During this webinar, Dr Harry Karpouzis, Founder & Scientific Director of IVF Pelargos Fertility Group, discussed the role of natural killer (NK) cells in IVF — separating fact from myth, and addressing the latest evidence on testing and treatment.
Dr Karpouzis explained the difference between peripheral and uterine NK cells, how they are measured, and what current research tells us about their impact on implantation and pregnancy outcomes.
Natural killer (NK) cells and reproductive immunology are among the most debated and least understood areas in fertility medicine. In this presentation, Dr Harry Karpouzis, Founder and Scientific Director of IVF Pelargos Fertility Group, explained what NK cells are, how they function in reproduction, and when they may play a role in IVF outcomes.
Dr Karpouzis repeatedly emphasised that immunology is not relevant for every IVF patient. Instead, it is a highly specialised field that should be approached cautiously, scientifically, and only in selected cases.
Dr Karpouzis began by explaining that there are two main types of natural killer cells relevant to fertility:
– natural killer cells circulating in the blood
– uterine natural killer cells, which are found in the lining of the womb
While they share a name, these two cell populations behave differently. Uterine natural killer cells are the dominant immune cells in the endometrium and play a crucial physiological role in implantation and early pregnancy.
Their primary function is not to attack the embryo. On the contrary, they support implantation by helping form new blood vessels, assisting placental development, and producing signalling molecules known as cytokines.
What really matters is what happens inside the uterus, Dr Karpouzis explained. Blood NK cells do not always reflect the immune environment of the endometrium.
Although uterine NK cells are essential for implantation, problems may arise when their activity becomes imbalanced. However, Dr Karpouzis highlighted a key challenge: there is no universal agreement on what constitutes “too much” or “too little” NK cell activity.
Some studies suggest an association between abnormal NK cell activity and recurrent implantation failure or recurrent miscarriage. Other studies find no consistent link. This lack of consensus is the main reason why major professional bodies, such as ESHRE and the American Society for Reproductive Medicine, do not currently recommend routine immunological testing or treatment.
The evidence is weak and inconsistent, Dr Karpouzis noted, but that does not mean immunology is irrelevant. It means it must be used correctly and selectively.
A central message of the presentation was that immunological testing should never be a first-line investigation.
Dr Karpouzis stressed that most IVF failures are caused by more common factors, particularly embryo chromosomal abnormalities. Chronic endometrial infections, anatomical problems, and hormonal issues are also far more frequent causes of implantation failure and miscarriage.
Before considering immunology, clinicians must first ensure:
– embryos are chromosomally normal
– uterine anatomy has been properly assessed
– chronic endometritis and infections have been ruled out
– hormonal and metabolic factors are optimised
Chromosomally normal embryos are too precious to risk, he said. If a woman has only one euploid embryo, it makes sense to investigate everything, but only if there is a clear history that justifies it.
According to Dr Karpouzis, immunology should be considered only in specific clinical scenarios:
– recurrent implantation failure
– recurrent miscarriages, after common causes have been excluded
Even within these groups, not all patients require immune testing. Testing is expensive, invasive, and lacks definitive evidence, so patients must be fully counselled before proceeding.
Immunological assessment can be performed using blood tests or an endometrial (uterine) biopsy.
Blood tests, such as those often referred to as the “CHICAGO test,” measure NK cell activity and cytotoxicity in the bloodstream. They may also assess autoimmune markers, antiphospholipid antibodies, thyroid-related antibodies, and TH1/TH2 cytokine balance.
However, Dr Karpouzis explained that blood results do not reliably reflect what is happening inside the uterus. For this reason, his team prefers uterine testing whenever possible.
Endometrial testing requires a biopsy and allows direct assessment of:
– uterine NK cell activity
– cytokine profiles
– immune balance at the implantation site
Although more invasive, this approach is considered more clinically relevant.
One of the most critical aspects of immune testing is timing.
Dr Karpouzis explained that the immune profile of the endometrium changes throughout the cycle and is influenced by estrogen and progesterone. Testing at a random time provides limited information.
At Pelargos IVF, immune testing is performed during a “mock” embryo transfer cycle, after estrogen and progesterone have been administered. This replicates the immune environment present at the time of actual embryo transfer.
Immunology can change within months,” he said. “Pregnancy, hormonal changes, endometriosis, PCOS, or infection can all alter the immune balance.
Personalisation is central to immunological treatment. Dr Karpouzis described three possible immune profiles at the time of implantation:
Overactive endometrium
This is characterised by increased NK cell activation and dominance of pro-inflammatory cytokines (TH1). In such cases, treatment may aim to reduce immune overactivity.
Underactive endometrium
Here, immune activity is insufficient to support implantation. Suppressing immunity further can be harmful.
Mixed profile
This involves elements of both overactivity and underactivity and requires careful balancing.
Giving steroids without knowing the immune profile can make things worse,
Dr Karpouzis warned.
For overactive immune profiles, potential strategies may include:
– corticosteroids
– intralipid therapy
– adjustment of progesterone and estrogen doses
– vitamin E in selected cases
Progesterone, Dr Karpouzis explained, is not only a hormone but also a powerful immunomodulator. Dose changes can significantly affect immune balance.
For underactive immune profiles, treatment may aim to stimulate immune activity, using approaches such as:
– luteal phase endometrial scratching in a previous cycle
– intrauterine procedures to promote local immune activation
– hormonal adjustments to support vascular development
In mixed cases, a combination of strategies may be required.
