Transforming Growth Factor-β: An Indirect Inducer of Th17 Cell Differentiation both in Human and Murine System

Since their first description only some years ago Th17 cells have become of vital 
importance in immunological research. However, the role of TGF-β in Th17 cell development is still a 
matter of controversial discussion. Predicted to have a key function in Th17 cell differentiation in the 
mouse TGF-β was shown to inhibit IL-17 production of human CD4+ T cells. Moreover, recent data 
indicate TGF-β signaling in T cells to be dispensable for Th17 cell differentiation in the murine 
system. Hence, rather being a specie-specific factor TGF-β is likely to act as an indirect inducer of 
Th17 cell differentiation both in mice and men.

In a number of inquiries using murine models of host defense Th17 cells have been described to be important in the pathogenesis of chronic inflammatory diseases including arthritis (Lubberts et al., 2005;Latham et al., 2005), colitis (Zhang et al., 2006;Ogawa et al., 2004), encephalitis (Komiyama et al., 2006;Hofstetter et al., 2005) as well as asthma and allergy (Hellings et al., 2003). In humans IL-17 producing T cells have been isolated from synovial membranes and synovial fluid of patients suffering from rheumatoid arthritis (Chabaud et al., 1999). At this, severity of inflammation is reported to be consistent to the IL-17 concentration detectable in serum and tissue of the arthritis patients. Furthermore, a correlation between IL-17 serum levels and the degree of inflammatory processes has been described in the context of autoimmune encephalomyelitis (Lock and Heller, 2003) and inflammatory bowel disease (Fujino et al., 2003). Thus, IL-17 especially if produced by T cells is a likely candidate for intervention in chronic inflammatory diseases in men.

