Luis A. Solchaga

My current research focuses on cell-based therapies and tissue engineering with special emphasis on mesenchymal stem cells. My most recent work extends in two parallel directions.
On the one hand, through collaborations with other researchers, I participate in the development of an integrated approach to cartilage tissue engineering; the goal of this research is to minimize the use of empirical approaches in favor of computational modeling to minimize the experimental variables and device a more focused experimental design.
Simultaneously, I am taking advantage of new technologies (e.g. microarray and bioinformatics) to better address fundamental questions of adult stem cell biology and advance the understanding of the role that these cells may play in the future of tissue engineering.

Stem Cell Biology:
Tissue Engineering holds the promise of biological replacements for damaged tissues. Most Tissue Engineering endeavors include a cellular component that is responsible for the genesis and maintenance of the bioengineered tissue. Tissue engineers have a choice between differentiated cells or progenitor cells. Both options have advantages and drawbacks. Fully differentiated cells already possess the appropriate phenotype but, in order to isolate these cells, it is necessary to obtain a biopsy of healthy tissue with the inherent morbidity of the harvest. Progenitor or stem cells, particularly bone marrow-derived stem cells, are easy to obtain with low morbidity and can be easily expanded in culture. However, one needs to drive their differentiation along the pathway of choice adding complexity over the intrinsic difficulties of any Tissue Engineering approach.
Recent progress in the isolation, expansion and characterization of progenitor cells derived from a variety of tissues has advanced stem cell biology and increased the interest on their potential therapeutic use. At present, stem cell preparations have still several shortcomings. Stem cell preparations are not established cell lines and the degree of donor-to-donor variability presents a considerable challenge. In addition, effective use of stem cells requires that they be expanded while retaining their potential to differentiate into specific phenotypes and the mechanism(s) by which stem cells retain (or loose) their potential is still largely unknown. Thus, there is a clear need for a better characterization of these cells and a more detailed understanding of the molecular mechanisms involved in regulating their multipotentiality during mitotic expansion in vitro, and in their subsequent commitment and differentiation along specific pathways.
It is well known that cytokines play a major role in the commitment and progression of progenitor cells along specific lineages during development. Several of these molecules (BMP2, TGF-β1) have been identified as inductive agents in adult mesenchymal stem cells. These molecules are capable of triggering lineage commitment and differentiation; while other molecules (FGF-2, PDGF-BB) are known regulators of proliferation and multipotentiality. Before these findings can be safely and reliably applied to clinical tissue engineering, the molecular mechanisms underlying these phenomena must be understood in much greater detail. My research addresses this under-explored area: the identification and analysis of the cellular mechanisms involved in the regulation of the multipotentiality of bone marrow-derived mesenchymal stem cells.
Human mesenchymal stem cells (hMSCs) expanded in the presence of cytokines show enhanced chondrogenic potential. Importantly, the chondrogenic potential of cytokine-treated hMSCs is sustained through a greater number of population doublings than in control cells. Preliminary gene expression profile data highlights several differentially-expressed genes involved in regulatory pathways that are potentially important for the regulation of these phenomena. The focus of this line of research is the evaluation of the functional effects of supplementation using quantitative analysis of standardized differentiation assays as a function of "in vitro aging". By exploiting gene expression profiling, I intend to identify the molecular pathways behind these phenomena and develop useful, predictive markers for stem cells.
The goals of this project are:
1. To develop media and culture conditions which allow rapid expansion of human MSCs while maintaining or enhancing their multipotentiality.
2. To identify the molecular mechanisms that control the stem nature of these cell preparations through the analysis of gene expression profiles of MSCs as function of "in vitro aging" and culture conditions.

