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'When we give a definition it is for the purpose of using it'. HENRI POINCARE in Science and Method A. Objectives The first version of this paper was written to introduce new students and fellows of my laboratory to the mysteries of herpesviruses. Consonant with this design sections dealing with well documented data were trimmed to the bone whereas many obscure phenomena, controversial data and seemingly trivial observations were discussed generously and at length. There is some doubt as to whether it was meant to be published, but it was not a review. The objective of reviews is frequently to bring order. But alas, even the most fluent summation of credible data frequently makes dull reading and too much plausible order, like very little entropy in chemical reactions, is not the most suitable environment on which to nurture the urge to discover. This version is more charitable but not less inbalanced. The bibliography reflects the intent of the paper and was updated last in December of 1968. It should be obvious without saying that no single account such as this can do justice or injustice, as the case may be, to the several hundred papers published on herpesviruses each year or to the many thousand papers published on herpesviruses since the first of the members of the family was experimentally transmitted to a heterologous host more than half a century ago (GRUTER, 1924). B. Definition 1.

Prominent progress in molecular biology was only made when it became possible to separate functionally distinct molecules by taking advantage of their biophysical properties. Likewise, the analysis of the functions of hetero­ geneous populations of immunocompetent cells, as to the functional properties of their various subpopulations, can not be done until these can be isolated in reasonably pure form by selective fractionation. During the last few years significant advances have been made in this field, and cells have been separated according to size, density or charge (MILLER et aI., 1969; SHORTMAN, 1968; ANDERSSON, 1973 c), or by taking advantage of more specific surface markers to allow selective depletion or enrichment of a given subpopulation of cells (WIGZELL and ANDERSSON, 1971). Although separation techniques have been used in a variety of cellular systems, they have been particularly useful in the study of reticuloendothelial cells and primarily in the study of cells partici­ pating in the immune responses. Quite extensive reviews have been written which well cover the methods used for separation of cells and the results obtained with the various approaches (WIGZELL and ANDERSSON, 1971; SHORTMAN, 1972). To review this work is becoming a more and more voluminous task. As data rapidly accumulate, we will not try to make such a complete review.

At the end of the last century and the beginning of this century, the prob­ lems of immunity in lower vertebrates and the influence of environmental temperature attracted attention for the first time (ERNST, 1890; WIDAL and SICARD, 1897; METCHNIKOFF, 1901). However, relatively little work has been done on this subject until recently. The early investigators were chiefly in­ terested in the immuno-pathological problems. They immunized various species of lower vertebrates essentially with bacterial vaccines; agglutinating, neutralizing and protective antibodies were detected in their blood. The in­ fluence of environmental temperature on the immune response was investigated, since this subject represented great economical and theoretical importance. Epizootic diseases were observed to occur in relation to the cold season of the year, when the decrease or spontaneous increase of water temperature occurred (SCHAPERCLAUS, 1965; BESSE et al., 1965; KLONTZ et al., 1965 WOOD,1966). The immunological deficiency of fish, caused by their natural or experimental stay in cold water, is now evident for both humoral and cellular immunity. In this review we will focus on two points: We shall attempt (1) to explain the mechanism by which the environmental temperature influences the immune resistance of fish to pathogens, (2) to determine the chronological location of this temperature-sensitive stage in the process of antibody formation, and to make some approaches to the general antibody formation mechanism.

This volume is dedicated to the memory of the late Professor WERNER BRAUN, one of the most devoted and active members of the Editorial Board of the Current Topics in Microbiology and Immunology, who passed away, after suffering a heart attack, in November 1972. Dr. WERNER BRAUN was born in Berlin, Germany, on November 16,1914. During his highschool days in Berlin he did research work on problems of genetics as a young guest in the Kaiser-Wilhelm-Institut fur Biologie, in the department of Prof. R. GOLDSCHMIDT. I remember his colourful description of his discussions during this period, while still a teen-ager, with OTTO WAR­ BURG. He studied biology and medicine at the University of G6ttingen and received a Ph.D. degree in biology in 1936. In the same year he left Nazi Germany and came to the United States first as a Guest Investigator in Genetics at the University of Michigan at Ann Arbor, and then in Berkeley, where he carried out his work in the Depart­ ments of Zoology and of Veterinary Science until 1948. He was engaged during this period in the study of problems concerned with physiological genetics, bacterial variation, immunology and biochemistry.

