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The central effector cells in the pathogenesis of atopic allergic diseases are type 2 T helper (Th2) cells, which display an aberrant cytokine profile dominated by type 2 cytokines. Initial reports from mouse studies indicated that established and committed Th2 cells are stable and unsusceptible to modulation. However, there is a growing awareness that in humans, established effector Th2 cells are more flexible and can be reverted to predominant Th1 phenotypes. In fact, the Th1-driving cytokine interleukin (IL)-12 is the crucial factor in this respect. IL-12 is mainly produced by dendritic cells (DC), which can be primed for high or low IL-12 production, depending on inflammatory and/or microbial signals they encounter during their residence in the peripheral tissues. Accordingly, both the regulation of and the priming for IL-12 production in DC form ideal targets for therapeutic intervention. The development of new therapies for atopic allergy now focuses on local IL-12-promoting substances to target both the development of new Th2 cells and the persistent population of established allergen-specific Th2 cells., (Copyright 2001 S. Karger AG, Basel)

IL-12 is a potent inducer of IFN-gamma production and drives the development of Th1 cells. Human polarized Th2 cells do not express the signaling beta2-subunit of the IL-12R and, therefore, do not signal in response to IL-12. The question was raised as to what extent the loss of the IL-12Rbeta2 chain in Th2 cells has bearing on the stability of the human Th2 phenotype. In the present report, we show that restimulation of human fully polarized Th2 cells in the presence of IL-12 primes for a shift towards Th0/Th1 phenotypes, accompanied by suppression of GATA-3 expression and induction of T-bet expression. These reversed cells are further characterized by a marked IL-12Rbeta2 chain expression and fully restored IL-12-inducible STAT4 activation. The IL-12-induced phenotypic shift proved to be stable as a subsequent restimulation in the presence of IL-4 and in the absence of IL-12 could not undo the accomplished changes. Identical results were obtained with cells from atopic patients, both with polyclonal Th2 cell lines and allergen-specific Th2 cell clones. These findings suggest the possibility of restoring IL-12 responsiveness in established Th2 cells of atopic patients by stimulation in the presence of IL-12, and that IL-12-promoting immunotherapy can be beneficial for Th2-mediated immune disorders, targeting both naive and memory effector T cells.

Optimal clearance of the various pathogen types encountered by the human body requires the selective activation of particular cellular and/or humoral immune responses. The orchestration of the types of effector responses is directed by Th cells through the production of type 1 (Th1 cell-associated) and type 2 (Th2 cell-associated) cytokines. The way in which the Th cell cytokine profile is matched to the type of invading pathogen, and why these profiles sometimes derail and lead to disease, is not well understood. Here, we will discuss the concept that antigen-presenting cells (APC) provide Th cells not only with antigen and costimulatory signals, but also with a polarizing signal (signal 3). This signal can be mediated by many APC-derived factors, but IL-12 and PGE2 seem to be of major importance. The Th2-biased responses in atopic allergy appeared to be associated with monocytes with a decreased IL-12/PGE2 ratio and, consequently, with the down-regulation of type 1 cytokine production in Th cells. As for Th cells, APC can be functionialy polarized. In vitro experiments with monocyte-derived dendritic cells (DC) showed that the presence of IFN-gamma during activation of immature DC primes for mature DC with the ability of high IL-12 production and, consequently, a Th1-driving capacity (APC1 or DC1). In contrast, PGE2 primes for a low IL-12 production ability and a Th2-driving capacity (APC2 or DC2). These findings suggest that pathogens provoke either Th1- or Th2-cell development by inducing the production of a certain pattern of inflammatory DC-polarizing mediators (e.g. IFN-gamma and PGE2) at the site of infection. The type of immune polarization will not only depend on the type of pathogen, but also varies with the type of infected tissue, i.e. that different tissues produce different mediators in response to the same pathogen. In the case of atopic allergy, this concept implies that the Th2-cell bias may be related to low levels of cross-regulatory infections, to Th1 cell-inducing pathogens, or to an aberrant function of stromal cells in peripheral tissues.

In order to investigate the impact of an inflammatory mediator PGE2 on the functions of maturing DC we used an in vitro model of DC generation from peripheral blood monocytes. Addition of PGE2 (10(-9) M-10(-6) M) to the cultures performed in the presence of GM-CSF and IL-4 did not alter the morphology nor high levels of expression of class II MHC and co-stimulatory molecules on arising DC, although at concentrations above 10(-8) M, the acquisition of CD1a was selectively prevented. Control DC and the DC maturing in the presence of PGE2 (PGE2-DC) induced a similar proliferation of naive Th cells. Control DC produced high amounts of IL-12, and only trace amounts of IL-10, whereas PGE2-DC produced no IL-12 and high levels of IL-10, when stimulated after the removal of PGE2. The deficient IL-12 production by PGE2-DC was observed after stimulation both in the absence and in the presence of IFN gamma, and was not compensated during further 48 h culture in the absence of PGE2. Compared to control DC, PGE2-DC induced development of Th cells secreting elevated amounts of IL-4 and IL-5, from naive precursors. These data indicate that elevated tissue levels of PGE2 may promote type 2 Th responses by impairing the ability of locally maturing DC to produce IL-12. Since Th2 responses mediate protection in Th1-related autoimmune disorders, the use of PGE2-DC in immunotherapy of such disorders may be considered.

