Escape tag

In the unlikely case that our therapeutic cells escape from the capsules and to prevent their dissemination through the body we designed a safety mechanism that would enhance the recognition and destruction of the escaped therapeutic cells by the innate immune system.

We introduced an escape tag that labels the cells with a surface protein that alerts natural killer cells of the host organism to recognize and destroy cells.

HEK293 cells expressing MICA protein were efficiently killed by human NK cells.

We considered several variants to ensure destruction of escaped cells such as:

1. quorum sensing which would provide the survival signal only for cell clusters within microcapsules,
2. conjugation of microcapsule material with a ligand for a receptor that provides a survival signal,
3. activation of apoptosis for escaped cells when they encounter the extracellular matrix and
4. tagging cells for the recognition and destruction by the cells of the immune system.

After considering many factors we selected the immuno-tagging variant because of its elegant simplicity and since it seems less sensitive to the spontaneous loss of the constructs and leaky apoptosis than other options.

The MICA/NKG2D system

Though our microencapsulation system is designed in such a way that the capsules are permeable only to nutrients, signalling molecules and produced protein therapeutics, we cannot completely exclude the possibility that some cells could escape from the microcapsules, e.g. due to mechanical damage. We designed a safety mechanism that would ensure that the escaped therapeutic cells could not survive outside microcapsules because of enhanced recognition by the innate immune system.

Innate immune cells, such as natural killer cells (NK cells) and the γδT cells, patrol the tissues of the organism and recognize foreign, infected or damaged cells and destroy them without inflicting dammage to the surrounding tissue. One of the proteins that marks cells for destruction is MICA, a distant homolog of the major histocompatibility complex class I. It has been demonstrated that some cancer cells downregulate MICA to escape recognition (Salih et al., 2002) and that the overexpression of this surface protein marks them for destruction (Groh et al.,1999). MICA is recognized by the activating receptor NKG2D, which is expressed on most natural killer cells, CD8+ T cells, and γδT cells (Bauer et al., 1999), and recognition activates the cytolytic response of NK cells (Steinle et al., 2001) (Figure 1).

Our cells should therefore express an abundant amount of MICA protein (equivalent role could also be fufilled by MICB), which has strong affinity for the NKG2D receptor expressed by NK cells that would recognize and induce apoptosis of the escaped therapeutic cells.

Figure 1. The principle of our "Escape-tag" safety mechanism.