The human "Treg MLR": immune monitoring for FOXP3+ T regulatory cell generation

Controversy exists about the conditions effecting the development of forkhead/winghead helix transcription factor P3 (FOXP3) expressing T cells and their relevance in transplant recipients.

We generated carboxy-fluorescein diacetate succinimidyl ester-labeled CD4+CD25 high FOXP3+ cells in mixed lymphocyte reactions (MLRs) ("the Treg MLR"), with varying human leukocyte antigen (HLA) disparities and cell components. Five color flow cytometry and H-thymidine uptakes were the readouts.

(1) Despite lower stimulation indices (SIs) than two DR-mismatched MLRs, 2 DR-matched MLRs generated more than twofold higher percentages when gating on proliferating CD4+CD25 high FOXP3+ cells; (2) Even with low numbers of proliferating cells, autologous and HLA identical MLRs generated the highest FOXP3+:FOXP3- cell ratios; (3) Elimination of either non-CD3+ responding cells (resulting in "direct presentation" only) or responding CD25+ (Treg generating) cells increased the SI but inhibited proliferating CD4+CD25 high FOXP3+ cell development; (4) MLR-generated CD4+CD25 high FOXP3+ cells added as third components specifically inhibited the same freshly set MLR SI and caused recruitment of new CD4+CD25 high FOXP3+ cells. As an example of the "Treg MLR" immune monitoring potential, addition of third component peripheral blood mononuclear cell containing high percentages of CD4+CD25 high FOXP3+ cells from an HLA identical kidney transplant recipient (in a tolerance protocol) caused donor-specific Treg MLR inhibition or recruitment. This was similar to the third component MLR Tregs generated entirely in vitro.

In the Treg MLR, the generation of CD4+CD25 high FOXP3+ cells is more pronounced in the context of self-recognition (HLA matching, indirect presentation). These cells can be assayed for MLR inhibitory and Treg recruitment functions, so as to immunologically monitor the allospecific regulation after transplantation.