The growing complexity of TGF-β as a T-cell regulator

When Mosmann et al. first classified mouse CD4 T-cell clones according to the cytokines that they produced¹ and showed that these subsets possessed different effector functions² a new era in our understanding of immunology had begun. Since then, substantial effort has gone into understanding how T-cell subsets function in health and disease, the factors that govern their differentiation and more recently the characterization and function of regulatory T cells that control immune responses. Transforming growth factor-beta (TGF-β) is a pleiotropic cytokine that regulates diverse biological processes during development and homeostasis. In this issue of Immunology Smeltz et al. describe an unexpected effect of this cytokine on CD4 T-cell differentiation and IFN-γ synthesis.³

Extensive studies on T-cell subsets revealed that T helper 1 (Th1) and T helper 2 (Th2) cells were mutually exclusive and that IFN-γ, the signature cytokine of Th1 cells, inhibited the growth of Th2 cells⁴ while IL-4, the cytokine that defines Th2 cells, was inhibitory for Th1 cells⁵. Furthermore, both of these subsets produced cytokines that served as autocrine growth factors—IL-2 for Th1 and IL-4 for Th2 cells^6,7. Historically IFN-γ was the first cytokine to be identified that promoted Th1 responses⁴. IFN-γ selectively induced naïve CD45RA, but not memory CD45RO, T cells to secrete IFN-γ⁸ and express the Th1-specific transcription factor T-bet⁹. IL-4 appeared to be essential for Th2 development⁷.

A further level of control was introduced by cytokines produced by antigen presenting cells (APC). APC derived IL-12^10,11 and IL-18¹² work in synergy to promote Th1 cell differentiation. Interestingly, comparable Th2 promoting cytokines from APC have not been described but APC that are low IL-12 producers, as in the lung, appear to preferentially induce Th2 cells¹³. The IL-4 that is necessary for Th2 development may be derived from a subset of natural killer (NK) cells¹⁴ or from the CD4 T cells themselves¹⁵. In addition to extracellular cytokines, the nature of the T-cell receptor stimulus and the presence, or absence, of a costimulus also regulate T-cell differentiation: high affinity T-cell receptor–MHC peptide interactions leading to a Th1 response while low affinity lead to a Th2 cytokine profile^16–19.

To control aberrant immune responses there are, in addition, regulatory T cells. These include T regulatory 1 (Tr1) cells that make IL-10 which inhibits both Th1²⁰ and Th2²¹ responses and T helper 3 cells (Th3) that promote IgA responses in the gastrointestinal tract via TGF-β²² and also inhibit Th1 and Th2 responses^23,24. TGF-β has also been shown to inhibit GATA-3 expression and Th2 development⁹ as well as airway Th2 cell induced inflammation²¹. It has also been shown that CD4-dominant negative TGF-βRII mice are unable to mount effective Th1 responses to pathogens such as Leishmania²⁵ and suffer impaired immunity to tumors²⁶.

In this issue of the journal, Smeltz et al. demonstrate that, under certain circumstances, TGF-β can actually promote Th1 cell differentiation.³ This runs counter to the prevailing vision of TGF-β as a Th1 as well as a Th2 inhibitory cytokine²⁵. Smeltz et al. suggest that TGF-β mediates this effect by inhibiting IL-4, which is inhibitory for Th1 responses. Indeed, Lingnau et al. have shown that a combination of TGF-β and IL-4 induced IFN-γ production in human CD4 T cells stimulated with antiαβ T-cell receptor (TcR) specific antibody²⁷. Consistent with other reports⁹, Smeltz et al. also report that TGF-β inhibited GATA-3 gene expression and IL-4 synthesis, but also found that TGF-β augmented the generation of IFN-γ producing cells.³ They showed and that this process was independent of IL-12 and signal transducer and activator of transcription-4 (STAT-4) signaling but was dependent on endogenous IFN-γ, itself an important inducer of T-bet²⁸.

One explanation for the different effects of TGF-β on Th1 cell differentiation observed by these investigators may be the way in which the Th1 cells were generated. Smeltz et al. stimulated transgenic D011.10 CD4 T cells with specific ovalbumin peptide with or without cytokines and re-stimulated after one week with anti-CD3.³ Other investigators have used anti-CD3/CD28 to mimic T-cell activation²⁵. It is also known that TGF-β signaling is mediated via the intracellular signaling molecules SMAD 2, 3 and 4²⁹. Hence an alternative explanation for the different effects of TGF-β observed could be based on the cell-specific phosphorylation of SMADs or in the stoichiometry of the SMAD signaling complex. Whatever the explanation, these findings serve to remind us that T cells do not develop in isolation but receive a complex array of signals whose balance determines their differentiation into effector cells.