Coculture with Late, but Not Early, Human Endothelial Progenitor Cells Up Regulates IL-1β Expression in THP-1 Monocytic Cells in a Paracrine Manner

Endothelial progenitor cells (EPCs) have been used in clinical trials to treat ischemic heart disease. Monocyte infiltration plays an important role in inflammation, angiogenesis, and tissue repair during tissue ischemia. It is important to understand the interactions between EPCs and monocytes. In this study, a human EPC/THP-1 monocytic cell coculture system was used to examine EPC effect on IL-1α, IL-1β, and TNF-α expression in THP-1 cells. Late, but not early, EPCs upregulated IL-1β expression at both mRNA and protein levels. In contrast, neither early nor late EPCs affected IL-1α or TNF-α expression. Coculture with human umbilical vein endothelial cells did not alter IL-1β expression. It has been shown that activation of integrin β2 in human neutrophils augments IL-1β synthesis; however integrin β2 was not involved in IL-1β expression in THP-1 cells. Addition of late EPC conditioned medium to THP-1 cell culture led to a modest increase of IL-1β mRNA levels, indicating that late EPCs upregulate IL-1β expression partly through a paracrine pathway. IL-1β, an important inflammation mediator, has been shown to promote EPC function. Our data therefore suggest that late EPCs can exert self-enhancement effects by interacting with monocytes and that EPCs might modulate inflammatory reactions by regulating IL-1β expression in monocytes. 1. Introduction Endothelial progenitor cells (EPCs), first described by Asahara et al. [1], represent a heterogeneous cell population derived from circulating CD34 positive or CD34 and KDR/VEGF receptor-2 (KDR/VEGFR2) double positive mononuclear cells. They have the ability to differentiate into endothelial lineage cells and have been shown to incorporate into newly formed vessels in animal models of ischemia. It has also been shown that CD133 positive cells demonstrated endothelial progenitor capacity [2, 3]. Extensive in vitro and in vivo studies have established that EPCs play an important role in vascular repair and regeneration [4–6], and several clinical trials have been completed that have assessed the efficacy of EPC therapy in patients suffering ischemic heart disease [7–10]. Currently, the most common protocol to obtain EPCs is through culture of peripheral blood derived mononuclear cells (PBMNCs) in endothelial cell (EC)-specific media [11]. This method has led to the identification of 2 distinct populations of EPCs, that is, early and late EPCs [12]. Early EPCs demonstrate a typical spindle-like morphology and are generated after culture of PBMNC for 4–7 days. They possess little proliferative ability but robustly