Analysis of Granulocyte-Macrophage Colony-Stimulating Factor-Producing T Helper Cells in a Mouse Model of Contact Hypersensitivity

The granulocyte-macrophage colony-stimulating factor (GM-CSF)-producing T helper cells-the Th-GM subset-has been emerging as a distinct subset of T helper cells in human and mice and is considered to comprise "GM-CSF-expressing only" (GM-CSF⁺ IFN-γ^– IL-17A^– IL-22^–) CD4 T cells identified by single-cell RNA analysis, mass cytometry, and GM-CSF fate mapper mice^1,2,3. In 2014, Sheng et al. reported signal transducer and activator of transcription 5 (STAT5) programming of the Th-GM subset and conceptualized the "Th-GM" subset for the first time^4,5. Th-GM cells are characterized by cytokine expression of GM-CSF, IL-2, TNF-α, IL-3, CCL20, and chemokine receptors C-X-C chemokine receptor type (CXCR) 4 or CXCR6^1,2. STAT and/or the NF-κB pathway are essential for Th-GM lineage differentiation. An in vitro method was established to differentiate naïve CD4 T cells into Th-GM cells using IL-7 in the presence of TCR stimuli⁶. Meanwhile, the cytokines IL-23 and IL-1β were shown to maintain the expression and pathogenicity of Th-GM cells ex vivo^3,7.

Elevation of Th-GM cells has been associated with several autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis^2,8,9, suggesting a potential role in the pathogenesis of autoimmunity¹⁰. Accumulating evidence suggests that GM-CSF can function as an inflammatory mediator. Mice genetically overexpressing Csf2 (gene encoding GM-CSF) in CD4⁺ T cells spontaneously developed neurological deficits accompanied by the infiltration of phagocytes into the central nervous system. In a T cell-transfer colitis model, the adoptive transfer of Csf2^−/− T cells into Rag1^−/− mice significantly reduced the clinical and histopathological features of the disease. However, there are few reports of the roles of the Th-GM subset in allergic diseases, such as ACD.

ACD is among the most common inflammatory dermatological conditions with high prevalence in work and life environments^11,12. It is a type IV delayed-type hypersensitivity response mediated by an intact immune circuit that develops in two temporally segmented phases: sensitization and elicitation. Human ACD is triggered by exposure to some chemicals (haptens or metals) that lead to sensitization. During this phase, a T cell-mediated response is primed by hapten-protein complexes presented by antigen-presenting cells. Upon subsequent exposure to the same hapten, hapten-specific effector and memory T cells are reactivated and localize to the skin, a process involving the infiltration of a variety of immune cell populations. This acute inflammatory response is known as elicitation, resulting in the full development of lesions¹³. Human ACD can be studied using animal models of contact hypersensitivity (CHS)¹⁴.

The CHS model induced by a reactive, low-molecular-weight, organic hapten, 2,4-dinitrofluorobenzene (DNFB), is a commonly used murine model that has been utilized in the study of the pathology as well as potential therapeutic interventions of ACD^15,16. Thus, this T cell-dependent model could be applied to study the generation of the Th-GM subset in allergic disease. Here, we induced a murine model of CHS with DNFB, analyzed the generation of Th-GM in lesions and draining lymph nodes, and found that the Th-GM subset was mainly expanded upon reexposure to the same hapten. This suggests that the Th-GM subset could be essential for ACD development and represents a specific therapeutic target in ACD.