Classically, helper CD4⁺ T cells were classified as Th1 or Th2. Th1 cells differentiate in the presence of IL-12 and secrete IFN-γ, TNF-α, and IL-2. Th1 cells play an important role in the cellular immunity against tumors and intracellular viral and/or bacterial infections. IL-12 is secreted by APC such as macrophages and dendritic cells (DC) . IL-12 activates STAT4 pathway through IL-12 receptor (IL-12R) signalin g and T-bet, a transcription factor which promotes specific gene expression profile including IFN-γ. This expression of IFN-γ is also one of the important factors for Th1 differentiation, because such differentiation can be inhibited by IFN-γ neutralization according to in vitro experiments. However, T-bet, a member of the T-box family, is thought to be the master regulator of the Th1 differentiation. While artificial transduction of T-bet in polarized Th2 cells converts them into Th1 cells, its absence causes disorder of Th1 differentiation in vitro and in vivo. T-bet prompts IL-12R expression and activates IFN-γ gene, which results in a positive feedback of Th1 polarization [4, 5].

Th2 cells are differentiated in the presence of IL-4 when specific antigens are presented to their TCR by APC, and these cells then start to secrete IL-4, IL-5, and IL-13. Th2 cells play an important role mainly in humoral immunity against parasites and some allergens. IL-4 activates STAT6 pathway through IL-4 receptor (IL-4R) signaling and promotes expression of GATA-3, which is a master regulator of the Th2 differentiation. In the absence of GATA-3, Th2 development is inhibited in vitro and in vivo, while transduction of GATA-3 in the polarized Th1 cells results in IL-4 secretion. GATA-3 induces IL-4 gene expression , which forms a positive feedback of Th2 polarization. IL-12 inhibits Th2 polarization, while IL-4 inhibits Th1 differentiation, which makes these subsets reciprocal [4, 5].

Th17 cells have been reported to produce L-17A, IL-17F, IL-21, IL-22, and TNF-α, and these cells play an important role in the protective immunity against the extracellular pathogens such as bacteria [6]. Th17 cells are differentiated in the presence of IL-6 and TGF-β at the antigen presentation through the expression of RORγt. While TGF-β drives Smad signaling, IL-6 activates STAT3 pathway , which promotes the expression of RORγt [2]. RORγt, a transcription factor expressed on double positive (DP) T cells in the thymus and type 3 innate lymphoid cells (ILC3) , is thought to be the master regulator of Th17 differentiation. Transduction of RORγt to CD4⁺ T cells results in significant IL-17A secretion, while depletion in Th17 cells results in decreased IL-17A production. In addition to RORγt, Th17 cells may also express RORα, which is upregulated by STAT3 pathway . RORα deficient mice are still able to produce normal level of IL-17A, while RORγt deficient mice have impaired production of IL-17A. However, IL-17A production in RORγt deficient mice is dependent on the expression of RORα, and thus RORα and RORγt double deficient mice cannot produce any IL-17A [7]. In addition, IL-21 is also important cytokine for the differentiation of Th17. IL-6 promotes the production of IL-21 from Th17 cells independent of RORγt, and subsequently IL-21 upregulates the expression of RORγt through the activation of STAT3 pathway . This process forms a positive feedback in the Th17 differentiation and is thus called “amplification”. Therefore, it is necessary for the amplification of Th17 cells although IL-21 is not essential for h17 polarization. In fact, IL-21 deficient mice show a reduction in the number of Th17 cells [8]. IL-23 is also important for the Th17 pathway. However, IL-23 receptor (IL-23R) is not originally expressed on naïve T cells. TGF-β signal mediates IL-23R expression on Th17 cells, which makes them responsive to IL-23. IL-23 is essential to the maintenance of the Th17 phenotype in long-term cultures. Therefore, the effect of IL-23 on Th17 is defined as “stabilization” [8]. IL-23 is highly expressed in mucosa of human ileum, and there are many Th17 cells in the human GALT. These Th17 cells play an important role for protective immunity against intestinal pathogens. Intestinal microbiota is essential for the development of Th17, since mice in germ-free condition show decreased number of Th17 cells [9].

Regulatory T cells (Treg) play a crucial role in peripheral tolerance to prevent autoimmune disease development and chronic inflammation. Thus, Treg is one of the CD4⁺ T cell subsets that inhibit other Th cells, and this subset consists of two distinct subpopulations, naturally occurring Treg (nTreg) and induced Treg (iTreg) [10]. While nTreg is generated during T cell development in the thymus, iTreg is differentiated from naïve T cells in the peripheral tissues during an immune response.

In the thymus, nTreg is identified as CD4⁺CD25⁺ auto-reactive T cells expressing TCR specific for auto-antigens. Although CD25 (also known as IL-2Rα chain) was previously thought to be an activation marker of effector T cells, it is notably expressed on nTreg in response to auto-antigens. nTreg may also express GITR and CTLA-4, as well as Foxp3 which is thought to be the master regulator of nTreg. Genetic depletion of Foxp3 leads to various autoimmune disease and chronic intestinal inflammation similar to that of IBD. Furthermore, induction of Foxp3 to CD4⁺ T cells causes inhibition of effector T cells [10].

The other Treg subset, iTreg, may be derived from peripheral naïve T cells in the presence of TGF-β and the absence of IL-6 through the expression of Foxp3. It is believed that retinoic acid (RA) and IL-2 promote iTreg differentiation. IL-2 is especially important for the survival factor of Treg. On the other hand, IL-6 prevents iTreg differentiation and promotes Th17 differentiation instead [4–6, 10]. In this manner, IL-6 may be an important cytokine that regulates the balance between Treg and Th17. However, these findings are according to in vitro assays, and whether they can be applied in vivo is still largely unknown.

In the past, CD was originally thought to be a Th1-mediated disease, while UC was Th2-mediated. Accordingly, the pathogenesis of animal models of IBD was understood to be either Th1- or Th2-mediated intestinal inflammation. However, many studies have already demonstrated that both Th1 and Th2 conditions can exist in most of animal models (Table 14.1) as well as IBD patients (Table 14.2). Moreover, recent studies also showed plasticity of Th cells differentiating into other subsets. Also, newly defined Th subsets, such as Th9 and Th22, have been added to the mix (Fig. 14.1).

It is known that naïve T cells derived from wild type mice can be differentiated into colitogenic effector T cells after transferring to the recipients such as RAG-deficient and SCID mice. Chronic colitis in this animal model was originally thought to be induced by Th1 inflammation. Thus, it has been reported that naïve T cells from T-bet deficient mice are unable to induce colitis in the recipient RAG-deficient mice , and the overexpression of T-bet in naive T cells results in exacerbation of colitis [11]. In addition, naïve T cells from STAT4 deficient mice cause less severe colitis in the recipients [12]. On the other hand, it is known that the lack of IL10 gene results in spontaneous chronic colitis which was also thought to be Th1-mediated. In fact, administration of antagonistic antibody against IFN-γ abrogates colitis in IL-10 deficient mice as well as T cell-reconstituted RAG-deficient mice [13, 14]. However, naïve T cells from IFN-γ deficient mice may induce colitis [12]. Furthermore, IL-10 and IFN-γ double-deficient mice develop colitis equally as IL-10 deficient mice when they are infected with H. hepaticus [15]. These findings suggest that IFN-γ may not be essential for the development of colitis in these models.