Mucosal-associated invariant T (MAIT) cells are abundant in humans and recognize bacterial ligands. Here, we demonstrate that MAIT cells are also activated during human viral infections in vivo. MAIT cells activation was observed during infection with dengue virus, hepatitis C virus and influenza virus. This activation—driving cytokine release and Granzyme B upregulation—is TCR-independent but dependent on IL-18 in synergy with IL-12, IL-15 and/or interferon-α/β. IL-18 levels and MAIT cell activation correlate with disease severity in acute dengue infection. Furthermore, HCV treatment with interferon-α leads to specific MAIT cell activation in vivo in parallel with an enhanced therapeutic response. Moreover, TCR-independent activation of MAIT cells leads to a reduction of HCV replication in vitro mediated by IFN-γ. Together these data demonstrate MAIT cells are activated following viral infections, and suggest a potential role in both host defence and immunopathology.

CD8⁺T lymphocytes play a key role in host defense, in particular against important persistent viruses, although the critical functional properties of such cells in tissue are not fully defined. We have previously observed that CD8⁺T cells specific for tissue-localized viruses such as hepatitis C virus express high levels of the C-type lectin CD161. To explore the significance of this, we examined CD8⁺CD161⁺T cells in healthy donors and those with hepatitis C virus and defined a population of CD8⁺T cells with distinct homing and functional properties. These cells express high levels of CD161 and a pattern of molecules consistent with type 17 differentiation, including cytokines (e.g., IL-17, IL-22), transcription factors (e.g., retinoic acid-related orphan receptor γ-t,P= 6 × 10⁻⁹; RUNX2,P= 0.004), cytokine receptors (e.g., IL-23R,P= 2 × 10⁻⁷; IL-18 receptor,P= 4 × 10⁻⁶), and chemokine receptors (e.g., CCR6,P= 3 × 10⁻⁸; CXCR6,P= 3 × 10⁻⁷; CCR2,P= 4 × 10⁻⁷). CD161⁺CD8⁺T cells were markedly enriched in tissue samples and coexpressed IL-17 with high levels of IFN-γ and/or IL-22. The levels of polyfunctional cells in tissue was most marked in those with mild disease (P= 0.0006). These data define a T cell lineage that is present already in cord blood and represents as many as one in six circulating CD8⁺T cells in normal humans and a substantial fraction of tissue-infiltrating CD8⁺T cells in chronic inflammation. Such cells play a role in the pathogenesis of chronic hepatitis and arthritis and potentially in other infectious and inflammatory diseases of man.

Currently, no vaccine exists for hepatitis C virus (HCV), a major pathogen thought to infect 170 million people globally. Many studies suggest that host T cell responses are critical for spontaneous resolution of disease, and preclinical studies have indicated a requirement for T cells in protection against challenge. We aimed to elicit HCV-specific T cells with the potential for protection using a recombinant adenoviral vector strategy in a phase 1 study of healthy human volunteers. Two adenoviral vectors expressing NS proteins from HCV genotype 1B were constructed based on rare serotypes [human adenovirus 6 (Ad6) and chimpanzee adenovirus 3 (ChAd3)]. Both vectors primed T cell responses against HCV proteins; these T cell responses targeted multiple proteins and were capable of recognizing heterologous strains (genotypes 1A and 3A). HCV-specific T cells consisted of both CD4⁺ and CD8⁺ T cell subsets; secreted interleukin-2, interferon-γ, and tumor necrosis factor–α; and could be sustained for at least a year after boosting with the heterologous adenoviral vector. Studies using major histocompatibility complex peptide tetramers revealed long-lived central and effector memory pools that retained polyfunctionality and proliferative capacity. These data indicate that an adenoviral vector strategy can induce sustained T cell responses of a magnitude and quality associated with protective immunity and open the way for studies of prophylactic and therapeutic vaccines for HCV.

