Several discoveries are noteworthy for allowing us to probe the recesses of the virus infected cell and to search for cryptic viral genomes which might provide clues in our studies of cancer etiology or developmental biology. One of the most notable was the dis covery of reverse transcriptase. This marked a momentous occasion in the history of molecular biology. Not only did it provide insight into the mechanism of persistence of retroviruses but it also provided us with an enzyme that could synthesize a DNA copy of any RNA. This DNA copy could then be used as a hybridization reagent to search for both complementary DNA and viral-specific RNA. Thus one could follow the course of any viral infection or probe in tumor cells for hidden viral genomes. Second, a great deal of credit must be given to the geneticists who isolated the various deletion mutants in the 'avian retrovirus system and thus provided us with the frrst means of isolating gene-spe cific probes. Finally, the laboratories which have mapped the genome have provided us with the framework in which to ask very specific questions with our gene-specific probes. Recently, numerous excellent reviews concerning various aspects of the retroviruses have appeared. In this review I shall not even attempt to present a comprehensive review of retroviruses."

Michael Potter, Fritz Melchers, Martin Weigert The second workshop on Mechanisms of B Cell Neoplasia was held in Bethesda, Maryland in Wilson Hall at the National Institutes of Health on March 5, 6, and 7, 1984. It followed a workshop on the same topic that was held at the Basel Institute for Immunology, March 15-17, 1983. That first meeting attempted to bring together cell biologists, experimental pathologists and molecular geneti- cists interested in B cells, to discuss pathogenetic processes in the development and maintenance of the neoplastic state. The impetus for this discussion emanated from two important developments: first, the discovery of the viral promoter insertion mechanism for acti- vating the myc oncogene in bursal lymphomatosis by Hayward, Neil, and Astrin;-second, the findings that the non-random chromosomal trans locations involving the immunoglobulin gene chromosomes occur- red in very high frequencies in murine plasmacytomas and human Burkitt's lymphomas. During the planning stages of that meeting Shen-Ong et al. discovered that non-random translocations activated the myc oncogene. Promoter insertions and non-random trans locations were-rwo mechanisms that caused transcription of the myc oncogene messages in three different kinds of well defined experimental and clinical B cell tumors. Unregulated myc gene transcription provided the first evidence of a specific bioChemical lesion in B cell neo- plasia.

Many of the fundamental concepts of animal virology originated from the study of the variola-cowpox-vaccinia virus system with vaccinia virus serving as the type species (Fen- nerand Burnet 1957; Burnet 1959; Fenner 1976a, b). The importance of the Poxviridae(Fen- ner 1979) for the study of viruses as biologic entities and in defIning the events which occur in virus-infected cells are exemplifIed by investigations which: (a) described the epidemiology of a virus disease in an animal population (Fenner1949, 1959b); (b) em- ployed electron microscopy to study virion structure (Peters 1956, Nagington and Home 1962, Dales and Siminovitch 1961) and to derme the morphologic stages of virion develop- ment in infected cells (Morgan et al. 1954, Dales 1963); (c) dermed and elaborated on the mechanism of nongenetic reactivation for an animal virus (Joklik et al. 1960a, Fenner and Woodroofe 1960, Hanafusa 1960); (d) described the intracellular uncoating of a viral genome (Joklik 1964a, b); (e) studied the antigenic structure and complexity of poxvirions (Loh and Riggs 1961, Woodroofe and Fenner 1962, Appleyard et al. 1964, Appleyard and Westwood 1964); (1) described the use of chemotherapy to treat viral infec- tions (Bauer et al. 1963); (g) fIrst demonstrated the presence of virion-coded enzymes encapsulated within virions (Kates and McAuslan 1967, Munyon et al. 1967); and (h) established the H -2 restriction of cytotoxic T-cell killing of virus-infected cells in the murine system (Doherty et al. 1976).

Binding of various ligands (hormones, neurotransmitters, immunological stimuli) to membrane receptors induces the following changes: 1. Receptor redistribution (clustering, "capping") 2. Conformational changes that can be detected by fluorescent probes 3. Alteration in membrane fluidity (spin label and fluorescence polarization probes) 4. Changes in fluxes of ions and metabolites 5. Increased phospholipid turnover (especially of phosphatidyl inositol) 6. Activation of membrane-bound enzymes (adenyl cyclase, ATPase, transmethylases). Some of the early changes resulting from or associated with the binding (adsorption) of virions to the host cell membrane are of the same type. Adsorption of animal viruses to cells is the ftrst step in a chain of events resulting in the production of progeny virus on the one hand and in damage to cells and tissues on the other. In the classical studies of viral infection, cells are adsorbed with virus, usually for 60 min, and the changes induced by the virus in the host cell are recorded thereafter. In the past decade, more and more studies have been aimed at the events occurring in these ftrst 60 min of the so-called adsorption period. These studies deal with the nature of adsorption, e. g. , the ligand-receptor type of interaction between the virus and the cell membrane. Many receptors for viruses were identifted and so were the viral proteins which take part in adsorption.

