Lymphocytes and MS

People with MS need to get up close and personal with their lymphocytes and lymphocyte counts and hence need to understand what the term lymphopaenia actually means.

Jul 14, 2021

What does a lymphocyte count mean?

If you have MS and are on disease-modifying therapy you may be asked to have regular blood counts to make sure your lymphocyte count is not too low. Do you know what your last lymphocyte count was? And do you know what is a safe lymphocyte count? Is it above 500, 800 or 1,000/mm3 or per µL?

Please note that lymphocyte counts may be given to you as 0.5, 0.8 or 1.0 x 10^9/L. These are the same thing.

Photo by National Cancer Institute on Unsplash

In your peripheral blood, you have circulating leukocytes or white blood cells which help fight infections. When you get your blood results back you often get told your total white cell count or WCC. However, the WCC is a composite and includes different populations of cells. The white cells can be divided into the lymphocyte and non-lymphocyte populations. The non-lymphocyte population or the innate (hard-wired) immune cells are often referred to as polymorphs (neutrophils, eosinophils and basophils) and monocytes (circulating macrophage precursors).

When it comes to lymphocytes we divide them into so-called B-cells and T-cells. B-cells come from the bone marrow; yes B is for Bone. In contrast, T-cells from the thymus; T is for Thymus, which is sweetbread if you like eating offal.

B-cells are specialised cells that use antibodies to recognise infections or cancers. On the other hand, T-cells use T-cell receptors for identifying infections or cancers. There are two broad categories of T-cells. CD4+ T-cells that generally react to foreign proteins (generally infections) and cancer proteins (onco-antigens) that circulate outside of cells (extracellular antigens). In contrast, the CD8+ pathway is for recognising foreign proteins that are expressed inside cells such as viruses. The CD4+ cells tend to orchestrate an immune response and help other cells clear the infection; for example, they stimulate and educate B-cells to make antibodies and produce molecules that attract macrophages to the site of inflammation. A small population of CD4+ cells are cytotoxic and kill target cells directly (cell-cell contact) or by producing soluble proteins that kill the target cell. Another population are regulatory CD4+ cells that help control the immune system; these are often referred to as T-reg cells. Some immunologists think that MS is a disease of T-reg function. In other words, deficient or abnormal T-reg function allows autoimmunity to develop; I am not one of those individuals who support the T-reg hypothesis of MS.

In comparison, CD8+ T cells are much more sensitive to being activated and typically do their own killing; this is why a subset of them are called cytotoxic T-lymphocytes (CTLs). CTLs are like military commandos; they survey the body and if they identify a cell that is infected with a virus they kill it there and then. This is why CD8+ CTLs are so important in fighting viral infections such as SARS-CoV-2 that causes COVID-19.

In a routine blood count, we generally don’t get back detailed numbers of the lymphocyte subpopulations, we simply get the absolute lymphocyte count (ALC), which includes both the B-cells and T-cells. To get back subset numbers requires a specific laboratory request.

So what is a normal lymphocyte count?

There are different ways of defining a normal laboratory range in medicine. The most popular way is to take thousands of healthy volunteers of all ages and sexes and measure their ALC and then work out the normal range using the 2.5th and 97.5th percentile on the assumption that 2.5% of the population has abnormally low counts and 2.5% have abnormally high counts. If you do this then you get a different normal range for different populations; for example, in a very large Danish study the normal range for Danish people, using this method is defined as 1.1–3.7×10^9/L or 1,100 – 3,700/mm3.

To simplify and standardise things the WHO (World Health Organisation) has defined the lower limit of normal as 1.0×10^9/L or 1,000/mm3. This is not necessarily correct because at a population level people with an ALC of 1000/mm3 are at a greater risk of having an infection and dying from it than someone with an ALC higher than this. What I am trying to say is that the so-called ‘normal’ cutoffs are not black and white boundaries and that the risk of infections is affected by many other factors, in particular age (immunosenescence) and comorbidities.

