Hepatitis C virus dynamics and pathology: the role of CTL and antibody responses

Modeling the adaptive immune response in HBV infection

The aim of this work is to investigate a new mathematical model that describes the interactions between Hepatitis B virus (HBV), liver cells (hepatocytes), and the adaptive immune response. The qualitative analysis of this as cytotoxic T lymphocytes (CTL) cells and the antibodies. These outcomes are (1) a disease free steady state, which its local stability is characterized as usual by R (0) < 1, (2) and the existence of four endemic steady states when R (0) > 1. The local stability of these steady states depends on functions of R (0). Our study shows that although we give conditions of stability of these steady states, not all conditions are feasible. This rules out the local stability of two steady states. The conditions of stability of the two other steady states (which represent the complete failure of the adaptive immunity and the persistence of the disease) are formulated based on the domination of CTL cells response or the antibody response.

The evolutionary dynamics of a rapidly mutating virus within and between hosts: the case of hepatitis C virus

Many pathogens associated with chronic infections evolve so rapidly that strains found late in an infection have little in common with the initial strain. This raises questions at different levels of analysis because rapid within-host evolution affects the course of an infection, but it can also affect the possibility for natural selection to act at the between-host level. We present a nested approach that incorporates within-host evolutionary dynamics of a rapidly mutating virus (hepatitis C virus) targeted by a cellular cross-reactive immune response, into an epidemiological perspective. The viral trait we follow is the replication rate of the strain initiating the infection. We find that, even for rapidly evolving viruses, the replication rate of the initial strain has a strong effect on the fitness of an infection. Moreover, infections caused by slowly replicating viruses have the highest infection fitness (i.e., lead to more secondary infections), but strains with higher replication rates tend to dominate within a host in the long-term. We also study the effect of cross-reactive immunity and viral mutation rate on infection life history traits. For instance, because of the stochastic nature of our approach, we can identify factors affecting the outcome of the infection (acute or chronic infections). Finally, we show that anti-viral treatments modify the value of the optimal initial replication rate and that the timing of the treatment administration can have public health consequences due to within-host evolution. Our results support the idea that natural selection can act on the replication rate of rapidly evolving viruses at the between-host level. It also provides a mechanistic description of within-host constraints, such as cross-reactive immunity, and shows how these constraints affect the infection fitness. This model raises questions that can be tested experimentally and underlines the necessity to consider the evolution of quantitative traits to understand the outcome and the fitness of an infection.

Rapidly mutating viruses, such as hepatitis C virus, can escape host immunity by generating new strains that avoid the immune system. Existing data support the idea that such within-host evolution affects the outcome of the infection. Few theoretical models address this question and most follow viral diversity or qualitative traits, such as drug resistance. Here, we study the evolution of two virus quantitative traits—the replication rate and the ability to be recognised by the immune response—during an infection. We develop an epidemiological framework where transmission events are driven by within-host dynamics. We find that the replication rate of the virus that initially infects the host has a strong influence on the epidemiological success of the disease. Furthermore, we show that the cross-reactive immune response is key to determining the outcome of the infection (acute or chronic). Finally, we show that the timing of the start of an anti-viral treatment has a strong effect on viral evolution, which impacts the efficiency of the treatment. Our analysis suggests a new mechanism to explain infection outcomes and proposes testable predictions that can drive future experimental approaches.

Simple scaling laws for influenza A rise time, duration, and severity

Simple scaling laws are developed for the severity and characteristic time scales of influenza A infection in man. The scaling laws are based on a model of the infection described by six coupled ordinary differential equations that describe the time courses of the numbers of infectious viral particles, activated cytotoxic T-lymphocytes, interferon molecules, infected cells, uninfected cells, and the subset of uninfected cells that are protected by interferon from viral infection. Computer simulations show that the disease can be regarded approximately as a two-stage process. In the first stage, the growth in the number of infected cells is determined primarily by the interferon-enhanced limitation in the available number of target cells. In the second stage, the bulk of the duration of the infection is determined mainly by the destruction of the infected cells by the cytotoxic T-lymphocytes. The severity and characteristic times of the infection are found to depend simply on the logarithm of the initial number of viruses.

Dynamics of recurrent viral infection

In chronic viral infection, low levels of viral replication and infectious particle production are maintained over long periods, punctuated by brief bursts of high viral production and release. We apply well-established principles of modelling virus dynamics to the study of chronic viral infection, demonstrating that a model which incorporates the distinct contributions of cytotoxic T lymphocytes (CTLs) and antibodies exhibits long periods of quiescence followed by brief bursts of viral production. This suggests that for recurrent viral infections, no special mechanism or exogenous trigger is necessary to provoke an episode of reactivation; rather, the system may naturally cycle through recurrent episodes at intervals which can be many years long. We also find that exogenous factors which cause small fluctuations in the natural course of the infection can trigger a recurrent episode. Our model predicts that longer periods between recurrences are associated with more severe viral episodes. Four factors move the system towards less frequent, more severe episodes: decreased viral infectivity, decreased CTL efficacy, decreased memory T cell response and increased antibody efficacy.