Th17 cell differentiation: Proceeding specie-specific?
In the murine system the synergistic action of TGF-β and IL-6 has been considered to be essential for Th17 cell development, so far (Veldhoen et al., 2006;Mangan et al., 2006;Bettelli et al., 2006). In contrast, differentiation of human naïve T helper cells into Th17 cells has been reported not to depend on TGF-β (Chen et al., 2007;Wilson et al., 2007;Acosta-Rodriguez et al., 2007;Manel et al., 2008;Santarlasci et al., 2009;Cosmi et al., 2008). Moreover, TGF-β has been predicted to act suppressive on IL-17 production of human CD4 + T cells (Acosta-Rodriguez et al., 2007;Manel et al., 2008). Due to the divergent observations made in murine and human systems Th17 cells development in mice and men has been assumed to be regulated differentially (Annunziato et al., 2009;Annunziato and Romagnani, 2009). However, recent data provide evidence that TGF-β is not directly required for the molecular orchestration of Th17 cell differentiation in the mouse as well (Das et al., 2009;Schumann, 2008). Thus, the situation seen in the murine system is closer related to the human situation as previously thought.
What role for TGF-β in Th17 differentiation? TGFβ initially has been published as a growth factor (De Larco and Todaro, 1978). Thereinafter it was found, that TGF-β not only influences differentiation of cells but is also crucial in inflammatory processes (Kehrl et al., 1986a;1986b). TGF-β regulates the innate and the adaptive immunity affecting macrophages (Ashcroft, 1999), dendritic cells (Strobl and Knapp, 1999), T cells (Cerwenka and Swain, 1999), B cells (Lebman and Edmiston, 1999) and NK cells (Horwitz et al., 1999). The cytokine acts both pro-and antiinflammatory depending on genetic background, cell type, state of differentiation, extracellular matrix as well as the presence of other regulatory factors (McCartney-Francis and Wahl, 1994;Wahl, 1994). Hence, TGF-β shows a distinctive pleiotropy playing a complex role in immunmodulation.
At present time the role of TGF-β in T helper cell differentiation has become of special interest. Depending on cytokine milieu naïve T helper cells differentiate into four known T helper cell subsets. TGF-β interacts in this process by several ways. Both Th1 (Gorelik et al., 2002;Lin et al., 2005) and Th2 cells (Chen et al., 2003a;Gorelik et al., 2000;Heath et al., 2000) are inhibited by TGF-β. Concomitantly, TGF-β represents the direct inducer of Treg cells, which themselves are considerable producers of the cytokine (Chen et al., 2003b).
Beside its clear role in Treg cell development conflicting data have been published concerning the relevance of TGF-β in Th17 cell generation in mice and men. In murine system cytokines critically for Th17 cell differentiation were propagated to be TGF-β and IL-6 (Veldhoen et al., 2006;Mangan et al., 2006;Bettelli et al., 2006). However, TGF-β has not been shown to directly promote the generation of IL-17 producing T cells from naïve precursors. Rather, a beneficial effect of TGF-β was seen, when naive CD4 + T cells were co-cultivated with Treg cells plus dendritic cells and stimulated with anti-CD3-antibody as well as Lipopolysaccharid (LPS) (Veldhoen et al., 2006). LPS is known as strong inducer of Th1 reactions via a release of IL-12 by dendritic cells (Pearce et al., 2006). The effect of TGF-β in such a setting might therefore rather be based on the prevention of a Th1 cell development. Consistently to this assumption, using a mouse model unable to generate Th1 and Th2 cells, the cytokine IL-6 has been reported to be sufficient to drive Th17 cell generation (Das et al., 2009). In addition, TGF-β was shown not to affect the expression of the retinoic acid receptor-Related Orphan Receptor γt (ROR γt), a Th17 cell specific transcription factor (Das et al., 2009). Beyond that, Th17 cells could be detected in a transgene mouse strain T cell specific insensitive for TGF-β signaling (Schumann, 2008). Visualization of Th17 cells succeeded both ex vivo after re-stimulation of lymph node cells of B. burgdorferiinfected mice as well as in vitro after T cell specific stimulation of splenocytes and purified CD4 + lymphocytes (Schumann, 2008). In contrast, stimulation of T cells over-expressing TGF-β resulted in decreased Th17 cell numbers in comparison to the wild type (Schumann, 2008). Hence, there is compelling evidence that TGF-β does not act as a direct inducer of Th17 cell development in the mouse.
The dispensability of TGF-β signaling on T cells for Th17 cell development in the murine system is consistent to publications dealing with effects of TGF-β on human T helper cell differentiation. In human system TGF-β has been shown to inhibit IL-17 production by T cells (Acosta-Rodriguez et al., 2007;Manel et al., 2008). According hitherto existing literature rather than TGF-β, cytokines such as IL-1β and IL-23 drive Th17 cell development from human CD4 + precursors (Chen et al., 2007;Wilson et al., 2007;Acosta-Rodriguez et al., 2007).

TGF-β affects Th17 cell differentiation in an indirect
way: TGF-β is part of a complex regulatory system critically in immune homeostasis. The cytokine is of vital importance in T helper cell differentiation influencing all four known T helper cell subsets. In addition to interference in T helper cell development TGF-β represents the main product of Treg cells thereby mediating the immunosuppressive and anti-inflammatory function of these cells (Chen et al., 2003b).
Of note, the cytokine accomplishes its effects both directly and indirectly. TGF-β directly induces Treg cell differentiation via promotion of the transcription factor FoxP3 (Chen et al., 2003b). TGF-β directly blocks Th1 as well as Th2 cell development via inhibition of T-bet (Gorelik et al., 2002;Lin et al., 2005) and GATA-3 (Chen et al., 2003a;Gorelik et al., 2000;Heath et al., 2000) expression, respectively. By contrast, Th17 cell differentiation is affected indirectly by TGF-β. IFN-γ and IL-4, the cytokines produced by Th1 and Th2 cells, are well known inhibitors of Th17 cell development. Hence, blocking of Th1 and Th2 cells linked with the absence of IFN-γ and IL-4 provides a milieu optimal for naïve CD4 + precursors to differentiate into Th17 cells (Veldhoen et al., 2006;Mangan et al., 2006;Bettelli et al., 2006;Infante-Duarte et al., 2000). Taken together, the impact of TGF-β in T helper cell specification both in human and murine system is due to its inhibitory action on Th1 and Th2 cells in addition to its direct promotion of Treg cell amplification.

Conflict of interest:
The author discloses any financial and personal relationships with other people or organizations that could inappropriately influence her work.