In Paper IV we performed a detailed characterization of peripheral blood MAIT cells in PSC. We observed reduced MAIT cell frequencies, phenotypic changes consistent with activation and possibly exhaustion, as well as impaired functional responses upon MR-1 dependent and independent stimulation. Furthermore, MAIT cells were found to be enriched in the biliary epithelium as compared to peripheral blood, and comparable to MAIT cell frequencies and numbers of non-PSC controls. This suggests that MAIT cells are retained in the compartment affected by chronic inflammation in PSC.
5.3.1 Influence of inflammation and liver disease on peripheral MAIT cells
MAIT cells are enriched in the healthy human liver (217) and have been studied to some extent in different liver diseases. In chronic hepatitis C virus infection (235) as well as in alcoholic liver disease (236) MAIT cells are lost from circulation and present with functional impairment. These findings are in line with our observations in patients with PSC. Thus, it appears that a common profile of the MAIT cell population in chronic liver inflammation is a
reduction in frequency combined with functional impairment. MAIT cell depletion and dysfunction in peripheral blood have also been described for several other human diseases where the immune system is chronically activated. Conditions affecting the MAIT cell population include both single organs, for example multiple sclerosis (237), rheumatoid arthritis (238), and IBD (239), as well as more general systemic inflammatory settings, such as chronic HIV-infection (240, 241), systemic lupus erythematosus (242), and in metabolic inflammation during obesity and type 2 diabetes (243). It is currently unknown whether a common underlying mechanism is accountable for MAIT cell loss and functional impairment in these conditions or whether specific mechanisms exist in each disease.
One hypothesis that could explain the abnormal MAIT cell population is loss of gut integrity and subsequent microbial translocation. Many of the above-described conditions associate with a “leaky” gut such as chronic HCV infection (244), alcoholic liver disease (245), and chronic HIV infection (246). Increased presence of intestinal bacteria and bacterial antigens and metabolites would then lead to MAIT cell activation, and over time MAIT cell
exhaustion and apoptosis as a consequence of the prolonged insult. Evidence for such a mechanism has been presented both for patients with chronic HIV infection as well as in alcoholic liver disease (245, 246). In the case of PSC, and except for the actual inflammation in the liver that could promote gut leakiness, many patients also have concomitant IBD further exacerbating the microbial translocation. Of note, we observed no significant
differences regarding the MAIT cell loss and functional impairment in patients with PSC and IBD, or in patients with PSC and IBD only. The exact process by which MAIT cells become activated (and exhausted) and eventually die still remains unknown. However, it is interesting to note that removal of the chronic insult, in the case of chronic HCV via treatment with direct acting antivirals (235), does not readily lead to a reinvigoration, suggesting that the insult inflicted on the MAIT cell population during chronic inflammation might be long- lasting and perhaps irreversible.
A recent report investigated MAIT cells in humans affected by different autoimmune liver diseases, including PSC (247). Our findings are in line with this report including loss of MAIT cells from circulation, an activated (and possibly exhausted) phenotype, as well as a functional impairment. Interestingly, the authors further suggested that MAIT cells could contribute to liver fibrosis development via production of IL-17 and subsequent hepatic stellate cell (HSC) activation (247). PSC-patients are known to have increased CD4 T cell Th17-responses (73). However, in this regard, we could not detect increased MAIT cell IL-17 levels, neither upon E.coli nor cytokine stimulation.
5.3.2 MAIT cells in the biliary epithelium
PSC is a disease of extra- and intrahepatic bile ducts. Although work in previous studies have tried to localize MAIT cells within the liver parenchyma, and have reported that the cells are present in portal tracts surrounding bile ducts (92, 247), it has been challenging to specifically assess MAIT cells in bile ducts. To overcome this, we established a novel method allowing sampling of bile duct tissue during ERCP procedures for subsequent MAIT cell analysis. In
line with the fact that MAIT cells are accumulated in peripheral organs, such as the liver as well as at mucosal barriers, we here report that MAIT cells are enriched four-fold in bile ducts as compared to peripheral blood. Intriguingly, and in contrast to the circulating MAIT cell compartment, we also observed retained levels of MAIT cells in bile ducts from PSC patients as compared to controls. The specific role for MAIT cells in the pathogenesis of PSC is currently unclear. However, the possibility to assess bile duct MAIT cells will allow for future studies on this topic. The recent development of SOC with high quality visualization is also interesting in this aspect (248). The possibility to visually identify bile ducts affected by inflammation may allow for a more precise sampling and detailed assessment of the
inflammatory process.
5.3.3 Limitations
Several limitations of our study have to be considered. First, we used a cross-sectional cohort with sampling at one time-point only. Second, we did not detect clear correlations between MAIT cells and parameters of disease severity. One reason for this might be that our cohort consisted of patients with relatively mild disease. Future studies should include larger patient populations and longitudinal sampling for identification of correlates between immune system function and PSC disease activity and progression. Third, and on a more
technological level, we identified MAIT cells with flow cytometry using the combination of antibodies against the TCR Vα7.2 chain and CD161. Although it would have been more advantageous to instead use an MR1-tetramer, this tool was not commercially available at the time of our study. To account for this, we restricted our analysis to CD8+ or CD4-CD8- Vα7.2+CD161+ T cells, since CD4+ Vα7.2+CD161+ T cells have been shown to contain more “non-MAIT” cells (249, 250).