Differentiation of Naive CD4+ T Cells Towards T Helper 2 Cells
An impaired differentiation of naive CD4 T cells towards Th2 cells may contribute to the chronic tissue-destructive T-cell activity in rheumatoid arthritis (RA). The differentiation of naive CD4 T cells into memory Th2 cells by IL-7 in comparison with that by IL-4 was studied in RA patients and in healthy controls. Naive CD4 T cells from peripheral blood were differentiated by CD3/CD28 costimulation in the absence of or in the presence of IL-7 and/or IL-4. The production of IFN-γ and IL-4 was measured by ELISA and by single-cell FACS analysis to indicate Th1 and Th2 cell activity. CD3/CD28 costimulation and IL-7 were early inducers of IL-4 production, but primarily stimulated IFN-γ production. In contrast, in short-term cultures exogenously added IL-4 did not prime for IL-4 production but suppressed IL-7-induced IFN-γ production. Upon long-term stimulation of naive CD4 T cells, IFN-γ production was differentially regulated by IL-7 and IL-4, but IL-4 production was increased by both IL-7 and IL-4. IL-7 and IL-4 additively induced polarization towards a Th2 phenotype. This susceptibility of naive CD4 T cells to become Th2 cells upon culture with IL-7 and IL-4 was increased in RA patients compared with that in healthy controls. These findings demonstrate that, in RA patients, differentiation of naive CD4 T cells towards a Th2 phenotype by CD3/CD28 costimulation, IL-7 and IL-4 is not impaired. The perpetuation of arthritogenic T-cell activity in RA therefore seems not to be the result of intrinsic defects of naive CD4 T cells to develop towards suppressive memory Th2 cells.

T cells and macrophages are considered to play an important role in the initiation and perpetuation of inflammatory responses in rheumatoid arthritis (RA). Stimulation of macrophages can be mediated by activated memory CD4 T cells that are abundantly present in the inflamed joints of RA patients. In this respect, many studies have focused on the balance of Th1 and Th2 cells. The Th1 subset has been defined by the specific production of IFN-γ and IL-2, and by the stimulation of cell-mediated immunity, whereas the Th2 subset specifically produces IL-4 and stimulates humoral immunity. Based on analysis of IFN-γ and IL-4 production, a dominance of Th1 cell activity over Th2 cell activity has been shown in the inflamed joints of RA patients. This imbalance of Th1/Th2 cells was shown to correlate with disease activity scores. Although IL-4 production by T cells from the peripheral blood of RA patients is increased compared with that of healthy controls, this Th2 activity seems to be insufficient to control Th1-associated inflammation in RA.

IL-4 and other suppressive cytokines that can be produced by Th2 cells (e.g. IL-10 and IL-13) suppress activity of several cell types that contribute to inflammation in the RA joints.In vitro and in vivo induction of Th2 cell activity has been associated with anti-inflammatory responses and disease suppression in RA. Induction of Th2 cell activity as well as administration of Th2 cytokines can offer protection against experimental collagen-induced arthritis. Prevention of joint destruction is shown to be the final result of such elevated Th2 activity. Together these data suggest that RA patients may benefit from therapies aimed at the regulation of the Th cell balance towards Th2 cell activity. It also implies that intrinsic defects in the responsiveness of T cells to factors that can support the generation of Th2 cell activity, in peripheral lymphoid tissues and at the inflammatory sites, could cause or contribute towards RA. The activation of naive CD4 T cells towards IL-4-producing Th2 cells has been shown to require signaling through the TCR/CD3 complex together with costimulation. Since memory cells are less dependent on such costimulation to produce IL-4, in particular the development of naive CD4 T cells towards Th2 cells may be disturbed in RA patients.

Circulating naive CD4 T cells can enter areas of primary T-cell stimulation and can interact with antigen-presenting cells. Here naive cells can differentiate into memory effector Th cells. Factors that drive the initial expression of IL-4 (as the major Th2-defining cytokine) in human naive CD4 T cells include costimulation via CD28 in concerted action with TCR engagement. It has been shown in humans and in mice that, in an autocrine way, the initial endogenous IL-4 production, or IL-4 from other sources, can stimulate the development of IL-4-producing CD4 T cells. To achieve this, naive CD3-activated T cells need to be stimulated in the presence of CD28 costimulation. This is in contrast to (human) memory CD4 T cells, which can produce IL-4 upon CD3 stimulation alone, but production is more pronounced when cultured in the presence of IL-4. IL-7, in contrast to IL-4, has recently been shown to prime human naive neonatal CD4 T cells for IL-4 production in the absence of CD28 costimulation. Thus, in humans, IL-7 produced by stromal cells in the peripheral lymphoid organs that play an important role in lymphocyte development, including the lymph nodes, the spleen and the mucosal lymphoid tissues, may be more effective (than IL-4) in driving naive CD4 T cells to a Th2 phenotype.

The shortage of suppressive Th2 activity in RA patients is assumed to be involved in the chronic inflammatory activity in these patients. This shortage may be the result of a defect in the capacity of naive CD4 T cells to respond adequately to stimuli such as TCR(CD3)/CD28 costimulation, IL-7 and IL-4, and by that to become Th2 cells. Recent studies have shown that IL-7 is produced by activated synovial fibroblasts from RA patients, and that circulating levels of IL-7 were shown to correlate with markers of disease. The present study evaluated whether circulating naive CD4 T cells displayed an impaired response to IL-7 (and IL-4), and demonstrates that such a defect is not present in RA patients. In contrast, differentiation by IL-7 and IL-4 towards Th2 cell activity was increased in RA patients.