CD28

CD28 binds to CD80, CD86 meaning fulminant T cell activation
Lines et al. 2014 Cancer Immunol Res 2(6): 510

CD28-CD86 interaction being the weakest
Walker & Sansom, 2011

[CD28] is expressed on T cell subsets and activated B cells as receptor for the co-stimulatory signal 2 (binding CD80 and CD86) to activate naive T cells
Murphy 2012 Garland Science ISBN 978-0-8153-4243-4

One dimer of CD28 engages just one dimer of CD80 and CD86, which is not a dimer
Murphy 2012 Garland Science ISBN 978-0-8153-4243-4

B7 binding (CD80 or CD86) induces CD28 phosphorylation, activating PI3kinase to produce Phosphatidylinositol-3,4,5-Trisphosphate (PIP3)
Murphy 2012 Garland Science ISBN 978-0-8153-4243-4

Activation of PIP3 activates (phosphorylates) PLC-gamma, resulting in the activation of multiple signaling pathways, all leading to increased production of IL-2
Murphy 2012 Garland Science ISBN 978-0-8153-4243-4

Activation of T cell subsets via CD28 results in increased synthesis of the IL-2 receptor alfa chain, making it a high-affinity receptor for IL-2
Murphy 2012 Garland Science ISBN 978-0-8153-4243-4

CD28 costimulation is the crucial step that drives increased glycolysis in T cells. This metabolic shift is essential to meet the energy demands of immune cell activation.
Frauwirth et al. 2002 Immunity 16: 769-777

Biggest difference between CD4+ and CD8+ Tregs is the expression of CD28: CD4+ high, CD8+ no

see also:
Immune cell metabolism / Immunometabolism