A nucleic acid-hydrolyzing antibody penetrates into cells via caveolae-mediated endocytosis, localizes in the cytosol and exhibits cytotoxicity

Many natural anti-DNA antibodies (Abs) have the ability to translocate across the plasma membrane and localize in the nucleus of mammalian cells, frequently leading to cytotoxicity to cells. Herein, we report detailed intracellular trafficking routes and cytotoxicity in HeLa cells for a single chain variable fragment (scFv) Ab, 3D8, which is an anti-DNA Ab capable of hydrolyzing both DNA and RNA. The intracellular penetration of 3D8 scFv occurred by caveolae/lipid raft endocytosis. The time-course chasing experiments revealed that 3D8 scFv escaped directly from the caveosome into the cytosol and remained in the cytosol without further trafficking into endosomes, lysosomes, endoplasmic reticulum, Golgi, or nucleus. The cytosolically localized 3D8 scFv maintained its nuclease activity to hydrolyze cellular RNAs, mainly mRNAs, eventually triggering apoptotic cell death. Our results demonstrate that 3D8 scFv has a unique intracellular trafficking route of localizing in the cytosol, thereby exhibiting cytotoxicity due to its nuclease activity.

Differential effects of primary human immunodeficiency virus type 1 nef sequences on downregulation of CD4 and MHC class I

We analyzed two primary nef sequences, KS2 (subtype B) and K306 (subtype D), each directly isolated from patients. Cell lines constitutively expressing respective Nef proteins were constructed using a retroviral vector. There were significant differences in the ability to downregulate surface CD4 and MHC class I proteins between different nef sequences. When the nef sequence from NL432 was used as a reference, KS2 Nef demonstrated the highest ability to downregulate MHC class I, whereas it appeared to lack the ability to downregulate CD4. On the contrary, Nef from K306 decreased the level of surface CD4 to a greater extent, but was less effective on downregulation of MHC class I. These results showed that the levels of downregulation of CD4 and MHC class I could significantly vary among HIV strains and that two well-known functions of Nef, downregulation of CD4 and MHC class I, would be separated.

Stable expression of human immunodeficiency virus type 1 Nef confers resistance against Fas-mediated apoptosis

To investigate the effect of Nef on Fas-mediated apoptosis, we compared T cells, both population and subclones stably expressing Nef from HIV-1(NL432), with Nef(-) control cells. Fas-mediated apoptosis was significantly delayed in Nef(+) cells as determined by annexin V staining and the percentage of apoptotic cells was lower in all Nef-expressing cells than in the control cells by a maximum of 10-fold. Next we measured cell surface levels of Fas to test whether the delayed apoptosis in Nef(+) cells was due to reduced cell surface expression of Fas. We found that there was no significant difference in the surface level of Fas between the Nef(+) and Nef(-) cells. To further define the steps affected by Nef in the Fas signaling pathway, the activation of caspase-3 and caspase-8 was investigated. A reasonable correlation was found between the magnitude of apoptosis measured by annexin V staining and the enzymatic activity of caspase-3. The overall level of caspase-8 activity in Nef(+) cells was also lower than in Nef(-) cells, although the extent of inhibition was not as significant as seen for caspase-3. Overall, our results indicate that long-term stable expression of Nef, which mimicks persistent or latent infection in vivo, confers resistance against anti-Fas Ab-induced apoptosis through inhibition of caspase-3 and caspase-8 activation.