A Point Mutation in the Juxtamembrane Wedge of CD45 Impacts Myeloid Development and Generates a Novel Model for Hemophagocytic Disorders in the Context of a MHC I-Restricted Transgene.

The receptor like-protein tyrosine phosphatase CD45 is highly expressed on all nucleated hematopoietic cells and functions by modulating the activity of Src family kinases. We previously generated mice containing a point mutation in the juxtamembrane wedge of CD45 that leads to constitutive phosphatase activity. Demonstrating the critical negative regulatory function of the wedge, the CD45E613R mutation led to a lymphoproliferative disorder, lupus-like autoimmune syndrome, and premature death at 9–12 months of age. To address the intrinsic effects of the wedge mutation on the CD8 T cell lineage, we introduced a CD8 restricted T-cell receptor (TCR) transgene, OT1, specific for the antigen ovalbumin, into CD45E613R mice. Surprisingly, 100% of the CD45E613R/OT1+ mice succumb by 6 weeks of age to a rapidly progressive disease characterized by severe cachexia, hemophagocytosis, and markedly elevated interferon gamma levels. This phenotype shares similarity to the human hemophagocytic syndromes hemophagocytic lymphohistiocytosis (HLH) and macrophage activation syndrome (MAS). The aims of this study are to identify the mechanistic basis and the cell type(s) responsible for this phenotype. Using a genetic approach, we demonstrate that the phenotype is maintained in CD45E613R/OT1+mice on a Rag1−/− background, thus implicating myeloid and/or CD8 T cells in disease pathogenesis. Analysis of myeloid cell development, phagocytic function, and signaling indicate that the wedge mutation operates in this lineage. Despite the intrinsic hyperresponsiveness of CD45E613R macrophages, adoptive transfer experiments demonstrate they are not essential for disease. However, adoptive transfer of purified CD45E613R/OT1+ CD8 T cells into either CD45 mutant or CD45 wildtype (wt) is sufficient to initiate disease. Interestingly, CD45E613R CD8 OT+ T cells are not dominant over CD45wt CD8+ cells. In addition we show that transfer of CD45wt CD8+ T cells into ill mice can delay disease progression. We hypothesize that, in the setting of the wedge mutation, the restricted repertoire of the CD8 T cells in CD45E613R/OT1+ mice might not be able to limit macrophage activation. We believe these mice may represent a useful model for understanding the molecular pathogenesis of hemophagocytic disorders, devastating and often fatal diseases in humans.