The Control of HLA Expression

Human Leucocyte antigens (HLA) present peptides to T cells, enabling recognition of pathogenic proteins and initiation of the host immune response. Classical Class I HLA are found on the surface of all nucleated cells and platelets, whereas Class II HLA expression is normally restricted to dedicated antigen presenting cells. Cell surface HLA levels are under tight control to reduce the risk of autoimmunity but allow for a rapid response against pathogens, which leads to an increase in Class I HLA and the induction of Class II expression on some cells. In addition, HLA coded on different HLA loci vary significantly in their normal expression with HLA-C antigens, for example, expressed at about one-fifteenth the level of HLA-A or HLA-B.

To enable a suitable response against pathogens the amino acid structure of the HLA proteins themselves are highly variable between individuals and mutation hotspots make HLA genes the most polymorphic in the human genome. Whilst this provides an effective population response against environmental pathogens, the structural differences between HLA proteins can themselves invoke an unregulated immune reaction in the transplant setting, where host T cells are activated by contact with the foreign HLA molecule and the presentation of a novel set of peptides. This leads to generation of cell mediated rejection and/or HLA specific antibody.

Techniques are available to reduce the risk of rejection including better HLA matching between donor and recipient, improved immunosuppression, and the measurement of circulating donor specific antibody (DSA). But whilst there has been significant investigation into the host response to transplantation, it is more difficult to assess the level of antigenic stimulus provided by donor HLA proteins. Antigens from DNA coded on different HLA loci vary in average expression (for example HLA-A antigens are about ten times more abundant than HLA-C on the cell surface, but less is understood about the variation between allotypes and the magnitude of HLA response following cell stimulation.

The experiments described in this thesis use human umbilical vein endothelial cells as a model of endothelium. High concentrations of cytokines were used to investigate the difference between HLA antigen levels when cells are unstimulated and simulated. HLA antigens increase up to forty times baseline levels with an initial slow increase in HLA expression for approximately twelve hours followed by a more rapid increase for the next twenty four (HLA-C antigens) to thirty-six hours (HLA-A). In addition monoclonal antibody DT-9 binds all HLA-C antigens and we demonstrate that the variation in HLA-C expression level is allotype specific when cells are both resting (P = .004) and activated (P < .001).

Knowledge of the resting expression of donor HLA antigens and the level of response to stimulation may help our prediction of rejection risk in solid organ transplantation, particularly following events such as cold ischaemia, infection and even chronic rejection which will lead to endothelial cell activation and an increase in HLA expression.

Outputs

Carey BS, Poulton KV, Poles A. Factors affecting HLA expression: A review. Int J Immunogenet. 2019;46(5):307-320. doi:10.1111/iji.12443

Carey BS, Poulton KV, Poles A. HLA expression levels of unstimulated and cytokine stimulated human umbilical vein endothelial cells. HLA. 2020;(January):1-11. doi:10.1111/tan.13808

Carey BS, Poulton K V., Poles A. HLA-C expression level in both unstimulated and stimulated human umbilical vein endothelial cells is defined by allotype. Hla. 2020;(February):1-11. doi:10.1111/tan.13852