Antigenic homology of HIV-1 GP41 and human platelet glycoprotein GPIIIa (integrin beta3)

Emerging Concepts in Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease defined by low platelet counts which presents with an increased bleeding risk. Several genetic risk factors (e.g., polymorphisms in immunity-related genes) predispose to ITP. Autoantibodies and cytotoxic CD8⁺ T cells (Tc) mediate the anti-platelet response leading to thrombocytopenia. Both effector arms enhance platelet clearance through phagocytosis by splenic macrophages or dendritic cells and by induction of apoptosis. Meanwhile, platelet production is inhibited by CD8⁺ Tc targeting megakaryocytes in the bone marrow. CD4⁺ T helper cells are important for B cell differentiation into autoantibody secreting plasma cells. Regulatory Tc are essential to secure immune tolerance, and reduced levels have been implicated in the development of ITP. Both Fcγ-receptor-dependent and -independent pathways are involved in the etiology of ITP. In this review, we present a simplified model for the pathogenesis of ITP, in which exposure of platelet surface antigens and a loss of tolerance are required for development of chronic anti-platelet responses. We also suggest that infections may comprise an important trigger for the development of auto-immunity against platelets in ITP. Post-translational modification of autoantigens has been firmly implicated in the development of autoimmune disorders like rheumatoid arthritis and type 1 diabetes. Based on these findings, we propose that post-translational modifications of platelet antigens may also contribute to the pathogenesis of ITP.

Human Immunodeficiency Virus Proteins Mimic Human T Cell Receptors Inducing Cross-Reactive Antibodies

Human immunodeficiency virus (HIV) hides from the immune system in part by mimicking host antigens, including human leukocyte antigens. It is demonstrated here that HIV also mimics the V-β-D-J-β of approximately seventy percent of about 600 randomly selected human T cell receptors (TCR). This degree of mimicry is greater than any other human pathogen, commensal or symbiotic organism studied. These data suggest that HIV may be evolving into a commensal organism just as simian immunodeficiency virus has done in some types of monkeys. The gp120 envelope protein, Nef protein and Pol protein are particularly similar to host TCR, camouflaging HIV from the immune system and creating serious barriers to the development of safe HIV vaccines. One consequence of HIV mimicry of host TCR is that antibodies against HIV proteins have a significant probability of recognizing the corresponding TCR as antigenic targets, explaining the widespread observation of lymphocytotoxic autoantibodies in acquired immunodeficiency syndrome (AIDS). Quantitative enzyme-linked immunoadsorption assays (ELISA) demonstrated that every HIV antibody tested recognized at least one of twelve TCR, and as many as seven, with a binding constant in the 10-8 to 10-9 m range. HIV immunity also affects microbiome tolerance in ways that correlate with susceptibility to specific opportunistic infections.

Modeling HIV-1 Induced Neuroinflammation in Mice: Role of Platelets in Mediating Blood-Brain Barrier Dysfunction

The number of HIV-1 positive individuals developing some form of HIV-associated neurocognitive disorder (HAND) is increasing. In these individuals, the integrity of the blood-brain barrier (BBB) is compromised due to an increase in exposure to pro-inflammatory mediators, viral proteins, and virus released from infected cells. It has been shown that soluble CD40L (sCD40L) is released upon platelet activation and is an important mediator of the pathogenesis of HAND but the underlying mechanisms are unclear, emphasizing the need of an effective animal model. Here, we have utilized a novel animal model in which wild-type (WT) mice were infected with EcoHIV; a derivative of HIV-1 that contains a substitution of envelope protein gp120 with that of gp80 derived from murine leukemia virus-1 (MuLV-1). As early as two-weeks post-infection, EcoHIV led to increased permeability of the BBB associated with decreased expression of tight junction protein claudin-5, in CD40L and platelet activation-dependent manner. Treatment with an antiplatelet drug, eptifibatide, in EcoHIV-infected mice normalized BBB function, sCD40L release and platelet activity, thus implicating platelet activation and platelet-derived CD40L in virally induced BBB dysfunction. Our results also validate and underscore the importance of EcoHIV infection mouse model as a tool to explore therapeutic targets for HAND.

