Immunogenicity of Ly5 (CD45)-antigens hampers long-term engraftment following minimal conditioning in a murine bone marrow transplantation model

Various techniques are available for distinguishing donor from host cells e valuating the efficacy of conditioning regimen for experimental bone marrow transplantation (BMT). Techniques include the use of extracellular immunol ogical markers, such as Ly5 (CD45), and intracellular biochemical markers, such as glucose-phosphate-isomerase (Gpi), Because Ly5 is an extracellular protein, the disparity between donor (Ly5.1) and host (Ly5.2) antigens may induce a weak immune response whereas with Gpi, no immune response is expec ted, This difference may be of particular concern in experimental transplan tation approaches that use minimal conditioning such as low-dose total body irradiation (TBI), Such mild conditioning mag not induce the immunosuppres sion required to overcome host rejection of Ly5 disparate cells. To compare the relative engraftment of Lp5.1 and Gpi-1(a) donor marrow, B6 (Gpi-1(h)/Ly5.2) mice were irradiated with low-level TBI (0-6 Gy) and trans planted with several hone marrow (BM) doses (2 x 10(6)-5 x 10(7) cells). At 8, 26, and, 52 weeks post-BMT, the level, of donor engraftment was measure d using now cytometry (Ly5) or Gpi-electrophoresis, Lower engraftment levels were found in mice transplanted with Ly5 congenic BM in groups given low-dose TBI (less than or equal to 4 Gy) and/or low dos es of BM cells (BMC) (2 x 10(6)), However, when higher TBI or BR IC doses w ere used, similar engraftment levels were found, suggesting sufficient immu ne suppression to allow equal engraftment of both sources of BM. These data suggest that even a minor phenotypic disparity between donor and host, such as Ly5, may necessitate high-dose TBI to prevent rejection, The combination of low-dose TBI or other nonmyeloablative conditioning strateg ies with small numbers of BMC may lead to reduced engraftment when extracel lular immunological markers such as Ly5 are used for transplantation studie s, Therefore, small immunological differences must be considered when using the Ly5 marker for engraftment.