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Background:  CD28 is a key co-stimulatory molecule for lymphocyte activation. In the absence of CD28 ligation, T cells may become anergic or undergo apoptosis once exposed to antigen. In HIV infection, decreased CD28 expression on CD8 cells has been recognized. Whether this affects immunity directed at non-HIV antigens, and potentially contributes to the known altered course of hepatitis C virus (HCV) disease in the setting of HIV co-infection is unknown.

Methods:  Peripheral HCV-specific interferon (IFN)-γ producing CD4 and CD8 cell frequencies were measured in subjects with chronic HCV infection (n = 15), HCV/HIV co-infection (n = 15), and healthy controls (n = 10) by ELISpot assay using 10 pools of 18-mer peptides (overlapping by 11 amino acids) spanning the entire HCV-1 genome. Peripheral blood mononuclear cells (PBMC) and CD8-depleted PBMC were assayed separately and responses were thereby assigned as CD4 or CD8 mediated. Capability of memory-effector cells to respond to co-stimulatory signal was assessed by performing assays in the absence and presence of CD28 agonistic antibody. Proportions of CD8 cells that expressed CD28 were enumerated by FACS.

Results:  HCV-specific IFN-γ-producing CD4 cells were identified in 46% and 33% of HCV- and HCV/HIV-infected subjects. In the presence of CD28 agonist, 66% and 53% of the same groups had detectable CD4 responses. In the CD8 compartment 40% and 26% of HCV- and HCV/HIV-infected subjects had detectable responses in the absence of CD28 agonist, while in the presence of agonist, greater proportions of HCV- (73%) than HCV/HIV-infected subjects (26%, p = 0.01) had CD8 responses. Additionally, in the presence of agonist, both cumulative frequency of reactive CD8 cells and numbers of peptide pools recognized were greater in HCV- than in HCV/HIV-infected hosts (p = 0.02 and p = 0.02). Moreover, CD28-mediated co-stimulation increased the numbers of peptide pools targeted by CD8 cells in HCV (p = 0.03), but not HCV/HIV-infected subjects (p = 1.0). Finally, proportionally more CD8 cells expressed CD28 in HCV versus HCV/HIV-infected subjects (median 75.1% ± 10.9% vs 48.6% ± 14.8%; p = 0.02).

Conclusions:  These results indicate a reduced responsiveness of memory-effector CD8 cells to CD28 co-stimulation in HCV/HIV infected, but not HCV infected hosts, possibly reflecting an overall effect of HIV infection on activation/differentiation state of non-HIV antigen specific CD8 cell populations. This functional defect may play a critical role in the ability of the HIV-infected host to control HCV infection.

Keywords: CD28 co-stimulation; HCV/HIV co-infection; Tcell IFN-g