IL-4: role in disease and regulation of production (2024)

IL-4 plays a pivotal role in shaping the nature of immune responses. Upon activation, naive peripheral CD4⁺ T cells begin to synthesize and secrete cytokines. These cytokines serve as autocrine growth and differentiation factors and, as a consequence, naive T cells proliferate and differentiate into effector cells. Based on the pattern of the cytokines which they secrete, distinct subsets of effector helper T (Th) cells can be distinguished. Type 1 Th cells (Th1 cells) secrete IL-2, interferon-gamma (IFN-γ) and tumour necrosis factor (TNF), whereas type 2 Th cells (Th2 cells) produce IL-4, IL-5, IL-6 and IL-13. IL-4 is a 15-kD polypeptide with pleiotropic effects on many cell types. Its receptor is a heterodimer composed of an α subunit, with IL-4 binding affinity, and the common γ subunit which is also part of other cytokine receptors. In T cells, binding of IL-4 to its receptor induces proliferation and differentiation into Th2 cells [1].

The existence of polarized T cell populations was discovered in mice [2], and subsequently observed in humans [3], although some T cells can produce overlapping cytokine profiles. The distinct cytokine profiles of Th cells reflect their different functions: Th1 cells are vital for cell-mediated immunity, whereas Th2 cells provide help for B cells and promote class switching from IgM to IgG1 and IgE. The need for IL-4-dominated immune responses may reflect the protective influence of Th2 cells during infection by gastrointestinal parasites [4].

The development of successful immune responses requires a balance between Th1 and Th2 subsets and inappropriately skewed responses are associated with pathology. For example, there is an association between high IL-4 production and atopy, the tendency to produce excessive IgE in response to allergens. Allergen-specific T cells from atopic subjects preferentially develop into IL-4-producing Th2 cells [5] and their CD4⁺ cells exhibit high IL-4 production when stimulated by antigens other than allergens [6]. Moreover, IL-4-deficient mice develop attenuated airways inflammation compared with wild-type mice [7]. Inappropriately high IL-4 production also has harmful effects in the context of infectious diseases. Mice infected by the intracellular parasite Leishmania major resist fatal disease if they can mount effective cell-mediated immune responses. Inbred strains such as BALB/c, which are predisposed to high IL-4 production from Th2 cells, succumb to infection whilst other strains such as C57Bl/6, C57Bl/10 and B10.D2, which can produce an appropriate Th1 response, are resistant [8]. The importance of IL-4 is emphasized by observations that administration to BALB/c mice of anti-IL-4 antibodies results in subsequent resistance [8] and that normally resistant mice become susceptible upon transgenic over-expression of IL-4 [9].

The role of IL-4 in prevention of autoimmunity by down-regulation of Th1 cells remains less clear. Low levels of IL-4 production are found in tissues affected by organ-specific autoimmune diseases, but this may simply reflect the relative dominance of inflammatory Th1 cells (reviewed in [10]). Nonetheless, it has been postulated that IL-4 serves a regulatory function, as cases of spontaneous recovery from experimental autoimmune encephalomyelitis are associated with an expansion of Th2 cells [11] and early treatment with IL-4 is able to ameliorate subsequent development of some autoimmune diseases [12]. Significantly, however, IL-4-deficient mice do not develop spontaneous autoimmunity and adoptive transfer of Th2 cells to immunodeficient mice can cause autoimmune disease [13], suggesting other cells are principally involved in regulating autoimmunity.

Th1 and Th2 cells are not pre-programmed as separate lineages in the thymus. Rather, they develop from a common peripheral precursor T cell [14] and become committed to a developmental pathway in response to the conditions of their microenvironment. Several factors are known to influence this post-thymic Th1/Th2 cell fate decision. The most important of these is the cytokine milieu at the time of activation. Many in vivo and in vitro studies have established a clear role for IL-4 in driving Th2 differentiation and inhibiting development of Th1 cells, whilst IL-12 induces production of Th1 cells at the expense of Th2 cells (reviewed in [15]).

