Cells of the thymus, known as thymocytes, play a critical role in the development of T-cells alongside a myriad of other specialized cells. Unfortunately, the thymus, a crucial organ for the adaptive immune system, is prone to damage and reduces in size with age. These effects have problematic implications for T-cell production and can reduce effective response to therapeutics. However, the thymus has a remarkable ability to self-regenerate. Members of Dr Jarrod Dudakov’s Laboratory in the Fred Hutch Clinical Research Division set out to better understand the mechanisms at play in thymic regeneration by focusing on two key factors, IL-23 and BMP4, that are thought to efficiently facilitate the regeneration process. Their work, recently published in Cell Reports, highlights NOD2 as a fundamental suppressor of IL-23 and BMP4, in addition to identifying a potent pharmacological treatment for reinstating thymic regeneration and T-cell production. As Dr Sinéad Kinsella, lead author on the study, explained “our work identifies an innovative therapeutic strategy to enhance endogenous immune regeneration and provides a platform to increase the numbers of circulating naïve T cells.”
In order to better understand the transcriptional landscape of thymic regeneration, the authors first examined gene expression in mouse models that experienced thymic injury via sub-lethal irradiation. They assessed expression profiles, prior to and 4 days post irradiation, and identified NOD2 - an important player in innate immunity - as being highly expressed across damage response pathways. Repeating this analysis in thymic dendritic cells, NOD2 remained highly expressed. They hypothesized that NOD2 could potentially be suppressing thymic regeneration factors and set out to further understand this effect in NOD2 knock-out mouse models. Indeed, in a NOD2 depleted setting, the experimental mice exhibited higher levels of the regeneration factors IL-23 and BMP4 than mice with intact NOD2. The authors then investigated the role of a specific microRNA, mir29, which is known to reduce IL-23 expression. In their NOD2 knockout mouse model, expression of mir29 was decreased. Using a technique that retained the integrity of isolated thymic endothelial cells, the authors were able to assess the function of mir29 in an in vitro setting and noted an inverse relationship between mir29 and BMP4 which suggested an additional role of mir29 in suppressing regeneration.
Building on their previous research, the authors next hypothesized that cells undergoing apoptosis were causing a decrease in expression of IL-23 and BMP4. Using in vitro co-culture assays, they compared thymocytes undergoing apoptosis to thymocytes in which apoptosis had been blocked, and observed increased levels of IL-23 and BMP4 in the apoptosis-inhibited cells. Further, blockade of the TAM receptors, tyrosine kinase receptors involved in the disposal of apoptotic cells, also increased the expression of IL-23 and BMP4, implying that obstructing the detection of apoptotic cells could improve regeneration effects. The authors then surmised, as TAM receptors and RHO GTPases are interlinked, that blocking members of the RHO GTPase family could potentially improve thymic regeneration. They observed that inhibition of Rac1 led to increased levels of BMP4, in addition to establishing improved regeneration after injury in Rac1 negative mice.
Finally, the authors investigated whether a therapeutic candidate, Rac1 GTPase inhibitor, could aid in improving thymic regeneration. After acute injury to the thymus in their mouse models, Rac1 GTPase inhibitor increased cellularity in the thymus while simultaneously decreasing mir29 activity and, excitingly, increasing IL-23 and BMP4 levels. Dr Kinsella described how these data have “the potential to improve responses to immunotherapy treatments and may prove beneficial for improving response to infection in aged and immunocompromised individuals. Additionally, this work uncovered a novel reparative response that may be applicable to other regenerative pathways, in organs such as the skin and gut.”
Looking ahead, next steps for the Dudakov Laboratory include determining how these findings can be actioned to better understand the ageing process. “Age-related immune decline, which is driven by thymic involution, results in poor responses to infection and cancer immunosurveillance. We are working on translating this regeneration boosting therapeutic strategy to aged mouse models to understand if we can improve immune function in this setting,’ said Dr Kinsella. “Although more work is needed, there is vast potential to understand if our immune enhancing therapy can improve responses to vaccines,” she continued.
This work was funded by grants from the National Institutes of Health and the NCI Cancer Center. Support was also received from a Scholar Award from the American Society of Hematology, the Mechtild Harf (John Hansen) Award from the DKMS Foundation for Giving Life, the Cuyamaca Foundation, the Bezos Family Foundation, the American Society for Transplantation and Cellular Therapy and Pilot Funding from the Cooperative Center for Excellence in Hematology.
UW/Fred Hutch Cancer Consortium member Jarrod Dudakov contributed to this work.
Kinsella S, Evandy CA, Cooper K, Iovino L, deRoos PC, Hopwo KS, Granadier DW, Smith CW, Rafii S, Dudakov JA. Attenuation of apoptotic cell detection triggers thymic regeneration after damage. Cell Rep. 2021 Oct 5;37(1):109789. doi: 10.1016/j.celrep.2021.109789. PMID: 34610317.