Influenza continues to be one of the major epidemic diseases of man and is, in fact, his only remaining pandemic disease (BEVERIDGE, 1969). This is largely because influenza virus undergoes extreme antigenic variation, the mechanism of which is still poorly understood. Two kinds of antigenic variation occur in influenza viruses, antigenic drift and major antigenic shifts; both involve chan­ ges in the hemagglutinin and neuraminidase antigens on the surface of the virus. Antigenic drift, which involves gradual changes in the surface antigens of influenza virus, is thought to result from the selection by an immune popula­ tion of mutant virus particles with altered antigenic determinants. These mutants therefore possess a growth advantage in the presence of antibody (FRAN­ CIS and MAASSAB, 1965; ARCHETTI and HORSFALL, 1950; HAMRE et aI., 1958). It has been shown that antigenic mutants isolated in vitro by selection with antibody have changes in amino acid sequence in the polypeptides of the hem­ agglutinin subunits (LAVER and WEBSTER, 1968) and it is likely that antigenic drift in the neuraminidase occurs by the same mechanism.

General aspects of nucleic acid uptake by mammalian cells have been the subject of several reviews during the last few years (PAGANO, 1970; BHARGAVA and SHANMUGAM, 1971; DUBES, 1971; RYSER, 1967). These reviews covered methods used for the infection of cells by viral nucleic acids as well as interaction of mammalian cells with non-viral nucleic acids. This article is restricted to a discussion of experiments with poliovirus RNA and focuses special attention on the steps following the uptake of RNA into a cell, aspects that were not discussed in earlier review articles. The fate of input RNA once inside the cell is determined by the host cell but experimental conditions can be chosen to favor the survival of input RNA and the induction of a virus growth cycle by interfering with host-cell meta­ bolism through events that, in the case of infection with intact virus, might be controlled by viral proteins.

The expression of many bacterial genes adapts itself in an almost in­ stantaneous and reversible way to specific environmental changes. More specifically, the concentration of a number of metabolites, a function of the amounts of enzymes involved in their synthesis or degradation, in turn retroacts on the rate of synthesis of these enzymes. The genetic bases for this regulation were established by JACOB and MONOD (1961). These authors also showed how the known elements of these regulatory mechanisms could be connected into a wide variety of circuits endowed with any desired degree of stability, in order to account for essentially irreversible processes like differentiation (MONOD and JACOB, 1961). The general principles used by JACOB and MONOD in their study of negative regulation were extended to positive regulation by ENGLESBERG et al. (1965). An independent approach permitted the discovery of positive controls in temperate bacteriophages (see below, III). Each control operation is mediated by a pair of complementary genetic elements (hereafter called'control cell'): a control gene which produces a l control (or regulator) protein and a control site which is the target for the regulator protein. Negative control means that the control protein (repressor) prevents gene expression. One deals with positive control when the control protein (activator) is necessary for this expression. It has become apparent that, as initially postulated by JACOB and MONOD, control of gene expression operates, at least to a large extent, at the transcriptional level.

Phenomena as diverse as tuberculin sensitivity, delayed sensitivity to soluble proteins other than tuberculin, contact allergy, homograft rejection, experimental autoallergies, and the response to many microorganisms, have been classified as members of the class of immune reactions known as delayed or cellular hypersensitivity. Similarities in time course, histology, and absence of detectable circulating immunoglobulins characterize these cell-mediated immune reactions in vivo. The state of delayed or cellular hypersensitivity can be transferred from one animal to another by means of sensitized living lymphoid cells (CHASE, 1945; LANDSTEINER and CHASE, 1942; MITCHISON, 1954). The responsible cell has been described by GOWANS (1965) as a small lymphocyte. Passive transfer has also been achieved in the human with extracts of sensitized cells (LAWRENCE, 1959). The in vivo characteristic of delayed hypersensitivity from which the class derives its name is the delayed skin reaction. When an antigen is injected intradermally into a previously immunized animal, the typical delayed reaction begins to appear after 4 hours, reaches a peak at 24 hours, and fades after 48 hours. It is grossly characterized by induration, erythyma, and occasionally necrosis. The histology of the delayed reaction has been studied by numerous investigators (COHEN et al., 1967; GELL and HINDE, 1951; KOSUNEN, 1966; KOSUNEN et al., 1963; MCCLUSKEY et al., 1963; WAKSMAN, 1960; WAKSMAN, 1962). Initially dilatation of the capillaries with exudation of fluid and cells occurs.