CD56(pos) natural killer (NK)/natural T (NT) cells are important innate effectors providing the first line of defense against viral infection. Enhanced NK activity has been shown to protect from human immunodeficiency virus-1 infection. However, the role played by these innate effectors in protection against or development of hepatitis C virus (HCV) infection is unknown. We characterized CD56(pos) populations in 11 injection drug users (IDUs) who remained uninfected despite being repeatedly exposed to HCV. NK profiles in exposed but uninfected (EU) individuals were compared with preinfection samples (median 90 days prior to HCV seroconversion) collected from 14 IDUs who were exposed and subsequently became infected (EI) and unexposed normal control subjects (n = 8). Flow cytometric analysis of CD56(pos) populations demonstrated that EUs had a higher proportion of CD56(low) mature (P = 0.0011) NK cells compared with EI subjects. Bead-isolated NKs ( > 90% purity) from EUs had significantly higher interleukin-2 (IL-2)-induced cytolytic activity against the NK-sensitive cell line K562 at an effector-to-target ratio of 10:1 (P < 0.0001). NKp30, a natural cytotoxicity receptor involved in NK activation, is highest on NK/NT cells in EUs relative to infected subjects. Using the JFH-1 infection system, we demonstrated that NKp30(high) cells in the absence of exogenous stimulation significantly reduce infection of hepatocytes.
Conclusion: CD56(pos) populations in EUs are enriched for effector NKs displaying enhanced IL-2-induced cytolytic activity and higher levels of the natural cytotoxicity receptor NKp30-activating receptor. In addition, NKp30(high) cells are more effective in preventing infection of Huh-7.5 cells than their NKp30(low/neg) counterparts. These data support the hypothesis that NK cells contribute to anti-HCV defense in vivo in the earliest stages of infection, providing innate protection from HCV acquisition.