Ecological and evolutionary principles in immunology

Experimental immunology has given rise to detailed insights into how immune cells react to infectious agents and fight pathogens. At the same time, however, the interplay between infectious agents and immune responses can be viewed as an ecological system in vivo. This is characterized by complex interactions between species of immune cells and populations of pathogens. This review discusses how an understanding of the immune system can be aided by the application of ecological and evolutionary principles: competition, predation, and the evolution of viruses in vivo. These concepts can shed light onto important immunological concepts such as the correlates of efficient virus control, immunodominance, the relationship between viral evolution and the development of pathology, as well as the ability of the immune system to control immunosuppressive infections.

Efficient induction of tumoricidal cytotoxic T lymphocytes by HLA-A0201 restricted, melanoma associated, L27Melan-A/MART-126–35 peptide encapsulated into virosomes in vitro

Cancer immunotherapy requires the induction of HLA class I restricted cytotoxic T lymphocytes (CTL) specific for tumor associated antigens (TAA). While a number of TAA have been identified, there is an urgent need for the development of adjuvants capable of stimulating CTL responsiveness. Previously, we reported the capacity of immunopotentiating reconstituted influenza virosomes (IRIV) to enhance CTL responses specific for synthetic peptides simultaneously added to cultures in soluble form. This effect was based on IRIV mediated activation of CD4(+) T cells. Here we investigated the "in vitro" immunogenicity of a novel virosome formulation coupling in a single reagent the adjuvant power of IRIV to the capacity of liposomes to efficiently encapsulate synthetic peptides. As a model epitope we chose L(27)Melan-A/Mart-1(26-35) HLA-A0201 restricted peptide from a melanoma-associated antigen widely used in tumor immunotherapy. The reagent thus developed induced the proliferation of CD4(+) T cells characterized by a T helper 1 cytokine profile and CXCR3 expression. Most importantly, it significantly enhanced the generation of L(27)Melan-A/Mart-1(26-35) specific CTL, as compared to soluble peptides, in particular at low nominal epitope concentrations (<1 microg/ml). These effector cells were able to efficiently kill HBL melanoma cells expressing Melan-A/MART-1 and HLA-A0201. The adjuvant effects observed were also detectable in the absence of CD4(+) T cells. Taken together our results suggest that this highly immunogenic antigenic formulation might qualify for clinical use in active, antigen-specific, melanoma immunotherapy.

The duration of in vitro stimulation with recall antigens determines the subset distribution of interferon-γ-producing lymphoid cells: A kinetic analysis using the Interferon-γ Secretion Assay™

Analyses of cellular immune responses during natural infections and following vaccination with established or candidate vaccines are becoming increasingly important and so are the research tools used to achieve this goal. During a recent evaluation of the analytical performance characteristics of one of these techniques, the interferon-gamma secretion assay, we noticed that following overnight incubation of PBMC with recall antigens (varicella-zoster antigen, Candida albicans antigen or hepatitis B surface antigen) NK cells are frequently the most predominant interferon-gamma-producing cell population. In this study, we monitored the subset distribution of interferon-gamma-producing cells following more extended in vitro culture periods and found that, irrespective of the antigen applied, the contribution of NK cells decreased whereas the importance of T cells and NKT cells rose. Analysis of the subset distribution showed that HBsAg stimulated CD4 cells predominantly whereas Candida antigen and varicella-zoster antigen were better inducers of CD8 responses. No correlation was found between the kinetics of total number of interferon-gamma-producing cells and the changes of concentrations of interferon-gamma in the culture supernatants. Interferon-gamma levels in culture supernatants correlated strongly with the kinetics of T(H) lymphocytes (CD3+, CD4+), CTL (CD3+, CD8+), and NKT cells (CD3+, CD56+). These observations lead us to conclude that methods that enumerate cytokine-secreting cells without determining their phenotype should be interpreted with great care and that an 'elispot' should not be directly considered as the footprint of a T lymphocyte.

Active Antigen-specific Immunotherapy of Melanoma: from Basic Science to Clinical Investigation

Advanced-stage melanoma here dismal prognosis, and novel therapeutic approaches are urgently required. The possibility of taking advantage of the immune response of patients for its treatment has been an appealing concept for almost a century. Only during the last decade, however, has the molecular identification of tumor-associated antigens (TAAs) offered the possibility of vaccinating patients (e.g., active induction of TAA-specific immune responses). Active antigen-specific immunotherapy (AASIT) is currently being investigated in a number of clinical centers as a treatment option for advanced-stage melanoma. A large number of melanoma TAAs have been molecularly characterized and are being used in vaccination trials in various molecular forms and according to various immunization protocols. Here we provide a short overview on melanoma TAAs, the technologies currently in use to induce specific cytotoxic T-lymphocyte (CTL) responses in vivo, and their monitoring. We also propose a tentative AASIT agenda for the next few years, aiming at improving the capacity to induce and monitor TAA-specific immune responses and to verify their clinical effectiveness.

