Antigens as immunoregulators

Antigen-Specific Binding of Multivalent Soluble Antigen Arrays Induces Receptor Clustering and Impedes B Cell Receptor Mediated Signaling

A pressing need exists for autoimmune disease therapies that act in an antigen-specific manner while avoiding global immunosuppression. Multivalent soluble antigen arrays (SAgAPLP:LABL), designed to induce tolerance to a specific multiple sclerosis autoantigen, consist of a flexible hyaluronic acid (HA) polymer backbone cografted with multiple copies of autoantigen peptide (PLP) and cell adhesion inhibitor peptide (LABL). Previous in vivo studies revealed copresentation of both signals on HA was necessary for therapeutic efficacy. To elucidate therapeutic cellular mechanisms, in vitro studies were performed in a model B cell system to evaluate binding and specificity. Compared to HA and HA arrays containing only grafted PLP or LABL, SAgAPLP:LABL displaying both PLP and LABL exhibited greatly enhanced B cell binding. Furthermore, the binding avidity of SAgAPLP:LABL was primarily driven by the PLP antigen, determined via flow cytometry competitive dissociation studies. Fluorescence microscopy showed SAgAPLP:LABL induced mature receptor clustering that was faster than other HA arrays with only one type of grafted peptide. SAgAPLP:LABL molecules also reduced and inhibited IgM-stimulated signaling as discerned by a calcium flux assay. The molecular mechanisms of enhanced antigen-specific binding, mature receptor clustering, and dampened signaling observed in B cells may contribute to SAgAPLP:LABL therapeutic efficacy.

Molecular Dynamics of Multivalent Soluble Antigen Arrays Support a Two-Signal Co-delivery Mechanism in the Treatment of Experimental Autoimmune Encephalomyelitis

Many current therapies for autoimmune diseases such as multiple sclerosis (MS) result in global immunosuppression, rendering insufficient efficacy with increased risk of adverse side effects. Multivalent soluble antigen arrays, nanomaterials presenting both autoantigen and secondary inhibitory signals on a flexible polymer backbone, are hypothesized to shift the immune response toward selective autoantigenic tolerance to repress autoimmune disease. Two-signal co-delivery of both autoantigen and secondary signal were deemed necessary for therapeutic efficacy against experimental autoimmune encephalomyelitis, a murine model of MS. Dynamic light scattering and in silico molecular dynamics simulations complemented these studies to illuminate the role of two-signal co-delivery in determining therapeutic potential. Physicochemical characteristics such as particle size and molecular affinity for intermolecular interactions and chain entanglement likely facilitated cotransport of two signals to produce efficacy. These findings elucidate potential mechanisms whereby soluble antigen arrays enact their therapeutic effect and help to guide the development of future multivalent antigen-specific immunotherapies.

Single-step grafting of aminooxy-peptides to hyaluronan: a simple approach to multifunctional therapeutics for experimental autoimmune encephalomyelitis

The immune response to antigens is directed in part by the presence or absence of costimulatory signals. The ability to coincidently present both antigen and, for example, a peptide that inhibits or activates the costimulatory pathway, would be a valuable tool for tolerization or immunization, respectively. A simple reaction scheme utilizing oxime chemistry was identified as a means to efficiently conjugate different peptide species to hyaluronan. Peptides synthesized with an aminooxy N-terminus reacted directly to hyaluronan under slightly acidic aqueous conditions without the need for a catalyst. The resulting oxime bond was found to rapidly hydrolyze at pH2 releasing peptide, but was stable at higher pH values (5.5 and 7). Two different peptide species, a multiple sclerosis antigen (PLP) and an ICAM-1 ligand (LABL) known to block immune cell stimulation, were functionalized with the aminooxy end group. These peptides showed similar reactivity to hyaluronan and were conjugated in an equimolar ratio. The resulting hyaluronan with grafted PLP and LABL significantly inhibited disease in mice with experimental autoimmune encephalomyelitis, a model of multiple sclerosis. Aminooxy-peptides facilitate simple synthesis of multifunctional hyaluronan graft polymers, thus enabling novel approaches to antigen-specific immune modulation.

