A novel transcription factor, T-bet, directs Th1 lineage commitment

Control of T helper cell differentiation--in search of master genes

Naïve T helper (T(H)0) cells can differentiate into one of two distinct populations: T(H)1 and T(H)2. Each population is characterized by the expression of specific cytokines and their ability to participate in cell-mediated or humoral immune responses. Recent efforts at identifying the molecular mechanisms through which T(H)0 cells become T(H)1 or T(H)2 cells have been promising. A number of transcription factors, including GATA-3 and T-bet, have been identified that promote the differentiation of T(H)0 cells and the maintenance of the differentiated cell phenotype. Dong and Flavell review recent findings on proteins that control the fate of T(H)0 differentiation, whether by promotion or inhibition, and discuss the role of epigenesis in the differentiation process.

Dendritic cell regulation of TH1-TH2 development

Understanding the control exerted by cytokines on T helper cell subsets 1 and 2 (TH1-TH2) development has progressed to a fairly satisfying knowledge of intracellular signals and transcription factors. Less is understood about the molecular basis of TH1-TH2 development exerted by other parameters, such as how the antigen presenting cell can influence this process. Recent work suggests that dendritic cell subsets contribute significant polarizing influences on T helper differentiation, but how this comes about is less clear. In some cases known pathways may be used, as in the dendritic cell subset 1 exerting TH1 polarization by interleukin 12 (IL-12) production and STAT4 activation. In others, the effects are still in need of explanation.

The p38 mitogen-activated protein kinase is required for IL-12-induced IFN-gamma expression

IL-12 is a central immunoregulatory cytokine that promotes cell-mediated immune responses and the differentiation of naive CD4+ cells into Th1 cells. We and others have demonstrated that the Stat4 is critical for IFN-gamma production by activated T cells and Th1 cells. However, several studies have suggested that other pathways may be involved in IL-12-stimulated IFN-gamma expression. In this report we demonstrate that IL-12 activates mitogen-activated protein kinase kinase 3/6 (MKK) and p38 mitogen-activated protein kinase (MAPK), but not p44/42 (ERK) or stress-activated protein kinase/c-Jun N-terminal kinase MAPK. The activation of p38 MAPK is required for normal induction of IFN-gamma mRNA and IFN-gamma secretion by IL-12 in activated T cells and Th1 cells. Importantly, IL-12-stimulated p38 MAPK effector functions occur through a Stat4-independent mechanism and correlate with increased serine phosphorylation of activating transcription factor-2. The requirement for p38 MAPK in IL-12 function suggests that this pathway may be an important in vivo target for the anti-inflammatory actions of p38 MAPK inhibitors.

The role of antigen and IL-12 in sustaining Th1 memory cells in vivo: IL-12 is required to maintain memory/effector Th1 cells sufficient to mediate protection to an infectious parasite challenge

IL-12 plays a central role in both the induction and magnitude of a primary Th1 response. A critical question in designing vaccines for diseases requiring Th1 immunity such as Mycobacterium tuberculosis and Leishmania major is the requirements to sustain memory/effector Th1 cells in vivo. This report examines the role of IL-12 and antigen in sustaining Th1 responses sufficient for protective immunity to L. major after vaccination with LACK protein (LP) plus rIL-12 and LACK DNA. It shows that, after initial vaccination with LP plus rIL-12, supplemental boosting with either LP or rIL-12 is necessary but not sufficient to fully sustain long-term Th1 immunity. Moreover, endogenous IL-12 is also shown to be required for the induction, maintenance, and effector phase of the Th1 response after LACK DNA vaccination. Finally, IL-12 is required to sustain Th1 cells and control parasite growth in susceptible and resistant strains of mice during primary and secondary infection. Taken together, these data show that IL-12 is essential to sustain a sufficient number of memory/effector Th1 cells generated in vivo to mediate long-term protection to an intracellular pathogen.

Cutting edge: IL-12 is required for the maintenance of IFN-gamma production in T cells mediating chronic resistance to the intracellular pathogen, Toxoplasma gondii

IL-12 is required for the development of IFN-gamma-dependent resistance to intracellular pathogens but is not thought to play a major role in its maintenance. To directly assess the requirement for continuous IL-12 signaling in long-term cell-mediated immunity, recombinant cytokine was transiently administered to IL-12 p40-deficient mice during the first 2 wk of infection with the intracellular pathogen Toxoplasma gondii. As expected, these animals survived the acute phase and established chronic infections. However, 4-6 wk after IL-12 withdrawal, the mice exhibited increased brain cyst burdens and succumbed to toxoplasmic encephalitis. Reactivation was associated with a loss of T-dependent IFN-gamma production without a concomitant increase in Th2 cytokine expression. Importantly, parasite Ag-induced IFN-gamma synthesis by purified T cells from these animals could be restored by in vitro exposure to IL-12. These results argue that endogenous IL-12 is required for the long-term maintenance of IFN-gamma-dependent resistance against intracellular pathogens.

Commitment factors for T helper cells

Cytokines are dominant factors in driving the development of T helper cell subsets via specific signaling pathways. Transcription factors specific to the T helper cell lineages have also been shown to determine the commitment of Th1 and Th2 cells to the production of distinct profiles of cytokines.