HLA monomers as a tool to monitor indirect allorecognition.

Recognition of donor antigens can occur through two separate pathways: the direct pathway (non-self HLA on donor cells) and the indirect pathway (self-restricted presentation of donor derived peptides on recipient cells). Indirect allorecognition is important in the development of humoral rejection; therefore, there is an increasing interest in the monitoring of indirect alloreactive T-cells. We have used an in vitro model to determine the optimal requirements for indirect presentation and assessed the risk for semidirect presentation in this system. HLA-typed monocyte-derived dendritic cells (moDCs) were incubated with cellular fragments or necrotic cells and incubated with either indirect or direct alloreactive T-cell clones. T-cell reactivity was measured through proliferation or cytokine secretion. HLA-typed moDC, monocytes, or PBMCs were incubated with HLA class I monomers, in combination with either direct/indirect T-cell clones. Although both were efficiently taken up, alloreactivity was limited to the semi-direct pathway, as measured by allospecific CD4 (indirect) and CD8 T-cell clones (direct) when cells were used. In contrast, HLA-A2 monomers were not only efficiently taken up but also processed and presented by HLA-typed moDC, monocytes, and PBMCs. Activation was shown by a dose-dependent induction of IFN-γ production and proliferation by the CD4 T-cell clone. Antigen presentation was most efficient when the monomers were cultured for longer periods (24-48 hr) in the presence of the T-cells. Using this method, no reactivity was observed by the CD8 T-cell clone, confirming no semidirect alloreactivity. We have developed a system that could be used to monitor indirect alloreactive T-cells.