MicroRNAs and the Immune System

This blog entry will introduce a fascinating class of post-transcriptional regulators (microRNAs!) and discuss some of their functional relevance to immunology within the context of a recent paper:

J Immunol. 2009 Mar 1;182(5):2578-82.

Cutting edge: the Foxp3 target miR-155 contributes to the development of regulatory T cells.

Kohlhaas S, Garden OA, Scudamore C, Turner M, Okkenhaug K, Vigorito E.

It would probably be difficult to find a biological process that is not regulated to some extent by microRNAs. Immunology is certainly no exception. MicroRNAs (miRNAs) are short, endogenous, single stranded RNA molecules, which combine with a protein complex to specifically downregulate gene expression by destabilizing, degrading, or blocking translation of target mRNAs. Because miRNAs act to maintain homeostasis of biological activities, their misexpression (either upregulation or downregulation, depending on the particular target) can result in disease. As we will see, miRNA misexpression can even contribute to autoimmunity. A better understanding of miRNAs and their individual functions could lead to therapeutic opportunities for many diseases. For example, miRNAs which target genes associated with disease could be either inhibited or increased to treat that disease.

What follows is an in depth example of how one microRNA (miR-155) interacts with the immune system and how this interaction was studied by Kohlhaas et al. As we have learned, regulatory T cells (Tregs) suppress the activation of helper T cells which is very important for self-tolerance… Without Tregs, we would have overactive immune responses and self-reactivity (autoimmunity).

The authors of this paper introduce their research by summarizing the role of Foxp3 as a transcription factor that regulates Treg differentiation and suppressor function. Foxp3 expression establishes the protein and miRNA expression patterns typical of a Treg cell. The deletion of miRNA processors in Treg cells causes mice to develop a wasting disease similar to that of Foxp3 knockouts. Basically, this tells us that Foxp3 regulates miRNA expression and that those miRNAs are responsible for Treg differentiation and function.

Although it is unknown which miRNAs are responsible for Treg function, the authors decided to study miR-155 because it is highly expressed in Treg cells and because Foxp3 binds its promoter. Interestingly, the number of Tregs in miR-155 deficient mice were found to be 2-3 fold lower than in normal mice. This decrease in Tregs, of course, could have been due to a decrease in their development, proliferation, or survival so the authors went on to investigate miR-155’s affect on these phenomenon.

Investigating miR-155’s role in development:

Rag2 knockout mice (which lack mature lymphocytes because they have no Rag2 and thus no V(D)J rearrangement) were injected with wild type or miR-155 deficient bone marrow cells. Analysis seven weeks later showed that wild type bone marrow cells were able to reconstitute the lymphocytes while miR-155 deficient bone marrow cells gave rise to a smaller number of miR-155 deficient thymocytes and splenocytes. These results demonstrate that miR-155 is required for Treg development.

Investigating miR-155’s role in proliferation and survival:

To determine whether miR-155 affects Treg proliferation and survival, the authors transferred either normal or miR-155 deficient Treg cells into lymphopenic mice (which lack lymphocytes). After five weeks, Treg cells in the spleen and lymph nodes were counted and there were similar numbers of the normal and miR-155 deficient Treg cells, meaning that miR-155 is not required for proliferation and survival of Tregs in vivo.

So far, the results suggest that miR-155 is required for thymic development of Treg cells but not for their proliferation and survival in the periphery. Next the authors wanted to investigate whether miR-155 was critical for the suppressive function of Treg cells.

Investigating miR-155’s role in Treg suppressive function:

In order to determine whether or not miR-155 deficient Tregs could still suppress helper T cells, the authors cotransferred CD4⁺CD45RB^high T cells (which cause Th1/Th2 mediated colitis) with either normal Tregs or miR-155 deficient Tregs into immune deficient recipients. Mice which had received either normal Tregs or miR-155 Tregs were both protected from the colitis which would have otherwise been caused by the CD4⁺CD45RB^high T cells. This result showed that miR-155 does not regulate the suppressor function of Treg cells.

In summary, the results of these experiments show that miR-155 is required for Treg development but not for the proliferation, survival, or suppressor function of Treg cells in the periphery.

Questions/comments are very welcome! Below are a few questions to get you thinking about miRNAs and the immune system:

1) How do you think miR-155 could contribute to the development of Treg cells? What might miR-155 target?

2) How could this information (that miR-155 is required for thymic development of Tregs) be used therapeutically?

3) What further experiments could be useful to study miRNA interaction with the immune system? What would you have done differently in any of the above experiments?

4) The authors explain that deletion of miRNA processors in Treg cells causes mice to develop a wasting disease similar to Foxp3 knockouts. Could this be due to a decrease in Treg cells (no miR-155)? Or is it the lack of other miRNAs (which ones???) regulating Treg function?