IL-10 is a multipurpose cytokine. The research suggest IL-10 dampens immune response. Many cell types including T cells, B cells or macrophages produce IL-10. IL-10 receptor is made of two subunits, alpha and beta. Alpha subunit is specific for IL-10 and beta subunit is shared with other cytokines, for example IL-22. Since many cell types produce and sense IL-10, it is not immediately clear how redundant is IL-10 system. The animal model mostly used to test the role of IL-10 is an equivalent of human inflammatory bowel diseases (IBD). These studies showed that IL-10 played critical role in “educating” immune system to tolerate the presence of gut microbiota. IL-10KO mice develop spontaneous IBD. However, germ-free IL-10KO mice has no IBD.
Recently, two back to back studies published in Immunity try to explain mechanistically how exactly IL-10 prevents IBD.
First study came from Steffen Jung's lab (1). In my opinion, this particular paper is very simple and it is unlikely it would have been accepted for publication in Immunity by itself. I will highlight only the most relevant data from this study.
First, the authors showed that in the absence of IL-10 gut resident macrophages, identified as CX3CR1-GFP positive cells, adopt pro-inflammatory phenotype. This is expected.
Next, the authors generated gut macrophage-specific IL-10 or IL-10Ralpha deficient mice by crossing CX3CR1-cre mice to IL-10 fl/fl or IL-10Ralpha fl/fl mice.
Interestingly, CX3CR1-cre IL-10 fl/fl mice develop normally and did not show any sign of IBD. This suggested that gut macrophage-derived IL-10 is dispensable for protection against IBD. This is a new finding.
However, CX3CR1-cre IL-10Ralpha fl/fl mice developed IBD, similar to total IL-10KO mice. This suggested that sensing of IL-10 by gut macrophage was essential for protection against IBD. This is a new and unexpected finding.
The most obvious weakness of this paper is the direct use of KO mice without adoptive transfer experiments. Since CX3CR1 can be expressed by other cell types (Dendritic cells, T cells), CX3CR1-cre IL-10Ralpha fl/fl mice phenotype could have been affected by function of non-macrophage population.
Luckily, second paper (2) from Scott Snapper's Lab provided necessary data to fill the holes in the story. It contains enormous amount of data. I will highlight the most important findings.
The authors showed that IL-10Rbeta KO mice develop IBD. However, RAG2 KO / IL-10Rbeta KO mice lacking T and B cells are healthy. This suggests that presence of adaptive immune system is required for IBD development in IL-10Rbeta KO mice.
Next, the authors showed that adoptive transfer of wild-type (WT) total T cells into RAG2 KO / IL-10Rbeta KO mice drives IBD. This suggests that IL-10 signaling defect in non-T cells (innate system, other tissues) system drives IBD.
Next, to further narrow down IL-10 signaling defect that drives IBD, the authors generated bone-marrow (BM) chimeric mice by reconstituting lethally irradiated RAG2 KO or RAG2 KO / IL-10Rbeta KO mice with RAG2 KO or RAG2 KO / IL-10Rbeta KO BM. Upon transfer of total WT T cells, only mice with hematopoietic cells derived from RAG2 KO / IL-10Rbeta KO BM developed IBD. This definitely showed IL-10 signaling defect in innate immune system drives IBD.
The authors also conducted several in vitro experiments and showed that transfer of WT BM-derived macrophages generated in M2 polarizing condition could prevent IBD development in RAG2 KO / IL-10Rbeta KO recipient mice following T cells transfer, whereas transfer of IL-10Rbeta KO BM-derived macrophages failed to do the same.
Next, the authors showed that adoptive transfer of total WT CD4 T cells into RAG2 KO / IL-10Rbeta mice leads to fewer Foxp3-positive T cell generation in lamina propria (LP) compared to RAG2 KO recipient mice.
One unusual result was the evidence that exogenous supplementation with IL-10 following transfer of WT CD4 T cells failed to prevent IBD development in RAG2 KO / IL-10Rbeta recipient mice.
In summary, we can imagine the following scenario how IL-10 prevents IBD: first, gut microbiota is internalized by gut dendritic cells or gut macrophages and presented to T cells (either to naive T cells or thymus derived Foxp3+ T cells). Second, this process generates local pool of regulatory T cells that secrete IL-10 and conditions local innate system or specifically macrophages to adopt anti-inflammatory phenotype. In absence of this feedback, however, macrophages acquire pro-inflammatory phenotype, overrides regulatory T cell effect and drives IBD through generation of colitogenic effector T cells. Of course, the question how initial T cells “know” to secrete IL-10 to condition gut macrophages is unclear.