Immune Privilege Sites Require a Higher Threshold Level of Membrane Fas Ligand to Stimulate Inflammation and Tumor Rejection

Abstract

Abstract: : Purpose:The constitutive expression of Fas Ligand (FasL) in the eye maintains immune privilege, in part, through the induction of apoptosis in infiltrating Fas+ T cells. FasL is a transmembrane protein that exists as either a membrane-bound (mFasL) or soluble (sFasL) protein. Our previous data indicates that the form of FasL is critical in regulating the induction of innate immunity: (i) mFasL initiates, and (ii) sFasL prevents inflammation. We hypothesize that FasL is critical in regulating the development of early innate immunity within the eye. In the present series of experiments, we examined whether there is a threshold levels of mFasL required to initiate inflammation and if the threshold is higher in privileged versus non-privileged sites. Methods:Four types of lymphoma cells containing different cDNA constructs were injected into the ocular anterior chamber of syngeneic DBA/2 mice: mFasL, wild-type FasL, sFasL, or no FasL. Clones of mFasL tumor cells that expressed low and high levels of mFasL were isolated using limiting dilution techniques. Tumor growth and survival was recorded. The ability of tumor cells to trigger neutrophil secretion of IL-1ß was determined in vitro by ELISA assay. Results:When injected into the non-privileged subcutaneous tissue, tumors expressing either wtFasL, mFasL_low, or mFasL_high were rejected. By contrast, only tumors expressing mFasL_high were rejected from the privileged anterior chamber; tumors expressing mFasL_low or wtFasL grew progressively. Histologically, mFasL_low tumors induced a significant, but mild inflammation as compared to mFasL_high tumors. In addition, mFasL_low tumors had a reduced ability to stimulate IL-1ß release from neutrophils. Conclusion:Immune privileged sites require a higher threshold level of membrane FasL in order to stimulate inflammation and tumor rejection, as compared with non-privileged sites. We predict this is due to either the presence of endogenous soluble FasL found within the aqueous humor, and/or the cleavage of membrane to soluble FasL.

Keywords: 433 immune tolerance/privilege • 435 immunomodulation/immunoregulation • 437 inflammation