Whereas it is clear that IL-12 is produced by macrophages and dendritic cells [16], the origin of the initial burst of IL-4 which directs Th2 subset development remains controversial. In vivo production of IL-4 has been postulated from at least three sources: mast cells and basophils; a subpopulation of T cells bearing NK1.1 markers and naive CD4⁺ T cells. Mast cells and basophils release IL-4 from cytoplasmic granules in response to several stimuli including ligation of allergen to specific surface-bound IgE. However, this IL-4 is unlikely to be significant in the early encounter between T cell and antigen-presenting cell (APC) which occurs within lymphoid tissue. Moreover, reconstitution of IL-4-deficient mice with purified wild-type CD4⁺ cells reveals that IL-4 production does not require non-T cells [17]. NK1.1⁺ CD4⁺ T cells also produce IL-4 in response to stimulation [18] and SJL mice, deficient in NK1.1⁺ cells, exhibit deficient IgE production [19]. However, in vivo studies show that mice depleted of NK1.1⁺ cells are able to mount normal Th2 responses [20], suggesting that NK1.1⁺ cells are also not vital for IL-4 production. Indeed, it has become apparent that naive CD4⁺ cells themselves may provide the early IL-4 required for Th2 cell differentiation [21].

Naive CD4⁺ T cells produce only low levels of IL-4 upon stimulation through the T cell receptor (TCR). However, upon repeated stimulation, murine [22] naive CD4⁺ cells can produce detectable IL-4. In this issue, Bullens et al. [23] report their observations on the effect of anti-IL-4 receptor antibody on IL-4 production from CD4⁺ cells, from non-atopic donors. They detected IL-4 by sandwich ELISA following only a single-stage culture of T cells with anti-CD3 and CD80 (B7-1) or CD86 (B7-2). This was attributed to the enhanced sensitivity afforded by the addition of anti-IL-4 receptor antibody to cultures, thus blocking consumption of endogenously produced IL-4. These findings were also reproduced when T cells were stimulated with soluble antigen and alloantigen. Receptor blockade eliminates the autocrine influence of IL-4 binding to its receptor, as shown by abrogated T and B cell proliferation in the presence of anti-IL-4 receptor antibody. Therefore, the measured IL-4 may have represented the ‘early burst’ produced in response to activation of the TCR complex. Whilst this study does not allow conclusions to be drawn about the exact origin of this IL-4, as the T cells used were too heterogeneous to distinguish effects of separate CD4⁺ subpopulations, it does demonstrate the utility of this novel assay.

Besides the cytokine milieu, other factors are known to regulate the Th1/Th2 decision. These include the influence of antigen dose [24], the strength of TCR binding by antigen–MHC complex [25] and the type of APC interacting with a T cell [26]. Transcription factors also influence Th1/Th2 differentiation. The opposed effects of IL-4 and IL-12 on naive CD4⁺ T cells are mediated via different signalling pathways. Responsiveness to IL-4 requires STAT6 (signal transducer and activator of transcription 6) and STAT6-deficient mice are unable to produce Th2 cytokines [27]. In contrast, IL-12-dependent Th1 cell development is dependent on STAT4 [28]. Other transcription factors which are selectively expressed in Th2 cells in response to antigen stimulation, c-maf [29] and GATA3 [30], appear to be important in mediating Th2 cytokine gene expression, but their precise roles remain to be determined.

It is also clear that genetic background has a profound effect on Th cell differentiation. Human studies have established linkage of atopy and IgE production to numerous loci. Linkage is significant at a region of chromosome 5q, containing a cluster of cytokine genes, including the IL-4 gene itself [31]. Indeed, an abnormal regulation of the IL-4 gene may be important in some atopics. The human IL-4 promoter exists in multiple allelic forms, one of which confers high transcriptional activity, causing over-expression of the IL-4 gene [32]. However, other genes can also influence the development of atopy. Mutant forms of the IL-4 receptor gene result in increased receptor signalling following IL-4 ligation, ultimately resulting in increased IgE production and atopic asthma [33]. The genetic control of Th cell development can be most easily studied in inbred strains of mice, where differences in susceptibility to certain autoimmune and infectious diseases are correlated with Th subset differentiation. Early genetic studies of Leishmania-infected mice and TCR-transgenic mice suggested control by a single locus. However, recent more extensive linkage studies have established that Th cell differentiation is controlled by multiple gene loci [34,35]. The genes involved have yet to be identified.

In conclusion, the proper regulation of post-thymic Th1/Th2 subset differentiation is important for the generation and regulation of immune responses. It is now clear that the Th1/Th2 cell fate decision is influenced by multiple factors. The identification of these factors and the elucidation of their interaction may ultimately allow therapeutic intervention in cases of abnormal regulation.

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