Multipotent mesenchymal progenitor cells (MPCs) can give rise to several types of specialized cells [1]. These fibroblast-like cells were first identified and isolated from the bone marrow (BM) and spleen, and were termed “bone marrow stromal stem cells” [2]. Presently, many other tissues such as the placenta, amniotic fluid, umbilical cord, adipose tissue, tonsils, and endometrium have been identified as sources of MPCs [310]. Nevertheless, the phenotype and differentiation potency of MPCs vary with respect to the tissue source from which they are isolated and the harvesting procedure [1113]. MPCs possess the potential to differentiate into multiple cell types including adipocytes (fat), chondrocytes (cartilage), and osteoblasts (bone) [14]. Characterization of MPCs includes adherence to standard tissue culture plastic, expression of various surface antigens, and in vitro differentiation potential [15]. Like other types of adult progenitor cells, MPCs remain quiescent (nondividing) in vivo for long periods. When activated, MPCs divide and differentiate to replace injured cells as well as secreting factors to prevent inflammation and promote tissue repair [16].

Cellular therapies hold great potential for the cure of a wide range of diseases and provide enhanced treatment modalities including immunomodulatory therapies, tissue regeneration, and cancer therapies. Using MPCs is an attractive approach for cell therapy as it avoids the ethical and practical issues of embryonic- and fetal-derived stem cells [17]. Currently, many clinical trials are testing MPCs obtained from different tissue sources for the cure of numerous conditions such as autoimmunity, heart disease, bone and cartilage disease, cancer, neuropathologies, and gastrointestinal diseases [18]. Furthermore, other than disease treatment, human MPCs are equally used in drug discovery applications as replacements for primary cells and animal models for initial toxicity and screening of new compounds [19].

MPCs, also classified as pericytes, reside on blood vessels [2022] and, therefore, the more vascularized the tissue is, the more rich the tissue is with MPCs. However, pericytes do not share a common embryonic precursor (reviewed in [23]). Moreover, recent studies have shown that pericytes from different anatomical sites, regarded as “MPCs”, differ widely in transcriptomic signatures and differentiation potential [24]. Therefore, the tissue source and the derivation procedure can affect the abundance, phenotype, and differentiation potency of MPCs [3, 1113, 25]. Historically, BM has been one of the major sources of MPCs. However, the derivation of progenitor/stem cells from patients and healthy donors is not always possible. BM extraction requires hospitalization and leads to considerable donor morbidity, including pain and bleeding, and other complications such as infection and risk for viral exposure [26, 27]. Therefore, efforts have been made to find alternative sources of MPCs for cell therapies. Another current major source for MPC derivation is adipose tissue. However, 100 ml of tissue and blood collected from lipoaspirates provides approximately 105 cells [28]. This is likely due to the mechanical trauma to the tissue incurred during the liposuction procedure, leading to a low yield and reduced cell viability [29]. Importantly, in order to achieve enough viable cells in the scale needed for clinical purposes, a large quantity of starting tissue material is needed which may result in considerable risk and donor morbidity. Therefore, for translational purposes, an accessible alternative source of MPCs is still needed.

Tonsils retrieved from tonsillectomy procedures have been shown to be a good source of MPCs [30]. For translational purposes, cells that can be retrieved from healthy donors without the risk of major complications and donor morbidity are preferred as a reliable source for cell therapies. To this end, we developed a procedure to generate highly proliferative multipotent progenitor/stem cells from a small fragment of normal tonsillar tissue. Tonsils are lymphoid tissue anatomically located at the entrance of the pharynx. Here we report a novel procedure for the isolation of MPCs from a biopsy-sized sample of human tonsil. We have isolated tonsillar MPCs (T-MPCs) from multiple donors across a spectrum of age, sex, and race, and successfully expanded them in culture. We characterized them by cell surface markers, as well as by in vitro expansion and differentiation potential. Overall, our study highlights tonsillar biopsy as an excellent source of MPCs and a viable alternative to currently used sources such as bone marrow and adipose tissue.