WSS enhances long-term secondary engraftment. Secondary recipients received 20,000 CD45.2⁺ bone marrow cells sorted from primary recipients plus 200,000 CD45.1⁺ CD45.2⁺ Rag2^−/− Il2rγ^−/− competitor bone marrow cells. (A) Engraftment in the periphery of secondary recipients was monitored for up to 10 mo (n = 5 static, n = 7 WSS from independent primary recipients). Peak chimerism was found long term in two WSS and two static recipients, with substantially higher donor contribution in the WSS recipients. Long-term engraftment was defined as CD45.2 positivity in the periphery for >20 wk (denoted by the dashed line) at any level above background. (B) Plots of peripheral blood collected from recipients 8 mo after transplantation illustrate sustained contribution of donor-derived CD45.2⁺ marrow cells. (C) Analysis of lineages present in the CD45.2⁺ gate of the engrafted WSS recipient depicted in panel B alongside adult peripheral blood from the donor and recipient strains. (D) Lineages present in CD45.2⁺ donor peripheral blood from individual secondary recipients at 4, 24, 36, 48, and 68 wk after transplantation. Recipient strain is represented by an untransplanted adult CD45.1⁺ CD45.2⁺ Rag2^−/− Il2rγ^−/−; donor strain is adult Swiss Webster.