Hepatitis C virus (HCV) is a blood-borne pathogen that infects hepatocytes and causes
widespread destruction to the host immune system. Estimates suggest that HCV has
infected ~200 million people worldwide, and ~350,000-500,000 people die every year
from HCV-associated hepatic and extra-hepatic complications. HCV has developed
numerous mechanisms to evade host immune responses to establish persistence. Due to
the lack of a preventive vaccine, the mainstay of current treatment is a combination
therapy with interferon-α and ribavirin. The molecular mechanisms underlying the
establishment of persistent HCV disease remain poorly understood. A better
understanding of these mechanisms would aid design newer therapeutic targets and
improve the quality of life of HCV-infected patients
Here, we aimed to investigate the role of spontaneous apoptosis of immune cells,
expansion of senescent and exhausted virus-specific T-cells, as well as potential depletion
of circulating mucosal-associated invariant T (MAIT) cells, with immune impairment in
chronic HCV (CHC) disease. We recruited 62 chronically-infected HCV patients and 62
healthy controls (HCs) to conduct a cross-sectional investigation. Peripheral blood
mononuclear cells (PBMCs) were isolated, and co-cultured with HCV antigens and
phytohaemagglutinin (PHA) individually prior to the investigations (except for
apoptosis). Flow cytometry, quantitative real-time PCR (qRT-PCR), ELISA and
QuantiGene Plex 2.0 analyses were employed to investigate apoptosis in immune cells.
Multiparametric flow cytometry approaches were utilized to examine the phenotypes of
immunosenescent T-cells and expression of co-inhibitory molecules on HCV-specific T
cells, together with frequencies of circulating MAIT cells. Expression of molecules
associated with T-cell inhibition was confirmed by qRT-PCR.
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The ability of HCV to induce apoptosis in immune cells correlated with the increase
of apoptotic cells (Annexin V+PI+) and cellular reactive oxygen species (ROS).
QuantiGene Plex 2.0 analysis showed differential regulation of apoptotic pathways
involved in mitochondrial or activation of death receptors. Besides, the onset of
immunosenescence was clearly evident from the up-regulation of HLA-DR, CD38, CD57
and CD127 on HCV-specific CD4+ and CD8+ T-cells of chronic HCV-infected patients.
Furthermore, chronic HCV-infected patients displayed relatively significant increase of
late-differentiated T-cells. Chronic HCV infection also resulted in significantly increased
expressions of PD-1, CTLA-4, CD160 and TRAIL on HCV-specific CD4+ and CD8+ T
cells suggestive of immune exhaustion. Increase in the levels of pro-inflammatory
cytokines was also observed in PBMC cultures of chronic HCV-infected patients. MAIT
cells of HCs expressed elevated levels of CCR5 and CCR6. Conversely, all these
receptors were down-regulated on the circulating MAIT cells of chronic HCV-infected
patients. Expression of PD-1 was also higher on MAIT cells of chronic HCV-infected
patients relative to controls.
In conclusion, our observation suggests the spontaneous onset of apoptosis signaling
in chronic HCV disease, and increased frequency of late-senescent T-cells that lack the
potential to survive, possibly contributing to viral persistence. These phenotypically
defective HCV-specific T-cells may likely contribute to inadequate virus-specific T-cell
responses. Decreased frequency of MAIT cells with elevated levels of PD-1 may result
in diminished mucosal defense attributes, and could potentially contribute to HCV disease
progression