The plasma membranes of eukaryotic cells are not uniform and possess distinct cholesterol- and sphingolipid-rich raft microdomains that are enriched in proteins known to be essential for cellular function. Lipid raft microdomains are important for T cell receptor (TCR)-mediated activation of T cells. However, the importance of lipid rafts on antigen presenting cells (APCs) and their role in major histocompatibility (MHC) class II-restricted antigen presentation has not been examined. MHC class II molecules were found to be constitutively present in plasma membrane lipid rafts in B cells. Disruption of these microdomains dramatically inhibited antigen presentation at limiting concentrations of antigen. The inhibitory effect of raft disruption on antigen presentation could be overcome by loading the APCs with exceptionally high doses of antigen, showing that raft association concentrates MHC class II molecules into microdomains that allow efficient antigen presentation at low ligand densities.

Rapid phagocytosis of apoptotic cells is thought to limit the development of inflammation and autoimmune disease. Serum enhances macrophage phagocytosis of apoptotic cells. Here we identified protein S as the factor responsible for serum-stimulated phagocytosis of apoptotic cells. Protein S is best known for its anti-thrombotic activity, serving as a cofactor for protein C. Purified protein S was equivalent to serum in its ability to stimulate macrophage phagocytosis of apoptotic lymphoma cells, and immunodepletion of protein S eliminated the prophagocytic activity of serum. Protein S acted by binding to phosphatidylserine expressed on the apoptotic cell surface. Protein S is thus a multifunctional protein that can facilitate clearance of early apoptotic cells in addition to regulating blood coagulation.

Simian virus 40 (SV40) entry leading to infection occurred only after the virus was at the cell surface for 1.5 to 2 h. SV40 infectious entry was not sensitive to cytosol acidification, a treatment that blocks endocytosis via clathrin-coated vesicles. Instead, SV40 infectious entry was blocked by treating cells with the phorbol ester PMA or nystatin, which selectively disrupts caveolae. In control experiments, transferrin internalization was sensitive to cytosol acidification but was not sensitive to PMA or nystatin. Also, absorbed transferrin entered cells within minutes. Finally, bound SV40 translocated to caveolin-enriched membrane complexes isolated by a Triton X-100 insolubility protocol. Treatment with nystatin did not impair SV40 binding but did block the partitioning of virus into the caveolin-enriched complexes.

Simian virus 40 (SV40) enters cells by atypical endocytosis mediated by caveolae that transports the virus to the endoplasmic reticulum (ER) instead of to the endosomal-lysosomal compartment, which is the usual destination for viruses and other cargo that enter by endocytosis. We show here that SV4O is transported to the ER via an intermediate compartment that contains ␤-COP, which is best known as a component of the COPI coatamer complexes that are required for the retrograde retrieval pathway from the Golgi to the ER. Additionally, transport of SV40 to the ER, as well as infection, is sensitive to brefeldin A. This drug acts by specifically inhibiting the ARF1 GTPase, which is known to regulate assembly of COPI coat complexes on Golgi cisternae. Moreover, some ␤-COP colocalizes with intracellular caveolin-1, which was previously shown to be present on a new organelle ( Simian virus 40 (SV40) enters host cells by an atypical endocytic process mediated by caveolae (1,10,44,53,58,65), rather than by clathrin-coated pits. Caveolae are small invaginations of the plasma membrane that are distinguished from clathrin-coated pits by their size (70 to 100 nm), distinctive flask-like shape, and lack of a visible coat in thin sections.