Disruption of CXCR4 Utilizing AMD3100 Bypasses Mobilization Deficiency Exhibited by CD26^−/− Mice and Results in Efficient Mobilization of Myeloid Progenitors

The exopeptidase CD26 (also known as DPPIV/dipeptidylpeptidase IV) cleaves dipeptides from the N-terminus of proteins that contain the required X-Pro or X-Ala motif. We have previously reported that inhibition or loss of CD26 activity results in a deficiency in normal granulocyte-colony stimulating factor (G-CSF) induced mobilization, suggesting that CD26 is a necessary component of mobilization (Christopherson, et al Blood 2003 and Christopherson, et al Exp Hematol 2003). The chemokine CXCL12 (SDF-1, stromal cell derived factor-1) contains the appropriate recognition sequence for CD26 induced cleavage. This combined with the importance of CXCL12 in the trafficking of hematopoietic stem and progenitor cells (HSC/HPC) suggests CXCL12 as a likely functional target of CD26 during G-CSF induced mobilization. For this reason we therefore decided to investigate whether genetically engineered mice lacking CD26 (CD26^−/−) could be mobilized utilizing the CXCR4 antagonist, AMD3100. To evaluate this, ten week old C57BL/6 and CD26^−/− mice (also on a C57BL/6 background) received a single subcutaneous injection of AMD3100 (1mg/1kg). One hour following injection the mice were euthanized by CO₂ inhalation. Peripheral blood was then obtained by heart stick with a 1.2 ml syringe containing EDTA as an anticoagulant. A complete blood count was taken for each peripheral blood sample. Following red blood cell lysis, cells were plated for myeloid colony formation in a standard 1% methylcellulose colony assay containing the appropriate cytokines. Following 7 days of incubation at 5% O₂, 5% CO₂ and 37°C plates were scored for colony-forming units-granulocyte macrophage (CFU-GM), burst-forming units-erythroid (BFU-E), and colony-forming units-granulocyte, erythroid, macrophage, and megakaryocytic (CFU-GEMM). Data is presented as the number of colonies per femur for the bone marrow and as the number of colonies per ml of whole blood for the peripheral blood. AMD3100 treatment resulted in an increase in white blood cell (WBC) counts from 5.05±0.48 × 10⁶/ml in untreated mice to 10.21±0.88×10⁶/ml in treated mice (p≤0.01). An increase in WBC counts was also observed during AMD3100 treatment in CD26^−/− mice from 7.77±1.28×10⁶/ml in untreated mice to 16.7 ±2.11 × 10⁶/ml in treated mice (p<0.01). AMD3100 treatment resulted in an increase in circulating myeloid progenitors in the peripheral blood of C57BL/6 and CD26^−/− mice as compared to untreated C57BL/6 and CD26^−/− mice respectively (p≤0.01). Specifically, a 2.38, 3.75, 12.33 fold increase in CFU-GM, BFU-E, and CFU-GEMM were observed in the peripheral blood of C57BL/6 mice after treatment. A 2.63, 5.48, 14.29 fold increase in CFU-GM, BFU-E, and CFU-GEMM were observed in the peripheral blood of CD26^−/− mice after treatment. Existing pre-clinical and clinical data suggest that the CXCR4 antagonist, AMD3100, rapidly mobilizes hematopoietic progenitor cells from the bone marrow into the periphery. The results presented here provide pre-clinical evidence that disruption of the interaction between the CXCR4 chemokine receptor and CXCL12, via sub-cutaneous injection of AMD3100, mobilizes significant numbers of myeloid progenitors in mice, even in the absence of CD26. These results support the notion that CD26 is downstream of G-SCF treatment. Additionally, these results support the potential use of AMD3100 to treat patients that may have an altered ability to respond to G-CSF treatment as a result of a reduction or loss in CD26 activity. Future studies are warranted to evaluate potential variations in CD26 levels or activity in the general population, in differing patient populations, and during different treatment regimens.