NFAT Reporter Jurkat Cells

The current paradigm is that full activation of T cells requires at least two signals upon contact with APCs [1, 2].  Signal 1 is delivered through the interaction of the TCR and a specific antigenic peptide associated with an MHC (major histocompatibility complex) molecule on APCs. Signal 2 is delivered through the interaction of CD28, the prototypical T cell co-stimulatory molecule, and its ligands, CD80 or CD86, expressed by the APC. 

Signal 1: TCR and [HLA::peptide]

The 'classical' and most represented TCR is an 80 to 90 kDa heterodimer composed of one α chain and one β chain. The αβTCR is a transmembrane protein expressed by developing and mature T cells. It features an extracellular ligand-binding pocket and a short cytoplasmic tail. Each αβTCR is restricted to a specific complex made of an antigenic peptide and a class I or class II MHC molecule. Human MHC molecules are also known as HLA (human leukocyte antigen). Because of its short cytoplasmic tail, the TCR, once engaged,  lacks the ability to signal and requires non-covalent association with the CD3 to trigger downstream intracellular signaling and T cell activation [1, 2]. Importantly, signal 1 without co-stimulation results in T cell unresponsiveness or 'anergy', a tolerance mechanism that guards against premature activation.

Signal 2: CD28 and CD80/86

CD28 is a homodimeric and transmembrane protein expressed by T cells. Nearly all human CD4+ T cells and 50% of human CD8+ T cells express CD28. The CD28 interaction with CD80 (aka B7-1) or CD86 (aka B7-2) on APCs, in conjunction with TCR engagement, triggers a co-stimulation signal (signal 2). It results in T cell proliferation, cytokine production, cell survival and cellular metabolism [1, 2].

NFAT activation:

Most NFAT proteins are controlled by calcium influx upon TCR stimulation. Calcium binds calmodulin, which in turn activates calcineurin, a calmodulin-dependent phosphatase. Calcineurin dephosphorylates NFAT proteins, leading to their translocation into the nucleus, where they regulate the expression of many genes, either alone or in cooperation with other transcription factors [3, 4]. The co-engagement of CD28 triggers the activation of the AKT kinase, which contributes to enhancing NFAT translocation into the nucleus [4].

References:

1. Budd R.C. & Fortner K.A., 2017. Chapter 12 - T Lymphocytes. Kelley and Firestein's Textbook of Rheumatology (Tenth Edition). pages 189-206.
2. Smith-Garvin J.E. et al., 2009. T Cell Activation. Ann. Rev. Immunol. 27:591-619.
3. Lee J-U., et al., 2018. Revisiting the Concept of Targeting NFAT to Control T Cell Immunity and Autoimmune Diseases. Front Immunol. DOI: 10.3389/fimmu.2018.02747.
4. Macian F., 2005. NFAT proteins: key regulators of T-cell development and function. Nat Rev Immunol. 5(6):472-484.