Essential role of membrane cholesterol in accelerated BCR internalization and uncoupling from NF-kappa B in B cell clonal anergy

Divergent hypotheses exist to explain how signaling by the B cell receptor (BCR) is initiated after antigen binding and how it is qualitatively altered in anergic B cells to selectively uncouple from nuclear factor kappaB and c-Jun N-terminal kinase pathways while continuing to activate extracellular signal-regulated kinase and calcium-nuclear factor of activated T cell pathways. Here we find that BCRs on anergic cells are endocytosed at a very enhanced rate upon binding antigen, resulting in a large steady-state pool of intracellularly sequestered receptors that appear to be continuously cycling between surface and intracellular compartments. This endocytic mechanism is exquisitely sensitive to the lowering of plasma membrane cholesterol by methyl-beta-cyclodextrin, and, when blocked in this way, the sequestered BCRs return to the cell surface and RelA nuclear accumulation is stimulated. In contrast, when plasma membrane cholesterol is lowered and GM1 sphingolipid markers of membrane rafts are depleted in naive B cells, this does not diminish BCR signaling to calcium or RelA. These results provide a possible explanation for the signaling changes in clonal anergy and indicate that a chief function of membrane cholesterol in B cells is not to initiate BCR signaling, but instead to terminate a subset of signals by rapid receptor internalization.

On Immunity Against Infections and Vaccines: Credo 2004

Resistance of vertebrate hosts against infections comprises important natural or innate resistance combined with adaptive immune responses of T and B cells. Viruses, bacteria or classical parasites all probe the limit of immune responses and of immunity. They, therefore, offer an excellent opportunity to assess the biology, physiology and molecular aspects of immune responses and help in characterizing the three basic parameters of immunology-- specificity, tolerance and memory. Various experiments are summarized that indicate that the rules of antiviral, antitumour, antiorgan graft and of autoimmune responses are basically the same. The practical specificity repertoire of T and B cells is probably in the order of 10(4)-10(5) specificities expressed by T cells or by neutralizing antibodies. Tolerance is best defined by rules of reactivity to eliminate infections while avoiding destruction of normal cells by complete elimination of T cells that are specific for antigens persisting within the blood and lymphatic (lymphohaemopoietic) system. Induction of a T-cell response is the result of antigens newly entering lymph nodes or spleen, initially in a local fashion and exhibiting an optimal distribution kinetics within the lymphohaemopoietic system. Antigen staying outside lymphatic tissues are immunologically ignored (e.g. are non-events). Thus immune reactivity is regulated by antigen dose, time and relative distribution kinetics. Memory is the fact that a host is resistant against disease caused by reinfection with the same agent. Memory correlates best with antigen-dependent maintenance of elevated antibody titres in serum and mucosal secretions, or with an antigen-driven activation of T cells, such that they are protective immediately against peripheral reinfections in solid tissues. While antibodies transferred from mother to offspring are a prerequisite for the survival of otherwise unprotected immuno-incompetent offsprings, activated memory T cells cannot be transmitted. Thus, attenuation of infections in newborns and babies by maternal antibodies is the physiological correlate of man-made vaccines. T cells not only play an essential role in maintaining T-help-dependent memory antibody titres, but also in controlling the many infections that persist in a host at rather low levels (such as tuberculosis, measles and HIV).

Uncertainties - discrepancies in immunology

The many immunological observations and results from in-vitro or in-vivo experiments vary, and their interpretations differ enormously. A major problem is that within a normal distribution of biological phenomena, which are measurable with many methods, virtually anything is possible. Within a coevolutionary context, the definition of biologically relevant thresholds is an important key to improve our understanding of weaknesses and strengths of the immune system. This review is a personal view, comparing textbook rules and experiments using model antigens with observations on immunity against infections or tumors to critically evaluate our perception and understanding of specificity, affinity maturation, antigen presentation, selection of the class of the immune response, immunological memory and protective immunity, positive selection of T cells and self/nonself discrimination.

