[Non-classical human leukocyte antigen (HLA) tissue types--from implantation to transplantation]

The classical and extremely polymorphic human leukocyte antigens (HLA) classes Ia and II have been studied in great detail and have significant importance in organ transplantation, autoimmune diseases and presentation of antigen peptides. However, in the human major histocompatibility complex (MHC), other functional HLA genes have been detected, the so-called non-classical HLA class Ib genes: HLA-E, -G and -F. They resemble the HLA class Ia antigens in many ways, but several major differences have been described. They are almost monomorphic and generally have a restricted pattern of expression. One function of HLA-E is to inhibit and modulate natural killer (NK) activity. HLA-G seems to be important in the modulation of the maternal immune system during pregnancy and thereby the maternal acceptance of the semi-allogenic foetus. HLA-G can already be detected in at least some blastocysts and probably has a role in implantation. A very strong expression of HLA-G is observed in the invasive trophoblast cells in the placenta. HLA-G may be involved in certain complications of pregnancy and the genetic predisposition to these. Finally, HLA-G expression has been associated with a reduced risk of rejection episodes in heart and kidney/liver transplants.