Dr Karpouzis addressed the frequent use of medications such as low molecular weight heparin (for example, Clexane). While commonly prescribed as a blood thinner, he explained that it also has immunomodulatory effects.
It can reduce NK cell cytotoxicity and inflammatory cytokines, but only when used appropriately. In underactive immune environments, it may be counterproductive.
Conditions such as endometriosis, PCOS, and chronic endometritis are associated with chronic inflammation, which can disrupt immune balance in the uterus.
In patients with these conditions, the threshold for investigating immunology may be lower, particularly if implantation failures persist despite treatment of the underlying condition.
Dr Karpouzis also discussed HLA and KIR testing, which examines interactions between maternal immune receptors and fetal antigens.
This testing is not routine and is reserved for highly selected cases where all other explanations have been excluded. Evidence remains limited, and major scientific societies do not strongly support its routine use.
Possible management strategies in selected cases may include:
– single embryo transfer
– donor selection in donor cycles
– use of growth factors such as G-CSF
– tailored hormonal support
Dr Karpouzis concluded by emphasising that immunology is neither a miracle solution nor something to be ignored.
There is no conclusive evidence, he said, but there is a strong biological theory behind personalised immune treatment.
The key, he stressed, is careful patient selection, correct timing, understanding how each medication affects immune balance, and avoiding empirical treatment without proper investigation.
Used thoughtfully and selectively, immunology may offer benefits to a small group of patients. Used indiscriminately, it may do more harm than good.
There are several tests available. NK cells can be checked in the blood or in the uterus. For the reasons I explained earlier, we prefer to check NK cells in the uterus. We also prefer to assess them after preparing the endometrium with hormones, so that we mimic the immune environment at the time of embryo transfer. Estrogen and progesterone can affect immunology.
When blood tests are performed, they are done at a random point in the cycle, and they do not necessarily reflect what is happening inside the endometrium. For this reason, in our unit, we prefer an endometrial biopsy. A test we use that is closely aligned with the work of French immunologists is called IMAP. This test allows us to assess NK cells as well as other immune factors, such as the TH1/TH2 balance, directly in the endometrium at the time of embryo transfer.
We do not perform these tests on everyone. We reserve them for patients with recurrent implantation failure or recurrent miscarriages, and only after all other more established causes have been investigated. It does not make sense to invest in immunological testing if embryos have not been checked chromosomally, as chromosomal abnormalities remain the most common cause of miscarriage.
When we talk about IVF and embryo transfer, we usually aim for TSH levels below 2.5. It is also important to check for thyroid antibodies to rule out Hashimoto’s disease, which is an autoimmune condition.
Autoimmune conditions are often associated with other immune disturbances. In women with autoimmune disease, we may see a higher incidence of immune-related issues, including altered NK cell activity. This does not mean that NK cell problems will definitely be present, but our threshold for investigating immune factors is lower in women with recurrent miscarriages or implantation failure.
If there is no history of miscarriages or implantation failure, NK cell testing is not routinely indicated. In this case, there was no embryo transfer because embryos did not reach the blastocyst stage, so the patient does not fall into the category where immunological testing is recommended.
Hashimoto’s disease is very common. The priority should be to optimise thyroid function and bring TSH levels below 2.5. The most important factor is to create a good-quality embryo. Extensive immunological testing is not necessary at this stage.
Yes, we do take them into consideration in selected cases. These antibodies are not assessed by the IMAP test and require separate testing. They may be investigated in women with unexplained implantation failure or recurrent miscarriages, after all other factors have been thoroughly evaluated.
There is no conclusive evidence supporting these tests, and there is no proven treatment if abnormalities are found. However, there is a scientific rationale behind them, and some approaches may help in practice. These include single embryo transfer, immunomodulation, adjusting progesterone support, and using medications that have shown benefit in some patients. We offer these tests only to a specific group of patients, not routinely.
We recommend an endometrial biopsy using the IMAP test. This is performed after hormonal preparation with estrogen and progesterone to replicate the protocol used for embryo transfer. This approach allows us to assess the immune environment under the same conditions that will be present during transfer and to personalise treatment based on the results.
We use immunological treatments selectively and prioritise medications with better safety profiles. Some medications can have side effects, such as increased white blood cell counts, and require monitoring. IVIG, for example, can cause allergic reactions and is not available for use in Greece.
Instead, we often use combinations of steroids or alternative medications with better safety profiles. We avoid using treatments that are not approved or have significant risk unless absolutely necessary.
No, they are different. Intralipid has a much better safety profile than IVIG and is easier to access. IVIG requires close monitoring by an immunologist and carries a higher risk of adverse reactions. For this reason, we do not use IVIG in our practice. We achieve good results using intralipid in combination with steroids or safer alternative medications.
There is always a small risk of reaction, so monitoring is required, but intralipid generally has a good safety profile. We do not use intralipid routinely. We use it when we know there is immune overactivity, either in combination with steroids or when steroids are not well tolerated.
Typically, intralipid is administered about 5 days before embryo transfer, again after transfer, and sometimes after a positive pregnancy test. There are different protocols depending on individual circumstances.
Immunology is not the most important factor in IVF, but it should be considered in selected cases. The key is to proceed step by step. First, exclude the most common causes of failure. If failures continue, then immunology can be investigated properly.
Testing should aim to understand what is happening inside the endometrium, and treatment should be personalised. Using immunological medication without knowing the underlying immune environment can lead to worse outcomes rather than better ones.
