A model of Th2 differentiation based on polarizing cytokine repression

Abstract

Conventional dendritic cells (cDCs) can integrate multiple stimuli from the environment and provide three separate outputs in terms of antigen presentation, costimulation, and cytokine production; this guides the activation, expansion, and differentiation of distinct functional T helper subsets. Accordingly, the current dogma posits that T helper cell specification requires these three signals in sequence. Data show that T helper 2 (Th2) cell differentiation requires antigen presentation and costimulation from cDCs but does not require polarizing cytokines. In this opinion article, we propose that the 'third signal' driving Th2 cell responses is, in fact, the absence of polarizing cytokines; indeed, the secretion of the latter is actively suppressed in cDCs, concomitant with acquired pro-Th2 functions.

Keywords: GATA3; IL-12; IL-2; IL-6; SOCS3; STAT5; T-bet; TGFβ; Th2 cells.

Figure 1.. cDC-mediated T cell activation and T helper cell differentiation.

cDCs provide three separate signals to naïve CD4⁺ T cells to induce their full activation and subsequent differentiation into distinct T helper (Th) cell subsets defined by differential expression of signature cytokines and lineage-specifying transcription factors. Signal-1 is equivalent to the binding of TCR to peptide MHC complex. Signal-2 is generated by the engagement of costimulatory receptors such as CD28. The combination of signal-1 and signal-2 is required to fully activate CD4⁺ T cells. Signal-3 is produced by the secretion of polarizing cytokines by cDCs that respond to homeostatic, danger, or pathogen-derived signals. Particular sets of polarizing cytokines induce the expression of the master transcription factors Foxp3, RORγT, or T-bet, guiding the differentiation of inducible Treg, Th17, and Th1 cells [59]. Although cDCs are necessary and sufficient to induce Th2 cells in mouse models [,–15], a cDC-derived cytokine that upregulates the transcription factor GATA-3 and promotes Th2 cell lineage commitment has yet to be found. Th cells that acquire a cytokine profile can then upregulate the transcription factor Bcl6 to control B cell responses [–64] or upregulate Blimp1 to fulfill effector Th functions in peripheral tissues [61,63]. cDC, conventional dendritic cell; IFNγ interferon-γ; IL, interleukin; MHC II, major histocompatibility complex class II; TCR, T-cell receptor; TGFβ, transforming growth factor; Tfh, follicular helper T cells; Tfr, T follicular regulatory cells; Treg, regulatory T cells. Figure created with Biorender.com.

Figure 2.. cDC-mediated Th2 cell priming (key figure)

(A) Ligation of TCR and CD28 receptors on naive CD4⁺ T cells induces IL-2 secretion and upregulates the expression of the alpha chain of IL-2 receptor (IL-2Rα or CD25), leading to surface expression of high-affinity IL2R [24]. IL-2-IL-2R interactions promote activation and proliferation and prepare CD4⁺ T cells to receive subsequent differentiation signals [24,25]. Prolonged IL-2 stimulation in the absence of polarizing cytokines favors Th2 cell lineage commitment by promoting the expression of the alpha chain of the IL-4 receptor (IL-4R) and IL-4, allowing for an IL-4-positive feedback loop that initiates and preserves the Th2 cell phenotype [21,26,27]. (B) Polarizing cytokines secreted by cDCs upon pathogen detection or under inflammatory or homeostatic conditions repress the Th2 cell differentiation program in mice. IL-12 suppresses Th2 cell priming by inducing T-bet and inhibiting GATA3 in CD4⁺ T cells [4,16,29,30]. IL-6 upregulates suppressor of cytokine signaling 3 (SOCS3) in CD4⁺ T cells, which inhibits IL-2 signaling and early Th2 cell commitment [21]. TGFβ signaling prevents the Th2 cell differentiation program by unknown mechanisms [20]. cDC, conventional dendritic cell; IL, interleukin; MHC II, major histocompatibility complex class II; STAT5, signal transducer and activation of transcription 5; TCR, T-cell receptor; TGFβ, transforming growth factor. Figure created with Biorender.com.

Figure 3.. Transcription factor-guided functional specialization of type-2 cDCs for Th2 cell priming.

Mouse research indicates that IL-13 activates STAT6 and stimulates the expression and functional activity of the transcription factors IRF4 and KLF4 in type-2 cDCs (cDC2s) [–15,46]; this ultimately promotes the functional specialization of cDC2s to support Th2 cell differentiation by reducing the ability of cDC2s to produce polarizing cytokines IL-12, IL-6, and IL-23 by three independent mechanisms: 1) IL-13/STAT6 signaling and IRF4 expression repress the expression of TLRs [14,15]; 2) IRF4 interacts with MyD88 and acts as a negative regulator of TLR signaling [47]; 3) KLF4 cooperates with STAT6 to sequester coactivators required for NF-kB activation downstream of TLR activation [48]. In contrast, the priming of Th2 cells by cDC2s is prevented by transcriptional regulation that enhances the production of polarizing cytokines. Induced expression of T-bet promotes sustained IL-12 production, suppressing Th2 cell priming and promoting a Th1-like profile [22,23]. Activation of CREB and RORγt represses IRF4 and KLF4 expression and promotes IL-6 and IL-23 production, thus promoting Th17-biased responses while suppressing Th2 cell priming [49,50]. AP1, activator protein 1; cDC, conventional dendritic cell; CREB, cAMP-response element binding protein; IRF4, interferon regulatory factor 4; IL, interleukin; IL-13R, IL-13 receptor; KLF4, krüppel-like factor 4; MyD88, myeloid differentiation primary response 88; NF-κB, nuclear factor-κB; STAT6, signal transducer and activation of transcription 6; TLR, toll-like receptor; TRAF6, TNF receptor associated factor 6. Figure created with Biorender.com.