Advances in generating HLA-Universal platelets for transfusion medicine

Objectives

‘Universal’ blood typically refers to blood components that can be donated to most individuals. Allogenic platelet refractoriness is often caused by incompatibility of human leukocyte antigens (HLA) and antibodies in the recipient binding donor epitopes. HLA-matched platelets are not always readily available for transfusions, hindered by a general rise in platelet demand, logistical challenges of HLA-selected platelet provisions and the limited HLA repertoire of current stocks. Several groups have therefore progressed towards making in vitro platelets lacking HLA class I molecules, to avoid immunogenic reactions. This review will outline the progress to date in establishing these ‘HLA-universal platelets,’ taking a comparative view of the gene-editing technologies used for their creation while also evaluating the resulting products and the immunogenic properties of HLA-deficient cells.

Key findings

Universal platelets have been differentiated from HLA class I knockdown or knockout progenitors, generated mainly through gene modification techniques targeting B2M, which encodes the β2m-light chain component of HLA class I antigens. B2M-knockdown through RNA interference technology reduces but does not completely deplete HLA class I expression, whereas B2M-knockout via endonucleases (TALENs or CRISPR/Cas9 systems) creates HLA-negative cells. Cultured HLA-silenced or ablated megakaryocytes and platelets are functional and can circulate in mouse models, while evading immune detection. HLA class I proteins serve as ligands for Natural Killer (NK) cell inhibitory receptors. A key concern for patient welfare, using HLA-universal cells, is the triggering of immune reactions including NK cytotoxicity; therefore, a possible approach to circumvent this has been to either retain or co-express alternative inhibitory or immunoregulatory molecules.

Conclusions

The collection of studies brought together and reviewed here show proof-of-concept that functional HLA-universal platelets, inert to immune responses, can be produced using powerful gene-editing techniques. CRISPR/Cas9, in particular, offers versatility to co-engineer multiple specific gene knockouts in combination with options to knock in immunomodulating proteins or platelet factors, to correspondingly further enhance immune tolerance or improve platelet functions. Together these data demonstrate the rapid development and feasibility of moving HLA-universal platelets towards the clinic.