CD28 Signaling in T Regulatory Precursors Requires p56lck and Rafts Integrity to Stabilize the Foxp3 Message

Abstract

Naturally occurring CD4+25highFoxp3+ T regulatory (T-reg) cells are critical for maintaining tolerance to self and non-self Ags. The Foxp3 master-regulatory gene and CD28 costimulation are both required for thymic development and suppressogenic function of CD4+25highFoxp3+ T-regs. Herein, we show that the sole CD28 stimulation of T-reg thymic precursors augments Foxp3 expression through the increase in Foxp3 mRNA span life by a mechanism involving p56lck and its binding motif on CD28 cytosolic tail, as well as the lipid rafts. We found that 1) the glycosphingolipids and cholesterol components of lipid rafts were highly expressed and unusually partitioned in T-reg thymic precursors as compared with the conventional T cell precursors, 2) the CD28 receptor density on cell membrane is proportional with the content of cholesterol in lipid rafts and with the level of Foxp3 mRNA expression in T-reg precursors, and 3) the CD28-mediated increase of Foxp3 mRNA span life was paralleled by an increased proliferative and suppressogenic capacity of terminally differentiated CD4+25highFoxp3+ T-reg precursors. Thus, the functional integrity of CD28 receptor p56lck and plasma membrane lipid rafts are all prerequisites for up-regulation and long-term expression of Foxp3 mRNA transcripts in CD4+25highFoxp3+ T-reg precursors.

Limited cholesterol depletion causes aggregation of plasma membrane lipid rafts inducing T cell activation

“…In several studies it was concluded that lipid rafts are involved in T cell signalling because acute cholesterol depletion abolished T cell signalling [10,11,40]. We found that these studies employed cholesterol depletion protocols where the bulk of the cells die, which means that the involvement of lipid rafts in the process cannot be assessed.…”

Novel inflammatory mechanisms of accelerated atherosclerosis in kidney disease

“…88 Part of the dysfunction of regulatory T cells may also be related to low cholesterol levels (which is known to associate paradoxically with higher cardiovascular mortality in dialysis patientsreverse epidemiology) as cholesterol components of lipid rafts are highly expressed in T-regulatory cells and are essential for their function. 89 Uremia also induces proinflammatory changes in the adipose tissue, 90 and adipose tissue macrophages might have an important role in atherogenesis. 91 Collectively, increased proinflammatory monocytes, mast-cell proliferation, T-lymphocyte dysfunction, and decreased T-regulatory cells might result in an immune dysfunction that facilitates accelerated atherogenesis in uremia.…”

Cholesterol and benign prostate disease

“…This membrane partitioning promotes interactions between potential protein binding partners by segregating protein subunits with—and away from—interacting proteins that process discrete classes of signals. The lipid raft model of membrane organization posits that cholesterol-rich microdomains channel extracellular stimuli down discrete biochemical pathways to the nucleus 15 .…”