MSC-tumor cell interactions: an ultrastructural analysis

Mesenchymal stromal cells (MSCs) have attracted great interest for their considerable therapeutic potential. When implanted in the site of injury or in a tumor, they exert their biological effects by a multitude of signals and complex interactions with resident healthy, damaged or transformed cells. Cell-cell interaction, in particular, may be achieved through some basic mechanisms like: 1) direct cell-cell interaction through the formation of mechanical adhesions, functional junctions or tunnelling nanotubes; 2) information exchange mediated by soluble trophic factors and signaling molecules; 3) transfer of molecules enclosed in membrane bounded vesicles (MVs). In spite of the large number of papers dealing with the functional aspects and implications of MSC interactions, a few studies investigated their morphological counterpart. In this work, the most relevant morphological features of MSC homotypic and heterotypic interactions evidenced by electron microscopy has been surveyed. Our observations on cultured MSCs, demonstrated that they are often reciprocally interconnected by adhesion structures and by small points of contact located on cell body as well as along cytoplasmic processes. These cellular protrusions vary greatly in length and either get in touch with each other or insert into cytoplasmic invaginations of contiguous and distant cells. As cells proliferate and reach a higher density, these processes drastically reduce in length and appear as short intercellular bridges. In order to explore the possible modulation of MSCs on tumor growth and progression, we recently performed a set of experiments aimed at investigating, morphologically, the relationships between MSCs and different type of tumor cells. In particular, the interaction between MSCs and the leukemia cell lines L1210 and Molt4, and between MSCs and the melanoma cell line B16 were analyzed by electron microscopy. Co-incubation of L1210 or Molt-4 with MSCs resulted in the formation of mesenchymal cell aggregates surrounded by a “crown” of leukemic cells that seemed to be attracted by MSCs. Even if areas of contact between leukemia cells and MSCs were observed, no apparent specialization of the plasma membrane and of the cytoplasm beneath the sites of contiguity or contact was seen; therefore, the presence of junctional structures or complexes that could suggest a strong mechanical or functional interaction at the cell-cell interface was excluded. When co-cultivated with B16, MSCs realize different kinds of interactions including adhesion and MV release. This latter aspect was revealed by the observation of a huge number of vesicles, shedding from the cell surface, and of exosomes, derived from multivesicular bodies located inside the cells. The presence of nanometric cell surface projections has been occasionally detected and has been interpreted as a possible variety of communication referable to tunnelling nanotubes.