For example, the older you get the greater the proportion of your lymphocytes in your peripheral blood become dedicated to fighting latent or dormant viruses such as cytomegalovirus (CMV), Epstein-Barr virus (EBV) and other herpes viruses. Yes, keeping these viruses latent and non-infectious takes its toll on our immune systems. This means older people, typically older than 55 years, have less naive lymphocytes to fight new infections. So a younger person with an ALC of 1,000/mm3 may have 10-20% of the peripheral T-cells dedicated to controlling CMV and EBV and someone over the age of 70 may be using 60-70% of their T-cells to keep CMV and EBV at bay. When the latter happens we refer to the T-cell repertoire (variability of all the T-cells in the blood) as being restricted and is indicative of immunosenescence, i.e. the majority of peripheral T-cell clones can’t be used for anything else other than controlling CMV and EBV. This may explain why an older age is such an important risk factor for developing severe COVID-19, PML, other infections, secondary cancers and poor vaccine responses.

The WHO has also created grades of lymphopaenia based on the ALC:

Grade 0 >= 1,000/mm3

Grade 1 = 800-999/mm3

Grade 2 = 500-799/mm3

Grade 3 = 200-499/mm3

Grade 4 < 200/mm3

I know that a lot of you are confused because some neurologists are saying that you are at high risk of severe COVID-19 if your ALC is less than 1,000/mm3, others like me are saying that you are only at increased risk if your counts are less than 800/mm3 and still others who are saying that you should only worry if your ALC is less than 500/mm3.

Apologies, for the confusion, but as with most things in medicine nothing is certain or definitive; it is a soft call and advice also needs to be pragmatic and generalisable to the wider MS population.

For example, if you are treated with alemtuzumab your counts may never get above 1,000/mm3 before the next course. It is clear that the infection risk post-alemtuzumab drops quite precipitously after about 3 months when most patients have ALC that haven’t even recovered above 500/mm3. So should we use 500/mm3 or less than 500/mm3 as the safe limit? I say no because most of the patients in the alemtuzumab trials were young and had no comorbidities. Therefore, this advice does not take into account immunosenescence and other factors. So then why not stick with the WHO and simply recommend 1,000/mm3? I personally think this is too conservative and means people will be hyper-cautious when they don’t necessarily have to be.

To try and explain the subtleties to you I have hacked the data from the large Danish study I mentioned and have inserted it into this newsletter below to show that infection risk increases linearly below an ALC of ~1,700/mm3. Even at a WHO grade zero or ‘normal’, there is a 26% higher risk of infection, at Grade 2 (800/mm3 cut-off) there is a 44% increase in risk and with Grade 3 (500/mm3) it starts to increase rapidly (+76%).

I hope you now understand the complexities of setting a normal lymphocyte range and advice about what is safe. Since I was taught how to use azathioprine, one of the original immunosuppressive drugs used in solid organ transplantation, I have always used 800/mm3 as my target cut-off for pragmatic reasons. I think the overall evidence supports this position, particularly in older pwMS, but I am sure many of my critics will have other opinions.

Which DMT are you on?

Interferon beta

Most patients on interferon-beta drop their ALC by about 20-30%. About 15% of interferon-treated patients drop their counts below normal or 1,000/mm3, ~1 in 20 (5%) below 800/mm3 and very few, in the order of about 1 in 100 treated patients (1%), below 500/mm3. If you sit tight the drop in lymphocyte counts on interferon-beta is often transient and it recovers to a safe level over several months.

Glatiramer acetate

Glatiramer acetate (GA) is not associated with any significant incidence of lymphopaenia. If you are on GA and have lymphopaenia there is usually another cause for the lymphopaenia.

Natalizumab

Based on its mode of action natalizumab, which blocks lymphocyte adhesion molecules and stops lymphocytes marginating, i.e. sticking to and rolling on the inside of blood vessels, natalizumab shifts this so-called marginating pool of lymphocytes into the circulation and increases the ALC by ~20-30%; this increase in lymphocyte count is not clinically significant.

S1P Modulators (fingolimod, siponimod, ozanimod, ponesimod)

Please note that if you are on an S1P modulator, which work by blocking trafficking or migration of lymphocytes out of lymph nodes and other lymphoid tissues (tonsils, adenoids, spleen), your ALC drops. The actual lymphocyte count, however, is not a good guide to the consequences of being treated with this class of agent.

In general, the ALC does not correlate with the efficacy of any specific S1P modulators nor with its infectious complications. This is why you can’t use the peripheral blood ALC to derisk the class. You simply have to acknowledge that these are continuous immunosuppressive therapies with long term consequences, i.e. infections, opportunistic infections, secondary malignancies and blunted vaccine responses, and hence all that you can do is be vigilant. Being vigilant means staying aware of infections, having vaccines that may or may not work and enrolling yourself into cancer surveillance programmes if you are eligible (cervical, breast, colon, prostate, lung and skin). Please note not all countries have cancer surveillance programmes and some programmes are not recommended, for example, prostate cancer in the NHS.

