Transient blocking of Lnk-mediated pathways as a potential approach to promote engrafting ability of hematopoietic progenitor cells

Lnk, an intracellular adaptor protein functions as a negative regulator of B lymphopoiesis, megakaryopoiesis and erythropoiesis by modulating signals through c-Kit, c-Mpl and Epo-R, respectively. In recent studies, we and other groups have shown that Lnk-/- mice display an increased number and enhanced repopulating capability of hematopoietic stem or progenitor cells (HSC/Ps), which is due to their hyperresponsiveness to thrombopoietin. Moreover, we have demonstrated that Src homology-2 (SH2) domain of Lnk is essential for the inhibitory function and that Lnk mutants with a point mutation in the SH2 domain potently act as dominant-negative mutant (DN-Lnk). Forced expression of DN-Lnk in HSC/Ps enhanced their repopulating capability in lethally irradiated recipient mice. Remarkably, transient expression of DN-Lnk also facilitated engraftment of HSC/Ps in immunodeficient animal under nonmyeloablative condition, leading to full reconstitution of lymphoid lineage cells. Transwell migration assay revealed that an interaction between HSC/Ps and vascular cell adhesion molecule-1 (VCAM-1) was augmented by Lnk deficiency or expression of DN-Lnk. These results suggest that Lnk modulates cell mobility of HSC/Ps in addition to their expansion. Lnk-mediated pathway therefore could become a potential target for enhancing both integrin-mediated engraftment and cytokine-dependent expansion of HSC/Ps.