Cellular Immunity in Fish as Measured by Lymphocyte Stimulation

Fish are capable of responding to a variety of antigens. In many instances the primary response has attributes similar to those of mammals or birds, but in other ways the immunologic responses of fish stand apart, e.g., fish produce only one major class ofimmunoglobulin (lgM). Fish appear to handle all antigens as if they were thymus-independent. Some species have a faulty or nonexistent immunologic memory. Although fish lymphocytes perform certain functions characteristic of Tor B cells of mammals, there is no clear-cut evidence that these are performed by specialized lymphocytes as opposed to lymphocytes with multiple functions. Several factors appear to be capable of regulating immune response. These include the IgM natural antibodies (some of which have nonspecific immunologic reactivity), immune complexes, and suppressor cells. All of these may combine to suppress certain responses. It therefore behooves the profession to undertake more extensive and intensive studies in fish immunology if the profession is to develop a better understanding of optimal modes and conditions for achieving protective immunity in fish. Improvements and refinements in methodologies of tissue culture, immunology, virology, and bacteriology have made possible the attainment of new knowledge regarding the diseases of fish. A deeper appreciation of the factors contributing to the health of these poikilothermic animals has come from immunological studies. More recently, various parameters of cellmediated immunity have come under scrutiny. It has been known for some time, and confirmed in our laboratory, that bony fishes are capable of rejecting allografts (Hildemann, 1957; Hildemann and Haas, 1960; Hildemann and Cooper, 1963). The rejection process in these animals is fairly acute. A more chronic process occurs in the shark (Hildemann, 1970). Allograft rejection in higher forms is mediated by 6 lymphocytes activated by histocompatibility antigens which can be demonstrated in vitro by the blastogenic reaction. This is a complex reaction which is manifested in a variety of ways increased permeability of lymphocyte membranes and elevation of the rate of synthesis of protein, RNA, and DNA. The biochemical changes are accompanied by an enlargement of cells whereby the lymphocytes revert to Iymphoblasts, which can then reproduce by mitosis. All these changes can be measured by biochemical or morphological methods, and the most commonly employed method measures DNA synthesis (incorporation of radioactive thymidine into DNA). Blastogenic transformation reactions occur as a result of activation of lymphocytes by histocompatibility antigens (antigens involved in graft rejection), as just mentioned, or as a result of lymphocyte stimulation by specific nontissue antigens, i.e., viral, bacterial, etc., or nonspecific mitogenic lectins such as phytohemagglutinin (PHA) and concanavalin A (Con A) or certain carbohydrates or lipopolysaccharides (LPS) in bacterial endotoxins. Antigens can bind to specific receptors present on a few lymphocytes in the nonimmunized host and to the clones of lymphocytes following immunization with this antigen. This is believed to be the mode of expansion of a clone of lymphocytes specific for the antigen, assuming that the antigen is bound to the recognition receptor on the lymphocyte membrane, and this event generates a signal for a blastogenic transformation culminating in lymphocyte proliferation. The progeny lymphocytes recognize and respond (with blastogenic transformation) to the same antigen that launched the initial selective stimulation of the progenitor lymphocyte. This is a simplified version of an immunologic pyramidal reaction which does not take into account a variety of regulatory factors: proliferative asymmetry, suppressor cells, temporary anergy, etc. Moreover, knowlThe authors are with the University of Miami School of Medicine, Miami, FL 33152. Marine Fisheries Review edge about blastogenic transfonnation is derived mainly from in vitro studies, and relatively little is known about its occurrence and character in vivo. While blastogenic transformation induced by an antigen expresses a specific response, blastogenic transformation evoked by lectins or LPS is a more general response of large segments of lymphocytes regardless of sped ficity, commitment, or immune status. The only rule that seems to apply here is that T lymphocytes respond to PHA and Con A, and B lymphocytes react to LPS. Following this introduction to blastogenic reactions let me return briefly to my initial comments about allograft rejection. As has been stated, fish reject allografts, and some fish reject them as rapidly as do mammals, but the analogy does not appear to extend to the blastogenic reaction. When the lymphocytes of two unidentical mice, A and B, are placed in culture they will react blastogenically A VS Band B VS A. In contrast, when the lymphocytes of two snappers, A and B, are placed in culture they do not react in this manner. The reason for this discrepancy is not known. However, fish lymphocytes do respond with blastogenic reaction to certain antigens, and I will discuss this after a brief statement about the immunoglobulins and an overview of the immunologic response in fish.