Immune System

The growing power of the clonal selection theory during the late 1960s created a magnetic storm in the minds of immunologists. The disturbance was worldwide, but the epicenter was at the Hall Insti­tute in Melbourne, where Bumet and his students Ada, Nossal₁ and Jacques Miller, all biologists, disen­tangled the life and work of lymphocytes.

The thymus was an organ whose histology was described in great detail in all the elementary text­books, but it had never been found to have any function. Thymectomy seemed to have no effect on a normal adult. In 1961 Bumet suggested to Miller that he try to discover the effect of thymectomy not on adults, but on newborns in whom, according to his theory, self-tolerance was still being established. Experiments on mice produced a striking effect: The thymectomized mice looked ill; they had constant diarrhea and infections; foreign skin grafts, even skin grafts from rats, flourished on them, and they could hardly produce any antibody (Miller 1962).

In 1963 Australian and U.S. investigators met for a conference on the thymus. It was arranged by the U.S. pediatrician Robert Good, who had been treat­ing children with immune deficiencies, most of whom had congenital absence of the thymus. Like Miller’s mice, they suffered from endless infections, and they neither rejected grafts nor produced anti­bodies. In some of them, the thymus looked normal and graft rejection was normal, but no antibodies were produced. The only known experimental model of this type of deficiency was the chick from which the bursa OfFabricius, an organ rather like the mam­malian appendix, had been removed.

Back in 1890, Robert Koch announced that he had found a cure for tuberculosis. He was injecting a secret substance, later admitted to be an extract of tubercle bacilli, which he called tuberculin. It pro­duced a striking flare-up of skin lesions in an in­fected patient. But disappointingly, it was not a cure (Koch 1891). The Viennese pediatrician Clemens von Pirquet, working on the “serum sickness” that often followed the injection of diphtheria serum, rec­ognized that the tuberculin reaction had some rela­tion to his concept of “allergy,” or altered reactivity, but it was independent of antibody and could not be transferred by serum. Others saw the same kind of lesion in chronic bacterial infections, and it came to be called “infectious allergy” (Schadewaldt 1979). The lesions were very characteristic: a hard red lump that often had a black, necrotic center, which developed slowly over a few days and took weeks to heal. The reaction was thought perhaps to be respon­sible for the tissue destruction in tuberculous lungs, and as “delayed hypersensitivity,” to be quite sepa­rate from protective immunity.

In an era that concentrated on serology and protec­tion against infection, something that had nothing to do with serum and seemed to be destructive rather than protective did not attract much interest.

It soon became clear that delayed hypersensitivity could take the form of autoimmune disease, and that this kind of disease could often be reproduced by injecting material from the target organ in Freund’s adjuvant. During the 1950s, laboratories in Britain and the United States worked on the problem using experimental allergic encephalitis as a model. Un­der the microscope, all the lesions looked very much the same: There was always a cuff of cells surround­ing the small blood vessels. Most of the cells were lymphocytes, with some phagocytic cells, the macro­phages. The New York immunologist Sherwood Law­rence insisted that he was able to transfer specific sensitization with a cell-free extract from sensitized cells; he called his putative agent “transfer factor.” Other immunologists thought that there was some­thing wrong with his experiments. There were no known soluble factors, other than antibody, or per­haps antigen - Burnet thought it must be antigen - that could transfer a specific sensitivity.

Throughout the 1960s, under the influence of the clonal selection theory, more and more attention came to be focused on lymphocytes. Byron Waksman and his group at Yale University found that they could wipe out delayed hypersensitivity with anti­lymphocyte serum, an experimental finding that could be used clinically to prolong the life of trans­plants as well as to dampen autoimmune disease (Waksman, Arbouys, and Arnason 1961). Tissue cul­ture techniques improved, so that immune lympho­cytes could be watched as they responded to antigen by beginning to proliferate.

which produced a soluble “migration inhibition fac­tor,” the first of many such factors to be described. A different group of T cells could be seen to kill the cells of foreign grafts or tumor cells. Activated macrophages, too, could kill normal cells, especially if the cells were coated with antibody. It was Wright’s opsonic effect brought back to life. At­tempts were made to apply this new knowledge to human pathology. Patients’ lymphocytes were tested for their behavior with suspected antigens in some diseases thought to be autoimmune.

As Anne-Marie Moulin (1989) has pointed out, the Cold Spring Harbor Symposium of 1967 marked the moment at which immunologists seem to have agreed to accept the clonal selection theory. During the 1960s, different domains of immunology had evolved under the guidance of its heuristic influence. The theory suggested new questions and demanded new techniques, particularly techniques for growing and manipulating cell cultures on the microscopic scale. These new techniques in turn laid bare dozens of new phenomena and relationships. An early indicator of the crossing over between the separate areas of immu­nology was the change of name around 1970 of de­layed hypersensitivity, first to delayed (cellular) hy­persensitivity, then to cell-mediated immunity, as the barriers disappeared between the work on antibody­producing clones Oflymphocytes and the work on the lymphocytes of delayed hypersensitivity.

Cell-mediated immunity in the 1970s was, as Waksman has noted, completely redefined. More than 100 descriptions of new cellular phenomena were published over the 10 years (Waksman 1989). Many experiments showed that relationships be­tween cells were mediated by soluble factors. Some of these turned out to be capable of performing in more than one experimental scenario.

Waksman has called the decade beginning in 1980 the “holocene era,” the era when everything is connected to everything else. That era had already begun in the 1970s, with the work on cell-cell inter­actions. The end of the decade saw interaction be­ginning between cell biologists with their culture techniques and immunochemists and their se­quence studies. The common meeting ground was now to involve not only cells and serum, but also genetics and molecular biology. The raw material was provided by the myeloma proteins, each repre­senting a different immunoglobulin that could be studied in isolation. By 1970 it was fully accepted that antibody diversity was genetically determined. The question was still, as it had been for half a century, how such tremendous diversity could be genetically controlled.

Sequence studies had shown that the variable re­gion on the globulin was confined to one end of each chain and that these sites were separately coded in the genome. The number of genes for the variable regions was estimated to be perhaps no more than a thousand.

Yet many workers had found that the 18 S globulin formed just after immunization seemed to show less variety of variable regions than the 7 S that came later, suggesting that somatic mutation could have occurred during clonal expansion. Evi­dence for somatic mutation came in 1981 from Lou Hood and his group at the California Institute of Technology. They used the hybridoma technique, de­veloped in the mid-1970s, to make a myeloma that produced antibody of a predetermined specificity. Cells from a mouse myeloma were hybridized with spleen cells from 19 different immunized mice. The variable-region sequences of the antibody globulins were compared with the germ-line sequence of mouse DNA related to the antibody binding site. Not all of the antibody sequences were identical: Half of them reflected the exact sequence as coded for in the germ line, but the others differed from one another by one to eight amino acids. The germ-line sequence was found in the 18 S globulin and the variants in the 7 S (Crews et al. 1981; Clark 1983). It appears that the variable region is encoded by the germ-line genes but that further variants can arise after birth and perhaps after stimulation by antigen. Clonal expansion seems to generate new mutants, some of which respond to the antigenic stimulus by produc­ing a shower of new clones. Some of these clones may produce an ever closer antigen-antibody match, rather as an animal species evolves to fill an ecologi­cal niche by natural selection.