First published online May 25, 2006
Stem Cells
Vol. 24 No.
9
September 2006, pp.
2158
-2159
doi:10.1634/stemcells.2005-0584; www.StemCells.com
© 2006 AlphaMed Press
Neutrophil Serine Proteases: Future Therapeutic Targets in Patients with Severe Chronic Neutropenia and Leukemia?
Daniel Garwicz
Division of Molecular Toxicology, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden, and Division of Hematology, Department of Medicine, Karolinska Institutet, Karolinska University Hospital Huddinge, Stockholm, Sweden
Correspondence: Daniel Garwicz, M.D., Ph.D., Division of Clinical Chemistry and Blood Coagulation, Department of Molecular Medicine and Surgery, Karolinska Institutet, Karolinska University Hospital, S-171 76 Stockholm, Sweden. Telephone: +46-8-517-73342; Fax: +46-8-310376; e-mail: Daniel.Garwicz{at}ki.se
Received on November 23, 2005;
accepted for publication on May 16, 2006.
First published online in STEM CELLS EXPRESS May 25, 2006.
It has recently been discovered that members of the hematopoietic serine protease superfamily, such as cathepsin G, neutrophil elastase, and proteinase 3, may play an important role in myeloid biology by suppressing granulopoiesis. Patients with hematological disorders, such as severe chronic neutropenia and myelogenous leukemia, may have mutations in genes encoding neutrophil serine proteases or alterations in the expression, localization, or activity of the proteins. If these findings will stand the test of time, we may have caught a glimpse of the future regarding improved diagnosis, classification, and treatment of these severe blood disorders.
Goselink et al. have investigated the proliferation of normal human bone marrow-derived CD34+ progenitor cells [1]. They report that, in the absence of serum and serine protease inhibitors (such as secretory leukocyte proteinase inhibitor and 
1-proteinase inhibitor), the proliferation of stem cells is suppressed by neutrophil serine protease family members, the production of which increases when the CD34+ cells are subjected to myeloid differentiation.
These findings are interesting in light of recent studies that do not use serum-free medium, suggesting that newly synthesized proforms of neutrophil granule proteins (including lysozyme and neutrophil serine proteases) as an alternative to being stored intracellularly may be routed for constitutive secretion from myeloid progenitor cells/granulocytic precursor cells. The secretion occurs in significant quantities during in vitro differentiation of human bone marrow-derived CD34+ cells with human recombinant granulocyte colony-stimulating factor (G-CSF) [2]. The proform of proteinase 3 has been hypothesized to be involved in a negative feedback loop on granulopoiesis, whereas this has not been shown for the proforms of cathepsin G or neutrophil elastase [3]. The amino-terminal sequence, in combination with the conformation of the catalytically inactive proform of proteinase 3 and several other members of the hematopoietic serine protease superfamily, including neutrophil azurocidin and granzymes (of cytotoxic T lymphocytes and natural killer cells), seems to play an important role in causing S-phase arrest in granulopoietic progenitors [4]. On the other hand, the enzymatically active form of neutrophil elastase, which is not constitutively secreted but instead stored in the azurophilic granules awaiting a signal for regulated secretion, is able to counteract the effect of G-CSF in vitro [5]. This may be of clinical significance for cyclic hematopoiesis and severe congenital neutropenia, conditions characterized by mutations in the ELA2 gene, which encodes neutrophil elastase [6, 7]. One of the reported mutations affects intracellular localization of this protease and triggers apoptosis when expressed in differentiating HL-60 (human acute myeloblastic leukemia) cells [8]. Evidence for excessive apoptosis of myeloid progenitor cells in the bone marrow of patients belonging to the original Kostmann kindred has previously been reported [9], but a correlation between increased cell death and the occurrence of ELA2 or other gene mutations remains to be firmly established.
Long-term follow-up of patients with severe congenital neutropenia reveals that approximately one in 10 patients develops myelodysplastic syndrome or acute leukemia [10, 11]. In these patients, mutations in one or several genes, including ELA2 and the receptor for G-CSF, are seen and may have both a diagnostic and a prognostic value. However, how much long-term G-CSF therapy per se contributes to accelerate the genetic predisposition for acquired mutations, and hence malignant transformation, is not entirely clear. It is apparent that patients with cyclic hematopoiesis, who have mutations in ELA2 in almost all reported cases, do not seem to have any increased risk for acute leukemia.
Of potential prognostic interest in other malignant blood disorders, such as chronic myelogenous leukemia, is the novel and very recent finding that a combination of low expression of CD7 in combination with high expression of proteinase 3 or neutrophil elastase, as measured by mRNA levels in CD34+ cells of these patients, correlates with longer patient survival [12]. The underlying molecular mechanisms remain to be clarified; however, it has previously been shown that cytotoxic T lymphocytes directed against a nonapeptide of proteinase 3 preferentially lyse myelogenous leukemia blasts, as opposed to normal bone marrow cells [13].
In conclusion, hematopoietic stem cell/granulopoietic progenitor cell biology may have entered a new era with the recent discoveries that neutrophil serine proteases possess several novel and important functions in addition to their established roles in innate immunity. One may envisage that gene therapy protocols or selective inhibition of the myelosuppressive functions of serine proteases in neutropenic patients, as well as targeted T-cell therapy against malignant hematopoietic stem cells, seemingly overexpressing neutrophil serine proteases [14, 15] may become part of the therapeutic arsenal in pediatric and adult hematology/oncology in the future.
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DISCLOSURES
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The author indicates no potential conflicts of interest.
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ACKNOWLEDGMENTS
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The author was supported by the Swedish Society for Medical Research. I am grateful to Associate Prof. Bengt Fadeel and colleagues at Karolinska Institutet/Karolinska University Hospital for valuable discussions and continual encouragement, and I also thank Prof. Urban Gullberg and colleagues at Lund University/Lund University Hospital for continued support.
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REFERENCES
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