Suppression of blastogenesis and proliferation of activated CD4+ T cells: intravenous immunoglobulin (IVIg) versus novel anti‐human leucocyte antigen (HLA)‐E monoclonal antibodies mimicking HLA‐I reactivity of IVIg

Activated CD4⁺ T cells undergo blastogenesis and proliferation and they express several surface receptors, including β2‐microglobulin‐free human leucocyte antigen (HLA) heavy chains (open conformers). Intravenous immunoglobulin (IVIg) suppresses activated T cells, but the mechanism is unclear. IVIg reacts with HLA‐Ia/Ib antigens but its reactivity is lost when the anti‐HLA‐E Ab is adsorbed out. Anti‐HLA‐E antibodies may bind to the peptides shared by HLA‐E and the HLA‐I alleles. These shared peptides are cryptic in intact HLA, but exposed in open conformers. The hypothesis that anti‐HLA‐E monoclonal antibodies (mAbs) that mimic HLA‐I reactivity of IVIg may suppress activated T cells by binding to the shared peptides of the open conformers on the T cell surface was tested by examining the relative binding affinity of those mAbs for open conformers coated on regular beads and for intact HLA coated on iBeads, and by comparing the effects on the suppression of phytohaemagglutinin (PHA)‐activated T cells of three entities: IVIg, anti‐HLA‐E mAbs that mimic IVIg [Terasaki Foundation Laboratory (TFL)‐006 and (TFL)‐007]; and anti‐HLA‐E antibodies that do not mimic IVIg (TFL‐033 and TFL‐037). Suppression of blastogenesis and proliferation of those T cells by both IVIg and the anti‐HLA‐E mAbs was dose‐dependent, the dose required with mAbs 50–150‐fold lower than with IVIg. TFL‐006 and TFL‐007 significantly suppressed blastogenesis and proliferation of activated CD4⁺ T cells, but neither the non‐IVIg‐mimicking mAbs nor control antibodies did so. The suppression may be mediated by Fab‐binding of TFL‐006/TFL‐007 to the exposed shared peptides. The mAb binding to the open conformer may signal T cell deactivation because the open conformers have an elongated cytoplasmic tail with phosphorylation sites (tryosine³²⁰/serine³³⁵).