Cutting Edge: CD47 Controls the In Vivo Proliferation and Homeostasis of Peripheral CD4+CD25+Foxp3+ Regulatory T Cells That Express CD103

Peripheral CD103+Foxp3+ regulatory T cells (Tregs) can develop both from conventional naive T cells upon cognate Ag delivery under tolerogenic conditions and from thymic-derived, expanded/differentiated natural Tregs. We here show that CD47 expression, a marker of self on hematopoietic cells, selectively regulated CD103+Foxp3+ Treg homeostasis at the steady state. First, the proportion of effector/memory-like (CD44highCD62Llow) CD103+Foxp3+ Tregs rapidly augmented with age in CD47-deficient mice (CD47−/−) as compared with age-matched control littermates. Yet, the percentage of quiescent (CD44lowCD62Lhigh) CD103−Foxp3+ Tregs remained stable. Second, the increased proliferation rate (BrdU incorporation) observed within the CD47−/−Foxp3+ Treg subpopulation was restricted to those Tregs expressing CD103. Third, CD47−/− Tregs maintained a normal suppressive function in vitro and in vivo and their increased proportion in old mice led to a decline of Ag-specific T cell responses. Thus, sustained CD47 expression throughout life is critical to avoid an excessive expansion of CD103+ Tregs that may overwhelmingly inhibit Ag-specific T cell responses.