Pak2 Regulates MDSC Development and Function

Myeloid derived suppressor cells (MDSCs) are a heterogeneous cell population at various stages of differentiation that can increase under various pathologic conditions such as cancer, infection or inflammation, displaying suppressive function. It is well recognized that MDSCs contribute to tumor evasion by suppressing cell-mediated immunity. Based on the differential expression of Ly6C and/or Ly6G in mice, MDSCs are characterized as granulocytic (CD11b⁺Ly6G⁺Ly6C^low) or monocytic MDSCs (CD11b⁺Ly6G^low/−Ly6C^hi). These subsets induce T-cell hyporesponsiveness and can have various functions and distribution depending on their environment. Although much research has focused on the tumorigenic effects of MDSCs, studies on the regulation of their development during hematopoiesis remain limited.

p21-activated kinases (Paks) are serine/threonine kinases that regulate diverse cellular activities including cytoskeletal remodeling, cell motility, proliferation, apoptosis and mitosis. Despite active research on pharmacological inhibition of group I Paks in treating solid tumors, few studies have examined the role of Paks in modulating normal hematopoiesis. Knowledge of the role of Pak2 in regulating long-term hematopoiesis and lineage commitment remains limited. Utilizing a conditional Pak2-KO murine model, we have previously demonstrated that Pak2 disruption in hematopoietic stem/progenitor cells (HSPCs) induces myeloid lineage skewing and CD11b⁺Gr1⁺ cell expansion in mice. Compared to mice reconstituted with wild type (WT) bone marrow (BM), mice transplanted with Pak2-KO BM displayed a significantly higher percentage of granulocyte-monocyte progenitors (GMPs) in the BM and higher numbers of CD11b⁺Gr1⁺ cells in the spleen.

In this study, we demonstrated that CD11b⁺Gr1^high cells isolated from the spleens of mice with Pak2-KO BM displayed significantly greater suppressive function on T cell proliferation in vitro, consistent with MDSC phenotype. There was a near 2-fold increase in the numbers of both granulocytic and monocytic splenic MDSCs in mice reconstituted with Pak2-KO BM. At HSPC level, Pak2-KO BM yielded greater than 3-fold more colonies in response to GM-CSF but not G-CSF or M-CSF when compared to WT cells, indicating selective hypersensitivity to GM-CSF. In parallel experiments, Pak2-KO and WT BM C-kit⁺ cells that were enriched for hematopoietic progenitor cells (HPCs) were cultured in liquid culture in the presence of GM-CSF. Pak2-KO BM C-kit⁺ cells yielded greater than 2-fold higher numbers of CD11b⁺Gr1⁺ MDSCs that displayed potent suppression on CD8⁺ T cell proliferation. These data demonstrate that Pak2 disruption increases HPC sensitivity to GM-CSF signaling and drives lineage commitment toward granulocyte-monocyte lineage thus promoting MDSC development. In addition, we have also found that Pak2 deficient MDSCs are more proliferative and more resistant to apoptosis when compared to WT CD11b⁺Gr1⁺cells, thus contributing to expansion of this population in vivo. Loss of Pak2 decreases MDSC sensitivity to apoptosis through differential regulation of multiple pro- and anti-apoptotic gene expression. Furthermore, Pak2 disruption down regulates the expression of IRF8, a well-described myeloid transcription factor. Together, our data indicate that loss of Pak2 promotes HPC myeloid lineage commitment and CD11b⁺Gr1⁺ MDSC proliferation while suppressing apoptotic cell death in these cells. Further studies are ongoing to determine the interaction between Pak2 and IRF8.