QnAs with Max D. Cooper and Jacques F. A. P. Miller

Anyone who has ever contracted chicken pox can thank the adaptive immune system for future protection against the disease. It is also thanks to this system that vaccines prevent diseases. The adaptive immune system provides organisms with a memory of past infections, enabling the body to quickly kill returning infections before they can do significant damage. Immunologists Jacques F. A. P. Miller and Max D. Cooper determined that adaptive immunity requires 2 distinct cell types that perform complementary functions. Miller’s findings, published in the early 1960s in Lancet (1) and Proceedings of the Royal Society (2), showed that the ability to distinguish one’s own cells from foreign cells, a key feature of the adaptive immune system, depends on lymphocytes, now known as T cells, matured in an organ called the thymus. Subsequently, Cooper reported in Nature (3) that antibody production depends on a separate set of lymphocytes, dubbed B cells. The division of labor between T and B cells is a fundamental organizing principle of the adaptive immune system, the discovery of which laid the groundwork for modern immunology and made possible many subsequent medical advances, including monoclonal antibody production, vaccine development, and checkpoint inhibition therapies for cancer. In recognition of their discoveries, Miller and Cooper, both members of the National Academy of Sciences, received the 2019 Albert Lasker Basic Medical Research Award. PNAS spoke with both researchers to commemorate the occasion.

Max Dale Cooper. Image courtesy of Georgia Research Alliance/Billy Howard.

Jacques F. A. P. Miller. Image courtesy of © 2019 The Walter and Eliza Hall Institute of Medical Research.

PNAS: How did each of you get involved in studying the immune system?

Miller: I was interested in doing medical research from an early age, because my sister died of tuberculosis a few years before streptomycin, and I got very curious as to why some people get some diseases and others don’t. Also, because I was a child during the Second World War and I did not have any stomach for killing people, I decided I’d rather do surgery and patch them up instead. After my medical training, I got a fellowship to take me to the United Kingdom, in London, where I did cancer research. I started working on leukemia and this gave me an interest in lymphocytes.

Cooper: I became interested through patients that I was taking care of: Children that had deficient immune capabilities and were susceptible to infections. Some of them couldn’t defend themselves against a simple fever blister, a herpes simplex virus. It spread quickly and killed them. Others would have repeated bacterial infections. It was clear that if we were going to be able to diagnose them more precisely and have any chance of understanding the pathogenesis of these deficiencies, and if we were going to be able to treat them, then we needed to know more about how the immune system developed and functioned.

PNAS: What was known about the thymus at the time you began your work?

Miller: The thymus was [then] considered to be a useless organ. Thymectomy, or removal of the thymus, from adult mice was not associated with any defects. Immunologists thought it was a graveyard for dying lymphocytes.

PNAS: How did you figure out that the thymus was in fact important for immune function?

Miller: For my work on mouse leukemia, I had to take out the thymus from newborn mice, which had not been done before. The neonatally thymectomized mice were highly susceptible to infection, and they usually started wasting and being very sick about 4 months of age. When they died, I opened them up and found that they had very few lymphocytes, in contrast to mice that had been thymectomized as adults. I knew that lymphocytes had been implicated in immune responses, so I tested their immune responses by putting on foreign skin grafts, which should normally be rejected. Incredibly, the foreign skin grafts were not rejected. They grew luxuriant tufts of hair. You had 4 different types of skin graft on each mouse, and even rat skin grafts were not rejected. And that was spectacular.

PNAS: And then you transplanted a foreign thymus into a host that had had its thymus removed. What did you observe?

Miller: The foreign thymus graft of course would not be rejected because neonatally thymectomized mice can’t reject foreign tissues, but the foreign thymus graft would restore the capacity to produce immune responses, except to itself. In other words, skin from the same strain as the foreign thymus graft would be tolerated while skin from other strains would be rejected.

PNAS: So why does thymus removal not cause immune problems in adults?

Miller: The thymus is putting out most of its cells from before birth until the age of 3 years in humans, and maybe 7 or 8 months in mice. Most of the lymphocytes that we need have already been made in early life and they have a long lifespan. They recirculate and are ready to attack invaders. So you could say that the thymus has done its job by about 3 or 4 years of age in humans.

PNAS: Meanwhile, Dr. Cooper, you were studying a lymphoid organ unique to birds, the bursa of Fabricius. As someone interested in research with clinical applications, why did you decide to study an organ that only exists in birds?

Cooper: I was interested in a group of children who had an inherited disease called Wiskott–Aldrich syndrome. At that time, the general idea was that the thymus generated small lymphocytes, as shown in Jacques Miller’s experiments, some of which became plasma cells, which by that point were known to make antibodies. Wiskott–Aldrich children had few lymphocytes, but lots of plasma cells and high levels of antibodies, which didn’t fit well with the single-lineage idea. Bruce Glick, a graduate student at Ohio State University back in the 1950s, and his colleagues showed that bursectomized chicks [whose bursae had been removed] were defective in making antibodies. It then became a question of whether the thymus and the bursa did the same thing or did different things. That was what triggered me to go back and revisit the avian model.

PNAS: Your key experimental innovation was irradiating bursectomized and thymectomized chicks, to get rid of any immune cells that might have developed before the chicks hatched. What happened to these chicks?

Cooper: Bursectomized and irradiated chicks, after recovery from the radiation—their thymus was normal, their thymus-dependent small lymphocyte population and all of their cell-mediated immune functions were intact. But the birds no longer made plasma cells or antibodies, whereas thymectomy and irradiation gave the reverse pattern. So that allowed us to draw a provisional map of how the immune system developed along these 2 lines.

PNAS: Humans and other mammals do not have a bursa but still produce B cells. How did you figure out where B cells originated in species other than birds?

Cooper: The most telling experiments came during a sabbatical in England, at University College London, where I was working with John Owen and Martin Raff, using a method designed by John Owen to grow fetal liver in culture. If we put fetal liver in culture before there were any B lymphocytes, cultured them for several days, and then looked again, we could find B cells. So they’d been generated in that tissue. Those results (4), together with experiments done by others, suggested that B cells were generated in hematopoietic tissues: Fetal liver and bone marrow.

PNAS: More recently, you have shown that T-like cells and B-like cells are also found in jawless vertebrates, lampreys, and hagfish (5). What does this imply about immune system evolution?

Cooper: This founding principle for the immune system of having T- and B-like cells for adaptive immunity seems to be an old invention, one that evolved in a common ancestor of both jawed and jawless vertebrates—it’s thought—more than 500 million years ago. However, one of the most surprising things we’ve found is that jawless vertebrates don’t use immunoglobulin gene segments to recombine and generate diversity and make their receptors and antibodies. They use leucine-rich repeat proteins to generate what appears to be an equally diverse repertoire of receptors for their T- and B-like cells and antibodies.

PNAS: Based on your foundational experience, what lessons do either of you have for the next generation of immunologists?

Miller: At first I was criticized because people didn’t believe in T cells and their function, but I persevered. So I think one of the lessons to learn is to be patient, because it takes a long time to get results. Second is that serendipity is very important in medical research, because a lot of great discoveries have been made from serendipity.

Cooper: Sometimes you hear people saying that we basically know everything, it’s just a matter of putting it all together. I think that’s totally wrong. Every time we think we know everything, it turns out it’s either wrong or incomplete, and there’s lots left to learn.