Fumarates (dimethyl fumarate, diroximel fumarate)

On average patients on fumarates drop their ALC by about ~30%, with 1 about 1 in 20 (5%) patients dropping to less than 500/mm3 and 1 in 7 (~15%) dropping below 800/mm3. The issue with fumarate-induced lymphopaenia is that if left unnoticed and is ignored for a prolonged period of time (> 6months) it can take many months for the ALC to recover to normal when the fumarate is stopped. It is also noteworthy that fumarates hit CD8+ lymphocytes more than other lymphocytes, which may explain the PML (progressive multifocal leukoencephalopathy) signal seen in patients with fumarate-induced lymphopaenia, particularly in the older MS population (>50 years).

Teriflunomide

Almost all patients on teriflunomide drop their ALC by about 10-20%, with 1 in 10 (10%) dropping below normal (1,000/mm3) and 1 in 50 (2%) below 800/mm3.

Anti-CD20 therapies (rituximab, ocrelizumab, ofatumumab, ublituximab)

Anti-CD20 therapies drop the ALC by about 15-20%, but the majority of treated pwMS maintain an ALC above 1,000/mm3. The main reason for the drop in ALC is not B-cell depletion, but the depletion of a subset of T-cells that express CD20 on their surface. CD8+ T-cells express more CD20 than CD4+ T-cells so the CD8+ numbers drop by ~15% and CD4+ T-cells by ~10%. Whether this drop in T-cells has anything to do with the mode of action of anti-CD20 therapies is a moot point. Most HCPs don’t think this drop in ALC is clinically significant, but I disagree. Why? People treated with anti-CD20 therapy, particularly ocrelizumab, which causes greater depletion than the other anti-CD20 therapies is associated with a herpes zoster or shingles signal. The higher incidence of shingles on ocrelizumab, which occurs in about 1 in 75 treated subjects (1.5%) is likely to be due to the drop in CD8+ cytotoxic lymphocytes.

Another cell type to worry about with anti-CD20 therapy is the neutrophil; a small number of treated subjects develop a late-onset, and in some cases profound, neutropaenia as an adverse event that can be associated with life-threatening infections. The incidence of mild, but transient, neutropaenia is common and occurs in about 10% of anti-CD20 treated patients. The more severe later-onset neutropaenia is to the best of my knowledge rare and not common enough to warrant regular monitoring of blood counts to detect. Therefore, it is best to be aware of what symptoms to look for if you develop severe neutropaenia, which are essentially related to infection, such as fever, particularly a high temperature (>38°C), chills, rigors, sweating, sore throat (pharyngitis/tonsillitis), mouth ulcers, toothache (periapical tooth abscess), abdominal pain (mesenteric infection), pain near the anus (ischiorectal abscess or a pilonidal sinus infection), sores around the anus, diarrhoea, cough and/or shortness of breath (pneumonia).

Immune reconstitution therapies (alemtuzumab and cladribine)

Both alemtuzumab and cladribine cause transient lymphopaenia in all patients treated; this is how the therapies work. Alemtuzumab causes rapid and more severe lymphopaenia within the first week of starting the infusions. The lymphocytes then recover very slowly over the next 6-12 months. Cladribine depletes lymphocytes much more slowly over months with a nadir between 3 and 4 months after starting treatment. The level of total lymphocyte count depletion with cladribine is on average relatively mild (median ALC ~800/mm3) and is only transient with about 1 in 10 (10%) dropping below 500/mm3. The latter is lower in real-life compared to the phase 3 trials due to adaptive dosing, i.e. you only start the second course of oral cladribine once your ALC recovers to above 800/mm3.

Lessons

What COVID-19 is teaching me is that the MS community is not comfortable with uncertainty, but as we live in an uncertain world you are going to have to adapt to conflicting advice. Until data emerges we have only opinions, this is just one opinion, which may differ from the opinion you were given last week.

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Note

Please be aware that I am planning to do detailed MS-Selfie Newsletters on each of the DMTs we use to treat MS. Lymphopaenia as a consequence of the mode of action of DMTs is only one factor you need to consider when choosing and starting a DMT.

Please let me know if this is too much detail for you. I have tried to keep it simple, but most people even neurologists find immunology hard.

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