27, 32, 35, 36 We found MAC387 expression to be highest in patients transplanted X-396 sooner following acetaminophen ingestion, which could suggest that the influx of monocyte-derived macrophages to inflammatory foci occurs in the earlier phases of liver injury.14, 27 Experimental models demonstrate that the interaction between CCL2 and its receptor

CCR2 promotes efflux of CCR2-expressing monocytes from the bone marrow into the circulation.24, 37, 38 Our data demonstrate that despite reactive monocyte progenitor hematopoiesis and markedly elevated circulating CCL2 levels, there is a profound reduction in the absolute number of circulating monocytes that is proportional to the severity of acute liver injury (Figs. 1 and 2). This suggests that circulating monocytes are being recruited to the inflamed liver at a rate that exceeds bone marrow production resulting in a reduction in their numbers in the circulation. However, our data do not exclude the possibility that the depletion of circulating monocytes may also be attributed to apoptosis39 or recruitment to other tissues. Consistent with the previously published experimental APAP models12-14, 18 and human studies of AALF,25, 27 our data support the role of CCL2 in recruitment of circulating monocytes to the

liver during AALF. In Fig. 6, we show that necrotic liver tissue may act as a source of CCL2 secretion, as evidenced by the significantly elevated levels of monocyte chemoattractants cAMP (CCL2, CCL3) in whole liver tissue, the chemokine gradient from necrotic to nonnecrotic tissue, and elevations in circulating levels of this chemoattractant. We also AZD0530 found that all three circulating monocyte subsets express CCR2, suggesting that all three populations could be recruited to the inflamed liver. Our study, however, does not exclude the involvement of other chemokines in recruiting monocytes to the liver, and further studies are warranted to assess this. We observed

marked proliferation of the resident KC population within areas of necrosis (Fig. 4); this finding is in contrast to monocyte-derived infiltrating macrophages, where less than 1% were proliferating. Previous reports support the existence of two macrophage populations with distinct functional capabilities and self-renewal characteristics during steady state and inflammation. One population derived from circulating monocytes with little self-renewal potential is rapidly recruited to inflammatory sites, giving rise to the classical inflammatory macrophages that cause tissue destruction and necrosis.40 There is a second resident population with self-renewal capabilities that characterize later phases of inflammatory insult when tissue repair and regenerative responses prevail.7-10 Recently, the anti-inflammatory cytokine IL-4 has been shown to be a pivotal driver of macrophage self-renewal and tissue repair during experimental tissue injury.