Extra-hematopoietic immunomodulatory role of the SCID-susceptibility gene DOCK-2 identified by stepwise maturation of human iPSCs into clonogenic mesodermal stromal progenitors
Stromal cells contribute to organ integrity as fibroblasts and to vascular stability as pericytes, in addition to their enigmatic niche function in many tissues. Their inherent immunomodulatory capacity attracted particular attention, initiating numerous clinical trials, particularly testing trophic regeneration and immunomodulation. Key stromal immune functions are still enigmatic. Here we show that dedicator of cytokinesis (DOCK-2) previously described for causing immune cell dysfunction plays a role in extra-hematopoietic immunity by regulating stromal/fibroblast immunomodulatory function. We used three independent strategies including iPSC-derived mesodermal stromal cell (MSC) lineage maturation, severe combined immunodeficiency (SCID) patient-derived cells and CRISPR/Cas9 knockout to support our findings. Human induced pluripotent stem cells (iPSCs) were generated from healthy bone marrow and umbilical cord blood-derived fibroblasts by Sendai virus-mediated transient expression of Yamanaka factors. After mesoderm induction, stromal differentiation was induced by platelet-derived growth factors under animal serum-free conditions. Under feeder-free defined conditions, iPSCs differentiated into expandable and cryo-preservable CD73+/CD105+/Tra-1-81- early iPS-MSCs lacking immunosuppressive potential. Successive maturation was required for reaching the canonical MSC phenotype and immunomodulatory competence over time, while maintaining clonogenicity, comparable to parental MSCs. Sequential RNAseq revealed acquisition of a spectrum of immune-related genes significantly expressed in mature iPS-MSCs and resembling parental MSCs immune gene expression. The DOCK-2 gene attracted our attention because mutations can cause SCID. Interestingly, SCID patient-derived fibroblast lines harboring bi-allelic DOCK-2 mutations showed significantly reduced immunomodulatory capacity compared to non-mutated control fibroblasts. CRISPR/Cas9-mediated DOCK-2 knockout in healthy iPSCs resulted in iPS-MSCs that also displayed reduced immunomodulatory capacity, thus confirming a role of DOCK-2 in stromal immune function. At a mechanistic level, DOCK-2 deficiency resulted in disturbed subcellular localization of CDC42. This provides first evidence for an extra-hematopoietic immunomodulatory role of DOCK-2 in stromal cells, previously considered restricted to hampering immune cell migration/function resulting in SCID. We may speculate that some of the signs and symptoms of persisting immune disease after successful hematopoietic stem cell transplantation in SCID patients could at least in part be due to mutations, like DOCK-2-/-, permissive outside the hematopoietic immune system, as evidenced also by the increased virus infection susceptibility of DOCK-2 deficient fibroblasts.