The study of the genetic regulation of immune response to natural multidetermi nant immunogens was undertaken by the method of bidirectional selective breed ing of High or Low antibody responder lines of mice. Five Selections are described: Selection I, carried out for agglutinin responsiveness to sheep erythrocytes and pigeon erythrocytes alternated in each generation. Selection II, carried out for agglutinin responsiveness to sheep erythrocytes repeated in each generation. Selection III and Selection IV performed respectively for agglutinin response to flagellar or somatic antigens of Salmonella typhimurium and Salmonella oranienburg alternated in each generation. Selection V, performed for passive agglutinin response to bovine serum albumin and rabbit gamma globulin alternated in each generation. In each Selection the character investigated is polygenic. High and Low responder lines diverge progressively during the selective breeding. The maximal interline separation (selection limit) is reached in the 7th-16th generations. High and Low responder lines at selection limit are considered homozygous for the character submitted to se ection. Their variance is therefore only due to environ mental effects. The difference in agglutinin titre between High and Low lines is 220-fold in Selection I, 103-fold in Selection II, 90-fold in Selection III, 85-fold in Selection IV and 275-fold in Selection V. The partition of genetic and environmental variances in the foundation popu lations of the five Selections is established. The proportion of genetic variance is 60% in Selection I; 49% in Selection II; 51% in Selection III; 47% in Selection IV and 76% in Selection V."

Immunoglobulin gene expression appears to include a number of unique features (Cohn, 1971; Gaily and Edelman, 1972; Hood et aI. , 1975). First, a variety of genetic and protein structural evidence suggests that two discrete genes - both a variable region gene and a constant region gene - specify each heavy chain and each light chain. This constitutes the twogene-one polypeptide hypothesis. Second, a single differentiated lymphocyte or plasma cell appears to express only one heavy chain allele and one light chain allele at a time. This is the only example of allelic exclusion known in mammalian cells except for X chromosome inactivation. Third, during the course of lymphocyte differentiation, there may be a switch of the heavy chain constant region gene expression but no change in the heavy chain variable region gene expression. Rarely, normal or malignant cells have been found which express two different heavy chain subclasses simul taneously (e. g. see Sledge et aI. , 1976). Fourth, the vast number of different antibodies which can be made by an individual animal has raised the question of whether the generation of diversity occurs during evolution or within the animal, i. e. germ line vs somatic variation. Other aspects of immunoglobulin gene ex pression may be similar to regulation of gene expression in many eukaryotic cells.

The cells of the immune system generate a large variety of binding sites which differ in their binding specificities and can therefore react specifically with a large variety of ligands. These binding sites are part of receptor molecules, enabling the system to react to the universe of antigens. The classical antigen receptor is the antibody molecule, and accord ingly the first session of this colloquium deals with a classical sub ject, namely antibody structure. Dramatic recent advances in this field make it possible to interrelate primary and three-dimensional struc ture both to each other and to function, i.e. the binding of antigen and possible reactions occurring in the antibody molecule upon antigen binding. The latter point is of particular interest since it may be relevant not only for effector functions of antibodies such as the binding of complement, but also for the triggering of a lymphocyte through its antibody receptor for antigen."

1.1 Classification of Togaviruses The family, Togaviridae, is composed of the alphaviruses, the flaviviruses, rubella (a rubivirus), and the pestiviruses (Fenner, 1976). Of these four genera, two (the alpha- and flaviviruses) are transmitted by blood-sucking arthropods, specif ically mosquitoes and ticks. Among the togaviruses, extensive studies of defective interfering (DI) particles have so far been carried out only with Sindbis virus (SV) and Semliki Forest virus (SFV), both members of the alphavirus genus. Since these viruses are so similar, in most cases it will be assumed that what is true of one is also true of the other. 1.2 Definition of Defective Interfering (DI) Particles Defective interfering viral particles, as defined by Huang (1973), have the follow ing properties: (1) they are deletion mutants and therefore lack large amounts of the genetic material present in the standard virus; (2) they contain the same viral structural proteins as standard virus; (3) they are unable to replicate alone; however, they are replicated in cells co-infected with standard virions; and (4) at the same time as they require standard virus to replicate, they inhibit the replication of standard virus and hence are interfering."

For more than ten years cell fusion techniques have been applied in studies on various lymphocyte functions. Ig expression was first studied in hybrids obtained by fusing myeloma cells with fibroblasts (1) or lymphomas (2), both of which do not produce Ig, and with Ig producing myelomas (3) or human blood lymphocytes (4). Kohler and Milstein (5) fused a myeloma with spleen cells from immunized mice. Up to 10% of the hybrids obtained secreted antibodies specific for the immunizing antigen. This suggested that plasma cells preferenti ally fused with the myeloma cells, a finding which was of enormous practical value. It was found that both Band T lymphocytes could be fused with the T cell tumor BW5147, which is however not permissive for Ig synthesis (6). A very large number of T cell hybridomas were generated by fusing BW5147 with cell populations containing in vivo or in vitro activated cells (7). The hybrids showed no specific T cell functions and binding assays for T cell receptors were not available. In particular, no hybrids were obtained which expreS1ed specific cytolytic activity that could be tested in short-term Cr release assays (8). However, the frustrations expressed about these failures, published in January, 1978 (9), were relieved by Taniguchi and Miller's publication a few months later of T cell hybridomas producing antigen-specific suppressor factors (10). Unfortunately, their hybrids rapidly lost factor production."