The genome of retroviruses contains three major coding regions for virion proteins, gag, pol and env. Gag encompasses information for nonglycosylated viral proteins that form the matrix, the capsid and the nucleoprotein structures. From pol derive reverse transcriptase and integrase, and env codes for the surface glycoproteins of the virion which consist of a transmembrane and a surface domain, linked by disulfide bonds. A viral protease is derived eitherfrom the gagorfrom the pol coding region, depending on the virus. Simple retroviruses contain only this elementary gag, pol, and env coding information. Once integrated, they are able to multiply efficiently, using the cellular transcriptional and replication machineries without intervention of viral transacting factors. Most oncogenic retroviruses belong in this category. Complex retroviruses, on the other hand, encode additional nonstructural proteins from multiply spliced messages. These proteins play important regulatory roles in the life cycle of the virus. They function as transacting factors that, in concert with cellular regulatory proteins, control viral gene expression and function and are essential components in the replication of complex retroviruses. To this category belong the lentiviruses, the spumaviruses and a group of oncogenic retroviruses that includes human T cell leukemia virus (HTLV) and bovine leukosis virus(BLV).

Borna disease was first described over 200 years ago, in what is now Southeastern Germany, as a fatal neurologic affliction of horses and was considered a curiosity for many decades. The causative agent was unknown, and the animal species infected in nature were limited to horses and sheep. Today, as described in this volume, the host range has extended to all warm-blooded animals, the genes and proteins of the virus have been identified, and many of the mechanisms responsible for behavioral disturbances are understood. Serologic studies suggest that BDV or related agents are likely to play a role in human neuropsychiatric diseases.

Lyssaviruses are the etiological agents of rabies, one of the oldest documented and feared maladies in medical history. The last century has been particularly fruitful in regard to progress in Iyssavirus phylogenetic affinities, diagnostics, pathogenesis, molecular virology and epidemiology, pro phylaxis and control. Yet, despite these academic and practical advances in research, the age-old horror evoked by rabies is still very real, with only four documented human recoveries once symptoms are realized. After decades of intense scrutiny and four recent books describing rabies and its viral relatives, there is still much to be learned. The great authority on rabies, Karl Habel, once related an incident of a very distraught elderly woman, who showed symptoms of neurological disease. She told Habel, "I don't need a physician. I know I have rabies. My beloved dog had rabies and died. Look", she exclaimed, while flinging down a goblet of water, "I have hydrophobia". Habel asked for her confinement for psychiatric examination.

Genetic / DNA immunization represents a novel approach to vaccine and immune therapeutic development. The direct injec tion of nucleic acid expression cassettes into a living host results in a limited number of its cells becoming factories for production of the introduced gene products. This host-inappropriate gene expression has important immunological consequences, resulting in the specific immune activation of the host against the gene delivered antigen. The recent demonstration by a number of laboratories that the induced immune responses are functional in experimental models against both specific infectious diseases and cancers is likely to have dramatic consequences for the develop ment of a new generation of experimental vaccines and immune therapies. This technology has the potential to enable the pro duction of vaccines and immune-based therapies that are not only effective immunologically but are accessible to the entire world (rather than just to the most developed nations). Vaccine Development Vaccination against pathogenic microorganisms represents one of the most important advances in the history of medicine. Vaccines, including those against polio, measles, mumps, rubella, hepatitis A, hepatitis B, pertussis and other diseases, have dramatically improved and protected more human lives than any other avenue of modern medicine. The vaccine against smallpox, for example, has been so successful that it is now widely believed that this malicious killer, responsible for more deaths in the twentieth century than World Wars I and II combined, has been removed from the face of the earth.

Nitric Oxide (NO) an endogenous free radical, has been shown recently to mediate several important biological effects. It plays a neuro-transmitter like role in vascular endothelium, a scond-messenger role in N-methyl-D-aspartate (NMDA) responsive neurons in the central nervous system (CNS), a neurotoxic role after its release from these neurons, and a cytotoxic role after its release by macrophages.
This volume reviews among other topics the basic chemistry and physical properties of S-nitrosothiols (RS-NO) and their biochemical mechanisms of action, NO synthase isozymes, NO synthase structure, mechanisms of NO synthesis, regulation of NOS expression and posttranslational modification, and mechanisms involving NO of CNS's damage in virus infections.

The humoral response of the immune system to a foreign antigen usually requires the recognition of two antigenic determinants. The one, called the carrier, is recognized by T-Iymphocytes, the other, called the hapten, by B-Iympho cytes. As a consequence, T - and B-Iymphocytes proliferate, B-Iymphocytes produce hapten-specific antibodies, and the system develops memory to the antigens. It was long thought that antigens would form a bridge to mediate the cooperation of T - and B-Iymphocytes. However, it now appears that antigens are broken down to fragments which then act as carrier determinants for T -lymphocytes. The cells which originally process antigen are called an tigen-presenting cells. They have phagocytic properties. They can take up and degrade antigens, in the case of pro teins to peptides. The peptides of protein antigens reappear on the surface of the antigen-presenting cells, where they must become associated with membrane proteins encoded by genes of the major histocompatibility complex (MHC) in order to be recognized by T-Iymphocytes. To activate helper T-Iym phocytes which cooperate in antibody responses, MHC class II molecules have to be expressed on the surface of the antigen-presenting cells. Once T -lymphocytes have be come activated, they are ready to cooperate with B cells."

The diversity of antigen-binding structures of antibody molecules is so vast that every conceivable antigen can be bound by an antibody molecule within the immune system. This is true even for the antigen binding sites of antibodies called idiotypes, which are bound by complementary bind ing sites of other antibodies called anti-idiotypes. Thus, anti-idiotypes are structural homologues of antigens. These idiotypic-anti-idiotypic interactions constitute a network within the immune system. Since one lymphocyte produces only one type of antibody molecule, this network is in fact a network of cells. We expect that the network is functional: the appearance of antigen will disturb the equilibrium of the network at the point where it competes with the anti idiotypic lymphocyte for binding to the idiotypic lympho cyte. It has been known for quite some time that anti idiotypic antibody can be used to prime the immune system for memory to an antigen that it has never seen. This phe nomenon is now being explored for possible use in immuni zation against viruses, bacteria, parasites and tumors as well as for the modulation of autoimmunity. The ability of anti-idiotypes to mimic, both antigenically and function ally, the corresponding biologically active molecules seen by an idiotypic antibody was first demonstrated for the hormone insulin and is now being observed in many other systems. The papers assembled in this volume. bring the reader to the cutting edge of the potential practical applica tions of the network theory of the immune system."