T-cell signaling: the importance of receptor clustering

T-cell receptors bound to peptide-MHC molecules undergo higher-order oligomerization in solution. This observation, the low-affinity-recognition properties of T-cell receptors, and other indications that such receptors undergo rapid, serial engagement by a single ligand suggest a dynamic clustering model of T-cell signaling.

Linomide suppresses both Th1 and Th2 cytokines in experimental autoimmune myasthenia gravis

Suppressive effects of the synthetic immunomodulatory drug Linomide have been shown in several autoimmune models, including antibody-mediated experimental autoimmune myasthenia gravis (EAMG), a model for human myasthenia gravis (MG). To define the mechanisms underlying EAMG suppression, we injected Linomide subcutaneously at different doses into Lewis rats immunized with Torpedo acetylcholine receptor (AChR) in complete Freund's adjuvant (CFA), and investigated AChR-specific T and B cell responses, and the levels of lymph node cells expressing mRNA of different cytokines after AChR stimulation in vitro. Both 160 and 16, but not 1.6, mg/kg/day of Linomide effectively suppressed clinical muscle weakness, accompanied by decreased AChR-induced T and B cell responses. Linomide also suppressed the mRNA expression of the Th1 cytokines IFN-gamma, IL-12 and TNF-alpha as well as the Th2 cytokines IL-4 and IL-10, which are important in the immunopathogenesis of EAMG by promoting antibody production. There were no differences for IL-1beta, IL-6, lymphotoxin or TGF-beta expression in Linomide-treated vs nontreated control EAMG rats. We conclude that Linomide suppresses clinical EAMG as well as B and T cell responses to AChR by counteracting the production of AChR-induced Th1 and Th2 cytokines.

B cells are exquisitely sensitive to central tolerance and receptor editing induced by ultralow affinity, membrane-bound antigen

To assess the sensitivity of B cell tolerance with respect to receptor/autoantigen affinity, we identified low affinity ligands to the 3-83 (anti-major histocompatibility complex class I) antibody and tested the ability of these ligands to induce central and peripheral tolerance in 3-83 transgenic mice. Several class I protein alloforms, including Kbm3 and Dk, showed remarkably low, but detectable, affinity to 3-83. The 3-83 antibody bound Kb with K lambda approximately 2 x 10(5) M-1 and bound 10-fold more weakly to the Kbm3 (K lambda approximately 2 x 10(4) M-1) and Dk antigens. Breeding 3-83 immunoglobulin transgenic mice with mice expressing these ultralow affinity Kbm3 and Dk ligands resulted in virtually complete deletion of the autoreactive B cells from the peripheral lymphoid tissues. These low affinity antigens also induced receptor editing, as measured by elevated RAG mRNA levels in the bone marrow and excess levels of id- variant B cells bearing lambda light chains in the spleen. Reactive class I antigens were also able to mediate deletion of mature B cells when injected into the peritoneal cavity of 3-83 transgenic mice. Although the highest affinity ligand, Kk, was consistently able to induce elimination of the 3-83 peritoneal B cells, the lower affinity ligands were only partially effective. These results demonstrate the remarkable sensitivity of the deletion and receptor-editing mechanisms in immature B cells, and may suggest a higher affinity threshold for deletion of peripheral, mature B cells.

Suppression of experimental autoimmune encephalomyelitis by oral administration of myelin basic protein. VI. Suppression of adoptively transferred disease and differential effects of oral vs. intravenous tolerization

