Rheumatoid arthritis (RA) is a chronic joint disease characterized by leukocyte invasion and synoviocyte activation followed by cartilage and bone destruction. Its etiology and pathogenesis are poorly understood. We describe a spontaneous mouse model of this syndrome, generated fortuitously by crossing a T cell receptor (TCR) transgenic line with the NOD strain. All offspring develop a joint disease highly reminiscent of RA in man. The trigger for the murine disorder is chance recognition of a NOD-derived major histocompatibility complex (MHC) class II molecule by the transgenic TCR; progression to arthritis involves CD4+ T, B, and probably myeloid cells. Thus, a joint-specific disease need not arise from response to a joint-specific antigen but can be precipitated by a breakdown in general mechanisms of self-tolerance resulting in systemic self-reactivity. We suggest that human RA develops by an analogous mechanism.

It has been proposed that during embryonic development haematopoietic cells arise from a mesodermal progenitor with both endothelial and haematopoietic potential called the haemangioblast1,2. A conflicting theory associates instead the first haematopoietic cells with a phenotypically differentiated endothelial cell with haematopoietic potential, i.e. a haemogenic endothelium3-5. Support for the haemangioblast concept was initially provided by the identification during embryonic stem (ES) cells differentiation of a clonal precursor, the blast colony-forming cell (BL-CFC), which gives rise to blast colonies with both endothelial and haematopoietic components6,7. Although recent studies have now provided evidence for the presence of this bipotential precursor in vivo8,9, the precise mechanism of generation of haematopoietic cells from the haemangioblast still remains completely unknown. Here we demonstrate that the haemangioblast generates haematopoietic cells through the formation of a haemogenic endothelium intermediate, providing the first direct link between these two precursor populations. The cell population containing the haemogenic endothelium is transiently generated during BL-CFC development. This cell population is also present in gastrulating embryos and generates haematopoietic cells upon further culture. At the molecular level, we demonstrate that the transcription factor Scl/Tal110 is indispensable for the establishment of this haemogenic endothelium population whereas the core binding factor Runx1/AML111 is critical for generation of definitive haematopoietic cells from haemogenic endothelium. Together our results merge into a single linear developmental process the two a priori conflicting theories on the origin of haematopoietic development.