Thymic functions and gene expression profile distinct double-negative cells from single positive cells in the autoimmune lymphoproliferative syndrome

Objective: Autoimmune lymphoproliferative syndrome is characterized by autoimmunity, enlarged lymph nodes and spleen as well as the presence of a rare T-cell population expressing TCRαβ ⁺CD3 ⁺CD4 ^-CD8 ^-, known as double-negative (DN) cells, in the peripheral blood. The origin and function of the double-negative cells are incompletely understood. Here we studied the origin of DN T cells found in patients with autoimmune lymphoproliferative syndrome. Methods: T-cell receptor diversity, T-cell receptor excision circles and ordered V(D)J DNA gene rearrangement levels were determined in DN as well as CD4 ⁺CD8 ^- and CD4 ^-CD8 ⁺ (single-positive, SP) cells isolated from peripheral blood of two patients with typical autoimmune lymphoproliferative syndrome. In addition, expression of genes associated with autoimmune lymphoproliferative syndrome was determined in these cells by the TaqMan Low-Density Array. Results: We found that T cell receptor diversity in patients with autoimmune lymphoproliferative syndrome was similar to that of normal controls. In contrast, DN cells of patients contained significantly less T-cell receptor excision circles than SP cells. Similarly, certain DNA rearrangements were markedly reduced in DN cells. The transcriptional profile of the patients' DN cells revealed significant changes in 37.7% of the tested genes, some closely related to the pathophysiology associated with the syndrome. Their gene expression signature was unlike that of both SP cells. Conclusion: We hypothesize that the double-negative T cells in autoimmune lymphoproliferative syndrome display features that differ, in both thymic function and gene expression profile, from SP T cells. These findings may explain some of the patients' chronic lymphoproliferation and breakdown of self-tolerance checkpoints.