Molecular mechanisms controlling the growth and functional characteristics of the neonatal and juvenile thymus microenvironments 2267575

Abstract Introduction Unlike most organs, thymus growth is untethered to body size, following an independent trajectory across the lifespan, peaking during the perinatal period, then leveling off and then declining at a juvenile age. These changes in thymus size correlate to changes in thymus function and the generation of stage-specific subsets of T cells which persist throughout the lifespan, resulting in the full complexity of the adult T cell complement. Methods We used single-cell RNAseq and functional and phenotypic validation approaches to identify and test molecular mechanisms that control the dynamic changes in thymus growth and function during the perinatal to juvenile period , working first in mice, and validating results in human. Results Differentially expressed gene and intercellular network analyses of temporally resolved scRNA-seq data revealed fibroblast-derived insulin-like growth factor 2 (IGF2) as a candidate driving neonatal thymic expansion. We demonstrated IGF2 activity promotes a cortical TEC-specific proliferation and is tightly regulated at the thymic growth transition, acting at least partly via regulation of the CyclinD1-RB-E2F-Foxn1 molecular pathway in thymic epithelial cells (TECs). We further demonstrated using both gain and loss of function genetic models that this pathway impacts not only proliferation but also function, tying the dynamics of growth to TEC differentiation and production of stage-specific T cells. We demonstrated that multiple components of the IGF2 pathway are differentially regulated across this time period in multiple cell types, implicating an extrinsic mechanism responsible for the timing of these transitions. Conclusion We have identified IGF2 as a critical regulator of thymus growth and function across the perinatal to juvenile stages, likely controlled by a factor(s) extrinsic to the thymus. Current efforts are focused on identifying the upstream mechanisms controlling these dynamic changes that are required for propoer immune system development. Funding Source NIH/NIAID Topic Categories Hematopoiesis and Immune System Development (HEM)