Bone marrow-derived hematopoietic stem and progenitor cells infiltrate allogeneic and syngeneic transplants.

Lineage (CD3e, CD11b, GR1, B220 and Ly-76) negative hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) infiltrate islet allografts within 24 h posttransplantation. In fact, lineage(negative) Sca-1(+) cKit(+) ("LSK") cells, a classic signature for HSCs, were also detected among these graft infiltrating cells. Lineage negative graft infiltrating cells are functionally multi-potential as determined by a standard competitive bone marrow transplant (BMT) assay. By 3 months post-BMT, both CD45.1 congenic, lineage negative HSCs/HPCs and classic "LSK" HSCs purified from islet allograft infiltrating cells, differentiate and repopulate multiple mature blood cell phenotypes in peripheral blood, lymph nodes, spleen, bone marrow and thymus of CD45.2 hosts. Interestingly, "LSK" HSCs also rapidly infiltrate syngeneic islet transplants as well as allogeneic cardiac transplants and sham surgery sites. It seems likely that an inflammatory response, not an adaptive immune response to allo-antigen, is responsible for the rapid infiltration of islet and cardiac transplants by biologically active HSCs/HPCs. The pattern of hematopoietic differentiation obtained from graft infiltrating HSCs/HPCs, cells that are recovered from inflammatory sites, as noted in the competitive BMT assay, is not precisely the same as that of intramedullary HSCs. This does not refute the obvious multi-lineage potential of graft infiltrating HSCs/HPCs.

Isolated CD39 expression on CD4+ T cells denotes both regulatory and memory populations.

Foxp3(+) regulatory T cells (Tregs) express both ectoenzymes CD39 and CD73, which in tandem hydrolyze pericellular ATP into adenosine, an immunoinhibitory molecule that contributes to Treg suppressive function. Using Foxp3GFP knockin mice, we noted that the mouse CD4(+)CD39(+) T-cell pool contains two roughly equal size Foxp3(+) and Foxp3(-) populations. While Foxp3(+)CD39(+) cells are CD73(bright) and are the bone fide Tregs, Foxp3(-)CD39(+) cells do not have suppressive activity and are CD44(+)CD62L(-)CD25(-)CD73(dim/-), exhibiting memory cell phenotype. Functionally, CD39 expression on memory and Treg cells confers protection against ATP-induced apoptosis. Compared with Foxp3(-)CD39(-) naïve T cells, Foxp3(-)CD39(+) cells freshly isolated from non-immunized mice express at rest significantly higher levels of mRNA for T-helper lineage-specific cytokines IFN-gamma (Th1), IL-4/IL-10 (Th2), IL-17A/F (Th17), as well as pro-inflammatory cytokines, and rapidly secrete these cytokines upon stimulation. Moreover, the presence of Foxp3(-)CD39(+) cells inhibits TGF-beta induction of Foxp3 in Foxp3(-)CD39(-) cells. Furthermore, when transferred in vivo, Foxp3(-)CD39(+) cells rejected MHC-mismatched skin allografts in a much faster tempo than Foxp3(-)CD39(-) cells. Thus, besides Tregs, CD39 is also expressed on pre-existing memory T cells of Th1-, Th2- and Th17-types with heightened alloreactivity.