Mucosal associated invariant T cells (MAIT cells) make up a subset of T cells that recognize bacteria metabolites presented by MCH class I- related protein (MR1). These cells can be found in the liver, mucosa, and blood and have an effector memory-like phenotype. Once activated, MAIT cells express IFN-g and granzyme B. In a paper published in JCI insight, the Prlic laboratory (Vaccine and Infectious Disease Division) at Fred Hutch worked to better understand MAIT cell trafficking in humans. Dr. Prlic explained the rationale for this study, “Immunologists have studied immune cell trafficking using animal model systems, but little to no human data are available that directly address immune cell trafficking. We specifically examined MAIT cells because these cells are a key player in barrier immunity and numerous studies reported that MAIT cells are decreased in blood in a wide range of diseases (chronic infections, autoimmune diseases, etc.). Thus, it has been postulated that MAIT cells do not leave tissues once they entered. Our collaboration with Mike Betts (UPenn) allowed us to study a unique (chylothorax) patient cohort and use state of the art RNAseq analysis to define the relationship of mucosal-associated invariant T (MAIT) cells in human lymph and blood and gain direct insight into human MAIT cell trafficking.”
The Prlic lab first identified MAIT cells in the blood and thoracic duct lymph by flow sorting for cells with CD161highVa7.2+CD3+ expression and found that most MAIT cells were CD8 positive and CCR7 negative. The group also looked at expression of CXCR3, CCR6, and CCR4 and showed that lymph MAIT cells expressed CXCR3 and CCR6. This suggests that lymph MAIT cells are able to respond to proinflammatory chemotactic cues (sensed by CXCR3) and undergo tissue-homing (mediated by CCR6). In contrast their blood counterparts only expressed CCR4. This suggests that the MAIT cells in the lymph migrated from the tissue. Next the researchers used the RNAseq analysis to look for transcriptional differences between the two groups. Cells from 4 donors were selected by expression of CD161highVa7.2+CD3+ CD8+ and 200 cells were sorted for analysis. Using the RNAseq data, the group looked for clonal relatedness between the compartments by TCR repertoire, the more similar the repertoire the more related the T cells. They found that TCRa and TCRb repertoires were very similar. Many TCRb clonotypes were shared between the blood and lymph for each donor. This suggests that clonotypes are able to exit tissue via the lymphatics.
On closer examination of the activation signature of the MAIT cells in blood and lymph, both PD-1 and CD27 expression was similar. These markers suggest that that MAIT cells in both locations have a similar activation status. However, when comparing the full transcriptome produced by RNAseq, 62 transcripts were highly expressed in the blood versus lymph and 15 were upregulated in the lymph compared to blood (see figure). Many transcripts found upregulated in the blood were part of the IFN-inducible gene family as expected. To further validate the differences seen between the MAIT cell locations, 27-color flow cytometry was used. Samples from three donors were analyzed and dimensionality reduction using t-stochastic neighbor embedding (t-SNE) was done. The t-SNE plots isolated MAIT cells from other populations and showed a phenotypic heterogeneity between the two MAIT cell groups (based on 17 surface biomarkers). Interestingly, all donors showed distinct donor specific separation between MAIT cell groups. Together, these data support the idea that MAIT cells from the different compartments are phenotypically distinct and that MAIT cells in the lymph entered through tissue egress. Recirculation between blood and lymph is most likely, supported by the fact that the TCR repertoires were similar and the clonotypes overlapped. When asked about future work, Dr. Prlic said, “We showed in this study that MAIT cells exit tissues and recirculate. However, that does not exclude the possibility that MAIT cells could cease to leave tissue in certain conditions. In that context, it would be really interesting to address next if MAIT cells are retained in the tissue when there is local inflammation, but this will likely have to be addressed in an animal model system.”
Voillet V, Buggert M, Slichter CK, Berkson JD, Mair F, Addison MM, Dori Y, Nadolski G, Itkin MG, Gottardo R, Betts MR, Prlic M. 2018. Human MAIT cells exit peripheral tissues and recirculate via lymph in steady state conditions. JCI Insight. Epub.
Funding provided by the National Institutes of Health.