2.1 Endothelial progenitor cells (EPCs)
The controversy surrounding EPC definition are beginning from the first
observations of blood vessel development in 1917, but after a long
period of discussion, the precise definition of EPC are still
unclear(Ferkowicz & Yoder, 2005). The most accessed definition of EPC
is that progenitor cells are a heterogeneous population which includes
different origins and several residing sites at different maturity
stages(Yang, Pan, Wang, Qiu, & Mao, 2018). Due to the different
features of cells, it is difficult to produce a worldwide accepted
characterization of EPCs(Richardson & Yoder, 2011). Additionally, due
to the different characterizations of EPCs, some contrary results have
emerged(Harper et al., 2019; Toshner et al., 2009; Zhou et al., 2013).
Balistreri et al. argued that EPCs are a type of stem cell(Bianconi et
al., 2018; Prokopi & Mayr, 2011). In their opinion, progenitor cells
are an intermediate stage between multipotent stem cells and mature
cells, and both stem cells and progenitor cells have the ability to
replicate and differentiate to mature cells, and even have the same
biological markers(Brown & McGuire, 2012). EPCs can adhere to matrix
molecules such as fibronectin and demonstrate dual positivity to
acetylated low-density lipoprotein (acLDL) and Ulex europaeusagglutinin I (UEA-1) lectin(Yang, Pan, Zhao, & Wang, 2013). These types
of cells commonly have some common positive protein such as CD34+ and
CD133+ (which are two unique markers of hematopoietic stem cells),
vascular endothelial growth factor receptor-2 (VEGFR2+, also called
kinase insert domain receptor (KDR)/fetal liver kinase-1 (Flk-1), which
is an important regulator of development of ECs and vasculature), and
von Willebrand factor+ (vWF+), CD31+, CD144+, CD146+, and endothelial
nitric oxide synthase+(eNOS+).
EPCs in different classifications are characterized by different
abilities and affinities. According to the number of culture days after
monocytes are extracted from blood(Hansmann et al., 2011), EPCs can be
divided into early-outgrowth EPCs and late-outgrowth EPCs(Pelosi,
Castelli, & Testa, 2014; Prokopi & Mayr, 2011). Early-outgrowth EPCs,
also called circulatory angiogenic cells (CACs)(Basile & Yoder, 2014;
Prater, Case, Ingram, & Yoder, 2007), are a spindle-shaped cell that
emerges after culturing for 4-7 days(Murohara, 2010). Its name indicates
that this type of EPC possesses little differentiation capacity, and
thus, these cells cannot become mature cells(Paneni, Costantino,
Kränkel, Cosentino, & Lüscher, 2016). However, early-outgrowth EPCs
have a significant capacity to release several cell growth factors and
cytokines such as vascular endothelial growth factor (VEGF), hepatocyte
growth factor (HGF), granulocyte colony stimulating factor (G-CSF),
granulocyte/macrophage colony-stimulating factor (GM-CSF), and
interleukin (IL)-8(Guber, Ebrahimian, Heidari, Eliopoulos, & Lehoux,
2018; J. X. Yang et al., 2018). Due to their unique biological
behaviors, early-outgrowth EPCs exert their function mainly by secreting
growth factors and cytokines to support the growth of adherent cells and
surrounding late-outgrowth EPCs. Except for custom EPC markers,
early-outgrowth EPCs also express CD45 and CD14(S. J. Zhang et al.,
2006).
Other cell groups emerged after 14-21 days of culture, and these
cobblestone-shaped cells are named late-outgrowth EPCs, and also are
called endothelial outgrowth cells (EOCs)(S. Liu et al., 2018), although
they are different from early-outgrowth EPCs. The late-outgrowth EPCs
display a significant capacity to replicate and differentiate to mature
ECs. Compared with typical EPCs, late-outgrowth EPCs do not express
hematopoietic stem cells markers, but instead, express vascular
endothelial-cadherin (VE-cadherin) and CD146(Minami et al., 2015).
Given their biological properties, some researchers recognize that
early-outgrowth EPCs and late-outgrowth EPCs are not single types of
cells in different stages, but they may differentiate from different
cells(Medina et al., 2010). There also exist a small proportion of EPCs
that remain in the microcirculation of lung vessels before occurrence of
injury(Schniedermann et al., 2010). Therefore, we called this type of
EPC resident EPCs (in contrast to circulating EPCs). Resident EPCs
display a significant replication and differentiation capacity,
suggesting that lung blood and lymphatic ECs may be derived from these
cells. However, based on our existing research results, we cannot
clearly distinguish this cell type, and therefore, further study is
required to discern the nature between resident EPCs and circulating
EPCs.