HLA Class I Cells
Class I molecules present peptides to CD8+ T cells and are composed of B2M paired with diverse heavy alpha chains. Human cells express two alleles each of three polymorphic HLA class I molecules (A,B and C) that must be matched to recipients, as well as the non-polymorphic HLA-E molecule. Universal Donor Cells do not express free B2M and lack surface expression of HLA-A, B and C. The B2M-HLA-E single chain fusion protein is expressed from the B2M locus to inhibit lysis by host NK cells, with or without a covalently attached antigenic peptide in the trimer and dimer forms respectively.
HLA-A, B and C are polymorphic class I proteins expressed by most nucleated cells that must be matched for histocompatibility. The Beta-2 Microglobulin (B2M) gene encodes a common subunit essential for cell surface expression of all HLA class I heterodimers (the other subunits are the heavy chains for HLA-A, B, C, E, F, or G), so B2M-/- cells are class I-deficient. Universal Cells edits both B2M genes to create human pluripotent cells that lack polymorphic class I proteins. These editing steps also introduce suicide gene(s) such as Thymidine Kinase (TK) to allow for in vivo elimination of transplanted cells.
A complete lack of class I expression can lead to lysis by Natural Killer (NK) cells. To overcome this “missing self” response, our Universal Donor Cells express a non-polymorphic class I gene such as HLA-E at the B2M locus through single chain technology. This provides a class I-positive signal to inhibit NK cells. These class I-engineered stem cells can serve as universal donor cells in applications where the differentiated cell product does not express HLA class II.