Although the physiological role of CLEC in Kupffer cell
Although the physiological role of CLEC2 in Kupffer cell activation and phagocytic activity is not completely understood, published studies support our hypothesis. Kerrigan et al., for example, generated a chimeric receptor bearing the extracellular and transmembrane domains of CLEC7a and the cytoplasmic tail of CLEC2 to investigate CLEC2 function on neutrophils (Kerrigan et al., 2009). Whether the capacity of the chimeric protein, when expressed in cell lines, reflects the actual endogenous function of an intact CLEC2 receptor was not resolved, but the results indicated that the cytoplasmic tail of CLEC2, at least, could mediate a phagocytic activity. In another series of studies, Chang et al. (2010) showed that rhodocytin specifically induced macrophage secretion of proinflammatory cytokines, and Mourao-Sa et al. (2011) demonstrated that cross-linking of an anti-CLEC2 agonistic antibody in myeloid cell cultures modulated the effects of toll-like receptor (TLR) agonists to potentiate anti-proinflammatory IL-10 production. Thus, several published reports point toward a role for CLEC2 in macrophage function.
Although the results suggest that liver Kupffer formyl peptide receptors are a potential target for Fc-CLEC2(ECD)-mediated suppression of inflammation and subsequent improvement of metabolic parameters, it remains possible that this effect is indirect. In addition, it is important to note that there may be additional Fc-CLEC2(ECD) targets. CLEC2 is expressed on the surface of many myeloid cells including dendritic cells and neutrophils (Mourao-Sa et al., 2011), thus, whether the improved metabolic phenotypes induced by Fc-CLEC2(ECD) are due to its effects on the function of CLEC2 receptor on those cells, or due to a gain-of-function effect independent of the CLEC2 receptor, will require additional investigation.
Conflict of Interest This work was funded by Amgen Inc. All authors were full time Amgen employees when the work was performed.
Introduction Extracellular haemoglobin (Hb) is readily oxidized into metHb, which is highly redox-active and cytotoxic due to its pseudoperoxidase activity (metHb-POX) (Jiang et al., 2007). Dangerous levels of cell-free metHb may persist in the plasma, for example in sickle cell anaemia (1.6mg/ml) and paroxysmal nocturnal hemoglobinuria (5–20mg/ml) (Schaer et al., 2013; Hartmann et al., 1966). The plasma metHb as well as pathogen-associated molecular patterns (PAMPs) are released into the plasma in an infection by haemolytic microbes, which may cause systemic inflammatory responses leading to multiple organ dysfunctions. metHb is normally rapidly scavenged by haptoglobin, scavenger receptor class (SR)-B1, and CD163. The internalized metHb undergoes detoxification and degradation (Subramanian et al., 2013; Schaer and Alayash, 2010). However, in severe haemolysis, massive levels of metHb overwhelm the capacity of metHb scavengers leading to excessive production of reactive oxygen species (ROS) by metHb-POX, which perturbs immune homeostasis (Olsson et al., 2012). metHb may bind to other damage-associated molecular patterns (DAMPs) and PAMPs, which are recognized by pattern recognition receptors such as TLRs in various immune cells, to trigger pro-inflammatory responses. When present in the plasma, metHb is a highly redox-reactive major DAMP that threatens the integrity of the white blood cells (WBCs), but its potentials to signal through TLRs are hitherto unclear (Lee and Ding, 2013). The methicillin-resistant strain of Staphylococcus aureus is a notorious haemolytic Gram-positive bacterium, which has become a major public health problem (Iwamoto et al., 2013; Stryjewski and Corey, 2014). Lipoteichoic acid (LTA) is the key immunostimulatory component of S. aureus that triggers TLR2-activating innate immune system of the host. Hb has been known to form a complex with S. aureus LTA to potentiate the immune stimulatory effect of LTA (Hasty et al., 2006). We previously reported the mechanism of ROS production by metHb-POX, showing that binding of LTA to metHb enhances the production of ROS, which not only kills the invading microbe, but is also harmful to the host blood cells (Jiang et al., 2007; Bahl et al., 2011; Du et al., 2010).