Stromal cells of the lamina propria

Stromal cells also termed mesenchymal stromal cells (MSCs) are a diverse and heterogeneous cell population of non-epithelial (EPCAM^-), non-hematopoietic (CD45^-) and non-endothelial cells (CD31^-) including all fibroblast and myofibroblast populations and immunological stromal cells of the lymphoid tissue. Ultimately stromal cells derived from mesenchymal stem cells (as well MSC) of the bone marrow (Owens 2015) but can also arise from endothelial or epithelial cells through endothelial-to-mesenchymal or epithelial-to-endothelial-to-mesenchymal transition (Zeisberg et al. 2007; Kalluri and Weinberg 2009). A growing body of evidence suggests that multiple stromal cell populations with distinct functions and localizations exist within the same tissue.

1.1 The human gastric mucosa

components e.g. fibronectin, laminin and collagen type I, III, IV and V, but also matrix metalloproteinases (MMPs) for ECM degradation to balance the ECM homeostasis, maintaining the structural framework of the tissue. Furthermore, stromal cells are involved in the regulation of epithelial differentiation and proliferation, regulation of inflammatory processes and wound repair. Fibroblasts can be activated by different stimuli mainly through tissue damage and the release of growth factors like transforming growth factor β (TGFβ), epidermal growth factor (EGF) or fibroblast growth factor 2 (FGF2) by damaged epithelial cells or infiltrating immune cells.

Activated fibroblasts are termed myofibroblasts and are characterized by the expression of α smooth muscle actin (α-SMA). Upon activation myofibroblasts express elevated amounts of MMPs and growth factors e.g. EGF, FGF2 and hepatocyte growth factor (HGF), which in turn influences adjacent epithelial cells (Kalluri and Zeisberg 2006) (Figure 2). Besides α-SMA stromal cells express vimentin and are negative for epithelial marker E-Cadherin and EPCAM and hematopoietic marker CD45 and endothelial marker CD31. However, characteristic and exclusive positive markers for stromal cells are still unknown. Unfortunately, many stromal cell markers are also expressed by other cell types. Thus, a panel of markers needs to be used to define stromal cells and to delimit them from other cell types by negative exclusion. The International Society for Cellular Therapy defined minimal criteria to identify and define multipotent MSCs. These criteria include that MSC are plastic-adherent in in vitro cultures. Furthermore, they must be positive for the surface markers CD73, CD90 and CD105 and negative for CD34, CD45, HLA-DR, CD14 or CD11b and CD79α or CD19. Finally, MSCs differentiate into adipocytes, osteoblasts or chondroblasts in vitro (Dominici et al. 2006). Additionally, Stro-1 was described to be one of the best-known stromal cell markers (Kolf, Cho, and Tuan 2007). Gastric stromal cells (GSC) are part of the lamina propria in the gastric mucosa, surrounding and separating the gastric glands. A further population of myofibroblasts is located in the muscularis mucosae below the gastric glands separating the mucosa from the submucosa. Almost nothing is known about the role of stromal cells in the gastric mucosa and the communication with adjacent cell types under physiological but also pathophysiological conditions like bacterial infections except the impact of stromal cells in cancer initiation and progression by cancer-associated fibroblasts and gastrointestinal stromal tumors. The research on intestinal stromal cells instead is more advanced. It was shown that the

1.1 The human gastric mucosa mesenchymal stromal cells close to the colon crypt base, express BMP pathway antagonists e.g. gremlin or chordin-like to protect the crypt stem cells from BMP as it inhibits the Wnt/β-Catenin pathway in crypt stem cells and induces differentiation (Kosinski et al. 2007). On the other hand stromal cells in the villus region of the crypt express BMPs, maintaining the differentiation in epithelial cells after leaving the stem cell compartment by migration (Powell et al. 2011). Moreover, mesenchymal cells along the crypt-villus axis in the small intestine as well as in the colon are a source of different Wnt signaling components including non-canonical Wnts e.g. Wnt2b, Wnt4, Wnt5a and Wnt5b and antagonists e.g. DKK3, sFRP1 (Gregorieff et al. 2005).

In vitro co-culture studies by Katano et al. (2015) with murine stromal and epithelial cells revealed a supporting role of stromal cells on epithelial stem cell activity and proliferation. Recently, Sigal et al. (2017) showed that murine gastric stromal cells of the antrum in proximity to the stem cell compartment express R-spondin 3 and thereby support the self-renewal of the stem cell niche in vivo. In a follow-up study the authors demonstrated that upon H. pylori infection, myofibroblastic R-spondin 3 activates Lgr5+ stem cells to differentiate into a secretory phenotype expressing antimicrobial factors (Sigal et al. 2019). This study reveals the active communication between stroma and epithelium under pathological conditions revoking once more the dogma that stromal cells are passive structural subjects.

1.1 The human gastric mucosa

Figure 2: Stromal cells can be activated into a secretory phenotype with an immunomodulatory effect and communicate with adjacent cell types. Schematic representation of a stromal cell in the inactive state and after activation by a stimulus like an injury. Stromal cells produce ECM constituents like fibronectin, laminin and different types of collagen. After activation, stromal cells express αSMA, vimentin, different MMPs and a panel of cytokines, chemokines, growth factors and cell surface molecules with pro- or anti-inflammatory effects depending on the microenvironment. Secreted molecules influence adjacent cell types like immune cells or epithelial cells. TIMP: Tissue inhibitor of matrix metalloproteinase; ICAM: Intercellular adhesion molecule 1; VCAM1: Vascular cell adhesion molecule 1. The figure was adapted and modified from Kalluri and Zeisberg (2006); Kalluri (2016) (by permission of Springer Nature Customer Service Centre GmbH).

It is accepted that stromal cells belong to the group of antigen-presenting cells and are declared as non-professional immune cells as they show immunomodulatory characteristics by secretion of chemokines and cytokines having an impact on tolerance or active immunity (Mueller and Germain 2009). Figure 2 represents the activation of stromal cells into a secretory phenotype influencing adjacent immune cells and epithelial cells. Through secretion of plethora panel of chemokines, cytokines and cell surface molecules MSCs actively interact with different types of immune cells having either pro-inflammatory or anti-inflammatory and immunosuppressive effects which indicate the high plasticity of this cell type (Hoogduijn 2015). During an early

1.2 Embryonic development and patterning of the stomach