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Abstract
The hepatitis C virus (HCV) currently infects 3% of the world’s population, with chronic infection in 50-80% of exposed individuals. A small subset of individuals who are exposed to HCV do not develop anti-HCV antibodies, persistent viraemia or chronic hepatitis despite generating HCV-specific CD4+ and CD8+ T cells. These individuals are believed to develop an immune response which rapidly clears viraemia prior to the induction of an antibody response. Circumstantial evidence supports the likelihood that some of these individuals may generate these same responses and outcomes on repeated occasions of HCV infection.
HCV-specific cellular immune responses in seronegative subjects have been the subject of only limited prior study, in part due to the lack of appropriate recombinant antigens and assay systems. Therefore, this thesis described the development and validation of an interferon-? (IFN-?) ELISPOT assay using overlapping peptides (n=441). Using this assay, HCV-specific cellular immune responses were detected in 5/10 (50%) of chronically infected subjects. Responses were identified more frequently, and were directed against more regions of the HCV genome, than with traditional assay systems.
This IFN-? ELISPOT assay, a comparable interleukin (IL)-2 ELISPOT assay, and a multiplex in vitro cytokine production assay were then used to evaluate HCV-specific cellular immune responses in three cohorts of seronegative subjects at high-risk of exposure to HCV – babies born to infected mothers, multiply-transfused subjects with thalassaemia, and high risk injecting drug users.
Cellular immune responses were evaluated in 23 infants born to HCV-antibody positive women. Responses were not detected in infants born to HCV-PCR negative mothers. IFN-? production was detected in 1/11 infants born to viraemic mothers using the ELISPOT assay, with cytokine production observed in an additional 3/5 infants studied using the in vitro cytokine production assay.
HCV-specific cellular immune responses were assessed in a cohort of multiply transfused subjects with thalassaemia using assays for cytotoxic T lymphocyte activity, IFN-? and IL-2 ELISPOT, as well as lymphocyte proliferation and in vitro cytokine production. Responses were detected in 6/13 chronically infected subjects (46%), 4/7 subjects who had cleared infection (71%), and 14/17 seronegative subjects (82%). The seronegative subjects had responses which were broader and higher in magnitude than those with chronic HCV infection, although lower and narrower than in subjects who had cleared prior HCV infection.
IFN-? and IL-2 ELISPOT assays, in additional to in vitro cytokine production assays, were performed on 41 injecting drug users (IDUs), with responses detected in 6 (15%). Seronegative IDUs with HCV-specific cellular immune responses had been injecting for a mean of 7.7 years, and reported multiple risk factors for exposure to HCV.
The combined data from these three cohorts indicate that the HCV-specific cellular immune responses detected in seronegative subjects were generally broad in specificity. Cytokine production was generally Th1-biased, a pattern which has previously been associated with an increased likelihood of clearance in primary infection. The findings also suggest that responses can be maintained for decades after exposure, and may provide protection against repeated exposures. In summary, cellular immunity against HCV is evident in some seronegative high risk subjects, suggesting that the cellular immune responses may efficiently facilitate viral clearance. Understanding the mechanisms of this immune response pattern will allow better understanding of the host response to HCV and may provide key insights into vaccine design.