Potent enhancement of cellular and humoral immune responses against recombinant hepatitis B antigens using AS02A adjuvant in healthy adults

Recombinant subunit protein vaccines generally elicit good humoral immune responses, weak helper T cell responses and no cytotoxic T cell responses. Certain adjuvants are known to enhance humoral and cellular immune responses. This study evaluated the humoral, CD4+ T helper and CTL responses induced by the recombinant SL* protein adjuvanted with AS02A in comparison with non-adjuvanted SL* in PBS in two groups of 15 healthy adult volunteers. The AS02A adjuvant contains monophosphoryl lipid A (MPL), QS21 and an oil in water emulsion. The adjuvanted vaccine induced fast and vigorous humoral and helper T cell responses of the Th1 type. Using a pool of overlapping 20mer peptides a cytotoxic response was detected in 6 out of 14 HLA-A2-positive (+) and HLA-A2-negative (-) recipients of the adjuvanted vaccine. All HLA-A2-positive subjects in the adjuvanted group and up to 30% of the subjects in the SL* PBS group displayed a CTL response against selected HLA-A2-restricted CD8+ T cell epitopes. The non-adjuvanted vaccine induced a very weak antibody response and no helper T cell responses. Local and general reactions were more frequently reported by AS02A recipients than in the non-adjuvanted group but the safety profile was considered acceptable. AS02A can be considered as a useful adjuvant that strongly enhances the cellular and humoral responses of subunit protein vaccines.

Recurrent Hepatitis C in Liver Allografts

RATIONALE AND DESIGN

The accuracy of a prospective histopathologic diagnosis of rejection and recurrent hepatitis C (HCV) was determined in 48 HCV RNA-positive liver allograft recipients enrolled in an "immunosuppression minimization protocol" between July 29, 2001 and January 24, 2003. Prospective entry of all pertinent treatment, laboratory, and histopathology results into an electronic database enabled a retrospective analysis of the accuracy of histopathologic diagnoses and the pathophysiologic relationship between recurrent HCV and rejection.

RESULTS

Time to first onset of acute rejection (AR) (mean, 107 days; median, 83 days; range, 7-329 days) overlapped with the time to first onset of recurrent HCV (mean, 115 days; median, 123 days; range, 22-315 days), making distinction between the two difficult. AR and chronic rejection (CR) with and without co-existent HCV showed overlapping but significantly different liver injury test profiles. One major and two minor errors occurred (positive predictive values for AR = 91%; recurrent HCV = 100%); all involved an overdiagnosis of AR in the context of recurrent HCV. Retrospective analysis of the mistakes showed that major errors can be avoided altogether and the impact of unavoidable minor errors can be minimized by strict adherence to specific histopathologic criteria, close clinicopathologic correlation including examination of HCV RNA levels, and a conservative approach to the use of additional immunosuppression. In addition, histopathologic diagnoses of moderate and severe AR and CR were associated with relatively low HCV RNA levels, whereas relatively high HCV RNA levels were associated with a histopathologic diagnosis of hepatitis alone, particularly the cholestatic variant of HCV.

CONCLUSIONS

Liver allograft biopsy interpretation can rapidly and accurately distinguish between recurrent HCV and AR/CR. In addition, the histopathologic observations suggest that the immune mechanism responsible for HCV clearance overlap with those leading to significant rejection.

Sequence evolution of putative cytotoxic T cell epitopes in NS3 region of hepatitis C virus

AIM: Quasispecies of hepatitis C virus (HCV) are the foundation for rapid sequence evolution of HCV to evade immune surveillance of hosts. The consensus sequence evolution of a segment of HCV NS3 region, which encompasses putative cytotoxic T cell epitopes, was evaluated.

METHODS: Three male patients, infected with HCV through multiple transfusions, were identified from clinical symptoms and monitored by aminotransferase for 60 months. Blood samples taken at months 0, 32, and 60 were used for viral RNA extraction. A segment of HCV NS3 region was amplified from the RNA extraction by RT-PCR and subjected to subcloning and sequencing. HLA types of these three patients were determined using complement-dependent microlymphocytotoxic assay. CTL epitopes were predicted using MHC binding motifs.

RESULTS: No patient had clinical symptoms or elevation of aspartate/alanine aminotransferase. Two patients showed positive HCV PCR results at all 3 time points. The other one showed a positive HCV PCR result only at month 0. A reported HLA-A2-restricted CTL epitope had no alteration in the HLA-A2-negative carrier over 60 months. In the HLA-A2-positive individuals, all the sequences from 0 month 0 showed an amber mutation on the initial codon of the epitope. Most changes of consensus sequences in the same patient occurred on predicted cytotoxic T cell epitopes.

CONCLUSION: Amber mutation and changes of consensus sequence in HCV NS3 region may be related to viral immune escape.