Research Highlights

Abstract

Single-cell RNA-seq Reveals New Types of Human Blood Dendritic Cells, Monocytes, and Progenitors Villani A-C, Satija R, Reynolds G, et al. Science. 2017; 356(6335). Research in immunology is often complicated by challenges in clear communication: when we publish our work, are we talking about the same cells? The field is often fluid, with new cell types announced every few months. The past decade alone has seen the arrival of Th17 and Th22 cells, regulatory B cells, and a raft of innate lymphoid cells. When investigating leukocyte subsets in basic or translational studies, or when immune phenotyping patient cells in clinical trials, it is often frustrating to find comparisons between different studies impossible. Markers used to phenotype cells are different, and the gating strategies diverse. Replicating studies can be problematic, and understanding how various cells sit in a classification system is virtually impossible. To overcome the biased strategy of identifying and classifying leukocyte populations using expression of a specific set of surface markers or transcription factors, Villani and colleagues used deep single-cell RNA sequencing (scRNA-Seq) analysis of approximately 2400 cells from healthy donors.1 In this study, the focus was on dendritic cells and monocytes, and therefore HLA-DR+ lineage− cells were used. Transcriptomics have revolutionised studies in immunology and the rapidly evolving technology has accelerated discoveries. scRNA-Seq in particular, has brought about a radical shift in cellular analysis. RNA-Seq is largely replacing gene expression microarrays, because it allows highly sensitive and powerful analysis of expression across the transcriptome, returning data on both known and novel RNA. Villani et al used this strategy coupled with functional analyses to completely reclassify dendritic cells and monocytes based on a specific cell surface molecules. The proposed new classification system includes 6 dendritic cell subsets and 4 monocyte subsets, termed DC1-6 and Mono1-4, respectively. This hugely simplifies and validates the classification of cells in what is often a confusing field and ensures that dialogue is not lost in translation between different studies. Such an approach may be applied to a number of other leukocyte populations, with the hope that one day, we all have a common immunological language. REFERENCE Villani AC, Satija R, Reynolds G, et al. Single-cell RNA-seq reveals new types of human blood dendritic cells, monocytes, and progenitors. Science. 2017;356. IL-2 Therapy Restores Regulatory T-cell Dysfunction Induced by Calcineurin Inhibitors Whitehouse G, Gray E, Mastoridis S, et al. Proc Natl Acad Sci U S A. 2017 ;114(27):7083–7088. Regulatory T (Treg) cells are one of the major contenders in the field of transplant tolerance, and with trials in full swing, an accessible clinical therapy is fast approaching. The reality of introducing new therapies to transplantation, however, is complicated by the presence of regimens which already produce excellent short-term results. Immunosuppression withdrawal may help reveal the effects of a new therapy but is risky. The lack of useful primary outcome measures other than biopsy-proven rejection, and the inability for clinical trials to monitor outcomes over the long-term further complicate the situation. The current status is therefore to introduce cellular therapies, such as Treg cells, into a cocktail of active immunosuppressants with the risk that these completely neutralize any beneficial effect. Understanding how Treg cells behave in these situations is therefore vital. In the experimental setting, the challenge is separating the effects of drug immunosuppression from those of Treg cells. Although there are some data to show that certain immunosuppressive regimens appear more facilitative, such as alemtuzumab and rapamycin, the data are limited. In general, there is some agreement that the majority of immunosuppressants are unlikely to spare Treg cells, with calcineurin inhibitors (CNIs) being obvious culprits. In the study from Whitehouse and colleagues, CNIs were shown to be specifically detrimental to the most effective Treg cells by reducing their access to IL-2.1 Treg cells from patients on tacrolimus have reduced CD25 expression, low sensitivity to low levels of IL-2, and increased cell death. In an important finding, exogenous IL-2 was found to restore the functionality of Tregs. IL-2 synergized with CNI treatment in an in vivo model, without enhancing the activity of effector T cells. Interest in IL-2 is not new, with low-dose IL-2 therapy showing significant promise in the treatment of autoimmune diseases.2 The study from Whitehouse et al. however provides a useful mechanistic explanation for these findings and a rationale for combined application of drug immunosuppression with Treg cellular therapy. REFERENCES Whitehouse G, Gray E, Mastoridis S, et al. IL-2 therapy restores regulatory T-cell dysfunction induced by calcineurin inhibitors. Proc Natl Acad Sci U S A. 2017;114:7083–7088. Klatzmann D, Abbas AK. The promise of low-dose interleukin-2 therapy for autoimmune and inflammatory diseases. Nat Rev Immunol. 2015;15:283–294.