CD26 Inhibitor Treated and CD26^−/− Recipient Mice Exhibit Improved Transplant Efficiency as Quantitated by Increased Short-Term Homing and Long-Term Engraftment

A firm understanding of the biology of hematopoietic stem and progenitor cell (HSC/ HPC) trafficking is believed to be critical for the development of methodologies to improve transplant efficiency and subsequently immune reconstitution during hematopoietic stem cell transplantation in the clinical setting. Through the use of CD26 inhibitors and CD26 deficient mice (CD26^−/−), we have previously generated data in mice suggesting that suppression of CD26/DPPIV (dipeptidylpeptidase IV) enzymatic activity on the transplant donor cell population can be utilized as a method of increasing transplant efficiency (Christopherson, KW 2^nd, et al, Science 2004. 305:1000–3). However, the clinical importance of the transplant recipient should not to be overlooked given the potential importance of the bone marrow microenvironment in regulating the transplant process. We therefore investigated here whether inhibition or loss of CD26 activity in recipient mice would have an effect on transplant efficiency utilizing an in vivo congenic mouse model of transplantation. The short-term homing and long-term engraftment of BoyJ donor cells (expressing CD45.1⁺) into lethally irradiated control C57BL/6, CD26 inhibitor (Diprotin A) treated C57BL/6, or CD26^−/− mice (expressing CD45.2⁺) was monitored by flow cytometric analysis of the bone marrow and peripheral blood at 24 hours and 6 months post-transplant respectively. Twenty-four hours post-transplant of 20×10⁶ BoyJ mononuclear cells, we observed 8.85±0.58%, 10.69±1.01%, and 12.45±1.33% donor derived Sca-1⁺lin⁻ cells in the bone marrow of recipient mice for control, Diprotin A treated, and CD26^−/− recipient mice respectively. As compared to control mice, this represents a 20.8% increase (p=0.01) with CD26 inhibitor treatment and a 40.7% increase (p£0.05) resulting from the use of a CD26^−/− recipient in short-term homing (N=5 mice per group). Six months post-transplant of 1×10⁵ BoyJ mononuclear cells, we observed 39.90± 4.38%, 70.22± 3.72%, and 92.51± 1.04% donor contribution to hematopoiesis in the peripheral blood of control, Diprotin A treated, and CD26^−/− recipient mice respectively. This represents a 76.0% increase (p£0.01) with CD26 inhibitor treatment and a 131.9% increase (p£0.01) as a result of the CD26^−/− recipient in long-term engraftment as compared to control recipient mice (N=14 mice per group). These results provide pre-clinical evidence of the importance of CD26 expression within the transplant recipient with regard to regulating hematopoietic stem cell homing and engraftment. Our results also support the potential use of CD26 inhibitors to treat transplant patients during hematopoietic stem cell transplantation as a method of improving transplant efficiency. Lastly, our use of inhibitor treated C57BL/6 and CD26^−/− recipient mice, which are also on a C57BL/6 background, in conjunction with a congenic model of transplantation provides a accurate and convenient model system for the in vivo testing of the efficacy of existing and new CD26 inhibitors in transplant recipients.