In the quarter century since the first description of T helper (Th)1 and Th2 cells, an enormous body of data has established the basic paradigm of CD4 T cell lineage heterogeneity, with each lineage having distinct molecular, cellular and functional properties.1, 2
The original Th1/Th2 dichotomy has expanded to include a minimum of four different CD4 T cell lineages: Th1, Th2, Th17, and induced T regulatory. Each lineage expresses a unique cytokine profile, which along with other expressed genes, results in its functional characteristics. Each of the first 3 lineages generates a characteristic inflammatory response; Th1, Th2 and Th17 cells respectively causing macrophage-rich, eosinophil-rich, and neutrophil-rich inflammation. The concept of neatly pigeonholed stable irreversibly differentiated lineages requires some reassessment with recent findings of plasticity among various CD4 subsets.3
However, the basic Th1/Th2 concept of functional modularity of CD4 immune responses has withstood the test of time.
Differentiation of naïve CD4 T cells into a given lineage is the product of multiple integrated signals, including T cell receptor signal strength, costimulatory and innate immune signals, and cytokine milieu.2
During T cell differentiation, key genes, for example those encoding cytokines and lineage specific transcription factors, undergo epigenetic changes, including changes in histone and transcription factor binding and DNA methylation. Specific epigenetic changes can activate or repress a given gene, ultimately resulting in the transcription of specific gene products that confer the functional properties of that lineage. For example, in Th2 cells, there is increased binding of H3K4me3
and decreased binding of H3K27me3
, respectively activating and repressing forms of histone 3, to Th2 gene promoters.4
In Th1 cells reciprocal histone binding patterns are observed. In this manner, heterogeneity at the epigenetic level could ultimately result in Th2 heterogeneity. In contrast to cytokine gene promoters, the promoters for lineage specific transcription factors, such as the Th2 specific transcription factor GATA-3, demonstrate “bivalent” markings, with both activating and repressive histone binding.4
These latter findings suggest that by maintaining their capacity to alter lineage specific transcription factor expression, differentiated T cells retain lineage plasticity.
Th2 cells were initially characterized as expressing IL-4, IL-5, and IL-13.5
Although Th2 cells can express many other cytokines, including IL-2, IL-3, IL-9, IL-10, GM-CSF, and TNF, these cytokines are also expressed by other CD4 lineages. Thus, the original three cytokines, IL-4, IL-5, and IL-13, remain the established and generally agreed upon Th2 cytokines. The three Th2 cytokine genes are located in a syntenic region in human chromosome 5q31 and mouse chromosome 11. The Il4
genes are adjacent to each other, whereas the Il5
gene is 120 kilobases telomeric of these genes and in the opposite orientation.
A number of Th2 markers have emerged in the past decade. CRTH2, the type 2 prostaglandin D2 receptor, is preferentially expressed on Th2 cells and is the most well accepted Th2 surface marker. Ligation of this receptor results in augmentation of Th2 cytokine expression and chemotaxis.6
Accordingly, CRTH2 antagonists are being examined in a number of diseases. CCR3, CCR4, and CCR8 are all preferentially expressed on Th2 cells and play a role in Th2 specific chemotaxis.7
In a mouse model of allergic asthma, CCR4 but not CCR8, was required for Th2 trafficking.8
Th2 heterogeneity in regard to chemoattractant receptor expression has not been studied. The IL-33R (IL-1 receptor like-1, IL1RL1, also known as ST2) and the IL-25R (IL-17RB) are highly expressed in activated, but not resting, Th2 cells.
Th2 cells have generally been approached as a homogeneous population; however, recent reports provide evidence for subpopulations within the Th2 lineage.9–12
Given the various experimental systems and methods used to study Th2 cells it is not surprising that different approaches provide varying evidence for Th2 heterogeneity. However, at what point do these subpopulations graduate from in vitro curiosities to immunologically robust therapeutic targets?
We propose the following criteria to establish a Th subset as a biologically relevant entity:
- Generalizability. The first level of proof requires that the Th candidate subpopulation be demonstrated in multiple disease states, model systems, and species, using several different experimental approaches.
- Disease association. Although not formal proof of function, disease association is certainly evidence that the proposed subset may have clinically relevant pathological function.
- Function. Lastly, demonstration of function either in vitro or in vivo is needed to truly demonstrate that a given Th subset may actually play a unique role in disease pathogenesis.
The following review examines the literature regarding Th2 heterogeneity using the above criteria, with an emphasis on the potential roles for Th2 subpopulations in disease pathogenesis.