HLA‐G has a concentration‐dependent effect on the generation of an allo‐CTL response

Abstract

Human leucocyte antigen (HLA) -G is expressed on trophoblast cells during pregnancy, suggesting a role in protection of the semiallogeneic fetus. Published data suggest that HLA-G protects a cell against natural killer cell lysis. It has been hypothesized that HLA-G may also protect the fetus by preventing allo-cytotoxic T lymphocyte (CTL) responses. To test this hypothesis, we assayed the effects of various concentrations of purified HLA-G on CTL response in a mixed lymphocyte culture (MLC) system. We found that concentrations ≥ 0·1 µg/ml of HLA-G suppressed the allo-CTL response by 30–100% over the control, but, paradoxically, concentrations of 0·01–0·05 µg/ml of HLA-G augmented the allo-CTL response by 25–50% over the control. Concentrations ≤ 0·001 µg/ml HLA-G had no effect. Addition of HLA-G to preprimed allo-CTL effector cells did not affect their killing ability. Allo-CTL suppressive doses of HLA-G induced a T helper type 2 (Th2) cytokine response, whereas allo-CTL-enhancing doses of HLA-G induced a Th1-type cytokine response. HLA-G purified from first-trimester placenta does not affect allo-proliferative responses nor does it alter the percentage of CD4⁺ or CD8⁺ T cells in MLCs. These findings support a potential role for HLA-G-mediated suppression of allo-CTL formation in normal pregnancies. In addition, the effects observed at lower concentrations of HLA-G may have interesting implications for the condition of pre-eclampsia in which concentrations of this HLA class I molecule are reduced.