Antigen-driven tolerance is an effective method of suppressing cell-mediated immune responses. We have previously shown that oral administration of myelin basic protein (MBP) suppresses experimental autoimmune encephalomyelitis (EAE) when it is actively induced by MBP emulsified in complete Freund's adjuvant. In order to further study antigen-driven tolerance in this model, we investigated the effect of oral tolerization on adoptively transferred EAE and compared oral tolerance to intravenously (i.v.) administered MBP in both actively induced EAE and adoptively transferred EAE. Although orally tolerized animals were not protected from adoptively transferred EAE, spleen cells from orally tolerized animals suppressed adoptively transferred EAE when co-transferred with encephalitogenic cells or when injected into recipient animals at a different site at the time encephalitogenic cells were transferred. This suppression was mediated by CD8+ T cells, correlated with suppression of DTH responses to MBP, and was associated with decreased inflammation in the spinal cord. Unlike oral tolerization, spleen cells from i.v. tolerized animals did not suppress adoptively transferred EAE when co-transferred with encephalitogenic cells although i.v. tolerized animals were protected from adoptively transferred EAE. MBP peptides were then utilized to further characterize differences between i.v. and oral tolerization in the actively induced disease model. Both orally and intravenously administered MBP suppressed actively induced EAE. However, EAE was only suppressed by prior i.v. tolerization with the encephalitogenic MBP peptide 71-90, but not with the non-encephalitogenic peptide 21-40, whereas prior tolerization with 21-40 did suppress actively induced EAE when administered orally. These results suggest a different mechanism of tolerance is initiated by oral vs. intravenous administered antigen. Specifically, oral tolerization suppresses primarily by the generation of active suppression whereas the dominant mechanism of suppression associated with i.v. tolerization appears most consistent with the elicitation of clonal anergy.

Profound specific suppression by antigen of persistent IgM, IgG, and IgE antibody production

Ongoing, high-titer T-cell-dependent immune responses in adult mice, consisting of IgM, IgG, and IgE anti-fluorescein antibodies, can be specifically and substantially reduced (90-99%) when the mice are injected with appropriate doses of fluoresceinated dextran of defined molecular weight and hapten valence. This suppressive form of the antigen is nontoxic and specific, as responses to other antigens are unaffected. The suppression is long lasting and reduces high-affinity antibodies most markedly. Moreover, plasma cell secretion of specific antibody is virtually eliminated. This demonstrates that the reduction in antibody titer is not simply due to masking of serum antibody by the suppressive polymer. The results are discussed with reference to proposed models of B-cell and T-cell tolerance. Extension of these findings to disease-related immunogens may yield effective antigen-specific treatments of human allergy and autoimmune diseases.

Cellular and molecular mechanisms of B lymphocyte tolerance

A paradox of immunology is that the immune system is distributed so widely in the body, as a large number of cells that discharge most of their effector functions as single cells; but, at the same time, the elements of the system are so very interdependent, not only via specialized cell clusters and microenvironments, but also by mobile feedback loops, cellular and molecular. The end result is that one cannot really understand one element of the system without understanding every other, at least to a degree. Certainly, tolerance cannot be isolated from immune activation, nor B cell from T cell tolerance, rendering the task of the reviewer somewhat thankless. This being said, the last few years have seen wonderful progress in our grasp of B cell tolerance, to which the transgenic revolution has contributed a great deal. The fact that B cell tolerance exists as an important component of self-tolerance has been firmly established, as have the limits of the process in terms of both the survival of low-affinity antiself clonotypes and the question of location and concentration of antigen required for tolerance induction. Two processes have been identified as key alternatives: clonal abortion/maturation arrest/deletion and induction of clonal anergy. The latter requires a less strong Ig receptor crosslinking signal, may be partial, and is reversible. Recognition of these facts has prompted both experimentation and speculation on possible functions of the anergic cell. One unsatisfactory area, which we have not addressed because nothing like a consensus has been reached, is T cell-mediated suppression and its possible effects on tolerant states, including anergy induction in B cells. The phenomenology of suppression is too striking to sweep under the carpet, and suppressor T cell memory in particular (Adelstein et al., 1990) requires much more investigation; however, suppression has not been shown to play a major role in any of the best-studied transgenic models. These can readily be explained on the basis of direct interactions between the B cell target for abortion or anergy and the self antigen in question. The biochemical basis of discrimination between immunity and tolerance has also progressed, but not as fast. This is understandable, as so many signaling pathways have to come together for full immune induction, and as immaturity of the signal transduction pathway plays a profound role that must be studied in normal cells, with all the attendant difficulties of cell separation.(ABSTRACT TRUNCATED AT 400 WORDS)