Escape of pathogens from the host immune response by mutations and mimicry. Possible means to improve vaccine performance

The ability of certain pathogens, such as human immunodeficiency, hepatitis C, herpes simplex, influenza viruses, Plasmodium falciparum, etc., to escape from host immune response is generally ascribed to high mutation rate of their genome. We challenge this assumption and propose that molecular mimicry of host antigens by these pathogens could also participate to this resistance. Several studies show that there is no correlation between the mutation rate value of a pathogen and the possibility to develop an effective vaccine. On the other hand, pathogens which do not respond to vaccine are usually reported to display host protein mimicry. We propose to suppress in the thymus the epitopes of the self which are in common with the pathogen. This could be achieved by intrathymic injection of antibodies against this microorganism. These antibodies would be obtained by vaccination of a foreign animal species. It is expected that the negative selection of the CD4(+) and CD8(+) T lymphocytes specific for these epitopes would be prevented, that the number of epitopes recognized as foreign to the host would be increased and that the immune response diversity would be enhanced.

HIV and SIV Associated Thrombocytopenia: An Expanding Role for Platelets in the Pathogenesis of HIV

Thrombocytopenia is common in HIV and SIV infection, and is often associated with disease progression. HIV and SIV-associated thrombocytopenia arise through multiple mechanisms, including decreased platelet production, increased platelet destruction due to HIV-mimetic anti-platelet antibodies, and increased use of activated platelets. Activated platelets have the potential to contribute to the pathogenesis of HIV and SIV by interacting directly with inflammatory cells and endothelium and by releasing soluble immunomodulatory cytokines.

Antiplatelet antibodies in cases of Glanzmann's thrombasthenia with and without a history of multiple platelet transfusion

Antiplatelet antibodies are known to be present in a wide spectrum of patients, which include chronic Idiopathic Thrombocytopenic Purpura (ITP), infections, etc., including Glanzmann's thrombasthenia (GT) patients who receive multiple platelet transfusions. The presence of natural antibodies to platelet receptors is not studied in cases of GT. We studied the antiplatelet antibodies in 23 patients with GT, 15 of which had received multiple transfusions and eight that had not received transfusions, along with 50 cases of chronic ITP. The prevalence and specificity of platelet-bound antibodies were detected by inhibition assays using O-group platelets on flow cytometry. The mean antiplatelet antibodies in 15 patients of GT who had not received transfusions and eight patients with multiple transfusions was 8427 + 2131.88 and 9038 + 2856 antibodies/platelet, respectively, while in case of the 50 ITP patients studied, it was 22166 + 5616 antibodies/platelet (Normal Range 1500–3200 antibodies/platelet). We conclude that GT patients who have not received transfusions may develop antiplatelet antibodies to the missing/abnormal receptor. Whether this is due to a molecular mimicry or due to some other mechanism needs to be explored.

Exclusion of HIV epitopes shared with human proteins is prerequisite for designing safer AIDS vaccines

BACKGROUND

The safety of a potential AIDS vaccine is an issue that will become critical at later stages of product development and needs to be addressed before it is too late.

OBJECTIVE

In order to design safer vaccine, the HIV antigens, to be deployed in it, should be free of regions that are either present in human proteins or exhibit pronounced structural similarity to proteins responsible for important physiological functions.

STUDY DESIGN

The approach is based on the use of an original matrix predicting the antigenic similarity of amino acids. This mathematical approach developed by us was applied for identification of fragments with similarity to human proteins within potentially immunodominant regions of HIV proteins. A potential self-sensitization by viral quasispecies with variants of hypervariable V3 region, generated as a result of immune pressure on the immunodominant region of envelope, was considered in detail.

