Model of hematopoietic feedback by SP(1-4).

SCF mediated the release of SP from BM stroma with concomitant expression of NK-1. SP–NK-1 interaction leads to hematopoietic stimulation (+). NEP, which is also induced in the SCF-stimulated stroma, uses SP as its substrate to produce SP(1-4). Interaction between SP(1-4) and NK-1 negatively regulates the stimulatory effect of SP on hematopoiesis (−). The fate of SP(4-11) is yet to be determined (?).