John Kanellis for helpful comments; Hillary Patuwo, Diane Shao, and Sanna Ronkainen for enumeration of fibrocytes; and Jeff Crawford for critical reading of the manuscript. REFERENCES 1. aggregated IgG inhibited fibrocyte differentiation. Using inhibitors of protein kinase enzymes, we show that Syk- and Src-related tyrosine kinases participate in the inhibition of fibrocyte differentiation. These observations suggest that H3/l fibrocyte differentiation can occur in situations where SAP and aggregated IgG levels are low, such as the resolution phase of inflammation. Keywords: monocytes, inflammation, cellular differentiation, serum amyloid P, FcR INTRODUCTION Tissue damage or the presence of pathogens leads to the recruitment and activation of peripheral blood monocytes [1, 2]. These cells differentiate into tissue macrophages and are a key component of the innate immune response essential for the control of many infections. After the removal of pathogenic organisms, macrophages remove apoptotic cells and promote tissue regeneration by stimulating fibroblast proliferation and extracellular matrix (ECM) production [3, 4]. The source of the fibroblasts responsible for the repair of wound lesions or the hyperplasia characteristic of chronic RX-3117 inflammation is still controversial. The conventional hypothesis is that local quiescent fibroblasts migrate into the affected area, produce ECM proteins, and promote wound contraction [5]. An alternative hypothesis is that circulating fibroblast-like cell precursors (fibrocyte precursors), present within the blood, migrate to sites of injury, where they differentiate into fibroblast-like cellsfibrocytesand mediate tissue repair [6C9]. Fibrocytes express markers of hematopoietic (CD45, major histocompatibility complex class II, CD34) and stromal cells (collagen I and III and fibronectin) [10, 11]. Fibrocytes, at sites of tissue injury, secrete inflammatory cytokines and ECM proteins and promote angiogenesis and wound contraction [6, 12]. Fibrocytes are also associated with the formation of fibrotic lesions after infection or inflammation and are implicated in fibrosis associated with autoimmune diseases [11, 13C17]. Fibrocyte precursors originate from ~10% of circulating CD14-positive/CD16-negative peripheral blood monocytes [6, 18, 19] (data not shown). At least two factors promote the differentiation of monocytes into fibrocytes. First, direct contact between CD14+ monocytes and T cells increases the number of fibrocytes [6]. Second, transforming growth factor- acts as a maturation factor for fibrocytes once differentiation has occurred [6, 19]. We have found that the differentiation of fibrocytes from monocytes is inhibited by serum amyloid P (SAP), which overrides the positive effect of T cells [18]. In the absence of serum or purified SAP, monocytes differentiate into fibrocytes within 3 days. SAP, a member of the pentraxin family RX-3117 of proteins, which includes C-reactive protein (CRP), is produced by the liver, secreted into the blood, and circulates in the blood as stable pentamers [20C23]. SAP appears to play a role in the initiation and resolution phases RX-3117 of the immune response [24C26]. SAP binds to sugar residues on the surface of bacteria, leading to their opsonization and engulfment [23, 24]. SAP also binds to free DNA and chromatin generated by apoptotic cells at the resolution of an immune response, thus preventing a secondary inflammatory response [25C27]. Receptors for the Fc portion of immunoglobulin G (IgG; FcRs) are found on the surface of a variety of hematopoietic cells. There are four distinct classes of FcR. FcRI (CD64) is the high-affinity receptor for IgG expressed by peripheral blood monocytes and binds monomeric IgG with a high affinity [28, 29]. FcRII (CD32) and FcRIII (CD16) are low-affinity receptors for IgG and only bind aggregated IgG efficiently. FcRII is expressed by peripheral blood B cells and monocytes, whereas FcRIII is expressed by natural killer cells and a subpopulation of monocytes [30C32]. Recently, a new FcR has been identified in mice [33]. FcRIV RX-3117 is present on murine peripheral blood monocytes and neutrophils, macrophages, and dendritic cells and binds murine IgG2a and IgG2b antibodies efficiently. There is a putative, human FcRIV RX-3117 gene, but the biological functions of the protein, such as ligand specificity and cellular expression, are as yet unknown [34]. Bacteria and proteins bound by SAP are removed by phagocytic cells, such as macrophages, as a result of the ability of SAP to bind to all three classical FcRs, with a preference for FcRI and FcRII [35, 36]. CRP appears to bind with a high affinity to FcRII and a lower affinity to FcRI but does not bind FcRIII [37C 41]. SAP and CRP initiate intracellular.