The distinctive differentiated states of the various CD4 effector/regulatory subpopulations are determined largely by the set of transcription factors they express and the genes they transcribe. The induction of the distinctive patterns of gene expression may be achievable in several ways, but in vitro the major determinants of the differentiated state of the cell are the set of cytokines present during the T cell receptor (TCR)-mediated activation process. Our understanding of this process has evolved over an extended period and is described in detail below.
As discussed above, it was first demonstrated that naive CD4 T cells could differentiate into IL-4-producing CD4 T cells if the cytokines IL-4 and IL-2 were present at the time of stimulation by cognate antigen (3
). That one of the key inducing cytokines is also a major product was a striking finding; this has proven not to be unique for Th2 differentiation. For Th1 differentiation, it was first shown that IL-12 (7
) played a central role and only somewhat later was it appreciated that IFN-γ also played an important role in the induction of Th1 cells (39
), of which IFN-γ is a signature cytokine. Indeed, in vitro neutralization of IFN-γ will often markedly diminish Th1 development.
Understanding of Th17 differentiation went through a complex evolution, beginning with the recognition of the existence of an IL-12 congener (IL-23) that shared one chain with IL-12 (p40) but expressed a unique chain (p19), distinct from IL-12 p35 (40
). This led to the recognition that, in much research that had relied on deleting p40 to block Th1 differentiation, the development/maintenance of both Th1 and Th17 cells were blocked and that IL-23 played an important role in the development and/or maintenance of Th17 cells. However, it was soon appreciated that IL-23 did not act on naive CD4 T cells, but rather was more important later in the Th17 priming process or in the maintenance of the Th17 phenotype.
Further analysis revealed that in vitro Th17 differentiation was most efficient when TGF-β and IL-6 were available (21
) but that IL-21 could mediate many of the functions of IL-6 (22
). IL-6, IL-21, and IL-23 can be regarded, at least at one level, as congeners since each mediates its function through the activation of STAT3. The relative efficacy of the three cytokines may be determined, at least in part, by the number of specific receptors that exist at any one time. For example, IL-23 receptors appear not to be expressed until after the naive cell has partially completed its differentiation to becoming a Th17 cell, and consequently IL-23 plays little part in the initial determination of Th17 differentiation (21
). In accord with the importance of products of the differentiated cells playing a role in differentiation, Th17 cells produce IL-21, and IL-21 can certainly propagate the Th17 differentiation process, even if it is less effective than IL-6 in initiating differentiation.
The induction of iTregs from naive CD4 T cells relies on T cell activation in the presence of TGF-β and IL-2. Since Tregs are good TGF-β producers, the principle that a major product of the differentiated cell plays a major role in induction is also applicable to iTregs.
Reliable means of developing Tfh cells in vitro are still being uncovered. It has been proposed that the inclusion of IL-6 or IL-21 together with a TCR-mediated stimulation will induce these cells (44
), but this idea is still controversial. Some of these difficulties may stem from an uncertainty as to the proper starting cells for such differentiation—be they naive CD4 T cells or already differentiated Th cells— and the possibility that environmental factors from the germinal center or provided by B cells may play an important role in such differentiation (33
Cytokines may also play a role in effector cytokine production by differentiated Th1, Th2, and Th17 cells (). At later stages of Th cell differentiation, a distinct member of the family of IL-1 receptors is selectively upregulated in each lineage. Together with a STAT5 inducer including IL-2, IL-7, or TSLP (thymic stromal lymphopoietin), the IL-1 analog IL-33 causes TCR-independent IL-13 production in a cyclosporine A–independent manner in Th2 cells (46
), suggesting an innate-like effector function of Th cells. TCR-independent cytokine production can also be induced in Th1 and Th17 cells by IL-12/IL-18 and IL-23/IL-1, respectively (46
Figure 1 Cytokines play critical roles in differentiation and effector functions of Th1, Th2, and Th17 cells. Upon TCR activation triggered by antigen-presenting cells, naive CD4 T cells differentiate into distinct Th lineages in the context of combinations of (more ...)