Cathepsin K maintains the number of lymphocytes in vivo

In this study, we investigated the influence of the loss of Cathepsin K ( Ctsk) gene on the hematopoietic system in vitro and in vivo . We found that cultures with Lineage- SCA1+ KIT+ (LSK) cells on Ctsk deficient stromal cells display reduced colony formation and proliferation, with increased differentiation, giving rise to repopulating cells with reduced ability to repopulate the donor LSK and T cell compartments in the bone marrow. Subsequent in vivo experiments showed impairment of lymphocyte numbers, but, gross effects on early hematopoiesis or myelopoiesis were not found. Most consistently in in vivo experimental settings, we found a significant reduction of (donor) T cell numbers in the bone marrow. Lymphocyte deregulation is also found in transplantation experiments, which revealed that Ctsk is required for optimal regeneration not only of T cells, but also of B cells. Interestingly, cell non-autonomous Ctsk regulates both B- and T cell numbers, but T cell numbers in the bone marrow require an additional autonomous Ctsk -dependent process. Thus, we show that Ctsk is required for the maintenance of hematopoietic stem cells in vitro , but in vivo, Ctsk deficiency most strongly affects lymphocyte homeostasis, particularly of T cells in the bone marrow. ### Competing Interest Statement The authors have declared no competing interest.