Role of TRAFs in Signaling Pathways Controlling T Follicular Helper Cell Differentiation and T Cell-Dependent Antibody Responses

Abstract

Follicular helper T (T_FH) cells represent a highly specialized CD4⁺ T cell subpopulation that supports the generation of germinal centers (GC) and provides B cells with critical signals promoting antibody class switching, generation of high affinity antibodies, and memory formation. T_FH cells are characterized by the expression of the chemokine receptor CXCR5, the transcription factor Bcl-6, costimulatory molecules ICOS, and PD-1, and the production of cytokine IL-21. The acquisition of a T_FH phenotype is a complex and multistep process that involves signals received through engagement of the TCR along with a multitude of costimulatory molecules and cytokines receptors. Members of the Tumor necrosis factor Receptor Associated Factors (TRAF) represent one of the major classes of signaling mediators involved in the differentiation and functions of T_FH cells. TRAF molecules are the canonical adaptor molecules that physically interact with members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF) and actively modulate their downstream signaling cascades through their adaptor function and/or E3 ubiquitin ligase activity. OX-40, GITR, and 4-1BB are the TRAF-dependent TNFRSF members that have been implicated in the differentiation and functions of T_FH cells. On the other hand, emerging data demonstrate that TRAF proteins also participate in signaling from the TCR and CD28, which deliver critical signals leading to the differentiation of T_FH cells. More intriguingly, we recently showed that the cytoplasmic tail of ICOS contains a conserved TANK-binding kinase 1 (TBK1)-binding motif that is shared with TBK1-binding TRAF proteins. The presence of this TRAF-mimicking signaling module downstream of ICOS is required to mediate the maturation step during T_FH differentiation. In addition, JAK-STAT pathways emanating from IL-2, IL-6, IL-21, and IL-27 cytokine receptors affect T_FH development, and crosstalk between TRAF-mediated pathways and the JAK-STAT pathways can contribute to generate integrated signals required to drive and sustain T_FH differentiation. In this review, we will introduce the molecular interactions and the major signaling pathways controlling the differentiation of T_FH cells. In each case, we will highlight the contributions of TRAF proteins to these signaling pathways. Finally, we will discuss the role of individual TRAF proteins in the regulation of T cell-dependent humoral responses.

Keywords: NF-κB; TCR signaling; TRAF; antibody response; costimulation signaling; cytokine signaling; follicular helper T cell.

Figure 1

Role of TRAFs in TCR-, costimulatory receptor-, and cytokine receptor-associated signaling pathways controlling T_FH differentiation. T_FH differentiation depends on a variety of signals received through the TCR, costimulatory receptors of the Ig superfamily (yellow boxes), costimulatory proteins of the TNF receptor superfamily (orange boxes), and cytokine receptors (blue boxes). TRAF1, −2, −3, and −6 regulate TCR signals but TRAF3 activation depends on additional CD28 signaling. TRAF2 contributes to activation of the canonical NF-κB pathway that is critical for T_FH differentiation. CD28 and ICOS plays a key role in T_FH differentiation and can both activate PI3K. PI3K activation by ICOS is critical for T_FH differentiation as compared to CD28-induced PI3K signals. Recently, we revealed that the TANK-binding kinase TBK1 functions downstream of ICOS to promote T_FH differentiation. TRAFs are not recruited to ICOS but the presence of a TRAF-like motif in the intracellular tail of ICOS bypasses the need for TRAFs to recruit TBK1. The TNFR family members OX40, GITR, and 4-1BB signal through TRAFs and also contribute to T_FH differentiation. TRAF2 is involved in recruitment and activation of PI3K by OX40. TRAF2 and−5 promote NF-κB pathway activation downstream OX40 and GITR while TRAF1 and−2 associate with 4-1BB to promote this pathway. TRAF3 plays a regulatory role in OX40 signaling. Signals through type 1 IFN, IL-6, and IL-21 receptors converge through STAT1 activation, positively regulating T_FH differentiation. Signaling through IL-6, IL-21, and IL-27 receptors activates STAT3 to promote T_FH differentiation. TRAF2 and−5 can both inhibit IL-6 mediated activation of STAT3. STAT5, a negative regulator of T_FH differentiation, can be activated by signals through the IL-2 and IL-7 receptors. TRAF3 and−6 both negatively regulate IL-2R-induced signaling. Finally, TRAF6 is involved in the suppression of IL-2 production in the presence of TGF-β and, thus, could indirectly promote early T_FH differentiation by limiting signals received through the IL-2R.

Figure 2

Role of TRAFs in the canonical and non-canonical NF-κB signaling pathways. NF-κB can be activated via the canonical or non-canonical signaling pathways. The canonical pathway is controlled by TAK1 kinase activation, which activates the IKK complex and leads to ubiquitylation and proteasomal degradation of IκB family members, resulting in the release and nuclear translocation of NF-kB1/p50–RelA/p65 and NF-κB1/p50–c-Rel dimers. RelA/p65 is dispensable for T_FH differentiation but c-Rel regulates the expression of IL-21 and CD40L and is required for T_FH differentiation. TRAF2 and−6 favor IKK complex activation by TAK1 and TRAF1 and−5 are required for optimal IκB degradation. The activation of the non-canonical NF-κB pathway depends on the NF-κB-inducing kinase NIK. NIK can phosphorylate and activate IKKα, which in turn promotes p100 processing to generate NF-κB2/p52 and allows nuclear translocation of NF-κB2/p52 and RelB. In the absence of activating signals, constitutive ubiquitylation and degradation of NIK ensures repression of the non-canonical NF-κB pathway. NIK deficiency in T cells does not impact T_FH differentiation. TRAFs regulate the non-canonical NF-κB pathway by modulating NIK expression levels: TRAF6 is involved in transcriptional regulation of Nik whereas TRAF2 and−3 contribute to its degradation. TRAF1 restrains the non-canonical NF-κB pathway activation by inhibiting p100 processing.