25 Jan 2021
Issue #41: In the node - Part 1
Written by Nobel Laureate Professor Peter Doherty
Having discussed the interactions that allow naïve lymphocytes to enter the lymph nodes (#39), how the lymph nodes function in a broadly biological sense and what happens when effector and memory T cells and B cells exit, then head off around the body (#40) it’s time to backtrack a little and summarise what happens in the node. Why so much focus on lymph nodes? These are the specialised environments, the nurturing nurseries, where immune responses develop. Over the years, immunologists have progressively assembled an understanding of how this works and can give a reasonably coherent account.
That tale is, in fact, derived from a series of discoveries enabled by technological advances and intellectual insight. Added to that are a number of partially tested assumptions like, for instance, that the events occurring in the human lymph node are broadly equivalent to what experimentalists establish for a laboratory rat or mouse. Those two sets of perceptions are progressively becoming more convergent with, for example, the great advances in imaging (e.g. MRI and PET Scans) that help us to look directly at ‘the world within us’.
Inserting a human component makes any story more interesting, so I’ll tell a little of the history as we look ‘in the node’. In broad terms, what we are discussing here is the selection, differentiation and expansion – which go together – of antigen-specific lymphocytes, the B cells and T cells, to assemble an antigen-specific (#19, 20, 21 and 33) immune response that is fit-for-purpose in dealing with a particular pathogen. I should emphasise that any organ function is complex and multi-faceted. A haematologist who is basically interested in disorders of the blood, especially cancer, would come up with a narrative that is somewhat different from the immunology-oriented tale I tell here.
The first thing to keep in mind is that the ‘adaptive immune’ responder cells all start out as small lymphocytes. With a large nucleus and not much cytoplasm, the uninteresting lymphocytes just didn’t ring the bells of curiosity loudly for the early morbid anatomists and haematologists. If anything, the pathologists regarded lymphocytes as dangerous stem cells that, following mutational change, gave rise to the lymphomas and lymphoproliferative diseases that kill people. Basically, the lymphocytes were either boring, to the physiologists who were interested in defining normal bodily function - a major obsession through the first half of the 20th century – or bad, to the diagnostic microscopists and backroom types of the necropsy room.
As a consequence, while we knew quite a bit about what the various categories of white blood cells (WBCs) did (#7) by the first decade of the 20th century, it took another 50 years before we even began to understand the lymphocytes. The hero of this story was a tall, amiable and good humoured young medical graduate, James Learmonth (Jim) Gowans who, after graduating from London’s King’s College Hospital Medical School, decided that he didn’t want to spend his life treating patients but, instead, would focus on understanding the basics of disease. At age 80, I’ve been good friends with many prominent MD researchers across the planet who made that decision, which I came to from a veterinary background. Now, with the massive advances in molecular medicine, many younger medical scientists in both the human and animal sphere happily pursue both clinical and research-based themes as ‘physician investigators’.
After a long and productive life Jim Gowans died, aged 95, in April 2020. I met him from time to time over the years and, thinking I might write a book about the story of the WBCs from an immunology perspective, went to see him at his house in Oxford about a decade back. It was a great discussion. Some of the groundwork for that notional book is being used in this series of essays.
Trying to find a way forward, Jim – who always believed in going to the top – sought the career advice of pharmacologist Sir Edward Mellanby, the long-term (1933-1949) head of Britain’s Medical Research Council, a position later held by Sir James Gowans. Mellanby first sent him to the Pasteur Institute in Paris, then to Oxford University’s Sir William Dunn School of Pathology and the laboratory of its eminent and forceful Australian Director, Sir Howard Florey. Florey had shared the 1945 Nobel Prize for Medicine, for penicillin, with Ernst Chain and Alexander Fleming. Earlier, Mellanby helped organise much of the funding for Florey and Chain’s penicillin work.
Florey gave Gowans the job of finding out what the lymphocyte does. Here, he had a head start at Oxford because others there, as summarised in a 1970 book (Lymphatics, Lymph and the Lymphomyeloid Complex, Academic Press) by Joe Yoffey and Queenslander Colin Courtice, had developed surgical techniques for putting a tube (cannula) into the thoracic duct of the rat. This allowed them to drain away the clear lymph containing massive numbers of lymphocytes. But they were physiologists – lymphologists – and were not thinking in terms of immunity. Likely that was also true for Gowans when he did his ground-breaking experiment using a then new technique, autoradiography.
What Jim did was to incubate the lymphocytes with a radionucleotide, tritiated adenosinse, which was taken up by their RNA and DNA. He then injected these radiolabeled, donor lymphocytes back into other inbred, genetically identical, recipient rats. With one recipient group, he again inserted thoracic duct cannulas and showed that his labelled cells were coming back out in their lymph. The second group was ‘sacrificed’ at intervals to take tissue samples, which were processed and sectioned using a microtome – like a ham slicer that cuts 5-10 mM histological sections. These sections were then overlaid in the dark with photographic film, which was later developed and examined using an optical microscope. That allowed Gowans to count and localise the ‘spots’ where the radiation sent out by the reinfused, otherwise bland-looking, labelled lymphocytes had exposed the film, betraying their presence in a particular anatomical site.
What Gowans found was that his reinfused lymphocytes were accumulating in the lymph nodes, the spleen and the Peyer’s Patches in the gut. As his experiments evolved, he could see the lymphocytes penetrating through between the high endothelial venules of the lymph nodes (#39) within 15 minutes of injection. Published in the Journal of Physiology for 1959 (Volume 146, pp 54-69) this paper, The recirculation of lymphocytes from blood to lymph, marks the beginning of the discipline of cellular immunology and puts the lymph node at the centre of the action.
To be continued…