miR-142: RNA-mediated reprogramming of fibroblasts into dendritic cells without viruses
Small RNA variants of miR-142 improved the conversion of connective tissue cells into immune dendritic cells
On August 12, a Lund University team reported in a Cell Reports paper that several variants of the small regulatory RNA miR-142 work together to convert human fibroblasts into cDC1 cells. These dendritic cells present fragments of foreign or tumor proteins to T-cells. The authors also tested virus-free delivery of the same instructions using RNA.
Fibroblasts produce and maintain connective tissue. cDC1 cells have a different function: they capture protein fragments and present them to T-cells, allowing the T-cells to recognize a target. Three regulatory proteins, PU.1, IRF8 and BATF3, activate this immune cell program in fibroblasts, but the original fibroblast program remains active.
In a 2024 study, the same research group used these three proteins to give mouse tumor cells cDC1-like properties and induce an antitumor immune response. The current paper examines why these instructions alone are insufficient when a cell continues to follow its previous program.
The authors tested 15 small RNAs. miR-124 acted early and made the DNA regions needed to initiate the new program more accessible. miR-142 had a different role: it suppressed fibroblast features and helped the cells maintain cDC1 features. Direct reprogramming therefore requires the new role to be activated while the old one is weakened.
miR-142 has several mature variants whose sequences are shifted by one or more “letters” near the beginning of the molecule. This shift changes the short region through which the small RNA recognizes mRNA, which carries the cellular instructions for protein production. A mixture of the main miR-142 sequence and three such variants increased the proportion of cells expressing XCR1, a surface marker of cDC1 cells. In the selected XCR1-positive cells, the protein profile became more similar to that of natural cDC1 cells. After stimulation, these cells also released 2.6 times more interferon λ1, an immune signaling protein.
Disabling one predicted miR-142 target, the TGFBR1 gene, increased the proportion of reprogrammed cells but did not increase the proportion of XCR1-positive cells. Removing this single barrier did not reproduce the more precise acquisition of the immune cell identity that the authors observed with the mixture of miR-142 variants.
The team then repeatedly introduced mRNA into human dermal fibroblasts. These temporary instructions encoded PU.1, IRF8 and BATF3 and were delivered together with synthetic copies of the small RNAs. Partially reprogrammed cells appeared by day nine. This RNA cocktail affected more than half of the genes altered by lentiviral delivery, but it activated the immune program less strongly. In this system, the regulatory proteins direct the cell toward a new role, while the small RNAs help it move away from its previous one.