Differential FOXO1 Localization in SLE and Healthy Human Lymphocyte Subsets

Abstract

Systemic lupus erythematosus (SLE) is an inflammatory autoimmune disease characterized by elevated levels of circulating autoantibodies and multi-organ damage. Although SLE is a highly heterogeneous disease, one factor unifies it: lymphocyte hyperactivity driving immunopathogenesis. This involves CD4 helper T cells potentiating autoreactive B cells to produce pathogenic autoantibodies. In healthy individuals, lymphocyte activation is a closely regulated kinetic process controlled by key transcription factors (TF) signaling downstream of the T cell (TCR) and B cell receptor (BCR). Forkhead box O1 (FOXO1) is one such TF that integrates activation and differentiation signals in human lymphocytes. When active, it remains in the nucleus, but upon Akt phosphorylation downstream of TCR or BCR signaling, FOXO1 is inactivated and shuttles to the cytoplasm, linking FOXO1 localization to function. In SLE, both T and B cells are hyperactive, and respond more quickly and strongly to antigen, producing a disproportionate inflammatory response. Thus, we hypothesize that SLE lymphocytes will have altered FOXO1 localization, reflecting altered lymphocyte activation. To address this hypothesis, we first developed a method of examining dynamic native FOXO1 localization in human peripheral lymphocyte subsets using imaging flow cytometry (IFC). IFC combines the quantitative power of flow cytometry with the qualitative images of microscopy and can be performed with many fewer cells than are needed for the more traditional methods. We demonstrated that we can visualize native FOXO1 and detect significant kinetic differences in localization within user-defined subsets of HuT102 cells, a human CD4 T cell line with baseline nuclear FOXO1, as well as primary peripheral human T and B cells. We then used IFC to compare FOXO1 localization in SLE and healthy donor lymphocytes. Interestingly, we found that most T and B cell subsets have nuclear FOXO1 localization in both health and SLE. However, FOXO1 is significantly more cytoplasmic in SLE double negative (DN) atypical memory B cells. Based on our findings, we propose a model by which these DN B cells are highly active in disease flares and may serve as a death-resistant reservoir of autoreactive cells. Future experiments will be aimed at elucidating at how these cells persist in the periphery.