NEW APROACES IN ANTI HCV IMMUNE RESPONSE STUDIES
Dr. Flory Revnic *B.Sc (London) Ph.D., Dr. Bogdan Paltineanu* M.D.Ph.D., Dr.Gabriel Prada** M.D.Ph.D., Dr.Speranta Prada* M.D.Ph.D, Dr.Cristian Romeo Revnic ***M.D.Ph.D., Dr. Catalina Pena M.D.Ph.D.
*NIGG „Ana Aslan”
**UMF „Carol Davila”
***Ambroise Pare`Hospital, University of Medicine, Paris VI, France
In ciuda dificultatilor de a studia HCV, s-a facut un progres considerabil in caracterizarea raspunsului imun anti HCV. S-a estimat ca aproximativ 20% din indivizi sunt capabili sa se vindece spontan urmare a infectiei acute cu HCV. In vreme ce restul devin cronici. Studiile longitudinale a doua cohorte de pacienti in cursul si – post infectie acuta au dus la definirea corelatiilor imunologice care sunt asociate cu eliminarea virala .
Cuvinte cheie: hepatita cu virus C, raspuns imun, infectie acuta cu virusul hepatitei C.
Despite the challenges in studying HCV, considerable progress has been in made in characterizing anti-HCV immune responses. It has been estimated that approximately 20% of individuals are able to clear the infection spontaneously following acute HCV infection, whereas the rest progresses to chronicity. Longitudinal studies of the two cohorts of patients during and after acute infection have defined immunologic correlates that are associated with viral clearance.
Key words: hepatitis type C virus, immune response, acute infection with hepatitis C virus
A strong T-cell response, characterized by the production of effector cytokines including IFN-?, and broad epitope specifically correlate with the resolution of acute infection. After clearance of the acute infection, memory T cells are maintained, but whether they can provide protection against reinfection is incompletely understood.. While usually not resulting in sterilizing immunity, that is, prevention of acute infection after re-exposure especially to antigenically more divergent HCV strains, adaptive immunity protects against progression to chronic infection following repeated HCV exposure.. As chronic infection persists, the number of epitopes recognized decreases and T-cell responses are often lost. Since HCV-associated morbidity and mortality are caused by chronic infection, a vaccine, even if it only prevents viral persistence, would greatly ameliorate the problem. Although neutralizing antibodies are present during the chronic phase of infection, these antibodies are not able to clear the virus.[ Several mechanisms of viral escape from antibody-mediated neutralization have been postulated and tested (reviewed in). Recently, several human monoclonal antibodies against HCV envelope protein E1 or E2, which show crossneutralizing capability, were identified These antibodies were able to prevent infection of heterologous HCV in the HCV pseudoparticle and HCV cell culture model system, suggesting passive prophylaxis with exogenous neutralizing antibodies or eliciting high-affinity antibodies with similar specificity representing a viable strategy to prevent or more efficiently control HCV infections.
Although resolution of the infection is dependent on adaptive immunity, innate responses are also observed early after HCV infection. Type I interferons and interferon response pathways are induced in the liver at early stages of infection regardless of the clinical outcomes. The fact that various strategies have evolved in the HCV life cycle to interfere with the IFN response indicates that the innate response exerts a significant pressure on HCV. Moreover, recent genome-wide association studies have identified single-nucleotide polymorphisms in the IL-28B gene locus that correlate with spontaneous clearance of an acute HCV infection and predict to a certain extent how likely patients with a given combination of IL28B alleles are to respond to peg-IFN/RBV therapy.[6,7,]. Genetic analysis has also revealed the important role of natural killer (NK) cells, which produce IFN-? and are abundant in the liver. Genetic polymorphisms that affect the threshold of NK-cell activation influence the clinical outcomes of HCV infection. Recent genetic data suggest that taking into account the combination of polymorphisms within the loci of IL28B, HLA-C and its ligands, the killer immunoglobulin-like receptors has greater predictive value for clearance of HCV infection. These results not only highlight the importance of innate immunity during HCV infection, but also suggest the efficacy of a certain vaccine may depend on the genetic features of recipients.
