Identification of a scavenger receptor homologue on nonspecific cytotoxic cells and evidence for binding to oligodeoxyguanosine

In mammals scavenger receptors (SR) are expressed by monocytic-macrophage lineage cells and B-cells. Studies of various teleost species have indirectly demonstrated the presence of SR receptors on phagocytic or endothelial cells by showing the uptake of SR ligands (i.e. derivatised (acetylated) lipoproteins) by these cells. In the present study, nonspecific cytotoxic cells (NCC) were examined for membrane expression of an SR-like protein. Approximately 15-25% of purified NCC expressed scavenger receptor class A (SR-A) demonstrated by binding by a monoclonal (2F8) specific for mouse SR-A (types I, II). Flow cytometric analysis determined that SR binding cells had the same size and 'side scatter' characteristics as NCC. Two colour flow analysis of NCC demonstrated that only a subset of NCC expressed the SR-A-like protein and non-NCC were SR-A negative. Membrane expression of SR on NCC was confirmed by fluorescence microscopy. Analysis of the tissue distribution of SR bearing cells demonstrated that in both catfish and tilapia, SR-A was expressed by NCC in the peripheral blood, spleen and anterior kidney. Experiments were also done to determine if the ligands known to bind mammalian SR-A had a similar specificity for the teleost receptor. Cold competition binding experiments determined that anti-SR-A antibody competed with and reduced biotinylated polyguanosine 20-mer binding to NCC by approximately 40%. Two other types of ligands known to bind (mammalian) SR-A (i.e. polyvinyl sulphate and dextran sulphate) likewise decreased anti-SR-A antibody binding to NCC by 40%. These studies for the first time demonstrated that NCC express the teleost orthologue of mammalian SR-A, suggesting that NCC may participate in physiologic regulation of lipid metabolism in addition to functions of innate immunity.

MHC Class I-Restricted Presentation of Maleylated Protein Binding to Scavenger Receptors

Pathways for loading exogenous protein-derived peptides on MHC class I are thought to be present mainly in monocyte-lineage cells and to involve phagocytosis- or macropinocytosis-mediated antigenic leakage into either cytosol or extracellular milieu to give peptide access to MHC class I. We show that maleylation of OVA enhanced its presentation to an OVA-specific MHC class I-restricted T cell line by both macrophages and B cells. This enhanced presentation involved uptake through receptors of scavenger receptor (SR)-like ligand specificity, was TAP-1-independent, and was inhibited by low levels (2 mM) of ammonium chloride. No peptide loading of bystander APCs by maleylated (maleyl) OVA-pulsed macrophages was detected. Demaleylated maleyl-OVA showed enhanced MHC class I-restricted presentation through receptor-mediated uptake and remained highly sensitive to 2 mM ammonium chloride. However, if receptor binding of maleyl-OVA was inhibited by maleylated BSA, the residual presentation was relatively resistant to 2 mM ammonium chloride. Maleyl-OVA directly introduced into the cytosol via osmotic lysis of pinosomes was poorly presented, confirming that receptor-mediated presentation of exogenous maleyl-OVA was unlikely to involve a cytosolic pathway. Demaleylated maleyl-OVA was well presented as a cytosolic Ag, consistent with the dependence of cytosolic processing on protein ubiquitination. Thus, receptor-specific delivery of exogenous protein Ags to APCs can result in enhanced MHC class I-restricted presentation, suggesting that the exogenous pathway of peptide loading for MHC class I may be a constitutive property dependent mainly on the quantity of Ag taken up by APCs.