Table 5. Examples of approaches to modify stem cell products to reduce their immunogenicity.

Method	Rationale	Results	Ref.
CTLA-4-Ig and PD- L1 knock ins in human ESCs.	Stimulation of CTLA-4 and PD- 1 will inhibit CD8+ T-cell activity and proliferation, reducing CD8+ T-cell-mediated cytotoxicity.	Tumours formed by the introduction of a CTLA-4-Ig/PD-L1 knock-in human ESCs in a humanized mouse model displayed reduced T cell infiltration and formed larger tumours.	179
Beta-2-Microglobulin knockout	Beta-2-microglobulin forms a constitutive part of the MHC1 complex. Knock-out of Beta-2- Microglobulin inhibits the surface expression of MHC1.	Knock-out cells exhibited ~15% less T- cell activation in PBMC co-culture than wild-type cells219. Knock-out cells showed reduced T-cell activation in a mouse model of ischaemic hindlimb220.
MHCI knock-down	Immunogenicity in response to allogenic grafting is primarily consequential on MHC1 T cell activation. Silencing of MHC1 associated genes may inhibit its expression and diminish immune response.	Transplantation of knock-down human ESCs elicited significantly less T-cell activation and survived longer than wildtype cells.	221
Beta-2- Microglobulin and CIITA knock-out	A combined knock out of Beta- 2-Microglobulin and CIITA disrupts the surface expression of MHCI and MHCII, respectively. This would decrease the activation of CD8+ and CD4+ T cells, respectively.	Knock-out cultures shoed a reduced propensity (3-fold) to activate T cells, and produced significantly larger spheroids when co-cultured with peripheral-blood mononuclear cells (with respect to wildtype cells).	222
Disruption of HLA-A and HLA-B expressions with selective retention of HLA-C expression.	Cells non-expressive of MHCI are susceptible to NK- mediated cytotoxicity. Selective HLA-C expression can allow CD8+ T cell evasion while also diminishing cytotoxic NK cell responses.	HLA-C cells evaded both NK and CD8+ T- cell-mediated cytotoxicity in vitro and in vivo.	223