Engraftment of Splenic Tissue as a Method to Investigate Repopulation by Hematopoietic Cells from Host and Donor Marrow

The lymphohematopoietic function of the spleen in mice varies dependent on age and hematopoietic requirements. A method was developed to study splenic repopulation of mature and progenitor cell populations by grafting neonatal or adult spleen tissue under the renal capsule of splenectomized mice. Two weeks following implant of irradiated syngeneic neonatal spleens into B6-Ly 5.1 or B6-gfp recipients, host lymphoid (B220 + , CD4/8 + ) and myeloid cells (CD11b + ) had repopulated the splenic grafts and constituted the majority of cells contained in these heterotopic implants. Notably, the percentage of lymphoid and myeloid cells approximated adult levels in contrast to preimplant neonatal spleen levels. This observation indicated relatively rapid repopulation of the grafted tissue by adult host cells and suggests that the repopulation patterns were regulated by the host. Three months post-implantation, the cell composition in the graft remained comparable to adult levels. Microscopic examination demonstrated normal splenic architecture including follicles and red pulp. Lymphocytes within the graft were functional as indicated by their proliferation in response to lipopolysaccharide (LPS) and concanavalin A (ConA) stimulation. Progenitor cell activity determined by colony-forming unit interleukin-3 (CFU-IL-3) levels was also present in these grafts. Splenic implants were then assessed in transplant models following lethal irradiation and syngeneic or allogeneic bone marrow transplantation (BMT). Two weeks post-BMT, adult splenic tissue implants contained donor-derived B cells, T cells, and myeloid cell populations. As typically detected in the host spleen post-BMT, the grafted tissue also contained elevated levels of donor progenitor cells. By 3 months post-BMT, CFU-IL3 levels in the graft reflected the decreased levels characteristic of adult levels. The functional integrity of post-transplant splenocytes in the implants was also demonstrated by mitogenic responsiveness. In summary, this method should provide a useful model for the transfer of the splenic microenvironment to study the biology of the spleen in nontransplant and BMT settings.