Lymphatic endothelial cells, so much more to know!

Endothelial cells (green cell walls, magenta nucleus). From Becker et al, 2021 in Cells doi 10.3390/cells10051008

I love finding out new things about the lymph system, such a fascinating area of new discovery! This is partly because even basic research has lagged behind other biological systems, and we know much more about how the cardiovascular and renal systems function by comparison.

From Becker et al, 2021 in Cells doi 10.3390/cells10051008

So I get a little bit excited when someone writes up some results that illuminate more aspects of how this elegant system works. It's a bit like finding new rooms in your house that you knew were always there but no-one had ever flicked on the light.

The study looked at how the lymphatic endothelia cells behave when there is tissue hypoxia, so there are three interesting things here.

1. How do the cells become hypoxic?

2. What happens when they do?

3. What can we do about it?

1: What causes lymphatic endothelial cell hypoxia? There is a good description of this in the introduction which I was able to follow until the last sentence where they started talking about gene expressions. Briefly, in the early stages there is too much free fluid, this increases the distance respiratory gases need to diffuse between the capillary and the cells (the transit stretch) and if this distance becomes too great cells may suffer hypoxia. Then as the free fluid becomes bound to the hyaluronic acid and the thixotropic state moves too far into the gel state, that will slow down respiratory gas exchange further and as lymphoedema progress there is fibrosclerosis and increased tissue adiposity including thickening of the initial lymph vessel basement membrane, which is normally very thin and patchy.

2: How do the cells respond to hypoxia? Here is where the argument becomes circular, because what this research shows is that in a hypoxic state the lymphatic endothelial cells release factors that influence the connective tissue matrix, including factors that;

• increase collagen production

• reduce elastin production

• increase deposition of the basal lamina (basement membrane)

• are present in inflammatory skin disease

• cause thickening of skin

• induce skin alterations like papillomatosis

• are involved in the development of scar tissue

• are related to the relationship between obesity and lymphoedema progression

Movements of Dr Vodder's MLD are know to increase lymphmotoricity

3: How does this inform our practice? The study was done invitro so that's worth keeping that in mind, and also their purpose is to find drug targets which could be used to halt or slow lymphoedema progression. But for now, the opportunity that jumps out at me is to find ways to increase removal of these factors from the tissue so they don't have the opportunity to induce changes...

...and how are these factors removed? Via the lymph system like everything else.

So lets look at this by stage. In latent and very mild lymphoedema the hypoxia will be caused by excess tissue fluid slowing down respiratory gas exchange. Frequent MLD and self-massage will help the lymph system function better while light compression will reduce the overall capillary filtrate (MLD does this too by the way). So as always, intervention as early as possible may help a struggling system to recover. If we can keep the endothelial cells healthy and stop them from forming a thicker basement membrane, then there is an even greater the chance of full recovery.

But what if it's too late and lymphoedema has already progressed to the middle stages? We can still use MLD to support and maintain whatever lymphatic function does remain, we know that keeping lymph flowing is important for the health of the cells themselves, the proper formation of valves etc. I am yet to be convinced that anything does this as well as a Vodder trained MLD therapist. The lymphatic effleurage techniques that we teach our clients don't have the same effect in stimulating lymph-motoricity, but the frequency with which it can be done does keep the lymph flowing which is important.

The lymph system loves little bits of regular support.

If things have progressed to the gel state then MLD can be used to help normalise the turn over of hyaluronic acid and mobilise free fluid, some of which could then possibly be resorbed into veins as well as removed by lymphatics. If there are fibrous changes we have multiple tools at hand to reduce the accumulation of collagenous structures. As always once we are in the middle stages and beyond compression will play a big part in preventing everything from getting worse.

In the future, studies like this one may lead to drug therapies to reduce fibrosis in advanced lymphoedema, and they will have issues like all drug therapies.

For now MLD is still the best tool that we have to help the lymph vessels recover from stress and damage, and should therefore be applied as frequently as possible at the earliest possible stages to prevent progression to clinical disease.

Read the full research paper here.

1. Becker, J., et al. (2021). “Transcriptome Analysis of Hypoxic Lymphatic Endothelial Cells Indicates Their Potential to Contribute to Extracellular Matrix Rearrangement.” Cells 10(5). 2021/05/01

Abstract

Lymphedema (LE) affects millions of people worldwide. It is a chronic progressive disease with massive development of fibrosclerosis when untreated. There is no pharmacological treatment of lymphedema. The disease is associated with swelling of the interstitium of the affected organ, mostly arm or leg, impressive development of adipose tissue, fibrosis and sclerosis with accumulation of huge amounts of collagen, and Papillomatosis cutis. Malnutrition and reduced oxygenation of the affected tissues is a hallmark of lymphedema. Here, we investigated if the hypoxia of lymphatic endothelial cells (LECs) might contribute to fibrosis. We applied RNASeq and qPCR to study the concordant changes of the exome of three human foreskin-derived LEC isolates after 4 days of hypoxia (1% O2) vs. normoxia (21% O2). Of the approximately 16,000 genes expressed in LECs, 162 (1%) were up- or down-regulated by hypoxia. Of these, 21 genes have important functions in the production or modification of the extracellular matrix (ECM). In addition to the down-regulation of elastin, we found up-regulation of druggable enzymes and regulators such as the long non-coding RNA H19, inter-alpha-trypsin inhibitor heavy chain family member 5 (ITIH5), lysyl-oxidase (LOX), prolyl 4-hydroxylase subunit alpha 1 (P4HA1), procollagen-lysine 2-oxoglutarate 5-dioxygenase 2 (PLOD2), and others that are discussed in the paper. Initial lymphatics do not produce a continuous basement membrane; however, our study shows that hypoxic LECs have an unexpectedly high ability to alter the ECM.