Why are we adding the GM-CSF Cytokine to our Culture Media?

EmbryoGen® and BlastGen™ were the first sequential media commercially available to contain the cytokine, Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF).

In this article, Steven Fleming PhD, our Director of Embryology discusses the nature of GM-CSF, the rationale for using it and its clinical benefits. 

What exactly is GM-CSF?

As its name suggests, GM-CSF is a classical lymphohaematopoietic cytokine that is known to be a mediator of cell proliferation and differentiation in hematopoietic tissues.¹

Cytokines are potent cell-secreted intercellular signaling proteins that may have an autocrine, paracrine or endocrine effect at nanomolar to picomolar concentrations, binding to specific cell membrane receptors to activate intracellular signaling pathways.

GM-CSF is one of a family of pleiotropic cytokines also believed to be involved in the regulation of diverse reproductive functions including gamete maturation, ovulation, embryogenesis, implantation, pregnancy, and parturition.²

The rationale for adding GM-CSF to culture media

To validate the rationale for adding GM-CSF to culture media, it is necessary to demonstrate its secretion into the reproductive tract.  Also, it is necessary to demonstrate the presence of GM-CSF receptors on embryos and the dependence of embryo development and viability upon GM-CSF.

In this respect, GM-CSF has been shown to be produced and secreted from the human oviductal and endometrial epithelium.^3,4  Levels of GM-CSF reach a peak during the secretory phase of the menstrual cycle^4,5, which coincides with the time of conception and implantation.

The GM-CSF receptor is a dimer comprised of a cytokine-specific alpha subunit (GM-Rα) and a signal transducing beta subunit (βc), βc forming a high-affinity complex with ligand-coupled GM-Rα.⁶ However, GM-Rα can also act alone, for example, to mediate the effect of GM-CSF on glucose transport via a kinase-independent pathway.⁷ Indeed, GM-CSF has been shown to promote glucose transport in the embryo, which expresses the GM-Rα receptor throughout preimplantation development.^8,9

A beneficial role of GM-CSF during embryogenesis was first demonstrated 20 years ago in the cow.¹⁰

In a knock-out mouse model, GM-CSF was shown to be obligatory for normal embryo development, implantation, and fetal viability.¹¹

Furthermore, the addition of recombinant human GM-CSF (rhGM-CSF) to culture media enhances human blastocyst development and viability.^9,12

The clinical benefit of EmbryoGen/BlastGen

A large multi-center randomized controlled trial (RCT) of rhGM-CSF addition to culture media demonstrated improved ongoing implantation, clinical pregnancy, and live birth rates, particularly in women who had suffered a previous miscarriage.¹³

Also, in women experiencing repeated implantation failure (RIF) EmbryoGen was shown to increase the implantation and ongoing clinical pregnancy rate.¹⁴

Possible modes of action of GM-CSF

An early study, with autologous endometrial co-culture of embryos from women with RIF, demonstrated a significant positive correlation between clinical pregnancy rate and the level of GM-CSF secreted by the endometrial cells.¹⁵

The embryotrophic effect of GM-CSF suggested by this study correlates well with clinical benefits experienced after embryo culture in EmbryoGen and BlastGen.

In addition, GM-CSF is able to upregulate its own expression within the endometrium, suggesting that it also has an autocrine effect¹⁶ (Figure 1).

Figure 1: Embryo-maternal dialogue during conception and implantation

Interestingly, it has been reported that women suffering unexplained recurrent pregnancy loss (RPL) have markedly reduced levels of GM-CSF in their systemic circulation during the first and second trimesters of pregnancy.¹⁷

Although embryonic aneuploidy rates are higher in women experiencing RPL¹⁸, aneuploidy alone cannot explain its occurrence in all cases. Therefore, it is likely that RPL occurs in a significant proportion of women due to alternative factors of embryonic and/or endometrial origin.

Is EmbryoGen/BlastGen safe to use?

To date, there are no reports of any adverse events following transfer and live birth after use of EmbryoGen and BlastGen. In addition, GM-CSF has been found to have no negative impact on rates of aneuploidy or mosaicism in embryos.^19,20 Since it occurs naturally within the reproductive tract, it is unlikely that it should have any deleterious effect either upon the embryo or within the uterus.