Approaches of Vaccination
Along with the efforts in improving our understanding of the basic immunology of HCV infection, various approaches have been taken to develop vaccines. Specific approaches in the development of both prophylactic and especially therapeutic vaccines against HCV infection have been recently reviewed in great detail elsewhere.[ In this article, the authors limit the discussion on general principals pertaining to the different vaccination approaches and highlight candidate vaccination approaches that are in active clinical development.
Prophylactic vaccinations aim at preventing infection often through the induction of a pathogen-specific humoral immune response. However, the role of neutralizing antibodies in protection against HCV infection remains controversial Although only few founder viruses appear to initiate the HCV infection during transmission antigenically diverse viral variants are readily produced once HCV starts to replicate. The antigenic diversity poses further challenges to the prophylactic vaccination approach. Early attempts focused on inducing the production of neutralizing antibodies against envelope proteins of HCV, E1 and E2. This was inspired by the success of HBV vaccines, which induce antibodies against HBV surface antigens, thereby preventing viral entry and infection. Induction of HCV envelope-specific antibodies in naive chimpanzees by vaccination with recombinant E1 and E2 or DNA yielded protection from virus challenge.. Similarly, immunization of healthy human volunteers with HCV envelope glycoproteins elicits antibodies that crossneutralize heterologous virus strains in vitro A major challenge remains in the identification of suitable immunogens that elicit broadly neutralizing antibody responses. The major antigen determinants within the viral envelope are in the hypervariable-region 1 of the E2 glycoprotein, which, as the name implies, is not necessarily suitable to confer broad protection against antigenically diverse viruses. It has been speculated that more broadly shared epitopes will become accessible when the HVR1 region is deleted from the viral envelope. However, the idea of engineering the immunogenicity of HCV by exposing better-conserved epitopes remains to be tested. Furthermore, analysis of the structural details of (conformational) epitopes recognized by antibodies with broad neutralizing activity may provide a starting point for the design of immunogens capable of eliciting antibodies with similar activity.
Prophylactic vaccination approaches are not limited to those geared towards inducing neutralizing antibodies. Clinical trials are ongoing to assess the efficacy, safety and immunogenicity based on the sequential use of adeno- and/or modified vaccinia Ankara (MVA) vectors expressing HCV nonstructural proteins NS3-NS5BConceivably, combining the approaches that prime both humoral and cellular immunity would protect more efficiently against HCV challenge, although the concept remains to be tested in suitable animal models and/or clinical trials.
Immunotherapeutic Approaches. As an immunotherapeutic approach, immune cells with antiviral activities are transferred to enhance the endogenous immune response in the recipient. In a study, HCV-infected patients who have undergone liver transplantation were infused with lymphocytes extracted from the liver allografts. These lymphocytes include abundant NK and NK T cells, and were treated in vitro with IL-2/anti-CD3 mAbs before infusion. This treatment markedly lowered the HCV RNA titers in patients and completely prevented HCV infection in human liver-chimeric mice. Furthermore, it has been shown that CD56+ cells obtained from the peripheral blood mononuclear cells show anti-HCV activity. However, unlike T and B cells, NK cells lack an antigen-recognition receptor for distinguishing healthy and infected cells. Instead, their responses depend on the signals from inhibitory and activating receptors. A putative solution may be to guide NK cells to HCV-infected target cells using bispecific antibodies binding to an invariant domain of an activating receptor and viral antigens/antigen–MHC complexes on the target cell.
The efficacy of HCV vaccines may also be enhanced by immunomodulatory treatments. As overly activated T-cell response can cause excessive tissue damage, many regulatory mechanisms are in place to keep T-cell activity in check. For instance, T cells express the inhibitory receptor programmed death-1 (PD-1)[ and T-cell immunoglobulin and mucin domain-containing molecule 3 (Tim-3). Ligation of these regulatory receptors by their ligands induces negative signals to the responding cells and leads to reduction of cytokine production and cell proliferation Moreover, Foxp3+ Treg, which is a specialized CD4+ T cell, can suppress the function and proliferation of effector T cells. These regulatory mechanisms together limit T-cell responses during chronic infection, and also affect the efficacy of a vaccine. It has been shown that blockade of PD-1 or Tim-3 can enhance the proliferation and cytotoxicity of HCV-specific cytotoxic T lymphocytes. With this rationale, the effect of inhibitory receptor blockade or Treg depletion on the efficacy of a HCV vaccine needs to be examined.
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