Gorazd Krosl

Transplantation of Mesenchymal Stem Cells Promotes Tissue Regeneration in a Glaucoma Model Through Laser-Induced Paracrine Factor Secretion and Progenitor Cell Recruitment†‡§

Among bone marrow cells, hematopoietic and mesenchymal components can contribute to repair damaged organs. Such cells are usually used in acute diseases but few options are available for the treatment of chronic disorders. In this study, we have used a laser‐induced model of open angle glaucoma (OAG) to evaluate the potential of bone marrow cell populations and the mechanisms involved in tissue repair. In addition, we investigated laser‐induced tissue remodeling as a method of targeting effector cells into damaged tissues. We demonstrate that among bone marrow cells, mesenchymal stem cells (MSC) induce trabecular meshwork regeneration. MSC injection into the ocular anterior chamber leads to far more efficient decrease in intraocular pressure (IOP) (p < .001) and healing than hematopoietic cells. This robust effect was attributable to paracrine factors from stressed MSC, as injection of conditioned medium from MSC exposed to low but not to normal oxygen levels resulted in an immediate decrease in IOP. Moreover, MSC and their secreted factors induced reactivation of a progenitor cell pool found in the ciliary body and increased cellular proliferation. Proliferating cells were observed within the chamber angle for at least 1 month. Laser‐induced remodeling was able to target MSC to damaged areas with ensuing specific increases in ocular progenitor cells. Thus, our results identify MSC and their secretum as crucial mediators of tissue repair in OAG through reactivation of local neural progenitors. In addition, laser treatment could represent an appealing strategy to promote MSC‐mediated progenitor cell recruitment and tissue repair in chronic diseases. STEM Cells 2013;31:1136–1148

Photodepletion differentially affects CD4+ Tregs versus CD4+ effector T cells from patients with chronic graft-versus-host disease

Even the most potent immunosuppressive drugs often fail to control graft-versus-host disease (GVHD), the most frequent and deleterious posttransplantation complication. We previously reported that photodepletion using dibromorhodamine (TH9402) eliminates T cells from healthy donors activated against major histocompatibility complex-incompatible cells and spares resting T cells. In the present study, we identified photodepletion conditions selectively eradicating endogenous proliferating T cells from chronic GVHD patients, with the concomittant sparing and expansion of CD4(+)CD25(+) forkhead box protein 3-positive T cells. The regulatory T-cell (Treg) nature and function of these photodepletion-resistant cells was demonstrated in coculture and depletion/repletion experiments. The mechanism by which Tregs escape photodepletion involves active P-glycoprotein-mediated drug efflux. This Treg-inhibitory activity is attributable to interleukin-10 secretion, requires cell-cell contact, and implies binding with cytotoxic T-lymphocyte antigen 4 (CTLA-4). Preventing CTLA-4 ligation abrogated the in vitro generation of Tregs, thus identifying CTLA-4-mediated cell-cell contact as a crucial priming event for Treg function. Moreover, the frequency of circulating Tregs increased in chronic GVHD patients treated with TH9402 photodepleted cells. In conclusion, these results identify a novel approach to both preserve and expand Tregs while selectively eliminating CD4(+) effector T cells. They also uncover effector pathways that could be used advantageously for the treatment of patients with refractory GVHD.

Genetic interaction between Kit and Scl.

SCL/TAL1, a tissue-specific transcription factor of the basic helix-loop-helix family, and c-Kit, a tyrosine kinase receptor, control hematopoietic stem cell survival and quiescence. Here we report that SCL levels are limiting for the clonal expansion of Kit⁺ multipotent and erythroid progenitors. In addition, increased SCL expression specifically enhances the sensitivity of these progenitors to steel factor (KIT ligand) without affecting interleukin-3 response, whereas a DNA-binding mutant antagonizes KIT function and induces apoptosis in progenitors. Furthermore, a twofold increase in SCL levels in mice bearing a hypomorphic Kit allele (W41/41) corrects their hematocrits and deficiencies in erythroid progenitor numbers. At the molecular level, we found that SCL and c-Kit signaling control a common gene expression signature, of which 19 genes are associated with apoptosis. Half of those were decreased in purified megakaryocyte/erythroid progenitors (MEPs) from W41/41 mice and rescued by the SCL transgene. We conclude that Scl operates downstream of Kit to support the survival of MEPs. Finally, higher SCL expression upregulates Kit in normal bone marrow cells and increases chimerism after bone marrow transplantation, indicating that Scl is also upstream of Kit. We conclude that Scl and Kit establish a positive feedback loop in multipotent and MEPs.

