+) cells in this MHC-dependent state is not known, the corresponding fraction among CD4(+) T cells seems to be rather small. Overall, our analysis of the available data suggests that most or all mature CD4(+) (and perhaps also many CD8(+)) T lymphocytes do not depend on self-recognition for their viability in the periphery.

Animals↗

Imaging of T-cell interactions with antigen presenting cells in culture and in intact lymphoid tissue.

The development of an effective immune response requires cell-cell contact between T cells and antigen-bearing cells of several types (dendritic cells, B cells, infected tissue cells). Recent advances in light microscopy have led to intense investigation of the molecular events that accompany these cell interactions, especially the redistribution of membrane proteins into discrete organized subdomains within the zone of cell-cell contact termed the 'immunological synapse'. Here we discuss two aspects of our own studies in this area. First, we highlight results from our in vitro analysis of the role of the cytoskeletal ezrin, radixin, moesin adapter proteins in the exclusion of CD43 from the well-defined T cell receptor (TCR) and integrin-rich zones of the synapse. Based on the molecular mechanism uncovered in this work, we propose a new model for how TCR-signaled changes in cytoskeletal organization indirectly influence both protein distributions and the efficiency of signaling between T cell and presenting cell. We then discuss the development of a new method for dynamic visualization of T cell - dendritic cell interactions in intact lymphoid tissue. The remarkable longevity of monogamous lymphocyte-presenting cell interactions is discussed, differences between our observations and those of others are laid out in detail, and prospects for future application of this technical approach to analysis of early immune responses in lymphoid organs and of effector lymphocyte function in tissues are presented.

Animals↗

T-cell development and the CD4-CD8 lineage decision.

Cell-fate decisions are controlled typically by conserved receptors that interact with co-evolved ligands. Therefore, the lineage-specific differentiation of immature CD4+ CD8+ T cells into CD4+ or CD8+ mature T cells is unusual in that it is regulated by clonally expressed, somatically generated T-cell receptors (TCRs) of unpredictable fine specificity. Yet, each mature T cell generally retains expression of the co-receptor molecule (CD4 or CD8) that has an MHC-binding property that matches that of its TCR. Two models were proposed initially to explain this remarkable outcome--'instruction' of lineage choice by initial signalling events or 'selection' after a stochastic fate decision that limits further development to cells with coordinated TCR and co-receptor specificities. Aspects of both models now appear to be correct; mistake-prone instruction of lineage choice precedes a subsequent selection step that filters out most incorrect decisions.

Animals↗