Abstract 563: Axl Regulates Major Histocompatibility Complex Class II (MHC-II) Expression through STAT1 Pathway in Smooth Muscle Cells

Introduction Major histocompatibility complex class II (MHC-II) is upregulated in vascular smooth muscle cells (VSMCs) in response to injury. Previously we found that Axl, a receptor tyrosine kinase, is important for vascular remodeling via regulation of MHC-II in vivo . The aim of this study was to identify Axl-mediated intracellular signaling that is responsible for upregulation of MHC-II in VSMCs. Methods and Results Mouse aortic smooth muscle cells (MASMCs) were isolated from Axl wild type (Axl+/+) or knockout (Axl-/-) littermates from our colony. Cells were treated with interferon gamma (IFNγ; 1,000U/ml) or PBS for various times (10min-24hrs). There was no difference in apoptosis as measured by cleaved caspase-3 in Axl-/- vs. Axl+/+ after treatment with IFNγ. Likewise, phosphorylation levels of Akt were similar between the groups. Expression of MHC-II mRNA and protein was significantly lower in Axl-/- vs. Axl+/+ MASMCs. Levels of major histocompatibility class II transactivator (CIITA) mRNA was also significantly reduced in Axl-/- MASMCs. Reduced expression of number of cytokines and chemokines was observed in Axl-/- vs. Axl+/+ MASMCs after treatment with IFNγ. Similar to our in vivo results, a complement component 3 (C3) expression was drastically reduced in Axl-/- vs. Axl+/+ MASMCs. We found that STAT1, a major regulator for CIITA-MHC-II and C3 pathways, was not phosphorylated in Axl-/- vs. Axl+/+ MASMCs upon IFNγ treatment. Conclusions This is the first study that demonstrates that Axl affects STAT1-signaling in VSMCs. Our findings suggest that Axl contributes to vascular remodeling via enhancement of pro-inflammatory signals in smooth muscle cells.