Mesenchymal stem cells as a novel vaccine platform

Abstract

Vaccines are the most efficient and cost-effective means of preventing infectious disease. However, traditional vaccine approaches have thus far failed to provide protection against human immunodeficiency virus (HIV), tuberculosis, malaria, and many other diseases. New approaches to vaccine development are needed to address some of these intractable problems. In this report, we review the literature identifying stimulatory effects of mesenchymal stem cells (MSC) on immune responses and explore the potential for MSC as a novel, universal vaccination platform. MSC are unique bone marrow-derived multipotent progenitor cells that are presently being exploited as gene therapy vectors for a variety of conditions, including cancer and autoimmune diseases. Although MSC are predominantly known for anti-inflammatory properties during allogeneic MSC transplant, there is evidence that MSC can actually promote adaptive immunity under certain settings. MSC have also demonstrated some success in anti-cancer therapeutic vaccines and anti-microbial prophylactic vaccines, as we report, for the first time, the ability of modified MSC to express and secrete a viral antigen that stimulates antigen-specific antibody production in vivo. We hypothesize that the unique properties of modified MSC may enable MSC to serve as an unconventional but innovative, vaccine platform. Such a platform would be capable of expressing hundreds of proteins, thereby generating a broad array of epitopes with correct post-translational processing, mimicking natural infection. By stimulating immunity to a combination of epitopes, it may be possible to develop prophylactic and even therapeutic vaccines to tackle major health problems including those of non-microbial and microbial origin, including cancer, or an infectious disease like HIV, where traditional vaccination approaches have failed.

Keywords: MSC; adaptive immunity; antibodies; antigen delivery; vaccination.

Figure 1

Strategy of modified MSC vaccination and possible MSC functions during vaccination. (A) MSC isolated from the bone marrow of human donors can be expanded in culture and modified by transfection using antigen(s)-encoding plasmid to express and secrete soluble proteins, including both cancer and microbial antigens. Parenteral immunization of these modified MSC could then provide protective immunity. (B) These modified MSC may carry out several possible functions after vaccination. Primarily, it is expected that they serve as antigen delivery vehicles or even antigen depots following immunization. Based on the literature, it is clear that MSC can also take a more active role in induction of adaptive immunity, including cytokine secretion, like IL-6, and/or antigen presentation through phagocytosis and MHC-loading of antigen for presentation to lymphocytes expressing cognate T-cell/B-cell receptors. These immunostimulatory functions may also be involved in MSC-based vaccinations.

Figure 2

MSC can be modified to express viral protein gp120. MSC derived from the bone marrow of C57Bl/6 mice were isolated and validated by the Tulane Center for Stem Cell Research and Regenerative Medicine (New Orleans, LA) as previously described (Ripoll and Bunnell, 2009). A total of 1 × 10⁶ MSC were transfected by electroporation using the Invitrogen Neon system (Carlsbad, CA) with 2.5, 5, or 7.5 μg pSWTK-gp120, or empty vector, pSWTK (generously provided by Dr. V. S. Kalyanaraman of ABL Inc., Kensington, MD) according to the manufacturer's instructions. (A) Gp120 immunofluorescence staining of MSC transfected with 5 μg pSWTK or pSWTK-gp120 1 or 7 days post-transfection, and controls using secondary (2°) antibody only, was carried out as previously described at 63X (Tomchuck et al., 2008). (B) Western blot analysis of corresponding cell lysates (approximately 25 μg of protein) were probed with anti-gp120 as previously described (Lamarca et al., 2008). (C) 1 × 10⁵ transfected MSC were incubated 1–4 days and the harvested cell culture supernatants were analyzed by an HIV-1 gp120 ELISA according to the manufacturer's instructions (ABL Inc.) Data are presented as the mean ± standard error of the mean and analyzed by One Way ANOVA using the Tukey's post hoc test (GraphPad Prism Version 4). Statistical significance was determined by comparing pSWTK-gp120 and pSWTK groups. ^*p < 0.05; ^**p < 0.01; ^***p < 0.001.

Figure 3

Modified MSC expressing gp120 promote serum anti-gp120 antibody responses in mice after parenteral immunization. Groups of five female C57Bl/6 mice 6–8 weeks underwent a single immunization with 1 × 10⁶ MSC transfected with 7.5 μg pSWTK-gp120 (MSC-gp120; solid lines) 16 h post-transfection or 5 μg purified gp120 (a vector-corresponding recombinant protein provided by Dr. V. S. Kalyanaraman; broken lines), with naïve mice serving as a control (black). MSC and gp120 were diluted in DPBS and administered with a 0.5 ml syringe to deliver 100 μl per dose for intraperitoneal and subcutaneous injection, or 50 μl per dose for intramuscular injection. Mice were sacrificed 17 days post-immunization and sera collected. An ELISA for serum anti-gp120 IgG antibodies graphed as 405 nm absorbance versus sera dilution was preformed as previously described (Norton et al., 2011). Animal studies were approved by the Tulane University Institutional Animal Care and Use Committee.