RESULTS

Viral fragments occurring in normal human proteins as well as regions exhibiting high similarity to proteins responsible for physiological homeostasis were identified in every HIV protein at a frequency higher than expected. Most such regions contained either T-cell (CD8(+) CTL or CD4(+) Helper) or B-cell epitopes, or both of them simultaneously. The gained knowledge was applied in designing a synthetic immunogen containing multiple CTL epitopes. The synthesis of series of chimeric peptides representing hypervariable region of V3 loop of HIV envelope, to be used as a multi-epitope or mixotope vaccine candidate, has been achieved. Such a vaccine could theoretically pre-empt any escape mutant borrowing from antigenic diversity of hypervariable region of V3 loop of HIV envelope.

CONCLUSIONS

The epitopes shared by HIV and its host are likely to be implicated in the immunopathogenesis of AIDS through induction of cross-reacting effectors of the immune system. The prospect that 'house-keeping' immune mechanism can be foiled by molecular mimicry of HIV with physiologically important human proteins should be taken into consideration in safer vaccine design.

Prevalence of antiphospholipid and antiplatelet antibodies in human immunodeficiency virus (HIV)-infected Chilean patients

Antiphospholipid (aPL) and antiplatelet (aPlt) antibodies, found in patients with autoimmune diseases, are also detected in infectious diseases. The purpose of this study was to examine the prevalence of these antibodies in HIV patients and to evaluate an association of these antibodies with thrombocytopenia and/or thrombosis. Sixty-three HIV-seropositive patients and 52 normal controls were studied. Anti-cardiolipin (aCL), anti-beta(2) glycoprotein I (anti-beta(2)GPI), and antiprothrombin (aPT) antibodies were determined and the lupus anticoagulant (LA) test was performed. Antiplatelet antibodies (aPlt) were also determined. Seven out of 63 (12.7%) HIV patients were positive for aCL, four of 63 (6.3%) for anti-beta(2)GPI, and five of 63 (7.9%) for aPT. No patients studied were LA positive. Six out of 63 (9.5%) patients were positive for aPlt. One of them showed weak reactivity for GPIb-IX. The platelet count of patients (202+/-63 x 10(3) platelets/microL) was significantly lower than in the controls (343+/-6 x 10(3) platelets/microL) (P<0.001). There was no correlation between the presence of aPL and/or aPlt and thrombocytopenia. Of the HIV-infected patients, 22.2% presented aPL and 9.4% aPlt antibodies. In this study, the presence of aPL and aPlt antibodies was not associated with the development of thrombosis and/or thrombocytopenia.

Kinetics and characterization of cellular responses in the peritoneal cavity of mice infected with Taenia crassiceps

Changes in the leukocyte population of the peritoneal cavity ensue immediately after infection with Taenia crassiceps metacestodes. Basophils and neutrophils decrease, whereas macrophages, monocytes, and lymphocytes increase to reach only modest levels by 6 wk and then diminish to nearly disappear by 15 wk when the parasite begins rapid reproduction. Eosinophils also appear early in infection, but then abate to lower levels that persist. In late infections, when the mass of cysticerci equals that of the mouse, the cysticerci grow among surprisingly few inflammatory cells. Mingling with the peritoneal inflammatory cells is a number of odd-looking cells that could correspond to the metaplasic mesothelial cells of the host or be of parasite origin. These cells are multinucleated, they aggregate in varigerated clusters, and form cystic structures in vitro; they also bind specific anti-T. crassiceps antibodies and specific T. crassiceps DNA probes in their nuclei. When the peritoneal cell exudate is reinjected intraperitoneally into naive mice, the odd-looking cells subsist for months, inducing in the host the synthesis of specific anti-T. crassiceps antibodies and immune resistance to challenge but do not reassemble into cysticerci even after 6 mo of inoculation. The early appearance and the immunogenic and antigenic properties of these odd-looking cells suggest they are important protagonists in the early host-parasite confrontation when the outcome of infection is set.