NOD2 acts as a master regulator of regeneration pathways in the thymus

Abstract

Abstract Endogenous thymic regeneration is a crucial function that allows for renewal of immune competence following stress, infection and cytotoxic cancer treatments. Although there is continuous thymic involution and regeneration in response to stress and infection in the healthy state, prolonged T cell deficiency is common after profound thymic damage, such as that caused by the conditioning required for hematopoietic stem cell transplant. Our previous studies have revealed two pathways important for thymic repair, centered around the production of IL-22 and BMP4 by innate lymphoid cells and endothelial cells (ECs), respectively. However, the specific mechanisms that trigger these pathways are poorly understood. Here we reveal an unexpected role for the pattern recognition receptor Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) in governing multiple pathways of thymic regeneration. Mice deficient in NOD2 showed increased intrathymic levels of L-22, IL-23 and BMP4 and increased thymus cellularity after damage caused by total body irradiation (TBI). Although NOD2 is classically defined as a bacterial sensor, members of the Rho GTPase family have been implicated to promote NOD signaling, even in the absence of pattern recognition. Consistent with this, inhibition of RhoA or Rac1 can induce the production of BMP4 by ECs in vitro, and after TBI there is a significant reduction in these two Rho GTPases. These studies not only enhance our understanding of endogenous tissue regeneration, but in identifying a master regulator of multiple pathways, offers a superior therapeutic strategy to boost thymic function in patients whose immune system has been decimated due to age, infection or cancer therapies.