Immuno-Suppressive effects of MSCs
Allogeneic hematopoietic stem cell (HSC) transplantation is an effective therapy for a number of malignant and non-malignant hematologic diseases and an increasing number of patients are undergoing this procedure worldwide. Graft rejection and graft-versus-host disease (GVHD) remain significant obstacles to successful outcome in allogeneic cell therapy. We have generated data that suggests human bone marrow derived mesenchymal stem cells (MSCs) can be used with a therapeutic purpose during allogeneic cell transplantation, to facilitate engraftment and inhibit allogeneic rejection. Therefore MSCs may represent a prototype of "facilitator cells" in the setting of cellular therapy. Identification of the mechanisms involved in MSC mediated immune suppression and exploitation of this effect for development of clinical therapeutic strategies is the general goal of this project.
We have thus far demonstrated the feasibility of culturing and infusing large numbers of human MSCs into hematology patients, retroviraly transduced and tracked human MSCs in vivo using NOD-SCID mice, facilitated human HSC engraftment in this model by co-infusion of human MSCs and elucidated robust suppression of alloreactive T-cells by activated human MSCs.
We hypothesize that human MSCs inhibit T-cells in a non-specific manner within the local microenvironment. Furthermore, we hypothesize that optimal distribution and survival of co-transplanted MSCs will result in reduction of immune mediated rejection of therapeutic cells and reduce GVHD after allogeneic HSC transplantation.
Our specific aims are to:
1. Determine the effects of antigen presenting cells and inflammatory cytokines on human MSC mediated suppression of human T-cells.
2. Determine the effects of human MSCs on xeno-reactivity of human T-cells using RAG2-/-γc-/- double mutant mouse model of human GVHD.

Cartilage Tissue Engineering:
Degenerative joint disease is a major cause of chronic disability. Mature articular cartilage that has sustained full thickness damage has very limited inherent capacity for self repair, thus chondral lesions do not heal spontaneously. In fact, by altering the local mechanical environment in the adjacent cartilage, existing damage expands within the joint. In many cases, surgical joint replacement is the only treatment option. It would therefore be important to successfully repair cartilage defects at early stages in order to prevent progressive destruction of the articular surfaces. The incomplete natural regenerative response is likely due to the inability to recruit adequate numbers of reparative cells into the damaged area and/or the failure to establish the appropriate microenvironment for the repair process.
In animal studies, experimental lesions that penetrate the subchondral bone into the bone marrow space exhibit a limited reparative response, dependent upon the age of the animal, the location and extent of the lesion. This response is likely due to colonization of the site by marrow derived mesenchymal stem cells (MSCs), some of which then differentiate into chondrocytes.
These observations have led to the development of tissue engineering strategies combining cells, either differentiated or progenitor, and/or growth factors with delivery vehicles in order to fabricate implantable composites for the treatment of articular cartilage defect. Success to date using these purely cell- or signaling molecule-based approach has been modest and none of these strategies has attained the long term functional repair of articular cartilage.
It is our belief that these attempts to repair large cartilage defects using cells or cell-scaffold constructs fail because these constructs lack the characteristics of the mature cells and the matrix in which they are embedded both of which are prerequisites for them to thrive under the conditions of the joint environment.
The global hypothesis is that by exposing the cell/scaffold constructs, in a protected in vitro environment, to conditions that favor chondrogenic differentiation, a chondrogenic preconditioning will occur, during which these constructs will develop the properties required for survival after implantation in the joint.
The long-term goal is to optimize the fabrication of large tissue engineered cartilage implants for the treatment of clinically relevant lesions of the articular cartilage. We are to conducting experimentation in which MSCs are incorporated into appropriate delivery vehicles and are differentiated in vitro to obtain large tissue substitutes for in vivo implantation. We use a perfusion culture system where the diffusion of nutrients into the implant is enhanced by a continuous flow of culture medium in a closed air-permeable receptacle where the composite is cultured for variable periods of time.
The goal of this research is to produce stem cell-based implants for repairing musculoskeletal tissues. The work involves the development of:
1. Composite (cell-scaffold) implants in a bioreactor system
2. Animal models for testing these implants. These include models of both heterotopic and orthotopic tissue repair/generation such as bone, tendon and cartilage defects.

Digital photography, digital video recording and digital video editing; I have used these technologies to prepare novel multimedia teaching aids in the form of self-contained video clips demonstrating the technical details of some of the routine protocols utilized in our laboratory.