The innate immune response is not antigen-specific, and is composed of the interferon (IFN) system as well as cell-based anti-pathogen countermeasures that restrict the replication of pathogens. Understanding the innate immune response and the pathways that promote or restrict the kinetics of different steps of infection is essential for devising novel pharmacological strategies for therapeutic intervention during these infectious disease processes, to develop diagnostic tools or to prevent infection. Interferons exert their antiviral effects through the induction of Interferon Stimulated early response Genes (ISGs). An expanding family of such ISGs are the Schlafen (Slfn) genes, which in addition to IFN are also directly induced by pathogen contact with the cells. In our quest to define the biological function of Slfn proteins we discovered that huSlfn11 potently inhibits the production of retroviruses including HIV. Our studies revealed huSlfn11 expression had no effect on the early steps of the infection cycle, but that huSlfn11 protein acts at the very late stages of replication where new viral proteins are synthesized. A striking hallmark of huSlfn11 function is the selective inhibition of the synthesis of virus-encoded proteins, whereas production host cell proteins are apparently unhindered. Most recently, genome-wide screening efforts from two independent laboratories employing dozens of cancer cells lines identified an intriguing correlation between huSlfn11 expression and sensitivity of cells to DDAs. Using cell lines we generated during our previous studies which specifically lack huSlfn11, we find that indeed such cells are resistant to the cytotoxic effects of several classes of DDAs. It is therefore quite reasonable to consider that the development of resistance of tumor cells against DDAs involves the down- regulation of huSlfn11. Finally, our newest studies revealed that huSlfn 5 and 11 have the ability to bind and/or generate Z-DNA. As the structure of the Z-DNA double helix is considerably different from that of B-DNA it supports different interactivity with other molecules, and a few proteins have been identified that selectively bind to Z-DNA but not to B-DNA. It is also notable that unlike B-DNA, Z-DNA is very immunogenic which has not only allowed for the production of highly specific antibodies for research purposes, but has also given rise to anti-Z-DNA antibodies in autoimmune disease such as lupus erythematosus. Although the left-handed Z-DNA and Z-RNA forms were discovered decades ago, their biological role has remained a mystery, largely because little information exists on their formation under physiological conditions. So for several decades not much insight had surfaced on Z-DNA function. In fact, since the end of the 1980s the biology of Z-DNA was not receiving much attention from researchers or funding agencies, and with a few exceptions focused on the role of Z-DNA in genetic instabilities. Our discovery of Z-D/RNA generation by Slfns, a family of interferon-induced antiviral proteins, has the strong potential to place these left-handed nucleic acids into numerous immune response scenarios. Understanding of the biological roles of Z-D/RNA is elementary to exploit their prospective pharmacological possibilities in the development of vaccine adjuvants, anti-viral or anti-neoplastic chemotherapeutics.
Z-DNA had attracted the interest of a wide number of biochemists and physical chemists a couple of decades ago, but both interest and funding waned due to a lack of clear evidence for biological role of Z-DNA. The greatest hurdle had been the fact that Z-DNA could only be generated under extreme non-physiological conditions, thus all but preventing any biologically meaningful experiments. It is our hypothesis that each left- handed nucleic acid is a vital but unappreciated element in both the activation and the execution of the immune response. Z-DNA was a hot topic 30 yrs. ago, but research was abandoned as no biological roles for it could be identified with the technology available at the time. We have now discovered that several members of the Schlafen family (particularly huSlfn5 and huSLfn11) bind and/or generate Z-DNA under physiological conditions. huSlfn11 is already not only a potent HIV restriction factors, but also a crucial determinant of the sensitivity of cancer cells to DNA damaging agents. Now we can assign another extremely novel function to this unique protein family, as we suppose that Z- DNA contributes to both innate and adaptive immune responses. We arrive at this reasoning among other facts by the circumstance that all known Z-DNA binding proteins are either of viral origin, or those of mammalian derivation are exclusively products of interferon-inducible genes. We believe that it is crucial to revisit Z-DNA and its link to Schlafen proteins, with the benefit modern biological techniques not available 30 years ago.
