11 Jan 2021
Issue #39: Trafficking to the nodes
Setting it Straight - Issue 39
Written by Nobel Laureate Professor Peter Doherty
Having identified the discretely defined lymph nodes (#38) and the more ‘embedded’ mucosal associated lymphoid tissue (MALT: e.g. adenoids, tonsils) as specialised anatomical niches where ‘primary’ immune responses to a novel pathogen or vaccine develop, the next step is to look more closely at the key players, the immunologically naïve (or precursor) B cells and T cells (#18) that traffic selectively to those sites. The discussion focuses on the lymph nodes, as they are (for an experimentalist or pathologist) easy to remove and measure as single entities. The naive T cells that find their way to the nodes are the survivors of a selection process in the thymus that, though not perfect, functions to get rid of most autoreactive (#36) ‘entry candidates’ while at the same time ‘educating’ those that survive and progress to be responsive to peptide-induced changes in our MHC class I (CD8+ killers) or class II (CD4+ helpers) cell-surface glycoproteins (#29). A later essay will deal in more detail with how the thymus works but, for the moment, that can be set aside as we concentrate on understanding the complexities of a virus-specific immune response.
All the jawed vertebrates have a thymus located in the lower neck region, while birds further evolved a specific organ for precursor B cell development called the Bursa of Fabricius (#35). In us, we think that what happens in the avian Bursa goes on in our bone marrow. Positioned just inside the common cloacal opening at the end of the avian reproductive and gastrointestinal tracts, the function of the Bursa was established in 1956 when Philadelphia’s Bruce Glick surgically removed it from newborn chicks and thus destroyed their capacity to make antibodies. Similarly, the role of the thymus was discovered in 1961 when Australian Jacques Miller excised the thymus from neonatal mice and found that their immune systems were massively compromised.
After being physiologically ‘educated’ in the thymus or Bursa equivalent, the naïve T cells and B cells exit into the lymph (#9) or enter the blood by pushing through between the lining epithelial cells of the tiny post-capillary venules. Either way, they all end up in the venous blood circulation to be pumped (by the heart’s right ventricle) through the lungs to the more powerful left ventricle that drives oxygenated, arterial blood around the body. Here, as they pass from the superhighways of the major arteries, to the more minor roads of the arterioles, to the potential ‘exit points’ of the capillary circulation they will, if that blood is supplying a lymph node, encounter a ‘stop, leave and stay-awhile’ signal. How does that work?
Apart for ‘going with the flow’ in the bullet train of pumped arterial blood, the job of naïve T cells and B cells is to traffic to the right place, in their case the lymph nodes or the MALT (#38). Lymphocytes are single cells, so how do they know where to go? That answer is common through biology: it’s a chemical signalling system. They move along chemical gradients. Organisms as complex as soldier and worker termites have a brain but, like lymphocytes, they don’t have eyes that signal to their brain so they can decide whether to move towards (or away from) potential enemies or food sources. Both termites and their main predators, ants, make formic acid, but they also have other, unique chemical signatures. Sensing that ants are nearby, worker termites head for the safety of home, while at the same time blocking the tunnels into their residential termite mound as they leave their soldier colleagues to face the foe and die.
That sensing ability for lymphocytes reflects the function of cell-surface proteins with the capacity to detect (bind to) specific chemical signatures in fluid phase or on other cell-surface proteins. Such ‘receptor/ligand’ (or receptor/acceptor) binding events trigger molecular cascades within the individual lymphocyte that determine adhesion, penetration and movement. For air-breathing organisms like termites and us, the equivalent is the detection of aromatic chemicals, which we register as smells when the compound binds to specifc proteins expressed on the surface of olfactory neuons (#11). Either way, it’s a chemical sampling, ‘sniffer’ system that moves lymphocytes, termites, beagles and us around.
With regard to lymph node entry, the key ‘sniffer’ on the surface of naive T and B cells is a protein called CD62L (L-selectin) and the ‘smell’ it detects isn’t an aromatic chemical but another cell-surface molecule expressed on specialised ‘high endothelial venules’ (HEVs, the blood vessel lining cells look ‘tall’ under the microscope) in the lymph node’s vascular system. As naïve CD62Lhigh T and B lymphocytes enter the node’s blood capillary network, they encounter lots of a mucin-like (#10) glycoprotein called GlyCAM-1. Slowed by the ‘stopping to smell the roses’ experience of CD62L/GlyCAM-1 binding as they roll along the HEV walls, the immunologically naïve T cells and B cells push through the ‘looking glass’ of loose epithelial junctions into the wonderland of the lymph node interior. Another molecule, MadCAM-1 serves the same function in the MALT.
The differing concentrations (high to low) of ‘physiological’ receptors like CD62L – as distinct from the antigen specific TCRs (#33) on ‘killer’ T lymphocytes (#34) - can be influenced by secreted proteins, like the interferons, cytokines and chemokines produced early on in any infection. High concetrations at a local site of pathogen invasion drain via the lymph to promote the selective recruitment of naïve lymphocytes into the regional lymph nodes, where they will also encounter virus components (either free floating or on the surface of antigen-presenting dendrititic cells) entering via the incoming, ‘afferent lymphatics’. Exposure to the type 1 interferons that are made by virus-infected cells can increase the concentrations of CD62L on the surface of lymphocytes, while treating mice with an antibody that blocks type 1 interferon limits the recruitment of naïve lymphocytes into the lymph nodes.
Just giving a vaccine made up of killed virus (the early Salk polio vaccine) or as bits of virus (subunit flu vaccine) may not trigger cytokine/chemokine production to the extent that there is enhanced, naïve T and B cell extravasation and localisation into the lymphoid tissue. As a consequence, apart from live attenuated products like Sabin polio vaccine (the one you take on a sugar cube), vaccinologists may add an ‘adjuvant’ to induce elements of that innate response. Some who have reservations about vaccines worry about adjuvants: we’ll get to that later. Next week, the focus will be on what happens in the lymph nodes following infection or vaccination.