Modulation of tumor response to photodynamic therapy in severe combined immunodeficient (SCID) mice by adoptively transferred lymphoid cells

Photodynamic treatment, consisting of intravenous injection of PhotofrinR (10 mg/kg) followed by exposure to 110 J/cm2 of 630 plus or minus 10 nm light 24 hours later, cured 100% of EMT6 tumors (murine mammary sarcoma) growing in syngeneic immunocompetent BALB/C mice. In contrast, the same treatment produced no cures of EMT6 tumors growing in either nude or SCID mice (immunodeficient strains). EMT6 tumors growing in BALB/C and SCID mice showed no difference in either the level of PhotofrinR accumulated per gram of tumor tissue, or the extent of tumor cell killing during the first 24 hours post photodynamic therapy (PDT). In an attempt to improve the sensitivity to PDT of EMT6 tumors growing in SCID mice, these hosts were given either splenic T lymphocytes or whole bone marrow from BALB/C mice. The adoptive transfer of lymphocytes 9 days before PDT was successful in delaying tumor recurrence but produced no cures. A better improvement in PDT response was obtained with tumors growing in SCID mice reconstituted with BALB/C bone marrow (tumor cure rate of 63%). The results of this study demonstrate that, at least with the EMT6 tumor model, antitumor immune activity mediated by lymphoid cell populations makes an important contribution to the curative effect of PDT.

Tumor-associated macrophages (not tumor cells) are the determinants of photosensitizer tumor localization

The distribution of Photofrin and several other photosensitizers among major cellular populations contained in solid mouse tumors was examined using flow cytometry. Seven tumor models were included in the analysis: sarcomas EMT6, KHT, RIF, FsaR and FsaN, Lewis lung carcinoma and squamous cell carcinoma SCCVII. In all these tumors, the highest photosensitizer levels were found in a subpopulation of tumor associated macrophages consisting of activated cells (as suggested by their increased size, granularity, and the number of interleukin 2 receptors). There was no evidence of selective photosensitizer accumulation in malignant tumor cells. Results consistent with these observations were also obtained with the carcinogen induced squamous cell carcinoma growing in hamster cheek pouch.

Distinctive role of activated tumor-associated macrophages in photosensitizer accumulation

Cells dissociated from tumors (carcinomas and sarcomas) growing subcutaneously in mice that have been administered Photofrin or other photosensitizers were analyzed by flow cytometry. Monoclonal antibodies were used for identification of major cellular populations contained in these tumors. The results demonstrate that a subpopulation of tumor-associated macrophages (TAMs) is unique among tumor cell populations in that it excels in the accumulation of very high levels of photosensitizers. These macrophages showed an increased expression of interleukin 2 receptor, which is indicative of their activated state. since macrophages were reported to concentrate in the periphery of human neoplasms, it is suggested that activates TAMs are the determinants of tumor-localized photosensitizer fluorescence.

Potentiation of photodynamic therapy by granulocyte-macrophage colony stimulating factor immunotherapy

The murine squamous carcinoma cell line (SCCVII) was genetically engineered to produce high levels of granulocyte-macrophage colony stimulating factor (GM-CSF). Lethally irradiated GM-CSF producing cells were injected under the subcutaneously growing parental SCCVII tumor at various times before and/or after PDT. Even a single treatment with GM- CSF producing cells injected two days before PDT markedly enhanced the tumor cure rate when compared to the PDT treatment alone. Effective potentiation was observed with PDT mediated either by Photofrin or by benzoporphyrin derivative.