IL-12 is a powerful skewer of CD4+ T cell responses toward the Th1 phenotype by inducing IFN-gamma production in naive Th cells. In the present study we addressed the question of whether IL-12 can reverse established Th2 responses into Th1/Th0 responses by inducing IFN-gamma production in memory Th2 cells. To this aim, allergen-specific CD4+ T cell clones (TCC) were generated from the peripheral blood of three atopic patients, and their cytokine profiles were analyzed. The majority of these TCC exhibited a strongly polarized Th2 cytokine profile, and the production of IFN-gamma could not be induced by exogenous IL-12. Only those TCC with low IFN-gamma levels in the absence of IL-12 responded to IL-12 by additional enhancement of IFN-gamma production. The IL-12 nonresponsiveness of the Th2 clones was further evident by the total lack of IL-12-induced phosphorylation of STAT4 (signal transducer and activator of transcription-4), a transcription factor that is typically involved in IL-12 signaling. Consequently, IL-12 also failed to induce the DNA-binding activity of STAT4-containing complexes in the nuclei of these Th2 clones. All TCC expressed equal levels of the low-affinity IL-12R beta1 subunit. Our results indicate that human allergen-specific Th cells with strongly polarized Th2 cytokine profiles do not respond to IL-12 and, therefore, cannot be induced to produce IFN-gamma. The apparent high frequency of IL-12-nonresponsive Th cells within the allergen-specific populations in atopic patients predicts a limited skewing potential of IL-12 in the case of established Th2 responses, but only affecting newly recruited naive Th cells.

Several major pathological characteristics of atopic disease are causally related to CD4+ allergen-specific type 2 T-helper (Th2) cells with an aberrant cytokine secretion profile, comprising high levels of interleukin (IL)-4 and IL-5 and low levels of interferon (IFN)-gamma. Although the cytokine secretion patterns of CD4+ T-cells may be stable, they can be modulated by physiological factors which may be expected to be present during activation of these T-cells. In this review, we will focus on two secretion products of professional antigen presenting cells (APCs) and accessory cells with opposite modulatory effects on T-cell cytokine profiles, i.e. prostaglandin E2 (PGE2) and IL-12. PGE2 favours Th2-like cytokine secretion profiles by inhibiting the production of the Th1-associated cytokines, IL-2 and IFN-gamma, and in the presence of sufficient levels of IL-2, upregulating the production of the Th2-associated cytokines, IL-4 and IL-5, IL-12, on the other hand, induces and enhances IFN-gamma secretion in activated CD4+ T-cells, thereby promoting the generation of Th1 cells. PGE2 and IL-12 act via independent mechanisms and, therefore, do not mutually interfere with their modulatory effects. These data suggest that the relative contribution of PGE2 and IL-12 to the levels of secreted Th1- and Th2-associated cytokines are determined by their concentration ratio during T-cell activation.

CD30 is a member of the TNF receptor superfamily that is commonly used as a marker for Hodgkin and Reed-Sternberg cells in Hodgkin's disease. More recently, it has been proposed that CD30 is preferentially up-regulated on Th2-type human T cells. We analyzed regulation of CD30 expression on both peripheral blood T cells and T cell clones. In short-term culture, CD30 expression could be induced on T cells by Ags that elicit Th2-type responses (Schistosoma haematobium, adult worm Ag, and Toxocaria canis, excretory/secretory Ag) and Th0-type responses (tetanus toxoid), as well as Th1-type responses (tuberculin purified protein derivative). Moreover, simultaneous measurement of membrane phenotype and cytokine production showed that CD30-expressing cells can produce IFN-gamma. Finally, within panels of randomly generated as well as Ag-specific T cell clones, CD30 expression was found on Th0-, Th2-, and Th1-type clones. We conclude that induction of CD30 on activated T cells is not related to differentiation in Th0-, Th1-, or Th2-type cells.

Prostaglandin E2 (PGE2) favors T helper type 2 (Th2)-like cytokine secretion profiles in murine and human CD4+ T cells by inhibiting the production of the Th1-associated cytokines interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) and up-regulating the production of the Th2-associated cytokines IL-4 and IL-5 in a dose-dependent way. However, the potent inhibition of IL-2 production by PGE2 seems to be in contrast with the simultaneous up-regulation of IL-4 and IL-5 production, because the induction of these cytokines requires IL-2. We, therefore, investigated to which extent the net modulatory effect of PGE2 is determined by the availability of IL-2. To this aim, we examined the effects of PGE2 on the cytokine secretion profiles of a panel of human Th0 clones upon stimulation via different activation pathways, resulting either in high or low IL-2 production. The differential modulation of Th1 and Th2 cytokines by PGE2 was observed only upon modes of stimulation resulting in high IL-2 production. When IL-2 production was low, PGE2 inhibited the secretion of all four cytokines. These different modulation patterns were directly related to the IL-2 availability, because (i) neutralizing antibody to IL-2 abrogated the up-regulatory effect of PGE2 on IL-4 and IL-5 secretion in experiments with high endogenous IL-2 levels, (ii) lack of differential cytokine modulation by PGE2 in conditions with low levels of endogenous IL-2 could be restored with exogenous IL-2, and (iii) cell viability was comparable in all conditions. These results demonstrate that the net modulatory effect of PGE2 on the cytokine secretion profile of T cells critically depends on the availability of IL-2. Since this parameter varies with the experimental conditions and the T cell population studied, this finding may explain why certain immune responses may be either up- or down-regulated by PGE2 under different conditions.