Liver inflammation is greater in nonalcoholic steatohepatitis (NASH) than steatosis, suggesting that immune responses contribute to nonalcoholic fatty liver disease (NAFLD) progression. Livers normally contain many natural killer T (NKT) cells that produce factors that modulate inflammatory and fibrogenic responses. Such cells are relatively depleted in steatosis, but their status in more advanced NAFLD is uncertain. We hypothesized that NKT cells accumulate and promote fibrosis progression in NASH. We aimed to determine if livers become enriched with NKT cells during NASH-related fibrosis; identify responsible mechanisms; and assess if NKT cells stimulate fibrogenesis. NKT cells were analyzed in wildtype mice and Patched-deficient (Ptc+/−) mice with an overly active Hedgehog (Hh) pathway, before and after feeding methionine choline-deficient (MCD) diets to induce NASH-related fibrosis. Effects of NKT cell-derived factors on hepatic stellate cells (HSC) were examined and fibrogenesis was evaluated in CD1d-deficient mice that lack NKT cells. NKT cells were quantified in human cirrhotic and nondiseased livers. During NASH-related fibrogenesis in wildtype mice, Hh pathway activation occurred, leading to induction of factors that promoted NKT cell recruitment, retention, and viability, plus liver enrichment with NKT cells. Ptc+/− mice accumulated more NKT cells and developed worse liver fibrosis; CD1d-deficient mice that lack NKT cells were protected from fibrosis. NKT cell-conditioned medium stimulated HSC to become myofibroblastic. Liver explants were 2-fold enriched with NKT cells in patients with non-NASH cirrhosis, and 4-fold enriched in patients with NASH cirrhosis. Conclusion : Hh pathway activation leads to hepatic enrichment with NKT cells that contribute to fibrosis progression in NASH. (Hepatology 2010;)

Human mucosal associated invariant T (MAIT) CD8+ and Tc17 cells are important tissue-homing cell populations, characterized by high expression of CD161 (++) and type-17 differentiation, but their origins and relationships remain poorly defined. By transcriptional and functional analyses, we demonstrate that a pool of polyclonal, precommitted type-17 CD161++CD8αβ+ T cells exist in cord blood, from which a prominent MAIT cell (TCR Vα7.2+) population emerges post-natally. During this expansion, CD8αα T cells appear exclusively within a CD161++CD8+/MAIT subset, sharing cytokine production, chemokine-receptor expression, TCR-usage, and transcriptional profiles with their CD161++CD8αβ+ counterparts. Our data demonstrate the origin and differentiation pathway of MAIT-cells from a naive type-17 precommitted CD161++CD8+ T-cell pool and the distinct phenotype and function of CD8αα cells in man.

Nonalcoholic steatohepatitis (NASH) is a leading cause of cirrhosis. Recently, we showed that NASH-related cirrhosis is associated with Hedgehog (Hh) pathway activation. The gene encoding osteopontin (OPN), a profibrogenic extracellular matrix protein and cytokine, is a direct transcriptional target of the Hh pathway. Thus, we hypothesize that Hh signaling induces OPN to promote liver fibrosis in NASH. Hepatic OPN expression and liver fibrosis were analyzed in wild-type (WT) mice, Patched-deficient (Ptc+/−) (overly active Hh signaling) mice, and OPN-deficient mice before and after feeding methionine and choline–deficient (MCD) diets to induce NASH-related fibrosis. Hepatic OPN was also quantified in human NASH and nondiseased livers. Hh signaling was manipulated in cultured liver cells to assess direct effects on OPN expression, and hepatic stellate cells (HSCs) were cultured in medium with different OPN activities to determine effects on HSC phenotype. When fed MCD diets, Ptc+/− mice expressed more OPN and developed worse liver fibrosis ( P < 0.05) than WT mice, whereas OPN-deficient mice exhibited reduced fibrosis ( P < 0.05). In NASH patients, OPN was significantly up-regulated and correlated with Hh pathway activity and fibrosis stage. During NASH, ductular cells strongly expressed OPN. In cultured HSCs, SAG (an Hh agonist) up-regulated, whereas cyclopamine (an Hh antagonist) repressed OPN expression ( P < 0.005). Cholangiocyte-derived OPN and recombinant OPN promoted fibrogenic responses in HSCs ( P < 0.05); neutralizing OPN with RNA aptamers attenuated this ( P < 0.05). Conclusion: OPN is Hh-regulated and directly promotes profibrogenic responses. OPN induction correlates with Hh pathway activity and fibrosis stage. Therefore, OPN inhibition may be beneficial in NASH (Hepatology 2011)