Ph.D Thesis

Ph.D StudentDomev-Cohen Hagit
SubjectCharacterization of the Immunological Properties of
Pericytes Derived from Human Pluripotent Stem
DepartmentDepartment of Medicine
Supervisor PROFESSOR EMERITUS Joseph Itskovitz
Full Thesis textFull thesis text - English Version


Pericytes are vascular cells that encircle endothelial cells of small blood vessels, and function as regulators of vascular development, stabilization, and maturation. In-vivo, the main criteria to identify pericytes remain their anatomical localization and morphology, and since there are no specific markers of pericytes, the cells are identified by co-expression of several different markers such as CD146, and neuron-glial 2. In addition, pericytes display mesodermal multilineage differentiation potential following isolation and expansion. Therefore, pericytes represent a unique subtype of microvessel-residing perivascular cells with diverse angiogenic functions and multilineage developmental features of mesenchymal stem cells.

            Human pericytes can be isolated from multiple adult and fetal tissues as well as generated from human pluripotent stem cells (hPSCs), which are an alternative plentiful source for large scale generation of pericytes for basic and applicative research.

However, a major concern that may severely limit their use in the clinics is the immune processes that might be provoked against the pericytes after transplantation. Despite the growing interest in pericyte-based stem cell therapy, their immunogenicity and immunomodulatory effects on none-activated T cells are still poorly defined, with only one publication dealing with the immunogenicity of pericytes in steady state, and no information regarding the immunogenicity of vasculogenic pericytes derived from hPSCs.

We found that tissue embedded and unstimulated cultured hPSC- or tissue-derived pericytes (from full-term placenta and brain), constitutively expressed major histocompatibility complex (MHC) class I and the inhibitory programmed death ligand (PD-L1/2) molecules but not MHC class II or CD80/CD86 co-stimulatory molecules. Pre-treatment with inflammatory mediators failed to induce antigen presenting cell-like phenotype in stimulated pericytes. Furthermore, hPSC-pericytes did not induce activation and proliferation of allogeneic resting T cells independent of IFN-g pre-stimulation, similarly to pericytes from human brain or full-term placenta. Pericytes rather mediated a significant increase in the frequency of allogeneic CD25highFoxP3 regulatory T cells (Tregs) when co-cultured with none-activated peripheral blood T cells. When peripheral blood CD25high Tregs were depleted from isolated CD3 T cells, pericytes preferentially induced de-novo formation of CD4CD25highFoxP3CD127-, suppressive Tregs. Constitutive expression of PD-L1/2 and secretion of TGF-b by hPSC-pericytes directly regulated the generation of pericyte-induced Tregs. Pericytes co-transplanted with allogeneic CD25- T cells into immunedeficient mice, maintained non-immunogenic phenotype and mediated the development of functional Tregs.  

Altogether these findings reveal a novel feature of pericyte-mediated immunomodulation, shared by native tissue- and hPSC-pericytes and support the notion that pericytes can be applied for allogeneic cell therapy.