Background: 

The key issue of therapeutic stem cell approaches emerges to be the process of cardiac homing of stem cells via the SDF-1a-CXCR4 axis. Myocardial SDF-1a, which is crucial for incorporation of progenitors, is degraded by the extracellular protease DPP-IV/CD26. We hypothesized that genetic or pharmacological inhibition of CD26 leads to an increase of myocardial SDF-1a thus improving the homing of G-CSF-mobilized stem cells after myocardial infarction in a mouse model.

Methods: 

We induced acute myocardial infarction (MI) in 10–11 weeks old male CD26 knockout or C57BL/6 mice using surgical occlusion of the left descending artery (LAD). Mice were then treated either with G-CSF (100 mg/kg/d s.c.) in combination with Diprotin A (140/kg/d mg p.i., "G-CSF+DipA"), G-CSF or Diprotin A alone or saline ("control").

Findings: We show that genetic and pharmacological DPP-IV inhibition leads to decreased DPP-IV activity in the heart. Myocardial SDF-1a was enhanced after myocardial infarction as demonstrated by ELISA. Mass spectroscopy of heart lysates after MI revealed stabilization of myocardial SDF-1a. An increased amount of circulating CD45⁺/CD34⁺c-kit⁺, CD45⁺/CD34⁺Sca-1⁺, CD45⁺/CD34⁺CXCR-4⁺, CD45⁺/CD34⁺Flk-1⁺ as well as lin^-c-kit⁺Sca-1⁺ cells was attracted to the ischemic heart of G-CSF treated CD26 knockout or Diprotin-wildtype mice. This effect was reversible by adding the CXCR-4 antagonist AMD 3100 (1.25 mg/kg/day). Probably by parakrine means, these progenitor cells stimulated resident cardiac stem cells (CD45^-CD34^-c-kit⁺ and CD45^-CD34^-Sca-1⁺ cells) as we show by cardiac FACS. In our experiments, enhanced myocardial homing of stem cells and expansion of resident cardiac progenitor cells finally lead to enhanced neovascularization (increased number of CD31+ cells in the border zone) 6 days after MI. In parallel, CD26 inhibition combined with G-CSF application reduced the cardiac remodeling (decreased infarct size and increased left ventricular wall thickness 30 days after MI). These effects also translated into significantly improved left ventricular function (CD26 k.o. + G-CSF: 422%; wt + G-CSF + Dip: 361%; wt + saline: 161%; p<0.01) and decreased end-diastolic volume 30 days after MI (as measured by conductance catheter). Finally, survival significantly improved 30 days after MI by CD26 inhibition combined with G-CSF application (CD26 k.o. + G-CSF: 75%; wt + G-CSF + Dip: 70%; wt + saline: 30%; p<0.01; n=20 in each group).

Interpretation: This is the first study showing that genetic or pharmacologic DPP-IV/CD26 inhibition combined with G-CSF administration improves cardiac function and survival after myocardial infarction by an enhanced cardiac homing of stem cells due to stabilization of active myocardial SDF-1a.